Organic Electronics 7 (2006) 235–242

www.elsevier.com/locate/orgel

Charge formation and transport in bulk-heterojunctionsolar cells based on alternating polyfluorene

copolymers blended with fullerenes

Kim G. Jespersen a, Fengling Zhang b,*, Abay Gadisa b, Villy Sundstrom a,Arkady Yartsev a, Olle Inganas b

a Chemical Physics, Kemicentrum, Getingevagen 60, SE-22100 Lund, Swedenb Biomolecular and Organic Electronics, Center of Organic Electronics, Linkoping University, SE-58183 Linkoping, Sweden

Received 27 December 2005; received in revised form 1 March 2006; accepted 3 March 2006Available online 4 April 2006

Abstract

We investigate charge formation in bulk-heterojunction solar cells based on conjugated polymers in the form of alter-nating polyfluorene copolymers and the methanofullerene PCBM. Using transient absorption spectroscopy we show thatoptimal charge formation is obtained with 20–50 wt% PCBM. This is in contrast to the maximum short circuit currentdensity obtained at �80 wt% PCBM as determined by steady state current density–voltage characterization. Hence, weshow explicitly that the solar cell performance of these interpenetrating polymer networks containing PCBM is limitedby charge transport rather than by formation of charges.� 2006 Elsevier B.V. All rights reserved.

PACS: 73.50.Pz; 73.61.ph; 78.47.+p

Keywords: Transient absorption spectroscopy; Charge transport; Bulk-heterojunction solar cells

1. Introduction

The conversion of solar energy into electricalenergy using thin films of photovoltaic plastic hasshowed great potential as a renewable energy source[1]. Typical polymeric solar cells are based on inter-penetrating polymer networks in which a photosen-sitive hole-conducting polymer is combined with an

1566-1199/$ - see front matter � 2006 Elsevier B.V. All rights reserved

doi:10.1016/j.orgel.2006.03.001

* Corresponding author. Tel.: +46 13 281257; fax: +46 13288969.

E-mail address: fenzh@ifm.liu.se (F. Zhang).

electron-conducting acceptor moiety [2,3]. The twoco-continuous phases create a bulk-heterojunctionin which photoinduced charge separation is highlyefficient and where charge transport can take place.A common electron-conducting acceptor is the sol-uble methanofullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) [4]. The forwardelectron transfer from polymer to PCBM is very fast[5] (�45 fs as measured by Brabec et al. [6]), andgenerates a metastable charge separated state withls lifetime. The electron mobility of neat PCBM isreported to be 2 · 10�3 cm2/V s [7], while that of

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0.8R

S S

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orba

nce

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)

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Fig. 1. Absorption spectrum of neat APFO3 (solid), PCBM(dot), APFO3:PCBM in the weight ratio 4:1 (dash dot), 1:1 (dashdot dot) and 1:4 (dash). Inset shows the APFO3 repeat unit(R = C8H17 indicating ‘octyl’ side-groups) and the PCBMmolecular structure.

236 K.G. Jespersen et al. / Organic Electronics 7 (2006) 235–242

the conjugated polymer is typically an order of mag-nitude lower. In bulk-heterojunctions these valueshave a complex dependency on the relative PCBMconcentration and the morphology of the poly-mer–fullerene blend, i.e. the degree of phase separa-tion and percolation (see e.g. Ref [8]). Severalgroups have reported power conversion efficiencies(PCE) exceeding 2.5% under AM 1.5 solar illumina-tion [9–11]. An interesting common denominator inthese bulk-heterojunction solar cells is a rather highconcentration of PCBM. Typically, the ratio ofpolymer to PCBM is 1:4 by weight for optimalPCE. This is observed in PPV derivatives [9,12] aswell as in polyfluorene [13,14] and polythiophene[10] derivatives. In general, the highest PCE is acompromise between charge formation and the elec-tron and hole transport in the two compounds.Both charge formation and charge transport areassumed to be strongly influenced by the morphol-ogy of the blend. In previous photocurrent experi-ments [8] and device modelling [15] it has beensuggested that the efficiency is mostly limited bycharge transport.

