Review ArticleReview on Optical and Electrical Properties ofConducting Polymers
Manisha Bajpai1 Ritu Srivastava2 Ravindra Dhar1 and R S Tiwari3
1Soft Materials Research Laboratory Centre of Material Sciences Institute of Interdisciplinary StudiesUniversity of Allahabad Allahabad 211002 India2Physics for Energy Division National Physical Laboratory (Council of Scientific and Industrial Research)Dr K S Krishnan harvesting Road New Delhi 110012 India3Department of Physics Banaras Hindu University Varanasi 221005 India
Correspondence should be addressed to Manisha Bajpai mansa83gmailcom
Received 16 March 2016 Accepted 9 May 2016
Academic Editor Andres Sotelo
Copyright copy 2016 Manisha Bajpai et alThis is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
We reviewed optical and electrical properties of conjugated polymersThe charge transportmodels to describe the hole and electrontransport mechanism are also included in the electrical properties of conjugated polymers The effect of optical and electricalproperties after doping is also indexed in this paper
1 Introduction
It is well known that conjugated polymers (CPs) are themost promising candidate among the organic semiconductorworld due to their easy processing color tenability andso forth The devices based on structure CPs are com-posed of very simple structure that is an emissive polymersandwiched between low work function cathode and highwork function anode Since most of CPs shows trap-limitedelectron transport polymer LED device is hole dominatedsystem hence causing the polymer diode degrades So tofabricate a highly efficient polymer based LED device itis necessary to achieve balanced electron as well as holetransport To achieve a balanced electron and hole transportwe have incorporated the concept of doping The concept ofdoping has been first developed for organic small molecules[1 2] Thin films of these small molecules are deposited bythermal evaporation A dopant molecule is then coevap-orated with the host from another source In this waya homogeneous distribution of the doping in the host isachieved However conjugated polymers (CPs) are depositedfrom solution method [3] Adding a dopant to the solutioneasily leads to charge transfer and aggregation already in thesolutionmaking processing of thin films from such a solution
impossible Hence in this paper we have reviewed the opticalas well as electronic properties of conducting polymers and itis also shown how the optical and electronic properties havebeen changed via charge transfer occurred from doping
2 Optical Properties of Conjugated Polymers
21 Optical Absorption Formation of Excited States In smallmolecule containing an isolated double bond a pi electroncan be promoted from the lower energy state to the highestenergy state by the absorption of photon with energy greaterthan the energy gap (Eg) between the two orbitals Howevera similar molecule containing conjugated double bonds willhave a highest occupiedmolecular orbital (HOMO) higher inenergy and a lowest unoccupied molecular orbital (LUMO)lower in energy Since the orbital interactions resulted in adecreased energy gap a lower energy photon can promote api electron from HOMO to LUMO therefore in conjugatedpolymers the energy gap Eg can be even smaller
22 Optical Emission Relaxation of Excited States When asufficiently energetic photon (ℎ]) is absorbed by a semi-conducting polymer an electron can be promoted fromHOMO to LUMO and produces an electrostatically bounded
Hindawi Publishing CorporationIndian Journal of Materials ScienceVolume 2016 Article ID 5842763 8 pageshttpdxdoiorg10115520165842763
2 Indian Journal of Materials Science
Ener
gy
kICkIC
kIC
120576
120576 120576
(0 0)(0 1)
(0 2)
kF kp
S0
= 2
= 1
= 0
S1
= 2
= 1
= 0
S2
= 2
= 1
= 0
T1
= 2
= 1
= 0
S0
= 2
= 1
= 0
Figure 1 State energy diagram of some possible photophysical process in a typical fluorescent molecule
electron-hole pair called exciton [4 5] This excited statespecies can migrate from one location to another until itrelaxes by some deactivation process One of the most usefuldeactivation processes in conjugated polymers is lumines-cence (light emission) Luminescence is classified into twocategories fluorescence and phosphorescence dependingupon the spins of the electron involved in radiative transitionsand shown in Figure 1
If the electrons in the excited states have the same spinwith electron in the corresponding ground state orbital theemission of light is called phosphorescence If the excitedelectron has the opposite spin as the electron in the corre-sponding ground orbital the emission is called fluorescencePhosphorescence involves an electronic transition from atriplet excited state with unpaired electron spin to a singletground state with paired electron spin This transition isformally forbidden by quantum mechanical selection rule itoccurs at amuch slower rate than fluorescencewhich involvesan allowed transition between a singlet excited state to asinglet ground state [5] There are many other photophysicalprocesses that occur in electronic excited states and these canbe illustrated in a state energy diagram or Jablonski diagramas shown in Figure 1 The singlet ground states are definedas S0 and first and second excited states are denoted as S1and S2 respectively The first triplet excited state is denotedas T1 Each of these electronic energy levels contains its ownvibrational energy ] = 0 1 2 and so forth Absorptionof an energetic photon typically excites an electron fromS0 to S1 or S2 Usually excited electronic rapid relax byinternal conversion to the lowest vibration level of S1 Atthis excited state (S1 ] = 0 1 2 etc) the singlet excitonexists long enough to migrate over significant distances in aconjugated polymer Eventually the excited electron returnsto its ground state followed by fluorescence Fluorescence
involves electronic transitions from the lowest vibrationallevel (] = 0) of S1 to the vibrational level (] = 0 1 2) of theelectronic ground state (S0) and these radiative transitions aredenoted as (0 0) (0 1) (0 2) and so forth
3 Electrical Properties ofConjugated Polymers
The concept of band conduction by free electrons or holesis no more applied to describe the transport processes inconjugated polymers as these are strongly disordered systemIn order to make participation in transport charge carriersneed to hop from site to site in broad energy landscapeThe hopping probability between occupied 119894 to unoccupied119895 states depends upon energy difference Δ
119894119895between the two
states 119894 and 119895 and relative distance between corresponding twostates119877
119894119895The probability of hopping transition is given by [5]
This makes it possible by electric field term Now thetemperature dependence of mobility is given by [6]
120583 = 1205830expminus(
1198790
119879)
120574
(2)
120574 represents the field enhancement factor which reflects thefact that upon increasing temperature conductivity ariseswith increase in available states
31 Hole Transport in Conjugated Polymers In order toinvestigate the hole mobility the device consists of a single-polymer layer sandwiched between two electrodes (one hole
Indian Journal of Materials Science 3
injecting contact and one electron blocking contact) In thisstudy Au is used as electron blocking contact Generallycurrent in polymer based hole only (HO) diodes is spacecharge limited (SCL) and current density is characterized byChildrsquos law [7 8]
119869 =9
81205831199011205761199031205760
1198812
1198713 (3)
where 1205760120576119903is the permittivity of the material 119871 is the film
thickness and 120583119901is zero-field hole mobility which obeys the
following temperature and field dependence the thing thatwill be discussed in the next section
32 Charge Transport Models to Describe Hole Transport inConjugated Polymers Conjugated polymeric systems are notperfect systems because of chemical defects kinks twists andfinally conjugation breaks Hence band-to-band transportis not valid so far and charge transport is governed byhopping process It is difficult to reveal the charge transportin inorganic semiconductors Up till now there is no uniformway and many models have been designed to describe themobility In most of the models the mobility is found tobe dependent on charge carrier density electric field andtemperature Few of the models related to our work aredescribed below
321 Field and Temperature Dependence of Hole MobilityInitially at lower fields hole transport in conjugated polymersis well described by SCL currents However at higher fieldscurrent density normally shows an unusual behavior dueto an increase in hole mobility This suggests that carriermobility increases with electric field and this behavior hasbeen explained accurately within a space charge limitedconduction (SCLC) model taking into account a stretchedexponential field dependence of mobility [9]
is the electric field and temperature independenthole mobility Δ
0is the zero-field activation energy 119870
119861is
Boltzmannrsquos constant119879 is the temperature of the sample and120574 is field enhancement factor Equation (4) is used to describethe charge transport in a variety of semiconducting polymers[10] however it lacks theoretical justification
322 The Gaussian Disorder Model and the Correlated Dis-order Model Bassler proposed that the energy level (LUMOand HOMO) of the polymer can be approximated using a
Gaussian distribution [11] Under this Gaussian distributionapproximation the density of states (DOS) is given by
DOSGaussian =119873
radic2120587120590DOSexp(minus 119864
2
2120590DOS2) (5)
where119873 is the total density of states 120590DOS is the width of theGaussian density of states and 119864 is measured relative to thecenter of the DOS
The GDM model describes the carrier transport as abiased random walk among the hopping sites with Gaussiandistributed random site energies GDM predicted 119879 and 119865dependence of charge carrier mobility to be [12]
where 120590 and Σ are energetic disorder and positional disorderrespectively and 119862
0is the constant However GDM model
reproduced the PF-like field dependence of mobility overonly a relatively narrow range of electric field strengths Thishas led to believing that UGDM did not model disorderbehavior in disorder molecular organic system completelyRecent calculations and simulations demonstrated that thepresence of long range energy correlation gives rise toPoole Frenkel- (PF-) like mobility over a much broaderfield range Gartstein and Conwell resolved this discrepancyof field dependence between GDM and the experimentalresults by introducing a correlation between the energiesof spatially close sites with an empirical relation [13] Thisspatial correlation of energies included in correlatedGaussiandisorder model (CGDM) can be justified to arise from longrange energy correlation from the charge dipole interactionor correlation in thermal fluctuation inmolecular geometries[12]
and Γ are the parameters of the model Γcharacterizes the geometrical disorder and 119886 is the intersitehopping distance Poplavskyy and Nelson explained thehole transport in the organic small molecule material221015840771015840-tetrakis-(NN-di-4-methoxyphenylamino)-991015840-spi-robifluorene(methoxy-spiro) [14] using GDM In the recentstudy Redecker et al also described the hole transportbehavior using GDM in blue and white polymers [15]
323 Pasveerrsquos Model Further researchers have realizedthat one another important factor is overestimated whichalso affected the carrier mobility which is carrier densityIf we ignore carrier density dependence it will lead to anunderestimation of the hopping distance and the width of
4 Indian Journal of Materials Science
the density of states in these polymers Therefore Pasveeret al proposed a density dependent mobility model incombination with electric field and temperature in the formof the extended Gaussian disorder model (EGDM) [16] Inthis model they approachedmobility dependence on electricfield and charge carrier density that are factored in field anddensity enhancement functions
120583119901(119879 119901) = 120583
0exp [1
2(2minus ) (2119901119886
3)120575
]
where 1205830=1198862]0119890
120590
120583119901(119879 119901 119864) asymp 120583
119901(119879 119901) 119891 (119879 119864)
120575 = 2ln (2 minus ) minus ln (ln 4)
2
119891 (119879 119864) = exp
044 (32minus 022)
sdot [
[
radic1 + 08 (119864119890119886
120590)
2
minus 1]
]
(8)
where = 120590119870119861119879
The attractiveness of EGDM is that it includes both thedensity and field dependence of the mobility Additionally itrequires only three (temperature independent) parametersgreatly facilitating the fitting of experimental data Severalreports are available to describe the charge transport insuch type of materials using this model [17ndash20] Zhang et aldescribed the hole transport in poly[(99-di-n-octylfluorenyl-27-diyl)-alt-(benzo[2 1 3]thiadiazol-48-diyl)] (F8BT) bymaking an Ohmic hole contact on F8BT by using thehigh work function anode MoO
3as hole injection contact
[20]
4 Electron Transport in Conjugated Polymers
In the previous section an overview on hole transport inconducting polymers has been discussed However electrontransports are investigated by characterizing an electrononly (EO) diode consisting of a polymer layer sandwichedbetween two low work function electrodes In most of thePPV derivatives Malevich observed that the electron currentshows a strong field and thickness dependence and also theelectron current is small compared to hole current that it ischaracterized by a stronger voltage and thickness dependence[7] This is the characteristic of trap-limited conductionwhere charge carriers are trapped in localized states withinthe band gap An analytical description for the trap-limitedconduction in the presence of a discrete trap level wasobtained by Lampert and Mark [9] They proposed a trap-limited conduction model where the current density has thesame dependence as the trap-free SCL current only increased
by a factor 120579 and the density of trapped electrons is largerthan the density of free electrons
119869TLC =9
81205791205761199031205761205831198812
1198713 where 120579 =
119873119888
119873119905
exp [minus119864119905
119896119861119879] (9)
where 119873119888is the effective density of states in the LUMO 119873
119905
is the trap density and 119864119905is the trap depth This model is
only valid only if the traps are not fully filled But in mostof the organic semiconductors the trap states are generallyassumed to be exponentially distributed within the forbiddenband gap as obtained byMark and HelfrichThe exponentialdistribution by traps is given by the distribution [21]
119873119905 (119864) =
119873119905
119896119879119905
exp [119864 minus 119864
119888
119896119879119905
] (10)
where 119873119905is traps density of states at energy 119864 119864
119888is energy
of LUMO band 119873119905(119864) is total density of traps 119896119879
119905is energy
characterized and 119864 minus 119864119888is energy below the LUMO level of
the polymerThe trap distribution implies 119869-119881 characteristicsin trap filled limit [22]
119869 = 1199021minus119897120583119873V (
2119897 + 1
119897 + 1)
119897+1
(119897
119897 + 1
1205761199031205760
119867119887
)
119897119881119897+1
1198712119897+1 (11)
where 119869 is the current density 119881 is the applied voltage119902 is the elementary charge and 119871 is the thickness of thematerial films 120583 is the mobility of the material and 119865(119909)is the electric field 119873V is the effective density of states119867119887is the total trap density and 120576
1199031205760is the permittivity of
the material In case of SCLC and 119897 = 119879119888119879 in case of
TCLC 119879119888is the characteristic temperature of traps 119869-119881
characteristics follow square law (1198691205721198812) at lower bias butas the bias increases the slope of 119897 + 1 in log-log plot of 119869versus 119881 curve increases from 119897 = 2 Blom and Vissenbergexplained the transport of electrons in a poly(dialkoxy-p-phenylene vinylene) (PPV) derivative (MEH-PPV) [10] Theexperimental result was supported by trap-limited electrontransport with the energy of the trapping sites described by anexponential distribution Further their group demonstratedthat Gaussian density of states (GDOS) is the characteristic ofdisordered semiconductors for the mobile carriers It reducesthe temperature dependence of the trap-limited charge trans-port The reduction was governed by the width of the GDOSand originates from the equilibrium concentrations of themobile and trapped carriers [12]
41 The Doping Concept It has been realized that the oneimportant factor that affects the carrier mobility is the carrierdensity and it is also confirmed from the previous literaturesthat carrier mobility of such class of materials is very low[9 10 12ndash21] Hence to improve the carrier mobility wehave to increase the carrier density via doping Doping inpolymers provides the free charge carriers that leads to anenhancement of optical as well as electrical properties Thedoping can be done as 119901-type or 119899-type 119901-type dopantremoves an electron from HOMO of polymer and increasesthe hole carrier density in the matrix whereas 119899-type doping
Indian Journal of Materials Science 5
LUMO
Host
HOMODopant
(a)
Dopant
Host LUMO
HOMO(b)
Figure 2 (a) Schematic representation of 119901-type doping mechanismThe molecular dopant acts as acceptor in 119901-type dopant The energeticoverlap of matrix and dopant energy levels is important (b) Schematic drawing of 119899-type doping mechanism The molecular dopant acts asdonor in 119899-type doping The energetic overlap of matrix and dopant energy levels is necessary
provides electrons to the LUMO of the polymer which leadsto increase its electron carrier density [11 23ndash25] and finallyimproves the hole and electron mobility of the concernedmaterial These free charge carriers are increased by theapplication of electric fieldThus the carrier mobility is foundto be electric field and density dependent
Concept of doping in conjugated polymer is differentfrom that of inorganic semiconductor in which elements withefficient and deficient electrons are introduced In polymerdoping process involves both oxidation and reduction pro-cesses [25ndash27]The first method involves exposing a polymerto an oxidant such as iodine or bromine or a reluctantsuch as alkali metals The second is electrochemical dopingin which a polymer-coated electrode is suspended in anelectrolyte solution The polymer is insoluble in the solutionthat contains separate counter and reference electrodesBy applying an electric potential difference between theelectrodes counter ion from the electrolyte diffuses intothe polymer in the form of electron addition (119899 doping) orremoval (119901 doping) as shown in Figure 2 We have donedoping in polymers followed by second method
411 119901-Type Doping For 119901-type doping it is necessary thatLUMO of the dopant must match HOMO of the host toincrease free carrier concentration of holes (Figure 2(a))Organic materials like F
4-TCNQ TCNQ DDQ and C60 are
possible candidates for 119901-type doping depending on the hostmaterial
412 119899-Type Doping Up till now 119899-type doping is still achallenge For 119899-type doping the HOMO of the dopant mustbe adjacent to the LUMO of the host to provide more andmore electrons (Figure 2(b))
Alkali metals organic molecules which have a high-lyingHOMO and cationic salts are best 119899-type dopants
5 Review on Doping in Conjugated Polymers
The doping of 119901-type materials into conjugated polymers hasbeen realized in terms of enhanced hole injection into matrixfollowed by the modification of the interfaces Nollau et alreported a case study of doping of a 119901-type dopant tetraflu-orotetracyanoquinodimethane (F
4-TCNQ) with conjugated
polymers of wide range of the HOMO levels [28] They haveshown that the bulk conductivity and hole current increaseby several orders of magnitude with reduced turn-on voltageby the result of doping
