Thin Solid Films 519 (2011) 4201–4206

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Metal sulfide–polymer nanocomposite thin films prepared by a direct formationroute for photovoltaic applications

Eugen Maier a, Achim Fischereder a,b, Wernfried Haas b,c, Gernot Mauthner b,d, Jörg Albering a,Thomas Rath a,b, Ferdinand Hofer c, Emil J.W. List d,e, Gregor Trimmel a,b,⁎a Institute for Chemistry and Technology of Materials (ICTM), Graz University of Technology, Stremayrgasse 9, A-8010 Graz, Austriab Christian Doppler Laboratory for Nanocomposite Solar Cells, Graz University of Technology, Stremayrgasse 9, A-8010 Graz, Austriac Institute for Electron Microscopy and Fine Structure Research, Graz University of Technology (FELMI), Steyrergasse 17, A-8010 Graz, Austriad NanoTecCenter Weiz Forschungsgesellschaft mbH, Franz-Pichler-Straße 32, A-8160 Weiz, Austriae Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria

⁎ Corresponding author at: Institute for Chemistry(ICTM), Graz University of Technology, Stremayrgasse+43 316 87332281.

E-mail address: gregor.trimmel@tugraz.at (G. Trimm

0040-6090/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.tsf.2011.02.020

a b s t r a c t

a r t i c l e i n f o

Article history:Received 4 March 2010Received in revised form 6 October 2010Accepted 4 February 2011Available online 18 February 2011

Keywords:Hybrid materialsNanocompositesMetal sulfidesOrganic photovoltaicsTransmission electron microscopyX-ray diffraction

Blends of conjugated polymers and inorganic semiconductors are an interesting class of materials with variousapplications in the field of plastic electronics. This work presents a direct approach to obtain compositesconsisting of a conjugated polymer, poly(3-(ethyl-4-butanoate)thiophene) (P3EBT), and a sulfur-basedsemiconductor (i.e. CdS, PbS or ZnS) using an in-situ formation route. The metal sulfide semiconductor isformed by reaction of the correspondingmetal salt (cadmiumacetate, zinc acetate or lead thiocyanate) dispersedwithin the conjugated polymer matrix with thiourea at temperatures below 200 °C. Nanoscaled networks areformed in the caseof theCdS- andZnS-P3EBTcomposites as shownbyX-raydiffractionand transmissionelectronmicroscopy investigations, whereas the PbS-P3EBT blend exhibits inorganic structures on the μm-scale. Thematerials were used as active layer in bulk-heterojunction type hybrid solar cells. First photovoltaic devicescontaining an active layer of CdS- or ZnS-P3EBT show photovoltaic action, though efficiencies are low (≤0.06%).

and Technology of Materials9, A-8010 Graz, Austria. Tel.:

el).

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The need for energy resources alternative to fossil fuels makesphotovoltaics extremely attractive for industry and scientific research. Inrecent years polymer based solar cells have gained remarkable powerconversion efficiencies (PCEs) exceeding 6% [1,2] and their commercial-ization has already set in [3]. The use of polymer processing technologiesand the lowconsumptionofmaterials aremajor advantages,which allowthe roll-to-roll production of fully flexible photovoltaic modules on alarge scale. The best investigated and developed systems are based onblends of conjugated polymers and soluble fullerene derivatives [4,5].Theoretical considerations predict PCEs up to 10% for these systems [6,7].In these material combinations, the absorption of light leads to theformation of excitons [8,9]. Assuming an exciton binding energy of a fewtenths eVs, only an infinitesimal fraction of the excitons is directlyconvertible into extractable charge carriers in the absence of a suitableacceptor phase for electrons, and therefore, fullerenes are used as

effective electronacceptors. Amajor criterion for the bulk-heterojunctionapproach is an optimized phase separation on the nanoscale of the donor(polymer) and the acceptor phase (fullerene). The optimummorphologyconsists of an interpenetrating network ensuring a high interfacial areaneeded for charge separation and continuous pathways to the electrodesto optimize charge transport [10,11].