Recent publications on bulk-heterojunction solarcells based on P3HT:PCBM have shown that thePCE can be increased up to 5% by annealing. Some-what surprisingly the highest PCE was achievedwith relatively low PCBM concentrations of 0.8:1[16,17] and 1:1 [18]. These results indicate that animprovement of the transport properties by anneal-ing in P3HT:PCBM leads to an optimal PCBM con-centration that is significantly reduced as comparedto those reported in the past.

Here, we address the relative importance ofcharge formation and charge transport for the over-all performance of a polymer–fullerene bulk-hetero-junction. Using femtosecond time-resolvedspectroscopy and steady state photocurrent mea-surements, we show explicitly that a high concentra-tion of PCBM is not favorable in terms of thenumber of charges generated, and we give an esti-mate for the optimal PCBM concentration inbulk-heterojunctions based on alternating polyfluo-rene copolymers.

2. Experimental

The experiments are based on the alternatingpolyfluorene copolymer poly(2,7-(9-di-octyl-fluo-rene)-alt-5,5-(4 0,7 0-di-2-thienyl-2 0,1 0,3 0-benzothi-adiazole)) (APFO3) [19] in combination withPCBM (see repeat unit of APFO3 and molecular

structure of PCBM in the inset of Fig. 1). TheAPFO3 copolymer belongs to a new generation oflow-bandgap conjugated polymers with improvedoverlap between the absorption spectrum and thesolar radiation spectrum [14,20]. For a detailed studyof APFO3, see Ref. [21]. Samples for optical charac-terization, with APFO3:PCBM weight ratios 1:0,4:1, 1:1 and 1:4 were fabricated by spin coating onglass substrates from chloroform solutions. The sam-ples were subsequently sealed to prevent photo deg-radation. After the experiments, the samples wereopened and the film thickness was determined usinga standard profilometer. For electrical characteriza-tion, the samples were spin-coated onto ITO sub-strates and an Al electrode was evaporated (fordetails see Ref. [20]). For transport measurements,hole-only diodes were constructed by sandwichingthe active layer between ITO/PEDOT:PSS and pal-ladium (Pd) electrodes. Transport parameters areextracted from space charge limited current–voltagecharacteristics of the hole only diodes, which is mea-sured under dark condition. Measurement and trans-port description details can be found in Ref. [22]. InFig. 1 the absorption spectra of neat APFO3, PCBMand the APFO3:PCBM weight ratio 4:1, 1:1 and 1:4are plotted. The absorbance is linearly scaled to afilm thickness of 100 nm for comparison. This simplescaling approach is observed to be valid for the differ-ent blend ratios and thickness used in this work. Thestrong absorption below 400 nm in the 1:4 sample isdominated by PCBM absorption, while the region

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Fig. 2. Transient absorption spectra of APFO3:PCBM 1:0, 4:1,1:1, and 1:4. The spectra are scaled to 100 nm film thicknesses.Gray is at 0.5 ps; black is 466 ps.

K.G. Jespersen et al. / Organic Electronics 7 (2006) 235–242 237

above 500 nm is predominant absorption byAPFO3.

Time-resolved absorption experiments were per-formed using a transient absorption spectrometerwith 120 fs pulses. The experimental setup wasbased on a commercial 1 kHz Clark MXR CPA-2001 femtosecond laser pumping two non-colinearoptical parametric amplifiers (NOPA) for genera-tion of single color pump pulses and a broad probecontinuum. The excitation wavelength was 570 nmcorresponding to the red tail of the lowest absorp-tion band of APFO3. The probe continuum coveredthe spectral region from 550 nm to 750 nm. Thepump pulse was polarized at magic angle to theprobe polarization. To have approximately linearoptical excitation and thereby minimize the influ-ence of bimolecular annihilation we limited thepump intensity to �1014 ph/cm2/pulse.

Steady state photocurrent measurements wereperformed under low intensity monochromatic lightillumination from a halogen lamp passing through amonochromator with the intensity of 0.2 mW/cm2.The wavelength chosen to illuminate the diodeswas the same as for the excitation pulse in thetime-resolved absorption experiments. The currentdensity–voltage measurements were carried out byapplying negative potential on the Al electrodeand positive potential on the ITO electrode. Photo-current was measured using a Keithley 485 picoam-meter. All fabrications and characterizations wereperformed in an ambient environment without pro-tective atmosphere.

