Abstract Hybrid devices based on silicon nanowires(SiNWs) dispersed in a conjugated polymer poly(3-hexyl-thiophene) P3HT thin films have been realized. The carriertransport mechanism in inorganic/organic hybrid nancocom-posites consisting of SiNW dispersed in P3HT layer wasinvestigated by using I–V characteristics and impedancespectroscopy measurements. The conduction mechanismin these hybrid nanocomposites has been identified to bethermionic emission at the interfaces. The electrical param-eters of the structure have been investigated by modelizationof the I–V characteristics using an electrical equivalent cir-cuit and have been extracted for the different SiNW volumeratios. The barrier height, the series resistance and the shuntresistance values of the diodes have been calculated as about0.9 eV, several k� and several M�, respectively. The diodebehaves as non-ideal one because of the series resistance andthe Donor/Acceptor interface layer. The impedance spec-troscopy study, in the frequency range 100 Hz–100 kHz,shows a typical behavior of disordered materials and indica-tive of a hopping transport in the investigated temperature
S. AziziLMM, Ecole Supérieur des Sciences et techniques de Tunis,5 Avenue Tah Hussein, 1008 Tunis, Tunisia
M. Braik · C. Dridi (�) · H. Ben OuadaLIMA, Département de Physique, Faculté des Sciences deMonastir, Université de Monastir, 5019 Monastir, Tunisiae-mail: [email protected]
C. DridiInstitut Supérieur des Sciences Appliquées et de Technologie deSousse, Université de Sousse, BP 40, Cité Ettafala, 4003Ibn Khaldoun Sousse, Tunisia
A. Ryback · J. DavenasLMPB/IMP, EPUL, Université de Lyon1, 43 Bd du 11 Novembre1918, Villeurbanne cedex, France
range. The dc conductivity follows the Arrhenius law withan activation energy transition from 8.4 to 55.8 meV at about294 K.
1 Introduction
In the two last decades, organic nanomaterials have attractedgreat interest as semiconducting materials and have foundapplications as electroluminescent devices, field-effect tran-sistors, chemical sensors and solar cells [1–9]. Organic ma-terials have in particular been intensively studied for the de-velopment of photovoltaic systems at low cost. It has beenshown that hybrid organic/inorganic nanocomposite solarcells constitute a potential candidate to be an alternative tophotovoltaic (PV) devices based on semiconducting inor-ganic or only organic nanomaterials.
Moreover, nanostructured organic/inorganic compositeswhere the inorganic semiconductor is the electron-transportcomponent, offer a valuable alternative, highlighted bythe studies performed on nanostructures based on metal-lic nanoparticles [10], semiconductor nanocrystals [11] ormetal oxides [12–14] embedded in polymer matrices. Semi-conducting polymer and inorganic nanomaterials blends ex-hibit excellent photoconductive properties because the inor-ganic semiconductor can efficiently dissociate the photogen-erated excitons and have also high electron mobility. Differ-ent semiconducting polymer/nanostructures are at presentsuccessfully being studied in photovoltaic devices [14–19].
It has been shown in a previous work that hybridnanocomposites of PVK:n-TiO2 [1] can be used as the ac-tive layer of photovoltaic devices. However, photovoltaicproperties have been noticed to be degraded under il-lumination due to the combination of space charge for-mation and bipolar charge recombination. Recently [20],
a simulation of I–V characteristics for different organicnanomaterials/n-TiO2 sandwich structures has shown thatP3HT (poly(3-hexylthiophene)) could be a better matrix forhybrid nanocomposite-based devices. This is due to its bet-ter hole mobility and low band gap compared to other poly-meric semiconductors [21, 22]. Indeed, unlike most pho-toconductive polymers, where electronic transport is limitedthrough charge trapping by oxygen and oxidative impurities,efficient hole transport is shown by regioregular P3HT [23].Moreover, to extend the spectrum of the nanocomposite ab-sorption to IR region we will replace TiO2 nanoparticles bySi nanowires (SiNW) [24].