Zhang et al (University of Groningen Netherlands)addressed another approach to understand the effect ofdoping in organic semiconductor [29] Since conductivityof any material is the product of carrier mobility andnumber of charge carriers and if we dope the materialsconductivity rise will result in a simultaneous increase ofcarrier concentration and carrier mobility Generally thecarrier transport in semiconducting materials takes placevia hopping between the GDOS Consequently if we ignorecarrier density dependence it will lead to an underestimationof the hopping distance and the width of the density ofstates in these polymers Therefore they proposed a densitydependent mobility model in combination with electric fieldand temperature dependence
They have discussed different cases of controlled 119901-typeand 119899-type doping of poly[2-methoxy-5-(2-ethylhexyloxy)-14-phenylenevinylene] (MEH-PPV) deposited from solu-tion with tetrafluorotetracyanoquinodimethane (F
4-TCNQ)
and bis(pentamethylcyclopentadienyl)cobalt(II) (DMC) as119901- and 119899-type dopants respectively [29] They have demon-strated that by choosing suitable dopant solvents and adjust-ing the polarity of the solution aggregation can be preventedand doped films can be deposited with a controlled carrierdensity
As the electron transport in conducting polymers is char-acterized by exponential distribution of traps and resultantlyhole current is no longer equal to electron current it is foundthat the electron transport in MEH-PPV becomes similarto hole transport by deactivation of traps In this studyMEH-PPV was doped with the 119899-type dopant DMC Theyhave found a trap-free space-charge limited electron currentin MEH-PPV by filling the traps with electrons from theDMC donor For 119901-type and 119899-type doping greatly improvedcharge transport and Zhang et al showed that in MEH-PPVthe free-electron mobility is equal to the hole mobility [29]
The doping induced electrical properties have beenreported by Zhang and Blom [20] They have investigatedthe electron and hole transport in F8BT They have firststudied hole transport by resolving the injection barrier bythe use ofMoO
3as a hole injection contact Further they have
6 Indian Journal of Materials Science
studied the electron transport in F8BT that was found to betrap limited and these traps were then deactivated by 119899-typedoping of DMC
Recently electron transport studies of cesium carbon-ate (Cs
2CO3) doped tris(8-hydroxyquinolinato)aluminum
(Alq3) are reported [30]They form ohmic contact with Alq3by the use of an electron injection layer Cs
2CO3 Further they
have studied the effect of doping of Cs2CO3 this leads to
increase in conductivity as well as mobility
6 Effect of Doping on Optical andElectrical Properties
As discussed in Section 6 some of the generated free chargecarriers upon doping affect optical as well as electricalproperties In the following subsections a short description ofeffects of doping on the optical as well as electronic propertieswill be discussed
7 Doping Induced Optical Properties ExcitonQuenching by Charge Transfer Centers
Arkhipov and his group proposed a theory of quenching ofexciton in doped disordered semiconductors They proposedthat an exciton can dissociate into a geminate pair of chargecarriers if a deep trap (usually for electrons) is located nextto a molecule or segment visited by the exciton in the courseof its energy relaxation Since the spatial distribution of trapsis random and does not correlate with energies of the hostmolecules the probability for an exciton to encounter acharge transfer center is fully determined by the number ofsites visited by this excitonThey supposed that the possibilityof thermally assisted jumps of excitons to sites of higherenergies is disregarded implying that the time scale of energyrelaxation is longer than the exciton lifetime unless mostexcitons were generated within the deep tail of EDOS [31]
After every intermolecular jump an exciton can find itselfin a molecule that has a deep (electron) trap in its closeneighborhood They supposed that the density of deep elec-tron traps 119873
119886and the concentration of quenchers depends
upon the molecular configuration They have consideredin the analysis the exciton quenching in which excitonsare delocalized within conjugated molecular segments inthe polymer Most probably the deep traps are distributedhomogeneously and the probability 119908
119902of occupation for an
exciton quencher to a deep trap has been determined by thePoisson distribution as [31]
119908119902= 1 minus exp (minus1205871199032
119902119897119873119886) (12)
where 119897 is the conjugation length and 119903119902is the maximum
distance between a segment and a deep trap which still allowsfor quenching They have also supposed that the excitonwhich is already occupied by a quencher will still avoidfurther quenching The probability to be certainly quenchedat a quencher119882
119902 is given by [31]
119882119902=
120591119895
120591119902+ 120591119895
(13)
where 120591119902and 120591119895are the quenching and jump times respec-
tively To contribute to the photoluminescence an excitonmust avoid quenching during its entire lifetime Estimatingthe exciton jump time as 120591
119895= 120591119899 and using the Poisson
distribution of probabilities yield the following expressionfor the probability 120578 that an exciton is not quenched andeventually decayed radiatively [31]
They have shown that the radiative yield 120578 depends uponthe concentration of deep traps 119873
119886by using (14) together
with experimental data obtained on an alkoxy-substitutedpolyphenylenevinylene (PhPPV) doped by trinitrofluorene(TNF) [32]
71 Doping Induced Electrical Properties Doping affects theelectrical properties like carriermobility drift velocity and soforth of the organic semiconductors If we do 119901-type dopingthe electron transfers directly from the host level to thedopant molecule without the intermediate step through theshallow level Similar to the case of 119901 doping with 119899 dopingthe electrons in the donor level can drop into the electrontraps (empty defect levels) and make them inactive Hencehole and electron carrier density are increased upon dopingandmobility ratio is also changed accordingly [20 28 29 33]
The mechanism behind the enhancement of hole currentupon doping can be understood in Figure 3
As we know in most of conducting polymers the holetransport is governed by SCL Upon addition of 119901-typedoping free holes are introduced into the polymer Atlow voltages these additional free holes often termed asbackground density 119901
0 will largely outnumber the charges
that are injected from the contacts which are responsiblefor the SCLC as observed in undoped polymer Since thepositive charge of this background density 119901
0is compensated
by the negative charge of the corresponding acceptors andtherefore does not contribute to the built-up of space chargean Ohmic-like current will flow at low voltages [29]
It is also described in previous section that electrontransport in CPs is trap-limited When 119899-type doping is doneinto the polymer most of the traps are deactivated and SCLcurrent is obtained as shown in Figure 4 [33] As it is alreadydiscussed that for 119899-type doping HOMO of the dopant mustbe adjacent to the LUMO of the host hence the electroncurrent is initially trap-limited As dopant material is dopedinto polymer matrix some of the trap states present in thepolymer start filling approaching 119864tc asymp 0 eV it means most ofthe traps are filled by electron provided by donor and henceSCL current is achieved [29] A further increase in the dopantconcentration does not further enhance the electron currentsince the HOMO of dopant is not sufficient to add more andmore free electrons to the LUMO of polymer
8 Conclusions
In this paper we have presented a review on the optical andelectronic properties of CPs We have reviewed the effect
Indian Journal of Materials Science 7
Ener
gy
DOS
LUMODopant
120576t
120576F
E
(a)
Ener
gy
DOS
120576t
120576eq
120576F
(b)
Figure 3 Schematic representation of the density of states and the position of Fermi levels (a) in pristine polymer and (b) polymer dopedwith 119901-type dopant The red dashed line represents the LUMO level of dopant
DOS of traps
Ener
gy
DOS
HOMO of dopant
LUMO of host
Etc
Figure 4 Schematic presentation of energy level alignment ofGaussian DOS LUMO
of dopant on the optical as well as electronic properties ofCPs In essence this paper throws an adequate light on theoptoelectronic properties of conducting polymers to enableus to use it with a better understanding for the developmentof polymer based polymer light emitting diode (PLED)
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The author Manisha Bajpai gratefully acknowledges theUniversity Grant Commission New Delhi for the financial
assistance under Dr D S Kothari Postdoctoral FellowshipScheme (F no 4-22006(BSR)13-998(BSR))
References
[1] C K Chiang C R Fincher Jr Y W Park et al ldquoElectricalconductivity in doped polyacetylenerdquo Physical Review Lettersvol 39 no 17 pp 1098ndash1101 1977
[2] M Pfeiffer A Beyer T Fritz and K Leo ldquoControlled dop-ing of phthalocyanine layers by cosublimation with accep-tor molecules a systematic Seebeck and conductivity studyrdquoApplied Physics Letters vol 73 no 22 pp 3202ndash3204 1998
[3] W Gao and A Kahn ldquoControlled p-doping of zinc phthalo-cyanine by coevaporation with tetrafluorotetracyanoquin-odimethane a direct and inverse photoemission studyrdquoAppliedPhysics Letters vol 79 no 24 pp 4040ndash4042 2001
[4] M Yan L Rothberg B R Hsieh and R R Alfano ldquoExcitonformation and decay dynamics in electroluminescent poly-mers observed by subpicosecond stimulated emissionrdquo PhysicalReview B vol 49 no 14 pp 9419ndash9422 1994
[5] M Pollak and I Riess ldquoA percolation treatment of high-fieldhopping transportrdquo Journal of Physics C Solid State Physics vol9 no 12 article 2339 1976
[6] L Li G Meller and H Ksina ldquoTemperature and field-dependence of hopping conduction in organic semiconduc-torsrdquoMicroelectronics Journal vol 38 no 1 pp 47ndash51 2007
[7] V L Malevich ldquoOn the high-frequency electric field effect onthe two-phonon hopping transportrdquo Physica Status Solidi B vol163 no 2 pp K101ndashK105 1991
[8] H C F Martens P W M Blom and H F M Schoo ldquoCom-parative study of hole transport in poly(p-phenylene vinylene)derivativesrdquo Physical Review B vol 61 no 11 pp 7489ndash74932000
[9] M A Lampert and P Mark Current Injection in SolidsAcademic Press New York NY USA 1970
[10] P W M Blom and M C J M Vissenberg ldquoCharge transportin poly(p-phenylene vinylene) light-emitting diodesrdquoMaterials
8 Indian Journal of Materials Science
Science and Engineering R Reports vol 27 no 3-4 pp 53ndash942000
[11] D H Dunlap P E Parris and V M Kenkre ldquoCharge-dipolemodel for the universal field dependence of mobilities inmolecularly doped polymersrdquo Physical Review Letters vol 77no 3 pp 542ndash545 1996
[12] S V Novikov D H Dunlap V M Kenkre P E Parris and AV Vannikov ldquoEssential role of correlations in governing chargetransport in disordered organic materialsrdquo Physical ReviewLetters vol 81 no 20 pp 4472ndash4475 1998
[13] Yu N Gartstein and EM Conwell ldquoHigh-field hoppingmobil-ity in molecular systems with spatially correlated energeticdisorderrdquoChemical Physics Letters vol 245 no 4-5 pp 351ndash3581995
[14] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 39 no 1 article 3412003
[15] M Redecker D D C Bradley M Inbasekaran and E PWoo ldquoMobility enhancement through homogeneous nematicalignment of a liquid-crystalline polyfluorenerdquo Applied PhysicsLetters vol 74 no 10 pp 1400ndash1402 1999
[16] W F Pasveer J Cottaar C Tanase et al ldquoUnified descriptionof charge-carriermobilities in disordered semiconducting poly-mersrdquo Physical Review Letters vol 94 no 20 Article ID 2066012005
[17] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 93 no 1 pp 341ndash346 2003
[18] M A Parshin J Ollevier M Van Der Auweraer et al ldquoHoletransport in blue and white emitting polymersrdquo Journal ofApplied Physics vol 103 no 11 Article ID 113711 2008
[19] S L M Van Mensfoort and R Coehoorn ldquoDeterminationof injection barriers in organic semiconductor devices fromcapacitance measurementsrdquo Physical Review Letters vol 100no 8 Article ID 086802 2008
[20] Y Zhang and P W M Blom ldquoElectron and hole transport inpoly(fluorene-benzothiadiazole)rdquo Applied Physics Letters vol98 no 14 Article ID 143504 2011
[21] J C Blakesley H S Clubb andN C Greenham ldquoTemperature-dependent electron and hole transport in disordered semi-conducting polymers analysis of energetic disorderrdquo PhysicalReview B vol 81 no 4 Article ID 045210 2010
[22] P Mark and W Helfrich ldquoSpace-charge-limited currents inorganic crystalsrdquo Journal of Applied Physics vol 33 no 1 pp205ndash215 1962
[23] K C Kao and W Hwang Electrical Transport in SolidsPergamon Oxford UK 1981
[24] J Kido K Nagai and Y Okamoto ldquoBright organic electrolumi-nescent devices with double-layer cathoderdquo IEEE Transactionson Electron Devices vol 40 no 7 pp 1342ndash1344 1993
[25] J Kido and T Matsumoto ldquoBright organic electroluminescentdevices having a metal-doped electron-injecting layerrdquo AppliedPhysics Letters vol 73 no 20 pp 2866ndash2868 1998
[26] A G Werner F Li K Harada M Pfeiffer T Fritz and K LeoldquoPyronin B as a donor for n-type doping of organic thin filmsrdquoApplied Physics Letters vol 82 no 25 pp 4495ndash4497 2003
[27] F Li A Werner M Pfeiffer K Leo and X Liu ldquoLeuco crystalviolet as a dopant for n-doping of organic thin films of fullereneC60rdquo Journal of Physical Chemistry B vol 108 no 44 pp 17076ndash
17082 2004
[28] A Nollau M Pfeiffer T Fritz and K Leo ldquoControlled n-type doping of a molecular organic semiconductor naph-thalenetetracarboxylic dianhydride (NTCDA) doped withbis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF)rdquo Journal ofApplied Physics vol 87 no 9 pp 4340ndash4343 2000
[29] Y Zhang B de Boer and PWM Blom ldquoControllable molecu-lar doping and charge transport in solution-processed polymersemiconducting layersrdquo Advanced Functional Materials vol 19no 12 pp 1901ndash1905 2009
[30] P Tyagi R Srivastava A Kumar S Tuli and M NKamalasanan ldquoEffect of doping of cesium carbonate on electrontransport in Tris(8-hydroxyquinolinato) aluminumrdquo OrganicElectronics Physics Materials Applications vol 14 no 5 pp1391ndash1395 2013
[31] Y Zhang and P W M Blom ldquoField-assisted ionization ofmolecular doping in conjugated polymersrdquoOrganic Electronicsvol 11 no 7 pp 1261ndash1267 2010
[32] V I Arkhipov E V Emelianova and H Bassler ldquoQuenching ofexcitons in doped disordered organic semiconductorsrdquo PhysicalReview B vol 70 no 20 Article ID 205205 2004
[33] C Im J M Lupton P Schouwink S Heun H Becker andH Bassler ldquoFluorescence dynamics of phenyl-substitutedpolyphenylenevinylenendashtrinitrofluorenone blend systemsrdquoJournal of Chemical Physics vol 117 no 3 pp 1395ndash1402 2002
Figure 1 State energy diagram of some possible photophysical process in a typical fluorescent molecule
electron-hole pair called exciton [4 5] This excited statespecies can migrate from one location to another until itrelaxes by some deactivation process One of the most usefuldeactivation processes in conjugated polymers is lumines-cence (light emission) Luminescence is classified into twocategories fluorescence and phosphorescence dependingupon the spins of the electron involved in radiative transitionsand shown in Figure 1
If the electrons in the excited states have the same spinwith electron in the corresponding ground state orbital theemission of light is called phosphorescence If the excitedelectron has the opposite spin as the electron in the corre-sponding ground orbital the emission is called fluorescencePhosphorescence involves an electronic transition from atriplet excited state with unpaired electron spin to a singletground state with paired electron spin This transition isformally forbidden by quantum mechanical selection rule itoccurs at amuch slower rate than fluorescencewhich involvesan allowed transition between a singlet excited state to asinglet ground state [5] There are many other photophysicalprocesses that occur in electronic excited states and these canbe illustrated in a state energy diagram or Jablonski diagramas shown in Figure 1 The singlet ground states are definedas S0 and first and second excited states are denoted as S1and S2 respectively The first triplet excited state is denotedas T1 Each of these electronic energy levels contains its ownvibrational energy ] = 0 1 2 and so forth Absorptionof an energetic photon typically excites an electron fromS0 to S1 or S2 Usually excited electronic rapid relax byinternal conversion to the lowest vibration level of S1 Atthis excited state (S1 ] = 0 1 2 etc) the singlet excitonexists long enough to migrate over significant distances in aconjugated polymer Eventually the excited electron returnsto its ground state followed by fluorescence Fluorescence
involves electronic transitions from the lowest vibrationallevel (] = 0) of S1 to the vibrational level (] = 0 1 2) of theelectronic ground state (S0) and these radiative transitions aredenoted as (0 0) (0 1) (0 2) and so forth
3 Electrical Properties ofConjugated Polymers
The concept of band conduction by free electrons or holesis no more applied to describe the transport processes inconjugated polymers as these are strongly disordered systemIn order to make participation in transport charge carriersneed to hop from site to site in broad energy landscapeThe hopping probability between occupied 119894 to unoccupied119895 states depends upon energy difference Δ
119894119895between the two
states 119894 and 119895 and relative distance between corresponding twostates119877
119894119895The probability of hopping transition is given by [5]
This makes it possible by electric field term Now thetemperature dependence of mobility is given by [6]
120583 = 1205830expminus(
1198790
119879)
120574
(2)
120574 represents the field enhancement factor which reflects thefact that upon increasing temperature conductivity ariseswith increase in available states
31 Hole Transport in Conjugated Polymers In order toinvestigate the hole mobility the device consists of a single-polymer layer sandwiched between two electrodes (one hole
Indian Journal of Materials Science 3
injecting contact and one electron blocking contact) In thisstudy Au is used as electron blocking contact Generallycurrent in polymer based hole only (HO) diodes is spacecharge limited (SCL) and current density is characterized byChildrsquos law [7 8]
119869 =9
81205831199011205761199031205760
1198812
1198713 (3)
where 1205760120576119903is the permittivity of the material 119871 is the film
thickness and 120583119901is zero-field hole mobility which obeys the
following temperature and field dependence the thing thatwill be discussed in the next section
32 Charge Transport Models to Describe Hole Transport inConjugated Polymers Conjugated polymeric systems are notperfect systems because of chemical defects kinks twists andfinally conjugation breaks Hence band-to-band transportis not valid so far and charge transport is governed byhopping process It is difficult to reveal the charge transportin inorganic semiconductors Up till now there is no uniformway and many models have been designed to describe themobility In most of the models the mobility is found tobe dependent on charge carrier density electric field andtemperature Few of the models related to our work aredescribed below
321 Field and Temperature Dependence of Hole MobilityInitially at lower fields hole transport in conjugated polymersis well described by SCL currents However at higher fieldscurrent density normally shows an unusual behavior dueto an increase in hole mobility This suggests that carriermobility increases with electric field and this behavior hasbeen explained accurately within a space charge limitedconduction (SCLC) model taking into account a stretchedexponential field dependence of mobility [9]