3. Results and discussion

3.1. Charge generation

In Fig. 2 the transient absorption spectra ofAPFO3:PCBM are shown for different concentra-tion ratios. Here the raw data have been scaled toa 100 nm thick film for direct comparison of thespectra. The explicit scaling factor is given by theratio between the number of absorbed photons inthe measured film and in a corresponding film of100 nm thickness, and is given by ð1–10�AscaledÞ=ð1–10�AÞ, where A is the transient absorption ofthe film at the excitation wavelength, and Ascaled =A · 100 nm/d is the transient absorption at the exci-tation wavelength for a corresponding 100 nm thickfilm (d is the thickness of the measured film in nm).We show the spectra at 0.5 ps and 466 ps pump–probe delay. The upper panel shows the spectra of

neat APFO3 (1:0). Initially, the spectral featuresare due to stimulated emission (SE) (centered at700 nm) and photobleaching (PB) (centered at580 nm). The excited state dynamics is found to benon-exponential by fluorescence lifetime measure-ments (data not shown), and can be approximatedby three decay components, where the major contri-bution decays within 100 ps. Hence, at 466 ps theexcited state population is heavily reduced, suchthat there is only a negligible SE contribution to

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Fig. 3. Number of charged species generated (h), chargeformation efficiency (n), and the short circuit current densityJSC (d; dash). The data points are connected via an arbitraryfunction.

238 K.G. Jespersen et al. / Organic Electronics 7 (2006) 235–242

the transient spectrum, and what is left is a long-lived residual signal in the form of photoinducedabsorption (PA) together with a partly recoveredPB. When increasing the PCBM concentration(the three lower panels), the amplitude of thelong-lived feature increases and a faster quenchingof the excited state is observed, which is correlatedwith a faster formation of the long-lived PA feature.This is seen in the early time spectra, in the 4:1 sam-ple (second panel), the 0.5 ps spectrum indicates acontribution from SE decay and PA formation at700 nm, while for the 1:1 and 1:4 samples (thirdand fourth panels), the 0.5 ps spectra are very simi-lar to the long time spectra indicating that the majorpart of the excited state is quenched within 0.5 ps.The amplitude of the initial bleach for all PCBMconcentrations is approximately proportional tothe number of absorbed photons in the APFO3compound. At the excitation wavelength used inthe experiments, the total absorbance is primarilya result of absorption in APFO3 molecules. Thelong-lived PA signal is considered to be due toabsorption from the polymer cation in the case ofthe bulk-heterojunction, since PCBM is known tobe an effective electron scavenger. In neat copoly-mer we believe that the long-lived PA signal is dueto charge-separated species. This is corroboratedby the fact that the PA signal is located at the samespectral position as in the APFO3:PCBM blends.Moreover, long-lived PA signals in the visible spec-trum have been assigned to charge separated speciesin other conjugated polymers (see e.g. Ref. [23]). Inthe following we shall assume a linear correlationbetween the long-lived PA amplitude and the num-ber of generated charges in the case ofAPFO3:PCBM and the number of charge-separatedspecies in the case of neat APFO3. This is valid sincewe do not observe any significant spectral shifts thatcould result in signal amplitude ‘moving’ out of thedetection window of the spectrometer.

In order to compare charge formation in sampleswith different PCBM concentrations, we haveextracted (1) the efficiency of charge formation and(2) the relative number of generated charges fromthe amplitude of the long-lived PA. The charge for-mation efficiency (1) is defined by the number of gen-erated charges per absorbed photon in APFO3 andis calculated from the raw data by dividing thelong-lived PA amplitude at 700 nm by the numberof absorbed photons at the excitation wavelength.We note that the reflectivity is approximately thesame for samples of blend ratios 1:0, 1:4, and 0:1