In the present work, we have simply dispersed SiNW inP3HT to fabricate the active layer of a nanocomposites pho-tovoltaic device. The structural and optical properties of theused SiNW has been described earlier [25, 26] and we haveshown that the energy levels of P3HT and SiNW allow ex-citon dissociation [26] which is consistent with the work ofGoncher et al. [27]. Furthermore, the device performance ofhybrid solar cells depends on electrical and electronic char-acteristics of the metal/organic semiconductor junction. Inorder to obtain valuable information about charge transportproperties of hybrid solar cells, it is necessary to understandthe electronic properties and origin of their characteristics.The ideality factor which is called the curve shape factor forsolar cell is an important input parameter in the descriptionof the solar cell’s electrical behavior [28, 29]. These effectscause a dramatic deterioration of electronic properties of thesolar cell.
Moreover, charge collection from a nanowire via a semi-conducting polymer gives rise to two possible complica-tions: charge transport across the silicon-polymer interfacemust be favorable and carrier transport in the polymer (holesfor the device type investigated here) must be efficient. Thedevices fabricated in this investigation are designed to an-swer some of these questions.
Our purpose is to develop a detailed study of chargetransport phenomena evolution with Si nanowire volumeratio, frequency and temperature in hybrid nanocomposite-based devices: ITO/PEDOT:PSS/P3HT:SiNW/Al.
2 Experimental details
2.1 Materials
Tetrahydrofuran (THF) (HPLC, ≥99.9 %, Aldrich) andPoly-[3,4-(ethylenedioxy)-thiophene:polystyrene sulfonate(PEDOT:PSS) from Aldrich Fig. 1(a) were used withoutany purification. Silicon nanowires (SiNW) have been fab-ricated by oxide assisted growth (OAG) method [30] whichis a low cost and catalyst free one. Regioregular Poly(3-hexylthiophene-2,5-diyl) (P3HT) has been purchased fromAldrich (Fig. 1(b)).
Fig. 1 (a) Chemical structures of regioregular Poly(3-hexyl-thiophene-2,5-diyl) (P3HT). (b) Poly(3,4-ethylenedioxythiophene)(PEDOT)/poly(styrenesulfonate) (PSS)
2.2 Substrate preparation
ITO-coated glass substrates (ITO-thickness 100 nm, sheetresistance 20 �/sq) were used as anodes, for electricalcharacterizations, and were beforehand cleaned in an ace-tone (Chromasolv® Plus, for HPLC, ≥99.9 %, Aldrich) andethanol (ACS reagent, ≥99.5 %, Sigma-Aldrich) bath at100 ◦C, rinsed with isopropyl alcohol (Chromasolv® Plus,for HPLC, 99.9 %, Aldrich), and dried under nitrogen flow.Then PEDOT:PSS was casted, onto ITO, 4400 rpm during30 s and annealed at 100 ◦C during 15 min in air.
2.3 Device fabrication
ITO/PEDOT:PSS/P3HT:SiNW/Al device structures (repre-sented in Fig. 2), were fabricated according to the followingroute: solutions of P3HT (dissolved in THF at 45 ◦C dur-ing 24 h with magnetic stirring) and SiNWs (dissolved inTHF in ultrasonic bath for 15 min) were mixed in appro-priate ratio to obtain needed SiNW concentration and spin-coated at 2000 rpm during 60 s on the ITO/PEDOT:PSSsubstrate giving layers with a thickness of about 100 nmdetermined by Dektak profilometer, and then annealed (un-der vacuum) at 140 ◦C for 15 mn [31]. The morphology ofsuch P3HT:SiNW thin films is given by a Scanning Elec-tron Microscopy (SEM) image (inset of Fig. 2) and showsan homogeneous SiNW dispersion in P3HT matrix, whichcould results in a large donor/acceptor interface favorable toefficient charge transfer between P3HT and SiNW.
An aluminum contact was evaporated, as the top elec-trode, at pressure below 10−6 Torr using an Edwards E306A
Study of charge transport in P3HT:SiNW-based photovoltaic devices 101
Coating System. The active area of the diodes is limited bythe overlapping section of the electrodes which is approxi-mately 5 mm2. Gold wires were glued with silver paint to thesubstrate and the top electrode of the device. For the cyclicvoltammetry, the supporting electrolyte was 0.1M Bu4NPF6in dry acetonitrile, the redox couple ferrocene/ferriceniumion (Fc/Fc+) was used as external standard.