is the electric field and temperature independenthole mobility Δ
0is the zero-field activation energy 119870
119861is
Boltzmannrsquos constant119879 is the temperature of the sample and120574 is field enhancement factor Equation (4) is used to describethe charge transport in a variety of semiconducting polymers[10] however it lacks theoretical justification
322 The Gaussian Disorder Model and the Correlated Dis-order Model Bassler proposed that the energy level (LUMOand HOMO) of the polymer can be approximated using a
Gaussian distribution [11] Under this Gaussian distributionapproximation the density of states (DOS) is given by
DOSGaussian =119873
radic2120587120590DOSexp(minus 119864
2
2120590DOS2) (5)
where119873 is the total density of states 120590DOS is the width of theGaussian density of states and 119864 is measured relative to thecenter of the DOS
The GDM model describes the carrier transport as abiased random walk among the hopping sites with Gaussiandistributed random site energies GDM predicted 119879 and 119865dependence of charge carrier mobility to be [12]
where 120590 and Σ are energetic disorder and positional disorderrespectively and 119862
0is the constant However GDM model
reproduced the PF-like field dependence of mobility overonly a relatively narrow range of electric field strengths Thishas led to believing that UGDM did not model disorderbehavior in disorder molecular organic system completelyRecent calculations and simulations demonstrated that thepresence of long range energy correlation gives rise toPoole Frenkel- (PF-) like mobility over a much broaderfield range Gartstein and Conwell resolved this discrepancyof field dependence between GDM and the experimentalresults by introducing a correlation between the energiesof spatially close sites with an empirical relation [13] Thisspatial correlation of energies included in correlatedGaussiandisorder model (CGDM) can be justified to arise from longrange energy correlation from the charge dipole interactionor correlation in thermal fluctuation inmolecular geometries[12]
and Γ are the parameters of the model Γcharacterizes the geometrical disorder and 119886 is the intersitehopping distance Poplavskyy and Nelson explained thehole transport in the organic small molecule material221015840771015840-tetrakis-(NN-di-4-methoxyphenylamino)-991015840-spi-robifluorene(methoxy-spiro) [14] using GDM In the recentstudy Redecker et al also described the hole transportbehavior using GDM in blue and white polymers [15]
323 Pasveerrsquos Model Further researchers have realizedthat one another important factor is overestimated whichalso affected the carrier mobility which is carrier densityIf we ignore carrier density dependence it will lead to anunderestimation of the hopping distance and the width of
4 Indian Journal of Materials Science
the density of states in these polymers Therefore Pasveeret al proposed a density dependent mobility model incombination with electric field and temperature in the formof the extended Gaussian disorder model (EGDM) [16] Inthis model they approachedmobility dependence on electricfield and charge carrier density that are factored in field anddensity enhancement functions
120583119901(119879 119901) = 120583
0exp [1
2(2minus ) (2119901119886
3)120575
]
where 1205830=1198862]0119890
120590
120583119901(119879 119901 119864) asymp 120583
119901(119879 119901) 119891 (119879 119864)
120575 = 2ln (2 minus ) minus ln (ln 4)
2
119891 (119879 119864) = exp
044 (32minus 022)
sdot [
[
radic1 + 08 (119864119890119886
120590)
2
minus 1]
]
(8)
where = 120590119870119861119879
The attractiveness of EGDM is that it includes both thedensity and field dependence of the mobility Additionally itrequires only three (temperature independent) parametersgreatly facilitating the fitting of experimental data Severalreports are available to describe the charge transport insuch type of materials using this model [17ndash20] Zhang et aldescribed the hole transport in poly[(99-di-n-octylfluorenyl-27-diyl)-alt-(benzo[2 1 3]thiadiazol-48-diyl)] (F8BT) bymaking an Ohmic hole contact on F8BT by using thehigh work function anode MoO
3as hole injection contact
[20]
4 Electron Transport in Conjugated Polymers
In the previous section an overview on hole transport inconducting polymers has been discussed However electrontransports are investigated by characterizing an electrononly (EO) diode consisting of a polymer layer sandwichedbetween two low work function electrodes In most of thePPV derivatives Malevich observed that the electron currentshows a strong field and thickness dependence and also theelectron current is small compared to hole current that it ischaracterized by a stronger voltage and thickness dependence[7] This is the characteristic of trap-limited conductionwhere charge carriers are trapped in localized states withinthe band gap An analytical description for the trap-limitedconduction in the presence of a discrete trap level wasobtained by Lampert and Mark [9] They proposed a trap-limited conduction model where the current density has thesame dependence as the trap-free SCL current only increased
by a factor 120579 and the density of trapped electrons is largerthan the density of free electrons
119869TLC =9
81205791205761199031205761205831198812
1198713 where 120579 =
119873119888
119873119905
exp [minus119864119905
119896119861119879] (9)
where 119873119888is the effective density of states in the LUMO 119873
119905
is the trap density and 119864119905is the trap depth This model is
only valid only if the traps are not fully filled But in mostof the organic semiconductors the trap states are generallyassumed to be exponentially distributed within the forbiddenband gap as obtained byMark and HelfrichThe exponentialdistribution by traps is given by the distribution [21]
119873119905 (119864) =
119873119905
119896119879119905
exp [119864 minus 119864
119888
119896119879119905
] (10)
where 119873119905is traps density of states at energy 119864 119864
119888is energy
of LUMO band 119873119905(119864) is total density of traps 119896119879
119905is energy
characterized and 119864 minus 119864119888is energy below the LUMO level of
the polymerThe trap distribution implies 119869-119881 characteristicsin trap filled limit [22]
119869 = 1199021minus119897120583119873V (
2119897 + 1
119897 + 1)
119897+1
(119897
119897 + 1
1205761199031205760
119867119887
)
119897119881119897+1
1198712119897+1 (11)
where 119869 is the current density 119881 is the applied voltage119902 is the elementary charge and 119871 is the thickness of thematerial films 120583 is the mobility of the material and 119865(119909)is the electric field 119873V is the effective density of states119867119887is the total trap density and 120576
1199031205760is the permittivity of
the material In case of SCLC and 119897 = 119879119888119879 in case of
TCLC 119879119888is the characteristic temperature of traps 119869-119881
characteristics follow square law (1198691205721198812) at lower bias butas the bias increases the slope of 119897 + 1 in log-log plot of 119869versus 119881 curve increases from 119897 = 2 Blom and Vissenbergexplained the transport of electrons in a poly(dialkoxy-p-phenylene vinylene) (PPV) derivative (MEH-PPV) [10] Theexperimental result was supported by trap-limited electrontransport with the energy of the trapping sites described by anexponential distribution Further their group demonstratedthat Gaussian density of states (GDOS) is the characteristic ofdisordered semiconductors for the mobile carriers It reducesthe temperature dependence of the trap-limited charge trans-port The reduction was governed by the width of the GDOSand originates from the equilibrium concentrations of themobile and trapped carriers [12]
41 The Doping Concept It has been realized that the oneimportant factor that affects the carrier mobility is the carrierdensity and it is also confirmed from the previous literaturesthat carrier mobility of such class of materials is very low[9 10 12ndash21] Hence to improve the carrier mobility wehave to increase the carrier density via doping Doping inpolymers provides the free charge carriers that leads to anenhancement of optical as well as electrical properties Thedoping can be done as 119901-type or 119899-type 119901-type dopantremoves an electron from HOMO of polymer and increasesthe hole carrier density in the matrix whereas 119899-type doping
Indian Journal of Materials Science 5
LUMO
Host
HOMODopant
(a)
Dopant
Host LUMO
HOMO(b)
Figure 2 (a) Schematic representation of 119901-type doping mechanismThe molecular dopant acts as acceptor in 119901-type dopant The energeticoverlap of matrix and dopant energy levels is important (b) Schematic drawing of 119899-type doping mechanism The molecular dopant acts asdonor in 119899-type doping The energetic overlap of matrix and dopant energy levels is necessary
provides electrons to the LUMO of the polymer which leadsto increase its electron carrier density [11 23ndash25] and finallyimproves the hole and electron mobility of the concernedmaterial These free charge carriers are increased by theapplication of electric fieldThus the carrier mobility is foundto be electric field and density dependent
Concept of doping in conjugated polymer is differentfrom that of inorganic semiconductor in which elements withefficient and deficient electrons are introduced In polymerdoping process involves both oxidation and reduction pro-cesses [25ndash27]The first method involves exposing a polymerto an oxidant such as iodine or bromine or a reluctantsuch as alkali metals The second is electrochemical dopingin which a polymer-coated electrode is suspended in anelectrolyte solution The polymer is insoluble in the solutionthat contains separate counter and reference electrodesBy applying an electric potential difference between theelectrodes counter ion from the electrolyte diffuses intothe polymer in the form of electron addition (119899 doping) orremoval (119901 doping) as shown in Figure 2 We have donedoping in polymers followed by second method
411 119901-Type Doping For 119901-type doping it is necessary thatLUMO of the dopant must match HOMO of the host toincrease free carrier concentration of holes (Figure 2(a))Organic materials like F
4-TCNQ TCNQ DDQ and C60 are
possible candidates for 119901-type doping depending on the hostmaterial
412 119899-Type Doping Up till now 119899-type doping is still achallenge For 119899-type doping the HOMO of the dopant mustbe adjacent to the LUMO of the host to provide more andmore electrons (Figure 2(b))
Alkali metals organic molecules which have a high-lyingHOMO and cationic salts are best 119899-type dopants
5 Review on Doping in Conjugated Polymers
The doping of 119901-type materials into conjugated polymers hasbeen realized in terms of enhanced hole injection into matrixfollowed by the modification of the interfaces Nollau et alreported a case study of doping of a 119901-type dopant tetraflu-orotetracyanoquinodimethane (F
4-TCNQ) with conjugated
polymers of wide range of the HOMO levels [28] They haveshown that the bulk conductivity and hole current increaseby several orders of magnitude with reduced turn-on voltageby the result of doping
Zhang et al (University of Groningen Netherlands)addressed another approach to understand the effect ofdoping in organic semiconductor [29] Since conductivityof any material is the product of carrier mobility andnumber of charge carriers and if we dope the materialsconductivity rise will result in a simultaneous increase ofcarrier concentration and carrier mobility Generally thecarrier transport in semiconducting materials takes placevia hopping between the GDOS Consequently if we ignorecarrier density dependence it will lead to an underestimationof the hopping distance and the width of the density ofstates in these polymers Therefore they proposed a densitydependent mobility model in combination with electric fieldand temperature dependence
They have discussed different cases of controlled 119901-typeand 119899-type doping of poly[2-methoxy-5-(2-ethylhexyloxy)-14-phenylenevinylene] (MEH-PPV) deposited from solu-tion with tetrafluorotetracyanoquinodimethane (F
4-TCNQ)
and bis(pentamethylcyclopentadienyl)cobalt(II) (DMC) as119901- and 119899-type dopants respectively [29] They have demon-strated that by choosing suitable dopant solvents and adjust-ing the polarity of the solution aggregation can be preventedand doped films can be deposited with a controlled carrierdensity
As the electron transport in conducting polymers is char-acterized by exponential distribution of traps and resultantlyhole current is no longer equal to electron current it is foundthat the electron transport in MEH-PPV becomes similarto hole transport by deactivation of traps In this studyMEH-PPV was doped with the 119899-type dopant DMC Theyhave found a trap-free space-charge limited electron currentin MEH-PPV by filling the traps with electrons from theDMC donor For 119901-type and 119899-type doping greatly improvedcharge transport and Zhang et al showed that in MEH-PPVthe free-electron mobility is equal to the hole mobility [29]
The doping induced electrical properties have beenreported by Zhang and Blom [20] They have investigatedthe electron and hole transport in F8BT They have firststudied hole transport by resolving the injection barrier bythe use ofMoO
3as a hole injection contact Further they have
6 Indian Journal of Materials Science
studied the electron transport in F8BT that was found to betrap limited and these traps were then deactivated by 119899-typedoping of DMC
Recently electron transport studies of cesium carbon-ate (Cs
2CO3) doped tris(8-hydroxyquinolinato)aluminum
(Alq3) are reported [30]They form ohmic contact with Alq3by the use of an electron injection layer Cs
2CO3 Further they
have studied the effect of doping of Cs2CO3 this leads to
increase in conductivity as well as mobility
6 Effect of Doping on Optical andElectrical Properties
As discussed in Section 6 some of the generated free chargecarriers upon doping affect optical as well as electricalproperties In the following subsections a short description ofeffects of doping on the optical as well as electronic propertieswill be discussed
7 Doping Induced Optical Properties ExcitonQuenching by Charge Transfer Centers
Arkhipov and his group proposed a theory of quenching ofexciton in doped disordered semiconductors They proposedthat an exciton can dissociate into a geminate pair of chargecarriers if a deep trap (usually for electrons) is located nextto a molecule or segment visited by the exciton in the courseof its energy relaxation Since the spatial distribution of trapsis random and does not correlate with energies of the hostmolecules the probability for an exciton to encounter acharge transfer center is fully determined by the number ofsites visited by this excitonThey supposed that the possibilityof thermally assisted jumps of excitons to sites of higherenergies is disregarded implying that the time scale of energyrelaxation is longer than the exciton lifetime unless mostexcitons were generated within the deep tail of EDOS [31]
After every intermolecular jump an exciton can find itselfin a molecule that has a deep (electron) trap in its closeneighborhood They supposed that the density of deep elec-tron traps 119873
119886and the concentration of quenchers depends
upon the molecular configuration They have consideredin the analysis the exciton quenching in which excitonsare delocalized within conjugated molecular segments inthe polymer Most probably the deep traps are distributedhomogeneously and the probability 119908
119902of occupation for an
exciton quencher to a deep trap has been determined by thePoisson distribution as [31]
119908119902= 1 minus exp (minus1205871199032
119902119897119873119886) (12)
where 119897 is the conjugation length and 119903119902is the maximum
distance between a segment and a deep trap which still allowsfor quenching They have also supposed that the excitonwhich is already occupied by a quencher will still avoidfurther quenching The probability to be certainly quenchedat a quencher119882
119902 is given by [31]
119882119902=
120591119895
120591119902+ 120591119895
(13)
where 120591119902and 120591119895are the quenching and jump times respec-
tively To contribute to the photoluminescence an excitonmust avoid quenching during its entire lifetime Estimatingthe exciton jump time as 120591
119895= 120591119899 and using the Poisson
distribution of probabilities yield the following expressionfor the probability 120578 that an exciton is not quenched andeventually decayed radiatively [31]
They have shown that the radiative yield 120578 depends uponthe concentration of deep traps 119873
119886by using (14) together
with experimental data obtained on an alkoxy-substitutedpolyphenylenevinylene (PhPPV) doped by trinitrofluorene(TNF) [32]
71 Doping Induced Electrical Properties Doping affects theelectrical properties like carriermobility drift velocity and soforth of the organic semiconductors If we do 119901-type dopingthe electron transfers directly from the host level to thedopant molecule without the intermediate step through theshallow level Similar to the case of 119901 doping with 119899 dopingthe electrons in the donor level can drop into the electrontraps (empty defect levels) and make them inactive Hencehole and electron carrier density are increased upon dopingandmobility ratio is also changed accordingly [20 28 29 33]
The mechanism behind the enhancement of hole currentupon doping can be understood in Figure 3
As we know in most of conducting polymers the holetransport is governed by SCL Upon addition of 119901-typedoping free holes are introduced into the polymer Atlow voltages these additional free holes often termed asbackground density 119901
0 will largely outnumber the charges
that are injected from the contacts which are responsiblefor the SCLC as observed in undoped polymer Since thepositive charge of this background density 119901
0is compensated
by the negative charge of the corresponding acceptors andtherefore does not contribute to the built-up of space chargean Ohmic-like current will flow at low voltages [29]
It is also described in previous section that electrontransport in CPs is trap-limited When 119899-type doping is doneinto the polymer most of the traps are deactivated and SCLcurrent is obtained as shown in Figure 4 [33] As it is alreadydiscussed that for 119899-type doping HOMO of the dopant mustbe adjacent to the LUMO of the host hence the electroncurrent is initially trap-limited As dopant material is dopedinto polymer matrix some of the trap states present in thepolymer start filling approaching 119864tc asymp 0 eV it means most ofthe traps are filled by electron provided by donor and henceSCL current is achieved [29] A further increase in the dopantconcentration does not further enhance the electron currentsince the HOMO of dopant is not sufficient to add more andmore free electrons to the LUMO of polymer
8 Conclusions
In this paper we have presented a review on the optical andelectronic properties of CPs We have reviewed the effect
Indian Journal of Materials Science 7
Ener
gy
DOS
LUMODopant
120576t
120576F
E
(a)
Ener
gy
DOS
120576t
120576eq
120576F
(b)
Figure 3 Schematic representation of the density of states and the position of Fermi levels (a) in pristine polymer and (b) polymer dopedwith 119901-type dopant The red dashed line represents the LUMO level of dopant
DOS of traps
Ener
gy
DOS
HOMO of dopant
LUMO of host
Etc
Figure 4 Schematic presentation of energy level alignment ofGaussian DOS LUMO
of dopant on the optical as well as electronic properties ofCPs In essence this paper throws an adequate light on theoptoelectronic properties of conducting polymers to enableus to use it with a better understanding for the developmentof polymer based polymer light emitting diode (PLED)