at 570 nm, such that we may ignore deviations inreflection loss for samples with different blend ratios.The charge formation efficiency is plotted in Fig. 3(–n–). It indicates that maximum efficiency isobtained at a PCBM concentration close to50 wt%. The relative number of generated charges(2) in films of equal thickness is obtained from theraw data by scaling the thickness to 100 nm asdescribed above (see spectra in Fig. 2). Thus, the rel-ative number of charges formed in films of equalthickness is given directly by the long-lived PAamplitude at 700 nm. We note that the relative num-ber of charges can be considered as an overall effi-ciency that treats the sample as a whole includingglass substrate and active polymer. From Fig. 2 itis seen that the low PCBM concentration (4:1) leadsto a final PA amplitude close to that of the 1:1 sam-ple and clearly larger than the amplitude found inthe 1:4 sample. Hence, the total number of chargesgenerated is higher for samples 4:1 and 1:1 as com-pared to the sample with the APFO3:PCBM weightratio 1:4. This interesting observation is also shownin Fig. 3, where the amplitude (at 700 nm) of thelong-lived PA due to charges in samples of 100 nmthickness is mapped against PCBM concentration(–h–). The solid line connecting the data points ischosen arbitrarily, however, the maximum numberof generated charges is expected to lie in the range20–50 wt%. The reason for this is that a significant‘loss’ in the form of photoluminescence is observedat 20 wt% PCBM, while photoluminescence is effec-tively quenched at 50 wt% PCBM (data not shown).Furthermore, the charge formation efficiency curvein Fig. 3 (–n–) indicates that photoinduced charge

K.G. Jespersen et al. / Organic Electronics 7 (2006) 235–242 239

separation is more efficient at 50 wt% than at20 wt%. Hence, we expect the maximum number ofcharges to occur above 20 wt% PCBM.

3.2. Photocurrent collected in the devices

In general, the optimal PCBM concentration forbest solar cell performance should be seen as a com-promise between the number of charge carriers gen-erated in the polymer blend and the number ofPCBM molecules required to facilitate efficientcharge transport. Steady state photocurrent mea-surements show that the best performance ofAPFO3:PCBM solar cell devices was achieved inthe ratio of 1:4, which is in sharp contrast to theresults on photogeneration presented above. Photo-current collected in the devices is mainly determinedby three processes: photon absorption, exciton dis-sociation or charge generation, and charge carriercollection.

We measured the photocurrent collected indevices with different blend ratios as a function offilm thickness. The diodes were characterized undermonochromatic light at 570 nm, which is the samewavelength as the excitation pulse in the time-resolved absorption experiments. In Fig. 4 the shortcircuit current density (Jsc) of APFO3:PCBM sam-ples with composition ratios 4:1, 1:1, and 1:4 are dis-played. The diodes with the ratios 4:1, 1:1, and 1:4present very different behaviors. It shows that theJsc of the diodes with the ratio 4:1 and 1:1 decreaseswith increasing thickness, which indicates that the

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Fig. 4. The Jsc of APFO3:PCBM solar cells vs the thickness ofthe active layers under monochromatic illumination(570 nm,0.2 mW/cm2), where the square symbols are for the diodes withratio of 4:1, circles for 1:1 and up-triangles for 1:4.

transport property of the active-layer limits theJsc. Oppositely, it seems that the Jsc of the diodeswith the ratio 1:4 increases with increasing thicknessup to 255 nm, and thereafter decreases with thethickness. The fact that Jsc has a maximum impliesthat absorption limits Jsc at thinner active layers,and transport becomes a limitation for Jsc at thickerlayers.

In order to make a comparison between thecharge generation results of the previous sectionand the photocurrent density measurements pre-sented above it must be taken into account thatthe samples used in the electrical characterizationvary in thickness. That is, we consider the sampleswith ratio 4:1, 1:1, and 1:4 having thickness 130–160 nm, 150–170 nm, and 250–300 nm, respectively.This could make a comparison with the charge gen-eration results difficult. However, the absorptionspectra of the films with these varied thicknessesall present comparable absorption at 570 nm (thewavelength used in the time-resolved absorptionexperiments) as seen in Fig. 5. Thus, a similar num-ber of photons are absorbed by these specific filmsof different PCBM concentrations.