2.4 Instrumentation
The DC measurements (I–V characteristics) were madewith a Keithley 236 source measure unit.
The AC measurements were performed with a Volta-lab 40 system (Radiometer Analytical) constituted by aPotentiostat-Galvanostat PGZ 301. In general, for the G(ω)
characteristics, the excitation potential for AC measure-ments is given by
V = V0 + Vmod cos(ωt), (1)
with V0 is the DC bias and Vmod is the oscillation level andω/2π is the frequency. In our case, the measurements wereperformed in the following conditions: V0 = 0 V and Vmod
of 20 mV over a frequency range of 100 Hz–100 kHz. Allelectrical measurements were performed in dark and at dif-ferent temperatures.
3 Results and discussion
Charge transfer from donor to acceptor component, effectivecharge transport and charge injection into the electrodes areimportant parameters for optimization and design of hybridsolar cells. In this regard in section 3.1, an electrochemicalstudy is conducted to provide valuable information and al-low estimation of the HOMO and LUMO levels of the con-jugated polymer followed in section 3.2, by a dc electricalstudy to identify the role of interfaces in the transport mech-anism and finally an ac charge transport (section 3.3) inves-tigations give an insight into the relaxation processes arisingin hybrid nanocomposite materials.
3.1 Electrochemical study
Figure 3(a) represents cyclic-voltammetry plot of P3HT thinfilm measured versus saturated calomel electrode (SCE)from which we have estimated the onset and the peak valuesfor oxidation and reduction potentials of P3HT (Table 1).The corresponding HOMO and LUMO levels (Table 1) werecalculated using the relation [32]:
EHOMO/LUMO
= [−e(Eonset(vs.ESC) − Eonset(Fc/Fc+vs.ESC)] − 4.8 eV
including the ferrocene value of −4.8 V with respect to vac-uum level, which is defined as zero. Therefore, the obtainedelectrochemical band gap ECV
g value is about 2.06 eV lower
than the ECVg value of 1.67 eV reported by Al-Ibrahim and
al. [33]. In their work, they have determined a more impor-tant optical band gap of about 1.9 eV (close to the opticalband gap of our samples (1.89 eV) determined by the samemethod). Such relation between electrochemical and opticalband gaps is inconsistent with the physical aspects occur-ring inside such conjugated polymers. Indeed, in the case ofconjugated polymers ECV
g is in general larger than the Eoptg
and the difference is assigned to exciton binding energy ofconjugated polymers [34], as shown in Fig. 3(b), which isabout 170 meV in our case [35]. Therefore, we summarizein Fig. 3(c) the band energy diagram of the overall structurewith determined HOMO and LUMO levels of P3HT andSiNW [25, 26] in relation with the work functions of PE-DOT:PSS, ITO and Al. The HOMO of P3HT is positionedto inject holes into PEDOT:PSS and hence into the ITO elec-trode and should accept holes generated by light absorptionin the silicon nanowires. The LUMO of P3HT is well abovethe Fermi level of the n-Si nanowires and electron collec-tion should occur efficiently at the silicon interface. Elec-trons generated in the nanowires will be collected at the Alelectrode.
3.2 DC transport phenomena inITO/PEDOT:PSS/P3HT:SiNW/Al diodes
To study the junction properties, current-voltage character-istics (J –V ) have been made. These measurements, usually,provide a valuable source of information about the junctionproperties such as the diode quality factor (n), the reversesaturation current density (J0), the series (Rs) and the shunt(Rp) resistances. Analysis of the J –V characteristics is alsoextremely useful to identify the transport mechanisms con-trolling the conduction.
3.2.1 Forward bias: role of the interfaces
The influence of the interface on charge transport have beeninvestigated at low bias voltage (V < 1 V), where such pro-cess is dominant. The J –V characteristics, for pure P3HT
102 S. Azizi et al.
Table 1 Cyclic-voltammetry data, HOMO, LUMO levels and ECVg of P3HT
Eoxp vs. ECS
(V)
Eoxonset vs. ECS
(V)HOMO(eV)
Eredp vs. ECS
(V)
Eredonset vs. ECS
(V)LUMO(eV)
ECVg
(eV)
P3HT 1.19 0.84 3.03 −1.72 −1.22 5.09 2.06
Fig. 3 (a) Plot of cyclic voltammetry of P3HT. (b) The difference be-tween the electrochemical band gap energy and the optical band gapenergy. (c) Energy band diagram of ITO/PEDOT:PSS/P3HT:SiNW/Aldevice
and different volume ratios of SiNW (10, 20 and 50 vol. %),are reported in Figs. 4a and 4b.