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The author Manisha Bajpai gratefully acknowledges theUniversity Grant Commission New Delhi for the financial
assistance under Dr D S Kothari Postdoctoral FellowshipScheme (F no 4-22006(BSR)13-998(BSR))
References
[1] C K Chiang C R Fincher Jr Y W Park et al ldquoElectricalconductivity in doped polyacetylenerdquo Physical Review Lettersvol 39 no 17 pp 1098ndash1101 1977
[2] M Pfeiffer A Beyer T Fritz and K Leo ldquoControlled dop-ing of phthalocyanine layers by cosublimation with accep-tor molecules a systematic Seebeck and conductivity studyrdquoApplied Physics Letters vol 73 no 22 pp 3202ndash3204 1998
[3] W Gao and A Kahn ldquoControlled p-doping of zinc phthalo-cyanine by coevaporation with tetrafluorotetracyanoquin-odimethane a direct and inverse photoemission studyrdquoAppliedPhysics Letters vol 79 no 24 pp 4040ndash4042 2001
[4] M Yan L Rothberg B R Hsieh and R R Alfano ldquoExcitonformation and decay dynamics in electroluminescent poly-mers observed by subpicosecond stimulated emissionrdquo PhysicalReview B vol 49 no 14 pp 9419ndash9422 1994
[5] M Pollak and I Riess ldquoA percolation treatment of high-fieldhopping transportrdquo Journal of Physics C Solid State Physics vol9 no 12 article 2339 1976
[6] L Li G Meller and H Ksina ldquoTemperature and field-dependence of hopping conduction in organic semiconduc-torsrdquoMicroelectronics Journal vol 38 no 1 pp 47ndash51 2007
[7] V L Malevich ldquoOn the high-frequency electric field effect onthe two-phonon hopping transportrdquo Physica Status Solidi B vol163 no 2 pp K101ndashK105 1991
[8] H C F Martens P W M Blom and H F M Schoo ldquoCom-parative study of hole transport in poly(p-phenylene vinylene)derivativesrdquo Physical Review B vol 61 no 11 pp 7489ndash74932000
[9] M A Lampert and P Mark Current Injection in SolidsAcademic Press New York NY USA 1970
[10] P W M Blom and M C J M Vissenberg ldquoCharge transportin poly(p-phenylene vinylene) light-emitting diodesrdquoMaterials
8 Indian Journal of Materials Science
Science and Engineering R Reports vol 27 no 3-4 pp 53ndash942000
[11] D H Dunlap P E Parris and V M Kenkre ldquoCharge-dipolemodel for the universal field dependence of mobilities inmolecularly doped polymersrdquo Physical Review Letters vol 77no 3 pp 542ndash545 1996
[12] S V Novikov D H Dunlap V M Kenkre P E Parris and AV Vannikov ldquoEssential role of correlations in governing chargetransport in disordered organic materialsrdquo Physical ReviewLetters vol 81 no 20 pp 4472ndash4475 1998
[13] Yu N Gartstein and EM Conwell ldquoHigh-field hoppingmobil-ity in molecular systems with spatially correlated energeticdisorderrdquoChemical Physics Letters vol 245 no 4-5 pp 351ndash3581995
[14] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 39 no 1 article 3412003
[15] M Redecker D D C Bradley M Inbasekaran and E PWoo ldquoMobility enhancement through homogeneous nematicalignment of a liquid-crystalline polyfluorenerdquo Applied PhysicsLetters vol 74 no 10 pp 1400ndash1402 1999
[16] W F Pasveer J Cottaar C Tanase et al ldquoUnified descriptionof charge-carriermobilities in disordered semiconducting poly-mersrdquo Physical Review Letters vol 94 no 20 Article ID 2066012005
[17] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 93 no 1 pp 341ndash346 2003
[18] M A Parshin J Ollevier M Van Der Auweraer et al ldquoHoletransport in blue and white emitting polymersrdquo Journal ofApplied Physics vol 103 no 11 Article ID 113711 2008
[19] S L M Van Mensfoort and R Coehoorn ldquoDeterminationof injection barriers in organic semiconductor devices fromcapacitance measurementsrdquo Physical Review Letters vol 100no 8 Article ID 086802 2008
[20] Y Zhang and P W M Blom ldquoElectron and hole transport inpoly(fluorene-benzothiadiazole)rdquo Applied Physics Letters vol98 no 14 Article ID 143504 2011
[21] J C Blakesley H S Clubb andN C Greenham ldquoTemperature-dependent electron and hole transport in disordered semi-conducting polymers analysis of energetic disorderrdquo PhysicalReview B vol 81 no 4 Article ID 045210 2010
[22] P Mark and W Helfrich ldquoSpace-charge-limited currents inorganic crystalsrdquo Journal of Applied Physics vol 33 no 1 pp205ndash215 1962
[23] K C Kao and W Hwang Electrical Transport in SolidsPergamon Oxford UK 1981
[24] J Kido K Nagai and Y Okamoto ldquoBright organic electrolumi-nescent devices with double-layer cathoderdquo IEEE Transactionson Electron Devices vol 40 no 7 pp 1342ndash1344 1993
[25] J Kido and T Matsumoto ldquoBright organic electroluminescentdevices having a metal-doped electron-injecting layerrdquo AppliedPhysics Letters vol 73 no 20 pp 2866ndash2868 1998
[26] A G Werner F Li K Harada M Pfeiffer T Fritz and K LeoldquoPyronin B as a donor for n-type doping of organic thin filmsrdquoApplied Physics Letters vol 82 no 25 pp 4495ndash4497 2003
[27] F Li A Werner M Pfeiffer K Leo and X Liu ldquoLeuco crystalviolet as a dopant for n-doping of organic thin films of fullereneC60rdquo Journal of Physical Chemistry B vol 108 no 44 pp 17076ndash
17082 2004
[28] A Nollau M Pfeiffer T Fritz and K Leo ldquoControlled n-type doping of a molecular organic semiconductor naph-thalenetetracarboxylic dianhydride (NTCDA) doped withbis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF)rdquo Journal ofApplied Physics vol 87 no 9 pp 4340ndash4343 2000
[29] Y Zhang B de Boer and PWM Blom ldquoControllable molecu-lar doping and charge transport in solution-processed polymersemiconducting layersrdquo Advanced Functional Materials vol 19no 12 pp 1901ndash1905 2009
[30] P Tyagi R Srivastava A Kumar S Tuli and M NKamalasanan ldquoEffect of doping of cesium carbonate on electrontransport in Tris(8-hydroxyquinolinato) aluminumrdquo OrganicElectronics Physics Materials Applications vol 14 no 5 pp1391ndash1395 2013
[31] Y Zhang and P W M Blom ldquoField-assisted ionization ofmolecular doping in conjugated polymersrdquoOrganic Electronicsvol 11 no 7 pp 1261ndash1267 2010
[32] V I Arkhipov E V Emelianova and H Bassler ldquoQuenching ofexcitons in doped disordered organic semiconductorsrdquo PhysicalReview B vol 70 no 20 Article ID 205205 2004
[33] C Im J M Lupton P Schouwink S Heun H Becker andH Bassler ldquoFluorescence dynamics of phenyl-substitutedpolyphenylenevinylenendashtrinitrofluorenone blend systemsrdquoJournal of Chemical Physics vol 117 no 3 pp 1395ndash1402 2002
injecting contact and one electron blocking contact) In thisstudy Au is used as electron blocking contact Generallycurrent in polymer based hole only (HO) diodes is spacecharge limited (SCL) and current density is characterized byChildrsquos law [7 8]
119869 =9
81205831199011205761199031205760
1198812
1198713 (3)
where 1205760120576119903is the permittivity of the material 119871 is the film
thickness and 120583119901is zero-field hole mobility which obeys the
following temperature and field dependence the thing thatwill be discussed in the next section
32 Charge Transport Models to Describe Hole Transport inConjugated Polymers Conjugated polymeric systems are notperfect systems because of chemical defects kinks twists andfinally conjugation breaks Hence band-to-band transportis not valid so far and charge transport is governed byhopping process It is difficult to reveal the charge transportin inorganic semiconductors Up till now there is no uniformway and many models have been designed to describe themobility In most of the models the mobility is found tobe dependent on charge carrier density electric field andtemperature Few of the models related to our work aredescribed below
321 Field and Temperature Dependence of Hole MobilityInitially at lower fields hole transport in conjugated polymersis well described by SCL currents However at higher fieldscurrent density normally shows an unusual behavior dueto an increase in hole mobility This suggests that carriermobility increases with electric field and this behavior hasbeen explained accurately within a space charge limitedconduction (SCLC) model taking into account a stretchedexponential field dependence of mobility [9]
is the electric field and temperature independenthole mobility Δ
0is the zero-field activation energy 119870
119861is
Boltzmannrsquos constant119879 is the temperature of the sample and120574 is field enhancement factor Equation (4) is used to describethe charge transport in a variety of semiconducting polymers[10] however it lacks theoretical justification
322 The Gaussian Disorder Model and the Correlated Dis-order Model Bassler proposed that the energy level (LUMOand HOMO) of the polymer can be approximated using a
Gaussian distribution [11] Under this Gaussian distributionapproximation the density of states (DOS) is given by
DOSGaussian =119873
radic2120587120590DOSexp(minus 119864
2
2120590DOS2) (5)
where119873 is the total density of states 120590DOS is the width of theGaussian density of states and 119864 is measured relative to thecenter of the DOS
The GDM model describes the carrier transport as abiased random walk among the hopping sites with Gaussiandistributed random site energies GDM predicted 119879 and 119865dependence of charge carrier mobility to be [12]
where 120590 and Σ are energetic disorder and positional disorderrespectively and 119862
0is the constant However GDM model
reproduced the PF-like field dependence of mobility overonly a relatively narrow range of electric field strengths Thishas led to believing that UGDM did not model disorderbehavior in disorder molecular organic system completelyRecent calculations and simulations demonstrated that thepresence of long range energy correlation gives rise toPoole Frenkel- (PF-) like mobility over a much broaderfield range Gartstein and Conwell resolved this discrepancyof field dependence between GDM and the experimentalresults by introducing a correlation between the energiesof spatially close sites with an empirical relation [13] Thisspatial correlation of energies included in correlatedGaussiandisorder model (CGDM) can be justified to arise from longrange energy correlation from the charge dipole interactionor correlation in thermal fluctuation inmolecular geometries[12]
and Γ are the parameters of the model Γcharacterizes the geometrical disorder and 119886 is the intersitehopping distance Poplavskyy and Nelson explained thehole transport in the organic small molecule material221015840771015840-tetrakis-(NN-di-4-methoxyphenylamino)-991015840-spi-robifluorene(methoxy-spiro) [14] using GDM In the recentstudy Redecker et al also described the hole transportbehavior using GDM in blue and white polymers [15]
323 Pasveerrsquos Model Further researchers have realizedthat one another important factor is overestimated whichalso affected the carrier mobility which is carrier densityIf we ignore carrier density dependence it will lead to anunderestimation of the hopping distance and the width of
4 Indian Journal of Materials Science
the density of states in these polymers Therefore Pasveeret al proposed a density dependent mobility model incombination with electric field and temperature in the formof the extended Gaussian disorder model (EGDM) [16] Inthis model they approachedmobility dependence on electricfield and charge carrier density that are factored in field anddensity enhancement functions
120583119901(119879 119901) = 120583
0exp [1
2(2minus ) (2119901119886
3)120575
]
where 1205830=1198862]0119890
120590
120583119901(119879 119901 119864) asymp 120583
119901(119879 119901) 119891 (119879 119864)
120575 = 2ln (2 minus ) minus ln (ln 4)
2
119891 (119879 119864) = exp
044 (32minus 022)
sdot [
[
radic1 + 08 (119864119890119886
120590)
2
minus 1]
]
(8)
where = 120590119870119861119879
The attractiveness of EGDM is that it includes both thedensity and field dependence of the mobility Additionally itrequires only three (temperature independent) parametersgreatly facilitating the fitting of experimental data Severalreports are available to describe the charge transport insuch type of materials using this model [17ndash20] Zhang et aldescribed the hole transport in poly[(99-di-n-octylfluorenyl-27-diyl)-alt-(benzo[2 1 3]thiadiazol-48-diyl)] (F8BT) bymaking an Ohmic hole contact on F8BT by using thehigh work function anode MoO
3as hole injection contact
[20]
4 Electron Transport in Conjugated Polymers
In the previous section an overview on hole transport inconducting polymers has been discussed However electrontransports are investigated by characterizing an electrononly (EO) diode consisting of a polymer layer sandwichedbetween two low work function electrodes In most of thePPV derivatives Malevich observed that the electron currentshows a strong field and thickness dependence and also theelectron current is small compared to hole current that it ischaracterized by a stronger voltage and thickness dependence[7] This is the characteristic of trap-limited conductionwhere charge carriers are trapped in localized states withinthe band gap An analytical description for the trap-limitedconduction in the presence of a discrete trap level wasobtained by Lampert and Mark [9] They proposed a trap-limited conduction model where the current density has thesame dependence as the trap-free SCL current only increased
by a factor 120579 and the density of trapped electrons is largerthan the density of free electrons
119869TLC =9
81205791205761199031205761205831198812
1198713 where 120579 =
119873119888
119873119905
exp [minus119864119905
119896119861119879] (9)
where 119873119888is the effective density of states in the LUMO 119873
119905
is the trap density and 119864119905is the trap depth This model is
only valid only if the traps are not fully filled But in mostof the organic semiconductors the trap states are generallyassumed to be exponentially distributed within the forbiddenband gap as obtained byMark and HelfrichThe exponentialdistribution by traps is given by the distribution [21]
119873119905 (119864) =
119873119905
119896119879119905
exp [119864 minus 119864
119888
119896119879119905
] (10)
where 119873119905is traps density of states at energy 119864 119864
119888is energy
of LUMO band 119873119905(119864) is total density of traps 119896119879
119905is energy
characterized and 119864 minus 119864119888is energy below the LUMO level of
the polymerThe trap distribution implies 119869-119881 characteristicsin trap filled limit [22]
119869 = 1199021minus119897120583119873V (
2119897 + 1
119897 + 1)
119897+1
(119897
119897 + 1
1205761199031205760
119867119887
)
119897119881119897+1
1198712119897+1 (11)
where 119869 is the current density 119881 is the applied voltage119902 is the elementary charge and 119871 is the thickness of thematerial films 120583 is the mobility of the material and 119865(119909)is the electric field 119873V is the effective density of states119867119887is the total trap density and 120576
1199031205760is the permittivity of
the material In case of SCLC and 119897 = 119879119888119879 in case of
TCLC 119879119888is the characteristic temperature of traps 119869-119881
characteristics follow square law (1198691205721198812) at lower bias butas the bias increases the slope of 119897 + 1 in log-log plot of 119869versus 119881 curve increases from 119897 = 2 Blom and Vissenbergexplained the transport of electrons in a poly(dialkoxy-p-phenylene vinylene) (PPV) derivative (MEH-PPV) [10] Theexperimental result was supported by trap-limited electrontransport with the energy of the trapping sites described by anexponential distribution Further their group demonstratedthat Gaussian density of states (GDOS) is the characteristic ofdisordered semiconductors for the mobile carriers It reducesthe temperature dependence of the trap-limited charge trans-port The reduction was governed by the width of the GDOSand originates from the equilibrium concentrations of themobile and trapped carriers [12]
41 The Doping Concept It has been realized that the oneimportant factor that affects the carrier mobility is the carrierdensity and it is also confirmed from the previous literaturesthat carrier mobility of such class of materials is very low[9 10 12ndash21] Hence to improve the carrier mobility wehave to increase the carrier density via doping Doping inpolymers provides the free charge carriers that leads to anenhancement of optical as well as electrical properties Thedoping can be done as 119901-type or 119899-type 119901-type dopantremoves an electron from HOMO of polymer and increasesthe hole carrier density in the matrix whereas 119899-type doping
Indian Journal of Materials Science 5
LUMO
Host
HOMODopant
(a)
Dopant
Host LUMO
HOMO(b)
Figure 2 (a) Schematic representation of 119901-type doping mechanismThe molecular dopant acts as acceptor in 119901-type dopant The energeticoverlap of matrix and dopant energy levels is important (b) Schematic drawing of 119899-type doping mechanism The molecular dopant acts asdonor in 119899-type doping The energetic overlap of matrix and dopant energy levels is necessary
provides electrons to the LUMO of the polymer which leadsto increase its electron carrier density [11 23ndash25] and finallyimproves the hole and electron mobility of the concernedmaterial These free charge carriers are increased by theapplication of electric fieldThus the carrier mobility is foundto be electric field and density dependent
Concept of doping in conjugated polymer is differentfrom that of inorganic semiconductor in which elements withefficient and deficient electrons are introduced In polymerdoping process involves both oxidation and reduction pro-cesses [25ndash27]The first method involves exposing a polymerto an oxidant such as iodine or bromine or a reluctantsuch as alkali metals The second is electrochemical dopingin which a polymer-coated electrode is suspended in anelectrolyte solution The polymer is insoluble in the solutionthat contains separate counter and reference electrodesBy applying an electric potential difference between theelectrodes counter ion from the electrolyte diffuses intothe polymer in the form of electron addition (119899 doping) orremoval (119901 doping) as shown in Figure 2 We have donedoping in polymers followed by second method
411 119901-Type Doping For 119901-type doping it is necessary thatLUMO of the dopant must match HOMO of the host toincrease free carrier concentration of holes (Figure 2(a))Organic materials like F
4-TCNQ TCNQ DDQ and C60 are
possible candidates for 119901-type doping depending on the hostmaterial
412 119899-Type Doping Up till now 119899-type doping is still achallenge For 119899-type doping the HOMO of the dopant mustbe adjacent to the LUMO of the host to provide more andmore electrons (Figure 2(b))
Alkali metals organic molecules which have a high-lyingHOMO and cationic salts are best 119899-type dopants
5 Review on Doping in Conjugated Polymers
The doping of 119901-type materials into conjugated polymers hasbeen realized in terms of enhanced hole injection into matrixfollowed by the modification of the interfaces Nollau et alreported a case study of doping of a 119901-type dopant tetraflu-orotetracyanoquinodimethane (F
4-TCNQ) with conjugated
polymers of wide range of the HOMO levels [28] They haveshown that the bulk conductivity and hole current increaseby several orders of magnitude with reduced turn-on voltageby the result of doping
Zhang et al (University of Groningen Netherlands)addressed another approach to understand the effect ofdoping in organic semiconductor [29] Since conductivityof any material is the product of carrier mobility andnumber of charge carriers and if we dope the materialsconductivity rise will result in a simultaneous increase ofcarrier concentration and carrier mobility Generally thecarrier transport in semiconducting materials takes placevia hopping between the GDOS Consequently if we ignorecarrier density dependence it will lead to an underestimationof the hopping distance and the width of the density ofstates in these polymers Therefore they proposed a densitydependent mobility model in combination with electric fieldand temperature dependence