As an alternative, organic–inorganic hybrid materials consisting ofan organic donor and a nanostructured inorganic acceptor can be usedas active layer in plastic photovoltaics [12–14]. This approach allows analmost infinite number of combinations of inorganic and organicsemiconductors. Using inorganic nanostructures, additional tuningparameters are the shape of the semiconductor — i.e. spherical, rod-like, rectangular, and branched crystals— and the size of the structuresdue to quantum confinement effects [15]. Examples for already realizedcombinations are blends of a conjugated polymer and nanocrystals ofCdSe [16,17], CdS [18], ZnO [19,20], Zn1−xMgxO [21], CdTe [22,23], InAs[24], CuInS2 [25], CuInSe2 [26], PbS [27,28], PbSe [29], or TiO2 [30,31].Power conversion efficiencies of hybrid plastic solar cells are inferiorcompared to fullerene based devices reaching efficiencies of about 2.5%[12]. Very recently, PCEs of nanocomposite solar cells based on CdSe-nanostructures have exceeded values of 3% (3.13%) [32]. We attributethe lower PCEs of hybrid solar cells compared to fullerene-polymer solarcells to the following reasons: (1) preparation of semiconductornanoparticles and nanostructures often involves the application of a

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surfactant (capper) to prevent particle agglomeration, acting as a highbarrier for exciton dissociation and charge transport, which cannot beeasily removed in many cases; (2) a high surface-to-volume ratio leadsto high concentration of surface defects and, (3) compared to fullerene-polymer solar cells, only a fraction of manpower has worked on hybridphotovoltaics, especially when the manifold possibilities of materialscombinations are considered.

Most research groups pursue one of the following three synthesisstrategies for organic–inorganic hybrid-photovoltaic materials: (1) thesynthesis of inorganic nanocrystals, which are then mixed with theorganic semiconductor, (2) the creation of inorganic nanostructures onthe electrode surface, e.g. aligned nanorods, subsequently infiltratedwith the polymer, and (3) the in-situ formation of inorganicnanostructures within the polymer matrix. The major advantages ofthe latter one are that (1) no separate nanoparticle synthesis step isnecessary and (2) no capping agents to prevent agglomeration ofnanoparticles are used in this method.

In-situ-formation for hybrid-photovoltaic application has beendemonstrated for titania- [33] and zinc oxide-conjugated polymerblends [34,35]. In addition, several papers report on the opticalproperties of metal–sulfide-conjugated polymer composites using adirect synthesis route, e.g. (poly 3-hexylthiophene)-ZnS using Na2S assulfur source [36] or poly(3-octylthiophene)-MeS (Me=Zn, Cd, Cu)composites prepared by Langmuir–Blodgett-techniques and gaseousH2S as sulfurisation reagent [37]. Alternatively, thioacetamidewas usedas sulfur source, as e.g. realized in the preparation of CdS-polyvinylpyr-rolidone blends in a recent study [38]. Very recently, Liao et al. reportedCdS-poly(3-hexylthiophene) nanocomposite solar cells with PCEs up to3% by preparing CdS nanoparticles directly in the polymer solution fromcadmiumacetate and elemental sulfur. During the reaction, the polymeracts as capper and prevents extensive particle growth. These solutionswere then used for the preparation of solar cells [39].

This contribution introduces a facile preparation method to obtainthin films consisting of metal sulfide-poly(3-(ethyl-4-butanoate)thio-phene) (P3EBT)-blends starting frommetal salts and thiourea dispersedwithin thepolymer. The sulfur is suppliedby the thermal decompositionof thiourea, a well established and very commonly used method toobtain inorganic sulfide semiconductors [40,41]. The reaction takesplace at temperatures below 200 °C, which is compatible with manyelectroactive polymeric materials and flexible substrates. In this studywe focus on the preparation andmorphological characterization of ZnS,CdS and PbS–polymer nanocomposites. For one specific composite —

CdS-P3EBT—we also demonstrate the applicability of such compositesin hybrid solar cells.

2. Experimental details

2.1. Materials

Cadmiumacetate hydrate (Cd(Ac)2×nH2O, 99.99%), lead thiocyanate(Pb(SCN)2, 99.99%), zinc acetate (Zn(Ac)2), thiourea (TU, 99.0%) andpyridine (99.9%) were purchased from Sigma-Aldrich. Poly(3-(ethyl-4-butanoate)thiophene) (P3EBT) was acquired from Rieke Metals Inc.Indium-tin-oxide (ITO)-coated glass slides with a surface resistivity of20Ω/sq. were received from Delta Technologies Ltd. Water content incadmium acetate hydrate was determined to be 7.8%, corresponding toCd(Ac)2×1.08 H2O by Karl–Fischer titration. All chemicals were usedwithout further purification.