Fig. 4 (a) The J –V characteristic in dark of pure P3HT-based struc-ture. The inset shows the linear evolution of ln(J ) vs. V . (b) The J –V
characteristics in dark of nanocomposite-based devices containing 10,20 and 50 % volume ratios of SiNW. The inset shows the linear evolu-tion of ln(J ) vs. V for 50 % volume ratio
To study the mechanism of charge injection in the for-ward bias, we have first checked the (Space Charge Lim-ited Current) (SCLC) behavior. Nevertheless, we have notfound the classical lnJ = V n behavior for J –V characteris-tics [36–42], characterized by an ohmic contact for low bias,followed by a segment with n = 2 and (sometimes) a thirdsegment with n > 2 with increasing voltage. Several publi-cations and reports have shown for different polythiophenederivative-based sandwich structures that, in the low fielddomain, thermionic emission can occur [22, 43–45].
Study of charge transport in P3HT:SiNW-based photovoltaic devices 103
Table 2 Rectificationparameters for different SiNWvolume ratios
SiNW volume ratio (%) Ideality factor n J0 (nA cm−2) Barrier height φB (eV)
0 3.27 64.9 0.82
10 23.2 37 0.84
20 12 6.1 0.88
50 25.7 290 0.95
This is presently verified over a broad voltage domain,as shown in the inset of Fig. 4a for pure P3HT and in theinset of Fig. 4b for 50 % volume ratio of SiNW (the samebehavior is observed for 10 and 20 %). In the case of thethermally activated Schottky diode mechanism, the J (V ) re-lation [46, 47] is given by
J (V ) = J0 exp(qV/nkT ) (2)
where q is the electron charge, J0 is the saturation currentdensity and n is the ideality factor, which represents the de-viations from the ideal thermionic model and should varybetween 1 and 2, [45, 46]. The barrier height φB can be de-duced from the expression of the reverse saturation currentdensity [45]:
J0 = A∗T 2 exp(−qφB/kT ) (3)
where A∗ is the Richardson constant (= 120 A cm−2 K−2).The value of J0 extrapolated from ln(J ) vs. V plots,
when the potential tends toward zero, results in a mean bar-rier height for holes of about 0.9 eV (Table 2) for differentSiNW volume ratios. Such values have been reported for dif-ferent organic donor/acceptor devices [19, 22, 43, 46, 48].The height of the Schottky barrier φB depends on the Fermilevel of the nanocomposite and on the work function of theITO/PEDOT:PSS and metal electrodes, which are modifiedby interfacial chemistry and layers.
The ideality factor can be deduced from �V/� lnJ ,since n = (q/kT )(�V/� lnJ ) in the exponentially increas-ing current domain. This ideality factor should vary between1 and 2 [46]. As shown in Table 1, the n values estimatedfrom the slope of the curves lnJ = f (V ) are greater than 2and the contact seems to be non-ideal. This deviation fromthe ideal value can be due to barrier inhomogeneity, se-ries resistance, image force lowering, existence of interfaciallayers, recombination effect of charge-carrier drift and diffu-sion or tunneling currents through the barrier or a combina-tion of these effects. Thus, the calculation of ideality factorn should be reconsidered to take into account the interfaceeffects on injection processes.
3.2.2 Electrical equivalent circuit diagram over extendedbias domain
In real devices, the simple Schottky model must be modifiedto include:
Fig. 5 Equivalent electrical circuit of ITO/PEDOT:PSS/P3HT:SiNW/Al device in dark and in dc regime
– The series resistance (Rs) which affects the J –V charac-teristics at high voltages.
– The shunt resistance (Rp) which affects the J –V char-acteristics at low forward bias voltages and more signifi-cantly the reverse characteristics of diodes with high bar-rier height.