If we compare the Jsc of those diodes (Fig. 4)with similar absorption in Fig. 5, it is clear thatthe diode of blend ratio 4:1 (130 nm) presents muchlower Jsc (�4 lA/cm2) than the diode 1:1 (175 nm)(�21 lA/cm2), and 1:4 (280 nm) (�29 lA/cm2).The current density is displayed in Fig. 3 for visualcomparison. It is seen that Jsc increases monotoni-cally up to 80 wt% PCBM. Moreover, optimizationof the solar cell performance indicates that the high-est current density is indeed found at a ratio close to1:4, in good agreement with previous results onsolar cells based on other conjugated polymers

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Fig. 5. Absorption spectra of APFO3:PCBM in the ‘solar cell’with the ratio of 4:1 (dot line), 1:1 (dash line), and 1:4 (solid line).

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Fig. 6. The FFs of APFO3:PCBM solar cells vs the thickness ofthe active layers under monochromatic illumination(570 nm,0.2 mW/cm2), where the squares symbols are for the diodes withratio of 4:1, circles for 1:1 and up-triangles for 1:4.

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and PCBM. This is significantly different to theresults on charge generation. A notable point is thatthe small deviations in optical density for the spe-cific films as seen in Fig. 5 are insufficient to accountfor this difference.

The photocurrent peaks at higher PCBM concen-trations as compared to charge generation. There-fore, the fact that we observe a maximum chargeformation efficiency at �50 wt% and a maximumnumber of charged species at 20–50 wt% PCBM,while the short circuit current density peaks at�80 wt% argues that the solar cell performance ofAPFO3:PCBM is limited by charge transport ratherthan by charge formation.

3.3. Transport properties of the devices

To understand the different results betweencharge generation and photocurrent of the devices,we need to take into account charge transport inthe active layer because the photocurrent collectedin the devices not only depends on the charge gener-ation, but also the charge transport, that is, themobility. Charge transport properties in bulk-het-erojunction solar cells are very sensitive to the mor-phology of the active layer [17,18]. Bjorstrom et alinvestigated the morphologies of the films with samecomposition, but different stoichiometries (1:0, 1:1,1:2, 1:3, and 1:4) using atomic force microscopy[24,25]. AFM images presented little evidence forphase separation in all blends. There were no pro-nounced differences among those films with differentPCBM loadings. Fig. 6 presents the fill factors (FFs)of the same devices with the section above. It is obvi-ous that the FFs of the diodes show monotonicincrease with increasing PCBM concentration, irre-spective of thickness. The FF of diodes 4:1 is between0.23 and 0.27, for diode 1:1 it is 0.34 to 0.44, and fordiode 1:4 it is 0.63 and 0.72. This strongly suggeststhat the transport properties are improved whenadding more PCBM. Therefore, the transport prop-erty in the devices may be responsible for the devia-tion of photocurrent and charge generation.

It is tempting to assign this limiting process toelectron transport in the PCBM phase since onecould imagine that a high concentration of PCBMis required to facilitate efficient electron hopping.However, it has been reported that not only doeselectron mobility depend on PCBM concentrationbut also hole mobility seems to be increasing withPCBM concentration [26]. Tuladhar et al. found thatPCBM can support ambipolar transport, which can

explain the relative high PCBM to polymer rationeeded in previous studies [27]. This is in very good,keeping with the results on annealed P3HT:PCBMmentioned in the introduction. Annealing of theP3HT:PCBM blend results in self-organization[18,28] creating domains of crystalline polymerphase with high hole mobility. This reduces the roleof PCBM in hole transport in the annealed blendand makes it possible to approach a lower and morefavorable PCBM concentration for the solar cell per-formance. In this study we have measured the opti-mal PCBM contents to be 20–50 wt% for theAPFO3:PCBM bulk-heterojunction using femtosec-ond transient absorption spectroscopy, which is invery good agreement with the optimal ratio of1:0.8 (44 wt% PCBM) and 1:1 (50 wt% PCBM)found in the annealed P3HT:PCBM solar cells.

In order to understand the influence of PCBM ontransport, we have investigated hole transport inblend APFO3:PCBM films with weight ratio of1:1, 1:3, 1:4, and 1:5, using the space charge limitedcurrent–voltage characteristics of hole only diodes.The variation of hole mobility with acceptor con-centration is depicted in Fig. 7. The hole mobilityis enhanced as a consequence of varying PCBMconcentration from 50 to 83 wt.%. This confirmsthat the large amount of PCBM content in polymer-based solar cells results into facilitating chargetransport while charge generation is already com-plete at a small PCBM concentration. The influenceof PCBM loading on hole mobility was alsoobserved in field effect transistors (FET) based onthe same blends [29].