2.2. Preparation of the nanocomposite layers

For the precursor solutions, the corresponding metal salt wasdissolved in pyridine and stirred until a colorless, clear solution wasobtained [c(Cd(Ac)2×1.08H2O=41.5 mg/mL (0.17 mmol/mL); c(Pb(SCN)2=32.6 mg/mL (0.10 mmol/mL); c(Zn(Ac)2=45.3 mg/mL(0.25 mmol/mL)]. P3EBT was added to this solution, c(P3EBT)=6 mg/

mL). The mixture was heated to 50 °C until a clear, orange coloredsolution was obtained. Finally, fivefold excess of thiourea was added tothe metal salt - polymer solution (c(TU)=0.84 mmol/mL; c(TU)=0.50 mmol/mL; c(TU)=1.23 mmol/mL for the CdS, PbS and ZnSprecursor solution, respectively). In the case of the samples preparedfor X-ray diffraction and transmission electron microscopy measure-ments, the precursor solution was diluted 1/1 (vol.) with pyridine. Theprecursor solution was coated onto different substrates (glass-ITO,borosilicate glass, NaCl-single crystal, depending on experiment), andheated in a tube-furnace under vacuum to 180 °C (heating rate: roomtemperature→

3 min80 °C→

20 min180 °C→

10 min180 °C). Afterwards, the

tube was flooded with nitrogen, removed from the furnace and cooleddown to ambient temperature within 30 min.

2.3. Characterization methods

Powder-X-ray diffraction (XRD) measurements were performed ona Siemens D-5005 powder-diffractometer (theta-theta geometry, Cu-Kα-radiation). Rietveld analysis was performed using Bruker-TOPASsoftware. For sample preparation, precursor solutionswere drop-coatedonto borosilicate glass slides. Samples were heated according to theprocedure described above. As prepared layers were scraped off andplaced on a silicon sample holder.

The diameters of the crystallites were estimated according to thebroadening of the diffraction peaks using the Scherrer relationship(Eq. (1)):

DXRD ≈ K⁎ λΔ 2θð Þ⁎ cosθ

ð1Þ

with Δ(2θ) is the full width at half maximum (FWHM) of the peak inradians, θ is half of the scattering angle 2θ, λ the wavelength of the X-rays, and K is the shape factor (K=0.9 for spherical particles).

Transmission electron microscopy (TEM) investigations and selectedarea electron diffraction (SAED) patterns were acquired with a Tecnai 12microscope (FEI Company, 120 kV, LaB6 Cathode). High resolution TEM-micrographswereobtainedusing aTecnai F 20microscope (FEI Company,200 kV, Schottky emitter) equipped with a monochromator (FEICompany), a Gatan energy filter system and an UltraScan CCD camera.For sample preparation, the precursor solutions were spin coated ontoNaCl-single crystals which were consecutively heated as described. Aftercooling to ambient temperature, NaCl substrates were dissolved indistilledwater, and thefloating nanocompositefilmswere collectedusinga Ni-TEM grid.

Atomic forcemicroscopy (AFM) imageswere recordedusingaVeecoNanoMan VS Scanning Probe Microscope. Measurements were takendirectly from the surface of the nanocomposite solar cells in the areasbetween the metal contacts.

UV–Vis spectra were collected using a Shimadzu UV-1800spectrometer.

2.4. Preparation of nanocomposite solar cells

ITO-glass substrates were coated by spin coating of the precursorsolution at ambient conditions and subsequently heated as describedabove. After cooling down to ambient temperature, aluminum cathodes(thickness:200 nm;electrodearea=0.1 cm2)were evaporated at a basepressure of approx. 3×10−6 mbar using a Bal-TecMED-020 evaporationunit. Current–voltage curves were measured under N2-atmosphereusing a Keithley-2400 sourcemeter, samples were illuminated by ahalogen lamp (Pin=100 mW cm−2, the illumination power wasdetermined using a calibrated KippZonen CMP-11 pyranometer, nospectral mismatch correction).

Fig. 2. XRD pattern of a CdS-P3EBT nanocomposite. Mainly hexagonal CdS (wurtzite-structure) with diameters of approximately 4.3 nm (estimated by Scherrer equation)was obtained.

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3. Results and discussion

3.1. Preparation of the nanocomposite layers

Nanocomposite layers of a conjugated polymer and CdS, PbS orZnS, respectively, were prepared using an in-situ-formation processby reacting metal salts (cadmium acetate, lead thiocyanate, or zincacetate) and thiourea directly within a matrix of a conjugatedpolymer. As electroactive polymer we selected poly-(3-(ethyl-4-butanoate)thiophene) (P3EBT), a polythiophene with a polar sidegroup, which is necessary to ensure a homogeneous precursorsolution of the polymer and the inorganic salts. The concentrationsof the compounds were chosen in a way, that complete reaction of therespective metal salt with thiourea would yield 24 mg/mL of metal–sulfide and that the ratio of polymer to metal–sulfide is 1/4 (wt./wt.).The overall process is schematically depicted in Fig. 1.