The complete representation of the J –V characteristicsin dark is provided by the modified Shockley equation [45]corresponding to Fig. 5:
J = J0
(exp
(V − JRs
nVth
)− 1
)+ (V − JRs)
Rp(4)
where J is the current density, V the bias voltage and theterm JRs represents the potential drop across the series re-sistance Rs while Rp is the shunt resistance of the diodewhich reflects the main leakage processes in the cell, J0 isthe reverse saturation current density, n is the ideality factorand Vth = kT /q is the thermal voltage. The four electricaldiode parameters (J0, n, Rs and Rp) have been determinedusing the numerical method described in a previous work[19] and their values are summarized in Table 3 for differentvolume ratios of SiNW.
Taking into account the bias voltage dependence of theideality factor, we have no more the high n values obtainedfor the forward bias voltage, but 1.90, 1.0, 2.28 and 0.22for 0 %, 10 %, 20 % and 50 % SiNW volume ratios, re-spectively. The value of the ideality factor between 1 and 2obtained for pure P3HT indicates comparable diffusion andrecombination currents. For 50 % SiNW volume ratio de-vice, we notice an unusual value of ideality factor and veryimportant shunt resistance (which is an interesting fact forenhanced photodissociation processes). Therefore, we willinvestigate the ac transport properties of this device in thefollowing section.
104 S. Azizi et al.
Table 3 Extracted parameters from different ITO/PEDOT:PSS/P3HT:SiNW/Al solar cells in dark
Experimental conditions %SiNW (N)
Rs(k� cm2)
Rp
(M� cm2)n J0
(pA cm−2)
In dark0 0.265 1.394 1.90 295
10 740 5.694 1.00 23.82
20 6.956 273.972 2.28 3.662
50 152.93 797.7 × 103 0.22 58.656
Fig. 6 AC conductivity versus frequency for ITO/PEDOT:PSS/P3HT:SiNW/Al for 50 vol. % SiNW
3.3 AC transport phenomena inITO/PEDOT:PSS/P3HT:SiNW (1:1)/Al diodes
3.3.1 The conductivity σ(ω) characteristics
The conductivity characteristic of P3HT:50 vol. % SiNWnanocomposite versus frequency at ambient temperature isplotted in Fig. 6. The σ(ω) characteristic remain constantat low frequencies and a power-law behavior was observedat higher ones. Therefore, a hopping transport mechanismcan be postulated. Indeed, the trend of the conductivity withfrequency, in disordered materials is given in general by thefollowing relation [49, 50]:
σ(ω) = σdc + σac(ω) (5)
where σdc is the dc conductivity, and σac(ω) = Aωs withω the angular frequency of the applied excitation, A is aconstant and s the critical exponent 0 < s < 1. We have ob-tained 0.993 and 78.2 µS/m for the critical exponent and thedc conductivity, respectively. The latter is more importantby an order of magnitude than the reported one, at the sametemperature, for P3HT alone [51].
Fig. 7 (a) Typical Real Z (Z′) and Im Z (Z′′) evolution versus fre-quency of an ITO/PEDOT:PSS/P3HT: 50 vol. % SiNW/Al PV de-vice. (b) Equivalent electrical circuit for the ITO/PEDOT:PSS/P3HT:50 vol. % SiNW/Al PV device in dark and in ac regime
3.3.2 Relaxation mechanism: non-Debye behavior
In Fig. 7 the evolution of real (Z′) and imaginary (Z′′) partsof the impedance with frequency are represented. Only onerelaxation process is observed because the bulk and the junc-tion capacitance are not sufficiently differentiated. The re-sults have been modeled by an equivalent electrical circuitcomposed of a single parallel resistor Rp and a ConstantPhase Element (CPE) network placed in series with resis-tance Rs corresponding to the following equation [52]:
Z∗ = Z′ + jZ′′ = Rs − Rp
1 + (jω)αA0Rp(6)
Study of charge transport in P3HT:SiNW-based photovoltaic devices 105
Table 4 Fitted parametersprovided by (6) Series resistance (Rs)
�
Parallel resistor (Rp)
�
Constant Phase Element (CPE)µF
α R2
30.859 227.474 2.757 0.761 0.999
Fig. 8 (a) Typical σ(ω) characteristics of an ITO/PEDT:PSS/P3HT:50 vol. % SiNW/Al device for different temperatures (b) Evolution ofdc conductivity with 1000/T at low frequency for 50 vol. % SiNW
where A0 is the “true” inter-chain capacitance, the parame-ter α describes the distribution of the relaxation times of thesystem. The results show good agreement between the theo-retical and experimental data. The parameters obtained fromthe fit (Rs,Rp, α) are given in Table 4 with α different fromunity which implies a non-Debye relaxation process [53].