They have discussed different cases of controlled 119901-typeand 119899-type doping of poly[2-methoxy-5-(2-ethylhexyloxy)-14-phenylenevinylene] (MEH-PPV) deposited from solu-tion with tetrafluorotetracyanoquinodimethane (F
4-TCNQ)
and bis(pentamethylcyclopentadienyl)cobalt(II) (DMC) as119901- and 119899-type dopants respectively [29] They have demon-strated that by choosing suitable dopant solvents and adjust-ing the polarity of the solution aggregation can be preventedand doped films can be deposited with a controlled carrierdensity
As the electron transport in conducting polymers is char-acterized by exponential distribution of traps and resultantlyhole current is no longer equal to electron current it is foundthat the electron transport in MEH-PPV becomes similarto hole transport by deactivation of traps In this studyMEH-PPV was doped with the 119899-type dopant DMC Theyhave found a trap-free space-charge limited electron currentin MEH-PPV by filling the traps with electrons from theDMC donor For 119901-type and 119899-type doping greatly improvedcharge transport and Zhang et al showed that in MEH-PPVthe free-electron mobility is equal to the hole mobility [29]
The doping induced electrical properties have beenreported by Zhang and Blom [20] They have investigatedthe electron and hole transport in F8BT They have firststudied hole transport by resolving the injection barrier bythe use ofMoO
3as a hole injection contact Further they have
6 Indian Journal of Materials Science
studied the electron transport in F8BT that was found to betrap limited and these traps were then deactivated by 119899-typedoping of DMC
Recently electron transport studies of cesium carbon-ate (Cs
2CO3) doped tris(8-hydroxyquinolinato)aluminum
(Alq3) are reported [30]They form ohmic contact with Alq3by the use of an electron injection layer Cs
2CO3 Further they
have studied the effect of doping of Cs2CO3 this leads to
increase in conductivity as well as mobility
6 Effect of Doping on Optical andElectrical Properties
As discussed in Section 6 some of the generated free chargecarriers upon doping affect optical as well as electricalproperties In the following subsections a short description ofeffects of doping on the optical as well as electronic propertieswill be discussed
7 Doping Induced Optical Properties ExcitonQuenching by Charge Transfer Centers
Arkhipov and his group proposed a theory of quenching ofexciton in doped disordered semiconductors They proposedthat an exciton can dissociate into a geminate pair of chargecarriers if a deep trap (usually for electrons) is located nextto a molecule or segment visited by the exciton in the courseof its energy relaxation Since the spatial distribution of trapsis random and does not correlate with energies of the hostmolecules the probability for an exciton to encounter acharge transfer center is fully determined by the number ofsites visited by this excitonThey supposed that the possibilityof thermally assisted jumps of excitons to sites of higherenergies is disregarded implying that the time scale of energyrelaxation is longer than the exciton lifetime unless mostexcitons were generated within the deep tail of EDOS [31]
After every intermolecular jump an exciton can find itselfin a molecule that has a deep (electron) trap in its closeneighborhood They supposed that the density of deep elec-tron traps 119873
119886and the concentration of quenchers depends
upon the molecular configuration They have consideredin the analysis the exciton quenching in which excitonsare delocalized within conjugated molecular segments inthe polymer Most probably the deep traps are distributedhomogeneously and the probability 119908
119902of occupation for an
exciton quencher to a deep trap has been determined by thePoisson distribution as [31]
119908119902= 1 minus exp (minus1205871199032
119902119897119873119886) (12)
where 119897 is the conjugation length and 119903119902is the maximum
distance between a segment and a deep trap which still allowsfor quenching They have also supposed that the excitonwhich is already occupied by a quencher will still avoidfurther quenching The probability to be certainly quenchedat a quencher119882
119902 is given by [31]
119882119902=
120591119895
120591119902+ 120591119895
(13)
where 120591119902and 120591119895are the quenching and jump times respec-
tively To contribute to the photoluminescence an excitonmust avoid quenching during its entire lifetime Estimatingthe exciton jump time as 120591
119895= 120591119899 and using the Poisson
distribution of probabilities yield the following expressionfor the probability 120578 that an exciton is not quenched andeventually decayed radiatively [31]
They have shown that the radiative yield 120578 depends uponthe concentration of deep traps 119873
119886by using (14) together
with experimental data obtained on an alkoxy-substitutedpolyphenylenevinylene (PhPPV) doped by trinitrofluorene(TNF) [32]
71 Doping Induced Electrical Properties Doping affects theelectrical properties like carriermobility drift velocity and soforth of the organic semiconductors If we do 119901-type dopingthe electron transfers directly from the host level to thedopant molecule without the intermediate step through theshallow level Similar to the case of 119901 doping with 119899 dopingthe electrons in the donor level can drop into the electrontraps (empty defect levels) and make them inactive Hencehole and electron carrier density are increased upon dopingandmobility ratio is also changed accordingly [20 28 29 33]
The mechanism behind the enhancement of hole currentupon doping can be understood in Figure 3
As we know in most of conducting polymers the holetransport is governed by SCL Upon addition of 119901-typedoping free holes are introduced into the polymer Atlow voltages these additional free holes often termed asbackground density 119901
0 will largely outnumber the charges
that are injected from the contacts which are responsiblefor the SCLC as observed in undoped polymer Since thepositive charge of this background density 119901
0is compensated
by the negative charge of the corresponding acceptors andtherefore does not contribute to the built-up of space chargean Ohmic-like current will flow at low voltages [29]
It is also described in previous section that electrontransport in CPs is trap-limited When 119899-type doping is doneinto the polymer most of the traps are deactivated and SCLcurrent is obtained as shown in Figure 4 [33] As it is alreadydiscussed that for 119899-type doping HOMO of the dopant mustbe adjacent to the LUMO of the host hence the electroncurrent is initially trap-limited As dopant material is dopedinto polymer matrix some of the trap states present in thepolymer start filling approaching 119864tc asymp 0 eV it means most ofthe traps are filled by electron provided by donor and henceSCL current is achieved [29] A further increase in the dopantconcentration does not further enhance the electron currentsince the HOMO of dopant is not sufficient to add more andmore free electrons to the LUMO of polymer
8 Conclusions
In this paper we have presented a review on the optical andelectronic properties of CPs We have reviewed the effect
Indian Journal of Materials Science 7
Ener
gy
DOS
LUMODopant
120576t
120576F
E
(a)
Ener
gy
DOS
120576t
120576eq
120576F
(b)
Figure 3 Schematic representation of the density of states and the position of Fermi levels (a) in pristine polymer and (b) polymer dopedwith 119901-type dopant The red dashed line represents the LUMO level of dopant
DOS of traps
Ener
gy
DOS
HOMO of dopant
LUMO of host
Etc
Figure 4 Schematic presentation of energy level alignment ofGaussian DOS LUMO
of dopant on the optical as well as electronic properties ofCPs In essence this paper throws an adequate light on theoptoelectronic properties of conducting polymers to enableus to use it with a better understanding for the developmentof polymer based polymer light emitting diode (PLED)
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The author Manisha Bajpai gratefully acknowledges theUniversity Grant Commission New Delhi for the financial
assistance under Dr D S Kothari Postdoctoral FellowshipScheme (F no 4-22006(BSR)13-998(BSR))
References
[1] C K Chiang C R Fincher Jr Y W Park et al ldquoElectricalconductivity in doped polyacetylenerdquo Physical Review Lettersvol 39 no 17 pp 1098ndash1101 1977
[2] M Pfeiffer A Beyer T Fritz and K Leo ldquoControlled dop-ing of phthalocyanine layers by cosublimation with accep-tor molecules a systematic Seebeck and conductivity studyrdquoApplied Physics Letters vol 73 no 22 pp 3202ndash3204 1998
[3] W Gao and A Kahn ldquoControlled p-doping of zinc phthalo-cyanine by coevaporation with tetrafluorotetracyanoquin-odimethane a direct and inverse photoemission studyrdquoAppliedPhysics Letters vol 79 no 24 pp 4040ndash4042 2001
[4] M Yan L Rothberg B R Hsieh and R R Alfano ldquoExcitonformation and decay dynamics in electroluminescent poly-mers observed by subpicosecond stimulated emissionrdquo PhysicalReview B vol 49 no 14 pp 9419ndash9422 1994
[5] M Pollak and I Riess ldquoA percolation treatment of high-fieldhopping transportrdquo Journal of Physics C Solid State Physics vol9 no 12 article 2339 1976
[6] L Li G Meller and H Ksina ldquoTemperature and field-dependence of hopping conduction in organic semiconduc-torsrdquoMicroelectronics Journal vol 38 no 1 pp 47ndash51 2007
[7] V L Malevich ldquoOn the high-frequency electric field effect onthe two-phonon hopping transportrdquo Physica Status Solidi B vol163 no 2 pp K101ndashK105 1991
[8] H C F Martens P W M Blom and H F M Schoo ldquoCom-parative study of hole transport in poly(p-phenylene vinylene)derivativesrdquo Physical Review B vol 61 no 11 pp 7489ndash74932000
[9] M A Lampert and P Mark Current Injection in SolidsAcademic Press New York NY USA 1970
[10] P W M Blom and M C J M Vissenberg ldquoCharge transportin poly(p-phenylene vinylene) light-emitting diodesrdquoMaterials
8 Indian Journal of Materials Science
Science and Engineering R Reports vol 27 no 3-4 pp 53ndash942000
[11] D H Dunlap P E Parris and V M Kenkre ldquoCharge-dipolemodel for the universal field dependence of mobilities inmolecularly doped polymersrdquo Physical Review Letters vol 77no 3 pp 542ndash545 1996
[12] S V Novikov D H Dunlap V M Kenkre P E Parris and AV Vannikov ldquoEssential role of correlations in governing chargetransport in disordered organic materialsrdquo Physical ReviewLetters vol 81 no 20 pp 4472ndash4475 1998
[13] Yu N Gartstein and EM Conwell ldquoHigh-field hoppingmobil-ity in molecular systems with spatially correlated energeticdisorderrdquoChemical Physics Letters vol 245 no 4-5 pp 351ndash3581995
[14] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 39 no 1 article 3412003
[15] M Redecker D D C Bradley M Inbasekaran and E PWoo ldquoMobility enhancement through homogeneous nematicalignment of a liquid-crystalline polyfluorenerdquo Applied PhysicsLetters vol 74 no 10 pp 1400ndash1402 1999
[16] W F Pasveer J Cottaar C Tanase et al ldquoUnified descriptionof charge-carriermobilities in disordered semiconducting poly-mersrdquo Physical Review Letters vol 94 no 20 Article ID 2066012005
[17] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 93 no 1 pp 341ndash346 2003
[18] M A Parshin J Ollevier M Van Der Auweraer et al ldquoHoletransport in blue and white emitting polymersrdquo Journal ofApplied Physics vol 103 no 11 Article ID 113711 2008
[19] S L M Van Mensfoort and R Coehoorn ldquoDeterminationof injection barriers in organic semiconductor devices fromcapacitance measurementsrdquo Physical Review Letters vol 100no 8 Article ID 086802 2008
[20] Y Zhang and P W M Blom ldquoElectron and hole transport inpoly(fluorene-benzothiadiazole)rdquo Applied Physics Letters vol98 no 14 Article ID 143504 2011
[21] J C Blakesley H S Clubb andN C Greenham ldquoTemperature-dependent electron and hole transport in disordered semi-conducting polymers analysis of energetic disorderrdquo PhysicalReview B vol 81 no 4 Article ID 045210 2010
[22] P Mark and W Helfrich ldquoSpace-charge-limited currents inorganic crystalsrdquo Journal of Applied Physics vol 33 no 1 pp205ndash215 1962
[23] K C Kao and W Hwang Electrical Transport in SolidsPergamon Oxford UK 1981
[24] J Kido K Nagai and Y Okamoto ldquoBright organic electrolumi-nescent devices with double-layer cathoderdquo IEEE Transactionson Electron Devices vol 40 no 7 pp 1342ndash1344 1993
[25] J Kido and T Matsumoto ldquoBright organic electroluminescentdevices having a metal-doped electron-injecting layerrdquo AppliedPhysics Letters vol 73 no 20 pp 2866ndash2868 1998
[26] A G Werner F Li K Harada M Pfeiffer T Fritz and K LeoldquoPyronin B as a donor for n-type doping of organic thin filmsrdquoApplied Physics Letters vol 82 no 25 pp 4495ndash4497 2003
[27] F Li A Werner M Pfeiffer K Leo and X Liu ldquoLeuco crystalviolet as a dopant for n-doping of organic thin films of fullereneC60rdquo Journal of Physical Chemistry B vol 108 no 44 pp 17076ndash
17082 2004
[28] A Nollau M Pfeiffer T Fritz and K Leo ldquoControlled n-type doping of a molecular organic semiconductor naph-thalenetetracarboxylic dianhydride (NTCDA) doped withbis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF)rdquo Journal ofApplied Physics vol 87 no 9 pp 4340ndash4343 2000
[29] Y Zhang B de Boer and PWM Blom ldquoControllable molecu-lar doping and charge transport in solution-processed polymersemiconducting layersrdquo Advanced Functional Materials vol 19no 12 pp 1901ndash1905 2009
[30] P Tyagi R Srivastava A Kumar S Tuli and M NKamalasanan ldquoEffect of doping of cesium carbonate on electrontransport in Tris(8-hydroxyquinolinato) aluminumrdquo OrganicElectronics Physics Materials Applications vol 14 no 5 pp1391ndash1395 2013
[31] Y Zhang and P W M Blom ldquoField-assisted ionization ofmolecular doping in conjugated polymersrdquoOrganic Electronicsvol 11 no 7 pp 1261ndash1267 2010
[32] V I Arkhipov E V Emelianova and H Bassler ldquoQuenching ofexcitons in doped disordered organic semiconductorsrdquo PhysicalReview B vol 70 no 20 Article ID 205205 2004
[33] C Im J M Lupton P Schouwink S Heun H Becker andH Bassler ldquoFluorescence dynamics of phenyl-substitutedpolyphenylenevinylenendashtrinitrofluorenone blend systemsrdquoJournal of Chemical Physics vol 117 no 3 pp 1395ndash1402 2002
the density of states in these polymers Therefore Pasveeret al proposed a density dependent mobility model incombination with electric field and temperature in the formof the extended Gaussian disorder model (EGDM) [16] Inthis model they approachedmobility dependence on electricfield and charge carrier density that are factored in field anddensity enhancement functions
120583119901(119879 119901) = 120583
0exp [1
2(2minus ) (2119901119886
3)120575
]
where 1205830=1198862]0119890
120590
120583119901(119879 119901 119864) asymp 120583
119901(119879 119901) 119891 (119879 119864)
120575 = 2ln (2 minus ) minus ln (ln 4)
2
119891 (119879 119864) = exp
044 (32minus 022)
sdot [
[
radic1 + 08 (119864119890119886
120590)
2
minus 1]
]
(8)
where = 120590119870119861119879
The attractiveness of EGDM is that it includes both thedensity and field dependence of the mobility Additionally itrequires only three (temperature independent) parametersgreatly facilitating the fitting of experimental data Severalreports are available to describe the charge transport insuch type of materials using this model [17ndash20] Zhang et aldescribed the hole transport in poly[(99-di-n-octylfluorenyl-27-diyl)-alt-(benzo[2 1 3]thiadiazol-48-diyl)] (F8BT) bymaking an Ohmic hole contact on F8BT by using thehigh work function anode MoO
3as hole injection contact
[20]
4 Electron Transport in Conjugated Polymers
In the previous section an overview on hole transport inconducting polymers has been discussed However electrontransports are investigated by characterizing an electrononly (EO) diode consisting of a polymer layer sandwichedbetween two low work function electrodes In most of thePPV derivatives Malevich observed that the electron currentshows a strong field and thickness dependence and also theelectron current is small compared to hole current that it ischaracterized by a stronger voltage and thickness dependence[7] This is the characteristic of trap-limited conductionwhere charge carriers are trapped in localized states withinthe band gap An analytical description for the trap-limitedconduction in the presence of a discrete trap level wasobtained by Lampert and Mark [9] They proposed a trap-limited conduction model where the current density has thesame dependence as the trap-free SCL current only increased
by a factor 120579 and the density of trapped electrons is largerthan the density of free electrons
119869TLC =9
81205791205761199031205761205831198812
1198713 where 120579 =
119873119888
119873119905
exp [minus119864119905
119896119861119879] (9)
where 119873119888is the effective density of states in the LUMO 119873
119905
is the trap density and 119864119905is the trap depth This model is
only valid only if the traps are not fully filled But in mostof the organic semiconductors the trap states are generallyassumed to be exponentially distributed within the forbiddenband gap as obtained byMark and HelfrichThe exponentialdistribution by traps is given by the distribution [21]
119873119905 (119864) =
119873119905
119896119879119905
exp [119864 minus 119864
119888
119896119879119905
] (10)
where 119873119905is traps density of states at energy 119864 119864
119888is energy
of LUMO band 119873119905(119864) is total density of traps 119896119879
119905is energy
characterized and 119864 minus 119864119888is energy below the LUMO level of
the polymerThe trap distribution implies 119869-119881 characteristicsin trap filled limit [22]
119869 = 1199021minus119897120583119873V (
2119897 + 1
119897 + 1)
119897+1
(119897
119897 + 1
1205761199031205760
119867119887
)
119897119881119897+1
1198712119897+1 (11)
where 119869 is the current density 119881 is the applied voltage119902 is the elementary charge and 119871 is the thickness of thematerial films 120583 is the mobility of the material and 119865(119909)is the electric field 119873V is the effective density of states119867119887is the total trap density and 120576
1199031205760is the permittivity of
the material In case of SCLC and 119897 = 119879119888119879 in case of
TCLC 119879119888is the characteristic temperature of traps 119869-119881
characteristics follow square law (1198691205721198812) at lower bias butas the bias increases the slope of 119897 + 1 in log-log plot of 119869versus 119881 curve increases from 119897 = 2 Blom and Vissenbergexplained the transport of electrons in a poly(dialkoxy-p-phenylene vinylene) (PPV) derivative (MEH-PPV) [10] Theexperimental result was supported by trap-limited electrontransport with the energy of the trapping sites described by anexponential distribution Further their group demonstratedthat Gaussian density of states (GDOS) is the characteristic ofdisordered semiconductors for the mobile carriers It reducesthe temperature dependence of the trap-limited charge trans-port The reduction was governed by the width of the GDOSand originates from the equilibrium concentrations of themobile and trapped carriers [12]
41 The Doping Concept It has been realized that the oneimportant factor that affects the carrier mobility is the carrierdensity and it is also confirmed from the previous literaturesthat carrier mobility of such class of materials is very low[9 10 12ndash21] Hence to improve the carrier mobility wehave to increase the carrier density via doping Doping inpolymers provides the free charge carriers that leads to anenhancement of optical as well as electrical properties Thedoping can be done as 119901-type or 119899-type 119901-type dopantremoves an electron from HOMO of polymer and increasesthe hole carrier density in the matrix whereas 119899-type doping
Indian Journal of Materials Science 5
LUMO
Host
HOMODopant
(a)
Dopant
Host LUMO
HOMO(b)