The metal salt is dissolved in pyridine, then the polymer, andconsecutively thiourea is added to the solution. Thin films of theprecursor layers in the range of 50 nm to 300 nm can be prepared byspin coatingof this solutiononglass substrates aswell asdirectly on ITO-covered glass substrates. During the subsequent annealing process,these layers were heated up to a final temperature of 180 °C in vacuumleading to the decomposition of thiourea and the release of reactivesulfur-species, which react with the metal cations to the correspondingmetal sulfides. The resultingmaterials were investigated using XRD andTEM. Blends of CdS and P3EBTwere prepared using cadmium acetate asprecursor salt. The nanocomposite layers exhibit a yellow-orange color(optical spectra are presented in Fig. 8).

Fig. 2 shows the XRD pattern of a CdS-P3EBT layer. For XRD-measurements thicker films have been prepared by drop coating ontoglass substrates and were scraped off after the heating step to obtain asufficient amount of material for analysis. The positions of the XRD-reflections indicate the formation of hexagonal CdS (wurtzitestructure, reference file PDF 41-1049, from the Joint Committee onPowder Diffraction Standards, ASTM, Philadelphia, PA). The broadnessof the reflections implicates the existence of a nanostructuredmaterial. Using Scherrer relation and the (012)-reflection at 36.7°,the diameter of the primary crystallites is estimated to be 4.3 nm. Inaddition, a small quantity of cubic CdS (PDF 10-454) of approximately10% is present in the sample, estimated by a Rietveld fit of the XRDpattern.

These dimensions are in good accordance with the TEM-micro-graphs, presented in Fig. 3, which show a dense network of sphericallyshaped CdS nanoparticles within the polymer matrix with diametersranging from 5 to 10 nm. Selected area electron diffraction recordedfrom a representative area of the composite shows a feature-richdiffraction patternwith very good correlation to the obtainedXRD-data.As expected, there is no preferential orientation of the nanocrystals inthe layer resulting in diffraction rings. In addition, the HRTEM imageshows a single CdS particle with an elongated structure. The crystal

Fig. 1. Schematic illustration of the in-situ-f

plane distance found for this particle is 0.336 nmand corresponds to the(002) spacing of hexagonal CdS.

Analogous to CdS-P3EBT nanocomposites, ZnS-P3EBT layers areaccessible using zinc acetate as metal salt. An XRD pattern of such amaterial is shown in Fig. 4. The main peaks are in good accordancewith the reference file, PDF 5-566 for sphalerite ZnS. The broadeningof the peaks indicates that the primary crystallites are also in thenanometer range. From the half-width of the (113-reflection at 56.3°2-theta, a primary crystallite size of 4.6 nm is estimated using theScherrer equation. However, a thorough examination of the XRDpattern shows that the peak at approximately 52.4° 2-theta is not inagreement with sphalerite structure. Therefore a Rietveld analysis ofthe pattern was performed using the two main ZnS modifications —

cubic sphalerite and hexagonal wurtzite. The phase distribution wasestimated to be roughly 80% sphalerite and 20% wurtzite. Thecalculated pattern is also presented in Fig. 4 as well as the differenceof calculated and observed data.

The corresponding TEM-data (Fig. 5) show rather diffuse aggregatesof the ZnS nanoparticles and the polymer. Particles are of sphericalshape and have diameters of about 2–5 nm. SAED indicates sphaleritestructure, with no apparent reflections from thewurtzite phase becauseof its lowamount. As in the case of CdS, nopreferential orientation of theZnS nanocrystals within the layer is observed. HRTEMof single particlesreveals a plain distance of 0.315 nm corresponding to (111) spacing.

A different situation is encountered in case of the PbS-P3EBT blends.The XRD pattern presented in Fig. 6 shows sharp peaks withoutpronounced broadening indicating the presence of larger crystallitesthan in the former two cases. The positions of the reflections are in goodagreement with the reference data file PDF 5-0592 for cubic PbS

ormation of the nanocomposite layers.

Fig. 3. TEM-investigation of CdS-P3EBT nanocomposite layer. Top image: nanoparticles ofapprox. 5 to 10 nm are found; bottom left image: high-resolution TEM shows a latticeplane distance of 0.336 nm corresponding to (200)-spacing; bottom right image: selectedarea electron diffraction proves the presence of CdS.