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Fig. 7. Hole mobility as a function of PCBM concentration.

K.G. Jespersen et al. / Organic Electronics 7 (2006) 235–242 241

The above discussion suggests more precisely thathole transport in the polymer phase is the perfor-mance limiting process. It is well known that the elec-tron mobility of PCBM is 1–2 orders of magnitudelarger than the hole mobility in most polymers, whichmeans that the bottleneck for charge transport is holemobility in polymer. The hole mobility increase whenadding more PCBM, which also means the better bal-ance between hole and electron transport. This isconsistent with our observation from solar cells andalso supported by Rappaport et al. [30] who calcu-lated that in order to obtain an ideal fill factor thehole mobility in the polymer phase should increaseby two orders of magnitude compared to previousnon-annealed solar cells. In our work we find thatthe fill factor is very sensitive to the PCBM concen-tration; the more PCBM, the larger FF.

The transport limited performance in APFO3:PCBM blends suggests that the power conversionefficiency in APFO3:PCBM solar cells could beincreased if proper material processing leads to ahigh hole mobility. Polyfluorenes are known to haveliquid crystalline phases with an enhanced holemobility [31]. If this phase can be obtained, eitherby thermal treatment or by other means of process-ing, it would result in an increase in the solar cellperformance of APFO3:PCBM. A high PCE solarcell based on polyfluorene copolymers is attractivesince polyfluorenes are relatively stable against pho-tochemical degradation, compared to other conju-gated polymers.

4. Conclusion

In conclusion we have used time-resolved spec-troscopy to show that charge formation in

APFO3:PCBM bulk-heterojunctions is optimal at20–50 wt% PCBM concentration. On the otherhand, we found that Jsc is monotonically increasingup to 80 wt% PCBM. This proves that charge for-mation is not the performance limiting process.On the contrary, the results show that a high PCBMconcentration is a requirement for maximum shortcircuit current density and that charge transport islimiting the performance of the investigated bulk-heterojunction solar cell. The results indicate thatthe high PCBM concentration needed in the bestperforming APFO3:PCBM solar cells enhances theeffective hole transport in the blend. The solar cellperformance of the APFO3:PCBM blend is likelyto be increased by improving the hole transport inthe polymer phase.

Acknowledgements

We thank Prof. Mats Andersson for providingthe APFO3 copolymer, Dr. Nils-Krister Perssonfor valuable discussions. Financial support isacknowledged from the Center of Organic Electron-ics financed by the Swedish Strategic ResearchFoundation and the Swedish Research Council.

References

[1] P. Fairley, Technol. Rev. 107 (2004) 34.[2] J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia,

R.H. Friend, S.C. Moratti, A.B. Holmes, Nature 376 (1995)498.

[3] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger,Science 270 (1995) 1789.

[4] J.C. Hummelen, B.W. Knight, F. Lepeq, F. Wudl, J. Yao,C.L. Wilkins, J. Org. Chem. 60 (1995) 532.

[5] Q.H. Xu, D. Moses, A.J. Heeger, Phys. Rev. B 67 (2003)245417.

[6] C.J. Brabec, G. Zerza, G. Cerullo, S. De Silvestri, S. Luzzati,J.C. Hummelen, N.S. Sariciftci, Chem. Phys. Lett. 340 (2001)232.

[7] V.D. Mihailetchi, J.K.J. van Duren, P.W.M. Blom, J.C.Hummelen, R.A.J. Janssen, J.M. Kroon, M.T. Rispens,W.J.H. Verhees, M.M. Wienk, Adv. Funct. Mater. 13 (2003)43.

[8] S.A. Choulis, J. Nelson, Y. Kim, D. Poplavskyy, T.Kreouzis, J.R. Durrant, D.D.C. Bradley, Appl. Phys. Lett.83 (2003) 3812.

[9] S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, F. Padinger, T.Fromherz, J.C. Hummelen, Appl. Phys. Lett. 78 (2001) 841.