Figure 2 (a) Schematic representation of 119901-type doping mechanismThe molecular dopant acts as acceptor in 119901-type dopant The energeticoverlap of matrix and dopant energy levels is important (b) Schematic drawing of 119899-type doping mechanism The molecular dopant acts asdonor in 119899-type doping The energetic overlap of matrix and dopant energy levels is necessary
provides electrons to the LUMO of the polymer which leadsto increase its electron carrier density [11 23ndash25] and finallyimproves the hole and electron mobility of the concernedmaterial These free charge carriers are increased by theapplication of electric fieldThus the carrier mobility is foundto be electric field and density dependent
Concept of doping in conjugated polymer is differentfrom that of inorganic semiconductor in which elements withefficient and deficient electrons are introduced In polymerdoping process involves both oxidation and reduction pro-cesses [25ndash27]The first method involves exposing a polymerto an oxidant such as iodine or bromine or a reluctantsuch as alkali metals The second is electrochemical dopingin which a polymer-coated electrode is suspended in anelectrolyte solution The polymer is insoluble in the solutionthat contains separate counter and reference electrodesBy applying an electric potential difference between theelectrodes counter ion from the electrolyte diffuses intothe polymer in the form of electron addition (119899 doping) orremoval (119901 doping) as shown in Figure 2 We have donedoping in polymers followed by second method
411 119901-Type Doping For 119901-type doping it is necessary thatLUMO of the dopant must match HOMO of the host toincrease free carrier concentration of holes (Figure 2(a))Organic materials like F
4-TCNQ TCNQ DDQ and C60 are
possible candidates for 119901-type doping depending on the hostmaterial
412 119899-Type Doping Up till now 119899-type doping is still achallenge For 119899-type doping the HOMO of the dopant mustbe adjacent to the LUMO of the host to provide more andmore electrons (Figure 2(b))
Alkali metals organic molecules which have a high-lyingHOMO and cationic salts are best 119899-type dopants
5 Review on Doping in Conjugated Polymers
The doping of 119901-type materials into conjugated polymers hasbeen realized in terms of enhanced hole injection into matrixfollowed by the modification of the interfaces Nollau et alreported a case study of doping of a 119901-type dopant tetraflu-orotetracyanoquinodimethane (F
4-TCNQ) with conjugated
polymers of wide range of the HOMO levels [28] They haveshown that the bulk conductivity and hole current increaseby several orders of magnitude with reduced turn-on voltageby the result of doping
Zhang et al (University of Groningen Netherlands)addressed another approach to understand the effect ofdoping in organic semiconductor [29] Since conductivityof any material is the product of carrier mobility andnumber of charge carriers and if we dope the materialsconductivity rise will result in a simultaneous increase ofcarrier concentration and carrier mobility Generally thecarrier transport in semiconducting materials takes placevia hopping between the GDOS Consequently if we ignorecarrier density dependence it will lead to an underestimationof the hopping distance and the width of the density ofstates in these polymers Therefore they proposed a densitydependent mobility model in combination with electric fieldand temperature dependence
They have discussed different cases of controlled 119901-typeand 119899-type doping of poly[2-methoxy-5-(2-ethylhexyloxy)-14-phenylenevinylene] (MEH-PPV) deposited from solu-tion with tetrafluorotetracyanoquinodimethane (F
4-TCNQ)
and bis(pentamethylcyclopentadienyl)cobalt(II) (DMC) as119901- and 119899-type dopants respectively [29] They have demon-strated that by choosing suitable dopant solvents and adjust-ing the polarity of the solution aggregation can be preventedand doped films can be deposited with a controlled carrierdensity
As the electron transport in conducting polymers is char-acterized by exponential distribution of traps and resultantlyhole current is no longer equal to electron current it is foundthat the electron transport in MEH-PPV becomes similarto hole transport by deactivation of traps In this studyMEH-PPV was doped with the 119899-type dopant DMC Theyhave found a trap-free space-charge limited electron currentin MEH-PPV by filling the traps with electrons from theDMC donor For 119901-type and 119899-type doping greatly improvedcharge transport and Zhang et al showed that in MEH-PPVthe free-electron mobility is equal to the hole mobility [29]
The doping induced electrical properties have beenreported by Zhang and Blom [20] They have investigatedthe electron and hole transport in F8BT They have firststudied hole transport by resolving the injection barrier bythe use ofMoO
3as a hole injection contact Further they have
6 Indian Journal of Materials Science
studied the electron transport in F8BT that was found to betrap limited and these traps were then deactivated by 119899-typedoping of DMC
Recently electron transport studies of cesium carbon-ate (Cs
2CO3) doped tris(8-hydroxyquinolinato)aluminum
(Alq3) are reported [30]They form ohmic contact with Alq3by the use of an electron injection layer Cs
2CO3 Further they
have studied the effect of doping of Cs2CO3 this leads to
increase in conductivity as well as mobility
6 Effect of Doping on Optical andElectrical Properties
As discussed in Section 6 some of the generated free chargecarriers upon doping affect optical as well as electricalproperties In the following subsections a short description ofeffects of doping on the optical as well as electronic propertieswill be discussed
7 Doping Induced Optical Properties ExcitonQuenching by Charge Transfer Centers
Arkhipov and his group proposed a theory of quenching ofexciton in doped disordered semiconductors They proposedthat an exciton can dissociate into a geminate pair of chargecarriers if a deep trap (usually for electrons) is located nextto a molecule or segment visited by the exciton in the courseof its energy relaxation Since the spatial distribution of trapsis random and does not correlate with energies of the hostmolecules the probability for an exciton to encounter acharge transfer center is fully determined by the number ofsites visited by this excitonThey supposed that the possibilityof thermally assisted jumps of excitons to sites of higherenergies is disregarded implying that the time scale of energyrelaxation is longer than the exciton lifetime unless mostexcitons were generated within the deep tail of EDOS [31]
After every intermolecular jump an exciton can find itselfin a molecule that has a deep (electron) trap in its closeneighborhood They supposed that the density of deep elec-tron traps 119873
119886and the concentration of quenchers depends
upon the molecular configuration They have consideredin the analysis the exciton quenching in which excitonsare delocalized within conjugated molecular segments inthe polymer Most probably the deep traps are distributedhomogeneously and the probability 119908
119902of occupation for an
exciton quencher to a deep trap has been determined by thePoisson distribution as [31]
119908119902= 1 minus exp (minus1205871199032
119902119897119873119886) (12)
where 119897 is the conjugation length and 119903119902is the maximum
distance between a segment and a deep trap which still allowsfor quenching They have also supposed that the excitonwhich is already occupied by a quencher will still avoidfurther quenching The probability to be certainly quenchedat a quencher119882
119902 is given by [31]
119882119902=
120591119895
120591119902+ 120591119895
(13)
where 120591119902and 120591119895are the quenching and jump times respec-
tively To contribute to the photoluminescence an excitonmust avoid quenching during its entire lifetime Estimatingthe exciton jump time as 120591
119895= 120591119899 and using the Poisson
distribution of probabilities yield the following expressionfor the probability 120578 that an exciton is not quenched andeventually decayed radiatively [31]
They have shown that the radiative yield 120578 depends uponthe concentration of deep traps 119873
119886by using (14) together
with experimental data obtained on an alkoxy-substitutedpolyphenylenevinylene (PhPPV) doped by trinitrofluorene(TNF) [32]
71 Doping Induced Electrical Properties Doping affects theelectrical properties like carriermobility drift velocity and soforth of the organic semiconductors If we do 119901-type dopingthe electron transfers directly from the host level to thedopant molecule without the intermediate step through theshallow level Similar to the case of 119901 doping with 119899 dopingthe electrons in the donor level can drop into the electrontraps (empty defect levels) and make them inactive Hencehole and electron carrier density are increased upon dopingandmobility ratio is also changed accordingly [20 28 29 33]
The mechanism behind the enhancement of hole currentupon doping can be understood in Figure 3
As we know in most of conducting polymers the holetransport is governed by SCL Upon addition of 119901-typedoping free holes are introduced into the polymer Atlow voltages these additional free holes often termed asbackground density 119901
0 will largely outnumber the charges
that are injected from the contacts which are responsiblefor the SCLC as observed in undoped polymer Since thepositive charge of this background density 119901
0is compensated
by the negative charge of the corresponding acceptors andtherefore does not contribute to the built-up of space chargean Ohmic-like current will flow at low voltages [29]
It is also described in previous section that electrontransport in CPs is trap-limited When 119899-type doping is doneinto the polymer most of the traps are deactivated and SCLcurrent is obtained as shown in Figure 4 [33] As it is alreadydiscussed that for 119899-type doping HOMO of the dopant mustbe adjacent to the LUMO of the host hence the electroncurrent is initially trap-limited As dopant material is dopedinto polymer matrix some of the trap states present in thepolymer start filling approaching 119864tc asymp 0 eV it means most ofthe traps are filled by electron provided by donor and henceSCL current is achieved [29] A further increase in the dopantconcentration does not further enhance the electron currentsince the HOMO of dopant is not sufficient to add more andmore free electrons to the LUMO of polymer
8 Conclusions
In this paper we have presented a review on the optical andelectronic properties of CPs We have reviewed the effect
Indian Journal of Materials Science 7
Ener
gy
DOS
LUMODopant
120576t
120576F
E
(a)
Ener
gy
DOS
120576t
120576eq
120576F
(b)
Figure 3 Schematic representation of the density of states and the position of Fermi levels (a) in pristine polymer and (b) polymer dopedwith 119901-type dopant The red dashed line represents the LUMO level of dopant
DOS of traps
Ener
gy
DOS
HOMO of dopant
LUMO of host
Etc
Figure 4 Schematic presentation of energy level alignment ofGaussian DOS LUMO
of dopant on the optical as well as electronic properties ofCPs In essence this paper throws an adequate light on theoptoelectronic properties of conducting polymers to enableus to use it with a better understanding for the developmentof polymer based polymer light emitting diode (PLED)
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The author Manisha Bajpai gratefully acknowledges theUniversity Grant Commission New Delhi for the financial
assistance under Dr D S Kothari Postdoctoral FellowshipScheme (F no 4-22006(BSR)13-998(BSR))
References
[1] C K Chiang C R Fincher Jr Y W Park et al ldquoElectricalconductivity in doped polyacetylenerdquo Physical Review Lettersvol 39 no 17 pp 1098ndash1101 1977
[2] M Pfeiffer A Beyer T Fritz and K Leo ldquoControlled dop-ing of phthalocyanine layers by cosublimation with accep-tor molecules a systematic Seebeck and conductivity studyrdquoApplied Physics Letters vol 73 no 22 pp 3202ndash3204 1998
[3] W Gao and A Kahn ldquoControlled p-doping of zinc phthalo-cyanine by coevaporation with tetrafluorotetracyanoquin-odimethane a direct and inverse photoemission studyrdquoAppliedPhysics Letters vol 79 no 24 pp 4040ndash4042 2001
[4] M Yan L Rothberg B R Hsieh and R R Alfano ldquoExcitonformation and decay dynamics in electroluminescent poly-mers observed by subpicosecond stimulated emissionrdquo PhysicalReview B vol 49 no 14 pp 9419ndash9422 1994
[5] M Pollak and I Riess ldquoA percolation treatment of high-fieldhopping transportrdquo Journal of Physics C Solid State Physics vol9 no 12 article 2339 1976
[6] L Li G Meller and H Ksina ldquoTemperature and field-dependence of hopping conduction in organic semiconduc-torsrdquoMicroelectronics Journal vol 38 no 1 pp 47ndash51 2007
[7] V L Malevich ldquoOn the high-frequency electric field effect onthe two-phonon hopping transportrdquo Physica Status Solidi B vol163 no 2 pp K101ndashK105 1991
[8] H C F Martens P W M Blom and H F M Schoo ldquoCom-parative study of hole transport in poly(p-phenylene vinylene)derivativesrdquo Physical Review B vol 61 no 11 pp 7489ndash74932000
[9] M A Lampert and P Mark Current Injection in SolidsAcademic Press New York NY USA 1970
[10] P W M Blom and M C J M Vissenberg ldquoCharge transportin poly(p-phenylene vinylene) light-emitting diodesrdquoMaterials
8 Indian Journal of Materials Science
Science and Engineering R Reports vol 27 no 3-4 pp 53ndash942000
[11] D H Dunlap P E Parris and V M Kenkre ldquoCharge-dipolemodel for the universal field dependence of mobilities inmolecularly doped polymersrdquo Physical Review Letters vol 77no 3 pp 542ndash545 1996
[12] S V Novikov D H Dunlap V M Kenkre P E Parris and AV Vannikov ldquoEssential role of correlations in governing chargetransport in disordered organic materialsrdquo Physical ReviewLetters vol 81 no 20 pp 4472ndash4475 1998
[13] Yu N Gartstein and EM Conwell ldquoHigh-field hoppingmobil-ity in molecular systems with spatially correlated energeticdisorderrdquoChemical Physics Letters vol 245 no 4-5 pp 351ndash3581995
[14] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 39 no 1 article 3412003
[15] M Redecker D D C Bradley M Inbasekaran and E PWoo ldquoMobility enhancement through homogeneous nematicalignment of a liquid-crystalline polyfluorenerdquo Applied PhysicsLetters vol 74 no 10 pp 1400ndash1402 1999
[16] W F Pasveer J Cottaar C Tanase et al ldquoUnified descriptionof charge-carriermobilities in disordered semiconducting poly-mersrdquo Physical Review Letters vol 94 no 20 Article ID 2066012005
[17] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 93 no 1 pp 341ndash346 2003
[18] M A Parshin J Ollevier M Van Der Auweraer et al ldquoHoletransport in blue and white emitting polymersrdquo Journal ofApplied Physics vol 103 no 11 Article ID 113711 2008
[19] S L M Van Mensfoort and R Coehoorn ldquoDeterminationof injection barriers in organic semiconductor devices fromcapacitance measurementsrdquo Physical Review Letters vol 100no 8 Article ID 086802 2008
[20] Y Zhang and P W M Blom ldquoElectron and hole transport inpoly(fluorene-benzothiadiazole)rdquo Applied Physics Letters vol98 no 14 Article ID 143504 2011
[21] J C Blakesley H S Clubb andN C Greenham ldquoTemperature-dependent electron and hole transport in disordered semi-conducting polymers analysis of energetic disorderrdquo PhysicalReview B vol 81 no 4 Article ID 045210 2010
[22] P Mark and W Helfrich ldquoSpace-charge-limited currents inorganic crystalsrdquo Journal of Applied Physics vol 33 no 1 pp205ndash215 1962
[23] K C Kao and W Hwang Electrical Transport in SolidsPergamon Oxford UK 1981
[24] J Kido K Nagai and Y Okamoto ldquoBright organic electrolumi-nescent devices with double-layer cathoderdquo IEEE Transactionson Electron Devices vol 40 no 7 pp 1342ndash1344 1993
[25] J Kido and T Matsumoto ldquoBright organic electroluminescentdevices having a metal-doped electron-injecting layerrdquo AppliedPhysics Letters vol 73 no 20 pp 2866ndash2868 1998
[26] A G Werner F Li K Harada M Pfeiffer T Fritz and K LeoldquoPyronin B as a donor for n-type doping of organic thin filmsrdquoApplied Physics Letters vol 82 no 25 pp 4495ndash4497 2003
[27] F Li A Werner M Pfeiffer K Leo and X Liu ldquoLeuco crystalviolet as a dopant for n-doping of organic thin films of fullereneC60rdquo Journal of Physical Chemistry B vol 108 no 44 pp 17076ndash
17082 2004
[28] A Nollau M Pfeiffer T Fritz and K Leo ldquoControlled n-type doping of a molecular organic semiconductor naph-thalenetetracarboxylic dianhydride (NTCDA) doped withbis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF)rdquo Journal ofApplied Physics vol 87 no 9 pp 4340ndash4343 2000
[29] Y Zhang B de Boer and PWM Blom ldquoControllable molecu-lar doping and charge transport in solution-processed polymersemiconducting layersrdquo Advanced Functional Materials vol 19no 12 pp 1901ndash1905 2009
[30] P Tyagi R Srivastava A Kumar S Tuli and M NKamalasanan ldquoEffect of doping of cesium carbonate on electrontransport in Tris(8-hydroxyquinolinato) aluminumrdquo OrganicElectronics Physics Materials Applications vol 14 no 5 pp1391ndash1395 2013
[31] Y Zhang and P W M Blom ldquoField-assisted ionization ofmolecular doping in conjugated polymersrdquoOrganic Electronicsvol 11 no 7 pp 1261ndash1267 2010
[32] V I Arkhipov E V Emelianova and H Bassler ldquoQuenching ofexcitons in doped disordered organic semiconductorsrdquo PhysicalReview B vol 70 no 20 Article ID 205205 2004
[33] C Im J M Lupton P Schouwink S Heun H Becker andH Bassler ldquoFluorescence dynamics of phenyl-substitutedpolyphenylenevinylenendashtrinitrofluorenone blend systemsrdquoJournal of Chemical Physics vol 117 no 3 pp 1395ndash1402 2002
Figure 2 (a) Schematic representation of 119901-type doping mechanismThe molecular dopant acts as acceptor in 119901-type dopant The energeticoverlap of matrix and dopant energy levels is important (b) Schematic drawing of 119899-type doping mechanism The molecular dopant acts asdonor in 119899-type doping The energetic overlap of matrix and dopant energy levels is necessary
provides electrons to the LUMO of the polymer which leadsto increase its electron carrier density [11 23ndash25] and finallyimproves the hole and electron mobility of the concernedmaterial These free charge carriers are increased by theapplication of electric fieldThus the carrier mobility is foundto be electric field and density dependent
Concept of doping in conjugated polymer is differentfrom that of inorganic semiconductor in which elements withefficient and deficient electrons are introduced In polymerdoping process involves both oxidation and reduction pro-cesses [25ndash27]The first method involves exposing a polymerto an oxidant such as iodine or bromine or a reluctantsuch as alkali metals The second is electrochemical dopingin which a polymer-coated electrode is suspended in anelectrolyte solution The polymer is insoluble in the solutionthat contains separate counter and reference electrodesBy applying an electric potential difference between theelectrodes counter ion from the electrolyte diffuses intothe polymer in the form of electron addition (119899 doping) orremoval (119901 doping) as shown in Figure 2 We have donedoping in polymers followed by second method
411 119901-Type Doping For 119901-type doping it is necessary thatLUMO of the dopant must match HOMO of the host toincrease free carrier concentration of holes (Figure 2(a))Organic materials like F
4-TCNQ TCNQ DDQ and C60 are
possible candidates for 119901-type doping depending on the hostmaterial
412 119899-Type Doping Up till now 119899-type doping is still achallenge For 119899-type doping the HOMO of the dopant mustbe adjacent to the LUMO of the host to provide more andmore electrons (Figure 2(b))
Alkali metals organic molecules which have a high-lyingHOMO and cationic salts are best 119899-type dopants
5 Review on Doping in Conjugated Polymers