3.3.3 Temperature dependence
The temperature dependence of the ac conductivity is rep-resented in Fig. 8(a) and shows an increase with tempera-ture. This suggested that the ac conductivity is a thermallyactivated process, at least over limited temperature, and it
follows the Arrhenius law [54, 55] as
σ = σ0 exp
(−Ea
kBT
)(7)
where σ0 is a pre-exponential factor and is characteristic ofthe material, Ea is the activation energy for conduction, kB
is the Boltzmann constant and T is the absolute temperature.Thus, the activation energy for conduction can be calcu-lated from the slope of the straight line given by least-mean-square analysis of lnσ vs. 1000/T as shown in Fig. 8(b).We can observe an activation energy transition from 8.4 to55.8 meV at the equilibration temperature of about 294 K.Such transition of the activation energy has been observedearlier in P3HT and in porous silicon [56, 57] and can becorrelated to thermally activated relaxation processes fromlocalized to delocalized electronic states of; either P3HTnanocrystallites or SiNWs; below the Fermi edge.
4 Conclusions
We have fabricated P3HT:SiNW nanocomposite-based de-vices and we have also studied the dc and ac charge trans-port properties of these devices. The transport mechanismin the ITO/PEDOT:PSS/P3HT:SiNW/Al devices is limitedby the injection of a thermionic emission at the interfaceelectrode. The device’s dc properties have been modeled byan electrical equivalent circuit which parameters (n,Rs,Rp,and J0) have been extracted for different SiNW volume ra-tios. We have also shown that for 50 vol. % of SiNW: (i) theac conductivity shows a behavior typical of disordered ma-terials and indicative of a hopping transport in the investi-gated temperature range and (ii) an activation energy transi-tion from 8.4 to 55.8 meV at the equilibration temperature,about 294 K. This transition has been correlated to thermallyactivated relaxation processes from localized to delocalizedelectronic states of; either P3HT nanocrystallites or SiNWs;below the Fermi edge.
References
1. C. Dridi, V. Barlier, H. Chaabane, J. Davenas, H. Ben Ouada, Nan-otechnology 19, 375201 (2008)
2. B. Reeja-Jayan, A. Manthiram, Solar Energy Materials & SolarCells 94, 907–914 (2010)
3. J.C. Nolasco, R. Cabré, J. Ferré-Borrull, L.F. Marsal, M. Estrada,J. Pallarès, Journal of Applied Physics 107, 044505 (2010)
106 S. Azizi et al.
4. A.B. Daniel, M.O. Rosui, G.M. George, Organic Semiconduc-tors in Sensor Applications. Springer Series in Materials Science(2008)
48. C. Waldauf, P. Schilinsky, J. Hauch, C. Brabec, Thin Solid Films451–452, 503–507 (2004)
49. F. Michelotti, F. Borghese, M. Bertolotti, E. Cianci, V. Foglietti,Synth. Met. 111/112, 105–108 (2000)
50. A.K. Jonscher, Nature 256, 673 (1977)51. J. Obrzut, K.A. Page, Physical Review B 80, 195211 (2009)52. D. Zhu, J. Zhang, C. Xu, M. Matsuo, Synthetic Metals 161, 1820–
1827 (2011)53. A. Dey, S. De, A. De, S.K. De, Nanotechnology 15, 1277 (2004)54. M.M. EL-Nahass, A.F. EL-Deeb, F. Abd-El-Salam, Organic Elec-
Materials, 2nd edn. (Clarendon Press, Oxford, 1979)56. L.K. Pan, et al., J. Appl. Phys. 94, 2695 (2003)57. L. Pan, Z. Sun, J. of Phys. and Chem. of Solids 70, 1113 (2009)