[10] F. Padinger, R.S. Rittberger, N.S. Sariciftci, Adv. Funct.Mater. 13 (2003) 85.

[11] C.J. Brabec, A. Cravino, G. Zerza, N.S. Sariciftci, R.Kiebooms, D. Vanderzande, J.C. Hummelen, J. Phys. Chem.B 105 (2001) 1528.

242 K.G. Jespersen et al. / Organic Electronics 7 (2006) 235–242

[12] F.L. Zhang, M. Johansson, M.R. Andersson, J.C. Humme-len, O. Inganas, Synth. Met. 137 (2003) 1401.

[13] R. Pacios, D.C. Bradley, J. Nelson, C.J. Brabec, Synth. Met.137 (2003) 1469.

[14] O. Inganas, M. Svensson, F. Zhang, A. Gadisa, N.K.Persson, X. Wang, M.R. Andersson, Appl. Phys. A–Mater.79 (2004) 31.

[15] S. Lacic, O. Inganas, J. Appl. Phys. 97 (2005) 124901.[16] M. Reyes-Reyes, K. Kim, D.L. Carroll, Appl. Phys. Lett. 87

(2005) 083506.[17] W.L. Ma, C.Y. Yang, X. Gong, K. Lee, A.J. Heeger, Adv.

Funct. Mater. 15 (2005) 1617.[18] G. Li, V. Shrotriya, J.S. Huang, Y. Yao, T. Moriarty, K.

Emery, Y. Yang, Nature Mater. 4 (2005) 864.[19] M. Svensson, F.L. Zhang, S.C. Veenstra, W.J.H. Verhees,

J.C. Hummelen, J.M. Kroon, O. Inganas, M.R. Andersson,Adv. Mater. 15 (2003) 988.

[20] T. Yohannes, F. Zhang, M. Svensson, J.C. Hummelen, M.R.Andersson, O. Inganas, Thin Solid Films 449 (2004) 152.

[21] K.G. Jespersen, W.J.D. Beenken, Y. Zaushitsyn, A. Yartsev,M. Andersson, T. Pullerits, V. Sundstrom, J. Chem. Phys.121 (2004) 12613.

[22] A. Gadisa, X. Wang, S. Admassie, E. Perzon, F. Oswald, F.Langa, M.R. Andersson, O. Inganas, Organic Electronics, inpress.

[23] A. Ruseckas, M. Theander, M.R. Andersson, M. Svensson,M. Prato, O. Inganas, V. Sundstrom, Chem. Phys. Lett. 322(2000) 136.

[24] C.M. Bjorstrom, K.O. Magnusson, E. Moons, Synth. Met.152 (2005) 109.

[25] O. Inganas, F. Zhang, X. Wang, A. Gadisa, N-K. Persson,M. Svensson, E. Perzon, W. Mammo, M.R. Andersson, in:S. Sun, S. Sariciftci (Eds.), ‘‘Organic Photovoltaics: Mech-anisms, Materials and Devices’’, CRC, 2005 (Chapter 17).

[26] C. Melzer, E.J. Koop, V.D. Mihailetchi, P.W.M. Blom, Adv.Funct. Mater. 14 (2004) 865.

[27] S.M. Tuladhar, D. Poplavskyy, S.A. Choulis, J.R. Durrant,D.D.C. Bradley, J. Nelson, Adv. Funct. Mater. 15 (2005)1171.

[28] T. Erb, U. Zhokhavets, G. Gobsch, S. Raleva, B. Stuhn, P.Schilinsky, C. Waldauf, C.J. Brabec, Adv. Funct. Mater. 15(2005) 1193.

[29] L.M. Andersson, O. Inganas, Acceptor influence on holemobility in fullerene blends with alternating copolymers offluorene, Appl. Phys. Lett. 88 (2006) 082103.

[30] N. Rappaport, O. Solomesch, N. Tessler, J. Appl. Phys. 98(2005) 033714.

[31] M. Chen, C. Xavier, E. Perzon, M.R. Andersson, T.Pullerits, M. Andersson, O. Inganas, M. Berggren, Appl.Phys. Lett. 87 (2005) 252105.