The doping of 119901-type materials into conjugated polymers hasbeen realized in terms of enhanced hole injection into matrixfollowed by the modification of the interfaces Nollau et alreported a case study of doping of a 119901-type dopant tetraflu-orotetracyanoquinodimethane (F
4-TCNQ) with conjugated
polymers of wide range of the HOMO levels [28] They haveshown that the bulk conductivity and hole current increaseby several orders of magnitude with reduced turn-on voltageby the result of doping
Zhang et al (University of Groningen Netherlands)addressed another approach to understand the effect ofdoping in organic semiconductor [29] Since conductivityof any material is the product of carrier mobility andnumber of charge carriers and if we dope the materialsconductivity rise will result in a simultaneous increase ofcarrier concentration and carrier mobility Generally thecarrier transport in semiconducting materials takes placevia hopping between the GDOS Consequently if we ignorecarrier density dependence it will lead to an underestimationof the hopping distance and the width of the density ofstates in these polymers Therefore they proposed a densitydependent mobility model in combination with electric fieldand temperature dependence
They have discussed different cases of controlled 119901-typeand 119899-type doping of poly[2-methoxy-5-(2-ethylhexyloxy)-14-phenylenevinylene] (MEH-PPV) deposited from solu-tion with tetrafluorotetracyanoquinodimethane (F
4-TCNQ)
and bis(pentamethylcyclopentadienyl)cobalt(II) (DMC) as119901- and 119899-type dopants respectively [29] They have demon-strated that by choosing suitable dopant solvents and adjust-ing the polarity of the solution aggregation can be preventedand doped films can be deposited with a controlled carrierdensity
As the electron transport in conducting polymers is char-acterized by exponential distribution of traps and resultantlyhole current is no longer equal to electron current it is foundthat the electron transport in MEH-PPV becomes similarto hole transport by deactivation of traps In this studyMEH-PPV was doped with the 119899-type dopant DMC Theyhave found a trap-free space-charge limited electron currentin MEH-PPV by filling the traps with electrons from theDMC donor For 119901-type and 119899-type doping greatly improvedcharge transport and Zhang et al showed that in MEH-PPVthe free-electron mobility is equal to the hole mobility [29]
The doping induced electrical properties have beenreported by Zhang and Blom [20] They have investigatedthe electron and hole transport in F8BT They have firststudied hole transport by resolving the injection barrier bythe use ofMoO
3as a hole injection contact Further they have
6 Indian Journal of Materials Science
studied the electron transport in F8BT that was found to betrap limited and these traps were then deactivated by 119899-typedoping of DMC
Recently electron transport studies of cesium carbon-ate (Cs
2CO3) doped tris(8-hydroxyquinolinato)aluminum
(Alq3) are reported [30]They form ohmic contact with Alq3by the use of an electron injection layer Cs
2CO3 Further they
have studied the effect of doping of Cs2CO3 this leads to
increase in conductivity as well as mobility
6 Effect of Doping on Optical andElectrical Properties
As discussed in Section 6 some of the generated free chargecarriers upon doping affect optical as well as electricalproperties In the following subsections a short description ofeffects of doping on the optical as well as electronic propertieswill be discussed
7 Doping Induced Optical Properties ExcitonQuenching by Charge Transfer Centers
Arkhipov and his group proposed a theory of quenching ofexciton in doped disordered semiconductors They proposedthat an exciton can dissociate into a geminate pair of chargecarriers if a deep trap (usually for electrons) is located nextto a molecule or segment visited by the exciton in the courseof its energy relaxation Since the spatial distribution of trapsis random and does not correlate with energies of the hostmolecules the probability for an exciton to encounter acharge transfer center is fully determined by the number ofsites visited by this excitonThey supposed that the possibilityof thermally assisted jumps of excitons to sites of higherenergies is disregarded implying that the time scale of energyrelaxation is longer than the exciton lifetime unless mostexcitons were generated within the deep tail of EDOS [31]
After every intermolecular jump an exciton can find itselfin a molecule that has a deep (electron) trap in its closeneighborhood They supposed that the density of deep elec-tron traps 119873
119886and the concentration of quenchers depends
upon the molecular configuration They have consideredin the analysis the exciton quenching in which excitonsare delocalized within conjugated molecular segments inthe polymer Most probably the deep traps are distributedhomogeneously and the probability 119908
119902of occupation for an
exciton quencher to a deep trap has been determined by thePoisson distribution as [31]
119908119902= 1 minus exp (minus1205871199032
119902119897119873119886) (12)
where 119897 is the conjugation length and 119903119902is the maximum
distance between a segment and a deep trap which still allowsfor quenching They have also supposed that the excitonwhich is already occupied by a quencher will still avoidfurther quenching The probability to be certainly quenchedat a quencher119882
119902 is given by [31]
119882119902=
120591119895
120591119902+ 120591119895
(13)
where 120591119902and 120591119895are the quenching and jump times respec-
tively To contribute to the photoluminescence an excitonmust avoid quenching during its entire lifetime Estimatingthe exciton jump time as 120591
119895= 120591119899 and using the Poisson
distribution of probabilities yield the following expressionfor the probability 120578 that an exciton is not quenched andeventually decayed radiatively [31]
They have shown that the radiative yield 120578 depends uponthe concentration of deep traps 119873
119886by using (14) together
with experimental data obtained on an alkoxy-substitutedpolyphenylenevinylene (PhPPV) doped by trinitrofluorene(TNF) [32]
71 Doping Induced Electrical Properties Doping affects theelectrical properties like carriermobility drift velocity and soforth of the organic semiconductors If we do 119901-type dopingthe electron transfers directly from the host level to thedopant molecule without the intermediate step through theshallow level Similar to the case of 119901 doping with 119899 dopingthe electrons in the donor level can drop into the electrontraps (empty defect levels) and make them inactive Hencehole and electron carrier density are increased upon dopingandmobility ratio is also changed accordingly [20 28 29 33]
The mechanism behind the enhancement of hole currentupon doping can be understood in Figure 3
As we know in most of conducting polymers the holetransport is governed by SCL Upon addition of 119901-typedoping free holes are introduced into the polymer Atlow voltages these additional free holes often termed asbackground density 119901
0 will largely outnumber the charges
that are injected from the contacts which are responsiblefor the SCLC as observed in undoped polymer Since thepositive charge of this background density 119901
0is compensated
by the negative charge of the corresponding acceptors andtherefore does not contribute to the built-up of space chargean Ohmic-like current will flow at low voltages [29]
It is also described in previous section that electrontransport in CPs is trap-limited When 119899-type doping is doneinto the polymer most of the traps are deactivated and SCLcurrent is obtained as shown in Figure 4 [33] As it is alreadydiscussed that for 119899-type doping HOMO of the dopant mustbe adjacent to the LUMO of the host hence the electroncurrent is initially trap-limited As dopant material is dopedinto polymer matrix some of the trap states present in thepolymer start filling approaching 119864tc asymp 0 eV it means most ofthe traps are filled by electron provided by donor and henceSCL current is achieved [29] A further increase in the dopantconcentration does not further enhance the electron currentsince the HOMO of dopant is not sufficient to add more andmore free electrons to the LUMO of polymer
8 Conclusions
In this paper we have presented a review on the optical andelectronic properties of CPs We have reviewed the effect
Indian Journal of Materials Science 7
Ener
gy
DOS
LUMODopant
120576t
120576F
E
(a)
Ener
gy
DOS
120576t
120576eq
120576F
(b)
Figure 3 Schematic representation of the density of states and the position of Fermi levels (a) in pristine polymer and (b) polymer dopedwith 119901-type dopant The red dashed line represents the LUMO level of dopant
DOS of traps
Ener
gy
DOS
HOMO of dopant
LUMO of host
Etc
Figure 4 Schematic presentation of energy level alignment ofGaussian DOS LUMO
of dopant on the optical as well as electronic properties ofCPs In essence this paper throws an adequate light on theoptoelectronic properties of conducting polymers to enableus to use it with a better understanding for the developmentof polymer based polymer light emitting diode (PLED)
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The author Manisha Bajpai gratefully acknowledges theUniversity Grant Commission New Delhi for the financial
assistance under Dr D S Kothari Postdoctoral FellowshipScheme (F no 4-22006(BSR)13-998(BSR))
References
[1] C K Chiang C R Fincher Jr Y W Park et al ldquoElectricalconductivity in doped polyacetylenerdquo Physical Review Lettersvol 39 no 17 pp 1098ndash1101 1977
[2] M Pfeiffer A Beyer T Fritz and K Leo ldquoControlled dop-ing of phthalocyanine layers by cosublimation with accep-tor molecules a systematic Seebeck and conductivity studyrdquoApplied Physics Letters vol 73 no 22 pp 3202ndash3204 1998
[3] W Gao and A Kahn ldquoControlled p-doping of zinc phthalo-cyanine by coevaporation with tetrafluorotetracyanoquin-odimethane a direct and inverse photoemission studyrdquoAppliedPhysics Letters vol 79 no 24 pp 4040ndash4042 2001
[4] M Yan L Rothberg B R Hsieh and R R Alfano ldquoExcitonformation and decay dynamics in electroluminescent poly-mers observed by subpicosecond stimulated emissionrdquo PhysicalReview B vol 49 no 14 pp 9419ndash9422 1994
[5] M Pollak and I Riess ldquoA percolation treatment of high-fieldhopping transportrdquo Journal of Physics C Solid State Physics vol9 no 12 article 2339 1976
[6] L Li G Meller and H Ksina ldquoTemperature and field-dependence of hopping conduction in organic semiconduc-torsrdquoMicroelectronics Journal vol 38 no 1 pp 47ndash51 2007
[7] V L Malevich ldquoOn the high-frequency electric field effect onthe two-phonon hopping transportrdquo Physica Status Solidi B vol163 no 2 pp K101ndashK105 1991
[8] H C F Martens P W M Blom and H F M Schoo ldquoCom-parative study of hole transport in poly(p-phenylene vinylene)derivativesrdquo Physical Review B vol 61 no 11 pp 7489ndash74932000
[9] M A Lampert and P Mark Current Injection in SolidsAcademic Press New York NY USA 1970
[10] P W M Blom and M C J M Vissenberg ldquoCharge transportin poly(p-phenylene vinylene) light-emitting diodesrdquoMaterials
8 Indian Journal of Materials Science
Science and Engineering R Reports vol 27 no 3-4 pp 53ndash942000
[11] D H Dunlap P E Parris and V M Kenkre ldquoCharge-dipolemodel for the universal field dependence of mobilities inmolecularly doped polymersrdquo Physical Review Letters vol 77no 3 pp 542ndash545 1996
[12] S V Novikov D H Dunlap V M Kenkre P E Parris and AV Vannikov ldquoEssential role of correlations in governing chargetransport in disordered organic materialsrdquo Physical ReviewLetters vol 81 no 20 pp 4472ndash4475 1998
[13] Yu N Gartstein and EM Conwell ldquoHigh-field hoppingmobil-ity in molecular systems with spatially correlated energeticdisorderrdquoChemical Physics Letters vol 245 no 4-5 pp 351ndash3581995
[14] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 39 no 1 article 3412003
[15] M Redecker D D C Bradley M Inbasekaran and E PWoo ldquoMobility enhancement through homogeneous nematicalignment of a liquid-crystalline polyfluorenerdquo Applied PhysicsLetters vol 74 no 10 pp 1400ndash1402 1999
[16] W F Pasveer J Cottaar C Tanase et al ldquoUnified descriptionof charge-carriermobilities in disordered semiconducting poly-mersrdquo Physical Review Letters vol 94 no 20 Article ID 2066012005
[17] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 93 no 1 pp 341ndash346 2003
[18] M A Parshin J Ollevier M Van Der Auweraer et al ldquoHoletransport in blue and white emitting polymersrdquo Journal ofApplied Physics vol 103 no 11 Article ID 113711 2008
[19] S L M Van Mensfoort and R Coehoorn ldquoDeterminationof injection barriers in organic semiconductor devices fromcapacitance measurementsrdquo Physical Review Letters vol 100no 8 Article ID 086802 2008
[20] Y Zhang and P W M Blom ldquoElectron and hole transport inpoly(fluorene-benzothiadiazole)rdquo Applied Physics Letters vol98 no 14 Article ID 143504 2011
[21] J C Blakesley H S Clubb andN C Greenham ldquoTemperature-dependent electron and hole transport in disordered semi-conducting polymers analysis of energetic disorderrdquo PhysicalReview B vol 81 no 4 Article ID 045210 2010
[22] P Mark and W Helfrich ldquoSpace-charge-limited currents inorganic crystalsrdquo Journal of Applied Physics vol 33 no 1 pp205ndash215 1962
[23] K C Kao and W Hwang Electrical Transport in SolidsPergamon Oxford UK 1981
[24] J Kido K Nagai and Y Okamoto ldquoBright organic electrolumi-nescent devices with double-layer cathoderdquo IEEE Transactionson Electron Devices vol 40 no 7 pp 1342ndash1344 1993
[25] J Kido and T Matsumoto ldquoBright organic electroluminescentdevices having a metal-doped electron-injecting layerrdquo AppliedPhysics Letters vol 73 no 20 pp 2866ndash2868 1998
[26] A G Werner F Li K Harada M Pfeiffer T Fritz and K LeoldquoPyronin B as a donor for n-type doping of organic thin filmsrdquoApplied Physics Letters vol 82 no 25 pp 4495ndash4497 2003
[27] F Li A Werner M Pfeiffer K Leo and X Liu ldquoLeuco crystalviolet as a dopant for n-doping of organic thin films of fullereneC60rdquo Journal of Physical Chemistry B vol 108 no 44 pp 17076ndash
17082 2004
[28] A Nollau M Pfeiffer T Fritz and K Leo ldquoControlled n-type doping of a molecular organic semiconductor naph-thalenetetracarboxylic dianhydride (NTCDA) doped withbis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF)rdquo Journal ofApplied Physics vol 87 no 9 pp 4340ndash4343 2000
[29] Y Zhang B de Boer and PWM Blom ldquoControllable molecu-lar doping and charge transport in solution-processed polymersemiconducting layersrdquo Advanced Functional Materials vol 19no 12 pp 1901ndash1905 2009
[30] P Tyagi R Srivastava A Kumar S Tuli and M NKamalasanan ldquoEffect of doping of cesium carbonate on electrontransport in Tris(8-hydroxyquinolinato) aluminumrdquo OrganicElectronics Physics Materials Applications vol 14 no 5 pp1391ndash1395 2013
[31] Y Zhang and P W M Blom ldquoField-assisted ionization ofmolecular doping in conjugated polymersrdquoOrganic Electronicsvol 11 no 7 pp 1261ndash1267 2010
[32] V I Arkhipov E V Emelianova and H Bassler ldquoQuenching ofexcitons in doped disordered organic semiconductorsrdquo PhysicalReview B vol 70 no 20 Article ID 205205 2004
[33] C Im J M Lupton P Schouwink S Heun H Becker andH Bassler ldquoFluorescence dynamics of phenyl-substitutedpolyphenylenevinylenendashtrinitrofluorenone blend systemsrdquoJournal of Chemical Physics vol 117 no 3 pp 1395ndash1402 2002
studied the electron transport in F8BT that was found to betrap limited and these traps were then deactivated by 119899-typedoping of DMC
Recently electron transport studies of cesium carbon-ate (Cs
2CO3) doped tris(8-hydroxyquinolinato)aluminum
(Alq3) are reported [30]They form ohmic contact with Alq3by the use of an electron injection layer Cs
2CO3 Further they
have studied the effect of doping of Cs2CO3 this leads to
increase in conductivity as well as mobility
6 Effect of Doping on Optical andElectrical Properties
As discussed in Section 6 some of the generated free chargecarriers upon doping affect optical as well as electricalproperties In the following subsections a short description ofeffects of doping on the optical as well as electronic propertieswill be discussed
7 Doping Induced Optical Properties ExcitonQuenching by Charge Transfer Centers
Arkhipov and his group proposed a theory of quenching ofexciton in doped disordered semiconductors They proposedthat an exciton can dissociate into a geminate pair of chargecarriers if a deep trap (usually for electrons) is located nextto a molecule or segment visited by the exciton in the courseof its energy relaxation Since the spatial distribution of trapsis random and does not correlate with energies of the hostmolecules the probability for an exciton to encounter acharge transfer center is fully determined by the number ofsites visited by this excitonThey supposed that the possibilityof thermally assisted jumps of excitons to sites of higherenergies is disregarded implying that the time scale of energyrelaxation is longer than the exciton lifetime unless mostexcitons were generated within the deep tail of EDOS [31]
After every intermolecular jump an exciton can find itselfin a molecule that has a deep (electron) trap in its closeneighborhood They supposed that the density of deep elec-tron traps 119873
119886and the concentration of quenchers depends
upon the molecular configuration They have consideredin the analysis the exciton quenching in which excitonsare delocalized within conjugated molecular segments inthe polymer Most probably the deep traps are distributedhomogeneously and the probability 119908
119902of occupation for an
exciton quencher to a deep trap has been determined by thePoisson distribution as [31]
119908119902= 1 minus exp (minus1205871199032
119902119897119873119886) (12)
where 119897 is the conjugation length and 119903119902is the maximum
distance between a segment and a deep trap which still allowsfor quenching They have also supposed that the excitonwhich is already occupied by a quencher will still avoidfurther quenching The probability to be certainly quenchedat a quencher119882
119902 is given by [31]
119882119902=
120591119895
120591119902+ 120591119895
(13)
where 120591119902and 120591119895are the quenching and jump times respec-
tively To contribute to the photoluminescence an excitonmust avoid quenching during its entire lifetime Estimatingthe exciton jump time as 120591
119895= 120591119899 and using the Poisson
distribution of probabilities yield the following expressionfor the probability 120578 that an exciton is not quenched andeventually decayed radiatively [31]
They have shown that the radiative yield 120578 depends uponthe concentration of deep traps 119873
119886by using (14) together
with experimental data obtained on an alkoxy-substitutedpolyphenylenevinylene (PhPPV) doped by trinitrofluorene(TNF) [32]
71 Doping Induced Electrical Properties Doping affects theelectrical properties like carriermobility drift velocity and soforth of the organic semiconductors If we do 119901-type dopingthe electron transfers directly from the host level to thedopant molecule without the intermediate step through theshallow level Similar to the case of 119901 doping with 119899 dopingthe electrons in the donor level can drop into the electrontraps (empty defect levels) and make them inactive Hencehole and electron carrier density are increased upon dopingandmobility ratio is also changed accordingly [20 28 29 33]
The mechanism behind the enhancement of hole currentupon doping can be understood in Figure 3
As we know in most of conducting polymers the holetransport is governed by SCL Upon addition of 119901-typedoping free holes are introduced into the polymer Atlow voltages these additional free holes often termed asbackground density 119901
0 will largely outnumber the charges
that are injected from the contacts which are responsiblefor the SCLC as observed in undoped polymer Since thepositive charge of this background density 119901
0is compensated
by the negative charge of the corresponding acceptors andtherefore does not contribute to the built-up of space chargean Ohmic-like current will flow at low voltages [29]
It is also described in previous section that electrontransport in CPs is trap-limited When 119899-type doping is doneinto the polymer most of the traps are deactivated and SCLcurrent is obtained as shown in Figure 4 [33] As it is alreadydiscussed that for 119899-type doping HOMO of the dopant mustbe adjacent to the LUMO of the host hence the electroncurrent is initially trap-limited As dopant material is dopedinto polymer matrix some of the trap states present in thepolymer start filling approaching 119864tc asymp 0 eV it means most ofthe traps are filled by electron provided by donor and henceSCL current is achieved [29] A further increase in the dopantconcentration does not further enhance the electron currentsince the HOMO of dopant is not sufficient to add more andmore free electrons to the LUMO of polymer
8 Conclusions
In this paper we have presented a review on the optical andelectronic properties of CPs We have reviewed the effect
Indian Journal of Materials Science 7
Ener
gy
DOS
LUMODopant
120576t
120576F
E
(a)
Ener
gy
DOS
120576t
120576eq
120576F
(b)
Figure 3 Schematic representation of the density of states and the position of Fermi levels (a) in pristine polymer and (b) polymer dopedwith 119901-type dopant The red dashed line represents the LUMO level of dopant
DOS of traps
Ener
gy
DOS
HOMO of dopant
LUMO of host
Etc
Figure 4 Schematic presentation of energy level alignment ofGaussian DOS LUMO
of dopant on the optical as well as electronic properties ofCPs In essence this paper throws an adequate light on theoptoelectronic properties of conducting polymers to enableus to use it with a better understanding for the developmentof polymer based polymer light emitting diode (PLED)
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The author Manisha Bajpai gratefully acknowledges theUniversity Grant Commission New Delhi for the financial
assistance under Dr D S Kothari Postdoctoral FellowshipScheme (F no 4-22006(BSR)13-998(BSR))
References
[1] C K Chiang C R Fincher Jr Y W Park et al ldquoElectricalconductivity in doped polyacetylenerdquo Physical Review Lettersvol 39 no 17 pp 1098ndash1101 1977
[2] M Pfeiffer A Beyer T Fritz and K Leo ldquoControlled dop-ing of phthalocyanine layers by cosublimation with accep-tor molecules a systematic Seebeck and conductivity studyrdquoApplied Physics Letters vol 73 no 22 pp 3202ndash3204 1998
[3] W Gao and A Kahn ldquoControlled p-doping of zinc phthalo-cyanine by coevaporation with tetrafluorotetracyanoquin-odimethane a direct and inverse photoemission studyrdquoAppliedPhysics Letters vol 79 no 24 pp 4040ndash4042 2001
[4] M Yan L Rothberg B R Hsieh and R R Alfano ldquoExcitonformation and decay dynamics in electroluminescent poly-mers observed by subpicosecond stimulated emissionrdquo PhysicalReview B vol 49 no 14 pp 9419ndash9422 1994
[5] M Pollak and I Riess ldquoA percolation treatment of high-fieldhopping transportrdquo Journal of Physics C Solid State Physics vol9 no 12 article 2339 1976
[6] L Li G Meller and H Ksina ldquoTemperature and field-dependence of hopping conduction in organic semiconduc-torsrdquoMicroelectronics Journal vol 38 no 1 pp 47ndash51 2007
[7] V L Malevich ldquoOn the high-frequency electric field effect onthe two-phonon hopping transportrdquo Physica Status Solidi B vol163 no 2 pp K101ndashK105 1991
[8] H C F Martens P W M Blom and H F M Schoo ldquoCom-parative study of hole transport in poly(p-phenylene vinylene)derivativesrdquo Physical Review B vol 61 no 11 pp 7489ndash74932000
[9] M A Lampert and P Mark Current Injection in SolidsAcademic Press New York NY USA 1970
[10] P W M Blom and M C J M Vissenberg ldquoCharge transportin poly(p-phenylene vinylene) light-emitting diodesrdquoMaterials
8 Indian Journal of Materials Science
Science and Engineering R Reports vol 27 no 3-4 pp 53ndash942000
[11] D H Dunlap P E Parris and V M Kenkre ldquoCharge-dipolemodel for the universal field dependence of mobilities inmolecularly doped polymersrdquo Physical Review Letters vol 77no 3 pp 542ndash545 1996
[12] S V Novikov D H Dunlap V M Kenkre P E Parris and AV Vannikov ldquoEssential role of correlations in governing chargetransport in disordered organic materialsrdquo Physical ReviewLetters vol 81 no 20 pp 4472ndash4475 1998
[13] Yu N Gartstein and EM Conwell ldquoHigh-field hoppingmobil-ity in molecular systems with spatially correlated energeticdisorderrdquoChemical Physics Letters vol 245 no 4-5 pp 351ndash3581995
[14] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 39 no 1 article 3412003
[15] M Redecker D D C Bradley M Inbasekaran and E PWoo ldquoMobility enhancement through homogeneous nematicalignment of a liquid-crystalline polyfluorenerdquo Applied PhysicsLetters vol 74 no 10 pp 1400ndash1402 1999
[16] W F Pasveer J Cottaar C Tanase et al ldquoUnified descriptionof charge-carriermobilities in disordered semiconducting poly-mersrdquo Physical Review Letters vol 94 no 20 Article ID 2066012005
[17] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 93 no 1 pp 341ndash346 2003
[18] M A Parshin J Ollevier M Van Der Auweraer et al ldquoHoletransport in blue and white emitting polymersrdquo Journal ofApplied Physics vol 103 no 11 Article ID 113711 2008
[19] S L M Van Mensfoort and R Coehoorn ldquoDeterminationof injection barriers in organic semiconductor devices fromcapacitance measurementsrdquo Physical Review Letters vol 100no 8 Article ID 086802 2008
[20] Y Zhang and P W M Blom ldquoElectron and hole transport inpoly(fluorene-benzothiadiazole)rdquo Applied Physics Letters vol98 no 14 Article ID 143504 2011
[21] J C Blakesley H S Clubb andN C Greenham ldquoTemperature-dependent electron and hole transport in disordered semi-conducting polymers analysis of energetic disorderrdquo PhysicalReview B vol 81 no 4 Article ID 045210 2010
[22] P Mark and W Helfrich ldquoSpace-charge-limited currents inorganic crystalsrdquo Journal of Applied Physics vol 33 no 1 pp205ndash215 1962
[23] K C Kao and W Hwang Electrical Transport in SolidsPergamon Oxford UK 1981
[24] J Kido K Nagai and Y Okamoto ldquoBright organic electrolumi-nescent devices with double-layer cathoderdquo IEEE Transactionson Electron Devices vol 40 no 7 pp 1342ndash1344 1993
[25] J Kido and T Matsumoto ldquoBright organic electroluminescentdevices having a metal-doped electron-injecting layerrdquo AppliedPhysics Letters vol 73 no 20 pp 2866ndash2868 1998
[26] A G Werner F Li K Harada M Pfeiffer T Fritz and K LeoldquoPyronin B as a donor for n-type doping of organic thin filmsrdquoApplied Physics Letters vol 82 no 25 pp 4495ndash4497 2003
[27] F Li A Werner M Pfeiffer K Leo and X Liu ldquoLeuco crystalviolet as a dopant for n-doping of organic thin films of fullereneC60rdquo Journal of Physical Chemistry B vol 108 no 44 pp 17076ndash
17082 2004
[28] A Nollau M Pfeiffer T Fritz and K Leo ldquoControlled n-type doping of a molecular organic semiconductor naph-thalenetetracarboxylic dianhydride (NTCDA) doped withbis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF)rdquo Journal ofApplied Physics vol 87 no 9 pp 4340ndash4343 2000
[29] Y Zhang B de Boer and PWM Blom ldquoControllable molecu-lar doping and charge transport in solution-processed polymersemiconducting layersrdquo Advanced Functional Materials vol 19no 12 pp 1901ndash1905 2009
[30] P Tyagi R Srivastava A Kumar S Tuli and M NKamalasanan ldquoEffect of doping of cesium carbonate on electrontransport in Tris(8-hydroxyquinolinato) aluminumrdquo OrganicElectronics Physics Materials Applications vol 14 no 5 pp1391ndash1395 2013
[31] Y Zhang and P W M Blom ldquoField-assisted ionization ofmolecular doping in conjugated polymersrdquoOrganic Electronicsvol 11 no 7 pp 1261ndash1267 2010
[32] V I Arkhipov E V Emelianova and H Bassler ldquoQuenching ofexcitons in doped disordered organic semiconductorsrdquo PhysicalReview B vol 70 no 20 Article ID 205205 2004
[33] C Im J M Lupton P Schouwink S Heun H Becker andH Bassler ldquoFluorescence dynamics of phenyl-substitutedpolyphenylenevinylenendashtrinitrofluorenone blend systemsrdquoJournal of Chemical Physics vol 117 no 3 pp 1395ndash1402 2002
Figure 3 Schematic representation of the density of states and the position of Fermi levels (a) in pristine polymer and (b) polymer dopedwith 119901-type dopant The red dashed line represents the LUMO level of dopant
DOS of traps
Ener
gy
DOS
HOMO of dopant
LUMO of host
Etc
Figure 4 Schematic presentation of energy level alignment ofGaussian DOS LUMO
of dopant on the optical as well as electronic properties ofCPs In essence this paper throws an adequate light on theoptoelectronic properties of conducting polymers to enableus to use it with a better understanding for the developmentof polymer based polymer light emitting diode (PLED)
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The author Manisha Bajpai gratefully acknowledges theUniversity Grant Commission New Delhi for the financial
assistance under Dr D S Kothari Postdoctoral FellowshipScheme (F no 4-22006(BSR)13-998(BSR))
References
[1] C K Chiang C R Fincher Jr Y W Park et al ldquoElectricalconductivity in doped polyacetylenerdquo Physical Review Lettersvol 39 no 17 pp 1098ndash1101 1977
[2] M Pfeiffer A Beyer T Fritz and K Leo ldquoControlled dop-ing of phthalocyanine layers by cosublimation with accep-tor molecules a systematic Seebeck and conductivity studyrdquoApplied Physics Letters vol 73 no 22 pp 3202ndash3204 1998
[3] W Gao and A Kahn ldquoControlled p-doping of zinc phthalo-cyanine by coevaporation with tetrafluorotetracyanoquin-odimethane a direct and inverse photoemission studyrdquoAppliedPhysics Letters vol 79 no 24 pp 4040ndash4042 2001
[4] M Yan L Rothberg B R Hsieh and R R Alfano ldquoExcitonformation and decay dynamics in electroluminescent poly-mers observed by subpicosecond stimulated emissionrdquo PhysicalReview B vol 49 no 14 pp 9419ndash9422 1994
[5] M Pollak and I Riess ldquoA percolation treatment of high-fieldhopping transportrdquo Journal of Physics C Solid State Physics vol9 no 12 article 2339 1976
[6] L Li G Meller and H Ksina ldquoTemperature and field-dependence of hopping conduction in organic semiconduc-torsrdquoMicroelectronics Journal vol 38 no 1 pp 47ndash51 2007
[7] V L Malevich ldquoOn the high-frequency electric field effect onthe two-phonon hopping transportrdquo Physica Status Solidi B vol163 no 2 pp K101ndashK105 1991
[8] H C F Martens P W M Blom and H F M Schoo ldquoCom-parative study of hole transport in poly(p-phenylene vinylene)derivativesrdquo Physical Review B vol 61 no 11 pp 7489ndash74932000
[9] M A Lampert and P Mark Current Injection in SolidsAcademic Press New York NY USA 1970
[10] P W M Blom and M C J M Vissenberg ldquoCharge transportin poly(p-phenylene vinylene) light-emitting diodesrdquoMaterials
8 Indian Journal of Materials Science
Science and Engineering R Reports vol 27 no 3-4 pp 53ndash942000
[11] D H Dunlap P E Parris and V M Kenkre ldquoCharge-dipolemodel for the universal field dependence of mobilities inmolecularly doped polymersrdquo Physical Review Letters vol 77no 3 pp 542ndash545 1996
[12] S V Novikov D H Dunlap V M Kenkre P E Parris and AV Vannikov ldquoEssential role of correlations in governing chargetransport in disordered organic materialsrdquo Physical ReviewLetters vol 81 no 20 pp 4472ndash4475 1998
[13] Yu N Gartstein and EM Conwell ldquoHigh-field hoppingmobil-ity in molecular systems with spatially correlated energeticdisorderrdquoChemical Physics Letters vol 245 no 4-5 pp 351ndash3581995
[14] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 39 no 1 article 3412003
[15] M Redecker D D C Bradley M Inbasekaran and E PWoo ldquoMobility enhancement through homogeneous nematicalignment of a liquid-crystalline polyfluorenerdquo Applied PhysicsLetters vol 74 no 10 pp 1400ndash1402 1999
[16] W F Pasveer J Cottaar C Tanase et al ldquoUnified descriptionof charge-carriermobilities in disordered semiconducting poly-mersrdquo Physical Review Letters vol 94 no 20 Article ID 2066012005
[17] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 93 no 1 pp 341ndash346 2003
[18] M A Parshin J Ollevier M Van Der Auweraer et al ldquoHoletransport in blue and white emitting polymersrdquo Journal ofApplied Physics vol 103 no 11 Article ID 113711 2008
[19] S L M Van Mensfoort and R Coehoorn ldquoDeterminationof injection barriers in organic semiconductor devices fromcapacitance measurementsrdquo Physical Review Letters vol 100no 8 Article ID 086802 2008
[20] Y Zhang and P W M Blom ldquoElectron and hole transport inpoly(fluorene-benzothiadiazole)rdquo Applied Physics Letters vol98 no 14 Article ID 143504 2011
[21] J C Blakesley H S Clubb andN C Greenham ldquoTemperature-dependent electron and hole transport in disordered semi-conducting polymers analysis of energetic disorderrdquo PhysicalReview B vol 81 no 4 Article ID 045210 2010
[22] P Mark and W Helfrich ldquoSpace-charge-limited currents inorganic crystalsrdquo Journal of Applied Physics vol 33 no 1 pp205ndash215 1962
[23] K C Kao and W Hwang Electrical Transport in SolidsPergamon Oxford UK 1981
[24] J Kido K Nagai and Y Okamoto ldquoBright organic electrolumi-nescent devices with double-layer cathoderdquo IEEE Transactionson Electron Devices vol 40 no 7 pp 1342ndash1344 1993
[25] J Kido and T Matsumoto ldquoBright organic electroluminescentdevices having a metal-doped electron-injecting layerrdquo AppliedPhysics Letters vol 73 no 20 pp 2866ndash2868 1998
[26] A G Werner F Li K Harada M Pfeiffer T Fritz and K LeoldquoPyronin B as a donor for n-type doping of organic thin filmsrdquoApplied Physics Letters vol 82 no 25 pp 4495ndash4497 2003
[27] F Li A Werner M Pfeiffer K Leo and X Liu ldquoLeuco crystalviolet as a dopant for n-doping of organic thin films of fullereneC60rdquo Journal of Physical Chemistry B vol 108 no 44 pp 17076ndash
17082 2004
[28] A Nollau M Pfeiffer T Fritz and K Leo ldquoControlled n-type doping of a molecular organic semiconductor naph-thalenetetracarboxylic dianhydride (NTCDA) doped withbis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF)rdquo Journal ofApplied Physics vol 87 no 9 pp 4340ndash4343 2000
[29] Y Zhang B de Boer and PWM Blom ldquoControllable molecu-lar doping and charge transport in solution-processed polymersemiconducting layersrdquo Advanced Functional Materials vol 19no 12 pp 1901ndash1905 2009
[30] P Tyagi R Srivastava A Kumar S Tuli and M NKamalasanan ldquoEffect of doping of cesium carbonate on electrontransport in Tris(8-hydroxyquinolinato) aluminumrdquo OrganicElectronics Physics Materials Applications vol 14 no 5 pp1391ndash1395 2013
[31] Y Zhang and P W M Blom ldquoField-assisted ionization ofmolecular doping in conjugated polymersrdquoOrganic Electronicsvol 11 no 7 pp 1261ndash1267 2010
[32] V I Arkhipov E V Emelianova and H Bassler ldquoQuenching ofexcitons in doped disordered organic semiconductorsrdquo PhysicalReview B vol 70 no 20 Article ID 205205 2004
[33] C Im J M Lupton P Schouwink S Heun H Becker andH Bassler ldquoFluorescence dynamics of phenyl-substitutedpolyphenylenevinylenendashtrinitrofluorenone blend systemsrdquoJournal of Chemical Physics vol 117 no 3 pp 1395ndash1402 2002
Science and Engineering R Reports vol 27 no 3-4 pp 53ndash942000
[11] D H Dunlap P E Parris and V M Kenkre ldquoCharge-dipolemodel for the universal field dependence of mobilities inmolecularly doped polymersrdquo Physical Review Letters vol 77no 3 pp 542ndash545 1996
[12] S V Novikov D H Dunlap V M Kenkre P E Parris and AV Vannikov ldquoEssential role of correlations in governing chargetransport in disordered organic materialsrdquo Physical ReviewLetters vol 81 no 20 pp 4472ndash4475 1998
[13] Yu N Gartstein and EM Conwell ldquoHigh-field hoppingmobil-ity in molecular systems with spatially correlated energeticdisorderrdquoChemical Physics Letters vol 245 no 4-5 pp 351ndash3581995
[14] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 39 no 1 article 3412003
[15] M Redecker D D C Bradley M Inbasekaran and E PWoo ldquoMobility enhancement through homogeneous nematicalignment of a liquid-crystalline polyfluorenerdquo Applied PhysicsLetters vol 74 no 10 pp 1400ndash1402 1999
[16] W F Pasveer J Cottaar C Tanase et al ldquoUnified descriptionof charge-carriermobilities in disordered semiconducting poly-mersrdquo Physical Review Letters vol 94 no 20 Article ID 2066012005
[17] D Poplavskyy and J Nelson ldquoNondispersive hole transport inamorphous films of methoxy-spirofluorene-arylamine organiccompoundrdquo Journal of Applied Physics vol 93 no 1 pp 341ndash346 2003
[18] M A Parshin J Ollevier M Van Der Auweraer et al ldquoHoletransport in blue and white emitting polymersrdquo Journal ofApplied Physics vol 103 no 11 Article ID 113711 2008
[19] S L M Van Mensfoort and R Coehoorn ldquoDeterminationof injection barriers in organic semiconductor devices fromcapacitance measurementsrdquo Physical Review Letters vol 100no 8 Article ID 086802 2008
[20] Y Zhang and P W M Blom ldquoElectron and hole transport inpoly(fluorene-benzothiadiazole)rdquo Applied Physics Letters vol98 no 14 Article ID 143504 2011
[21] J C Blakesley H S Clubb andN C Greenham ldquoTemperature-dependent electron and hole transport in disordered semi-conducting polymers analysis of energetic disorderrdquo PhysicalReview B vol 81 no 4 Article ID 045210 2010
[22] P Mark and W Helfrich ldquoSpace-charge-limited currents inorganic crystalsrdquo Journal of Applied Physics vol 33 no 1 pp205ndash215 1962
[23] K C Kao and W Hwang Electrical Transport in SolidsPergamon Oxford UK 1981
[24] J Kido K Nagai and Y Okamoto ldquoBright organic electrolumi-nescent devices with double-layer cathoderdquo IEEE Transactionson Electron Devices vol 40 no 7 pp 1342ndash1344 1993
[25] J Kido and T Matsumoto ldquoBright organic electroluminescentdevices having a metal-doped electron-injecting layerrdquo AppliedPhysics Letters vol 73 no 20 pp 2866ndash2868 1998
[26] A G Werner F Li K Harada M Pfeiffer T Fritz and K LeoldquoPyronin B as a donor for n-type doping of organic thin filmsrdquoApplied Physics Letters vol 82 no 25 pp 4495ndash4497 2003
[27] F Li A Werner M Pfeiffer K Leo and X Liu ldquoLeuco crystalviolet as a dopant for n-doping of organic thin films of fullereneC60rdquo Journal of Physical Chemistry B vol 108 no 44 pp 17076ndash
17082 2004
[28] A Nollau M Pfeiffer T Fritz and K Leo ldquoControlled n-type doping of a molecular organic semiconductor naph-thalenetetracarboxylic dianhydride (NTCDA) doped withbis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF)rdquo Journal ofApplied Physics vol 87 no 9 pp 4340ndash4343 2000
[29] Y Zhang B de Boer and PWM Blom ldquoControllable molecu-lar doping and charge transport in solution-processed polymersemiconducting layersrdquo Advanced Functional Materials vol 19no 12 pp 1901ndash1905 2009
[30] P Tyagi R Srivastava A Kumar S Tuli and M NKamalasanan ldquoEffect of doping of cesium carbonate on electrontransport in Tris(8-hydroxyquinolinato) aluminumrdquo OrganicElectronics Physics Materials Applications vol 14 no 5 pp1391ndash1395 2013
[31] Y Zhang and P W M Blom ldquoField-assisted ionization ofmolecular doping in conjugated polymersrdquoOrganic Electronicsvol 11 no 7 pp 1261ndash1267 2010
[32] V I Arkhipov E V Emelianova and H Bassler ldquoQuenching ofexcitons in doped disordered organic semiconductorsrdquo PhysicalReview B vol 70 no 20 Article ID 205205 2004
[33] C Im J M Lupton P Schouwink S Heun H Becker andH Bassler ldquoFluorescence dynamics of phenyl-substitutedpolyphenylenevinylenendashtrinitrofluorenone blend systemsrdquoJournal of Chemical Physics vol 117 no 3 pp 1395ndash1402 2002
![arXiv:1807.10471v1 [cond-mat.mes-hall] 27 Jul 2018 · ley ground state. The rst excited state js CC 0i+ js C Ciis a triplet-like state and occupies a valley ground and excited state](https://static.documents.pub/doc/80x56/5e2b3a99510b8b42bd336720/arxiv180710471v1-cond-matmes-hall-27-jul-2018-ley-ground-state-the-rst-excited.jpg)