Fig. 5. TEM-investigation of a ZnS-P3EBT-nanocomposite layer; top image: represen-tative TEM image of the layer; bottom left image: HRTEM image of ZnS nanoparticles,the indicated spacing of d=0.315 nm corresponds to (111) spacing of sphalerite;bottom right image: selected area electron diffraction of the nanocomposite layer.

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(galena). The TEM pictures in Fig. 6 prove the presence of largercrystallites. The overview picture shows crystals in the range of 0.5 to1 μm in diameter. The SAED image on a single PbS crystal shows thesharp reflection of a single crystal in [0 0 1] orientation. The formation of

Fig. 4. Comparison of the measured XRD pattern (every 12th point is shown) of a ZnS-P3EBT nanocomposite with the calculated pattern using Rietveld analysis revealing amixture of sphalerite (80%) withwurtzite (20%). Bars at the bottom indicate the literaturevalues of sphalerite (PDF 5-566) and wurtzite (PDF 79-2204).

these larger particles could possibly result from the fact that we haveused lead thiocyanate instead of acetate. This was necessary, because itwas not possible to obtain stable solutionswith lead acetate. In the caseof lead acetate, the decomposition of thiourea already takes place atroom temperature immediately after mixing both compounds. Com-paring the preparation of all three materials, the herein presentedmethodology is suitable for the formation of metal sulfide–polymercomposites. In the case of ZnS and CdS-P3EBT-blends, the inorganicphase is in the nanometer regime and homogenously distributedwithinthematerial. However, in the case of PbS, large crystals up to 1 μmin sizeare formed. Having the average thickness of the photovoltaic activelayer in hybrid solar cells in mind, which is usually ranging from 60–200 nm [17], it is obvious that the obtained PbS-P3EBT composites arenot suited for solar cell applications.

3.2. Investigation of the photovoltaic activity

The investigated material combinations are of general interest forapplication in organic–inorganic hybrid solar cells, as the hereinpresented process is a very simple and a low cost process. Thephotovoltaic activity of our materials was probed choosing a simpledevice structure, keeping the number of interfaces andmaterial-specificinteractions low (compare Fig. 7). The precursor solutionswere directlyspin coated onto glass-ITO substrates. The resulting precursor blendswere then converted to the photovoltaic active material by theannealing step. Finally, aluminum electrodes were evaporated on topof this layer.

Fig. 6. TEM (top), SAED (inset) and XRD (bottom) data of the PbS-P3EBT composite.Micrometer-sized cubic PbS crystals are formed.

Fig. 8. Top image: comparison of the UV–Vis spectra of P3EBT (squares) with CdS-P3EBT composites (circles: CdS/P3EBT=1/1; triangles: CdS/P3EBT=4/1); bottomimage: J–V curves of a CdS-P3EBT solar cell (CdS/P3EBT=4/1) under dark conditionsand under illumination with 100 mW cm−2.

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Fig. 8, top image, shows the absorption spectra of CdS-P3EBT layersof different composite concentrations. The Pi–Pi* absorption of thepolymer at 520 nm is still the dominating feature of the 1/1(wt./wt)CdS-P3EBTcomposite (circles),whereas the overall absorptionbehaviorof the 4/1 composite (triangles) is already strongly dominated by theinorganic phase. Note that the nanoparticles present within the blendlead to considerable scattering in the transmission measurement,causing an offset in the graph. The current density–voltage plot of aCdS-P3EBT device is shown in the bottom image of Fig. 8. Whereas thedevices show reasonable open circuit voltages, the obtained photo-currents aswell as thefill factors are quite low compared to other hybrid

Fig. 7. Schematic view of the device architecture.

solar cell systems. The observed overall power conversion efficiency isfound to be below 0.1%.We have also been able to detect a photovoltaiceffect in ZnS-P3EBT-nanocomposite devices. Such devices even showeda higher open circuit voltage of up to 610 mV, however, almostnegligible current densities have been achieved and thus leading tosimilar efficiencies as pure P3EBT devices (see Table 1). Due to themicrometer size of the inorganic phase, PbS-P3EBT solar cells show nophotovoltaic activity. Table 1 summarizes the parameters of theobtained nanocomposite solar cells.

The surface morphology of the nanocomposite layers was investi-gated by AFM, as shown in Fig. 9. The films exhibit a thickness of about200 nm. In the case of the CdS-P3EBT blend, deep craters are formed

Table 1Parameters of hybrid solar cells (VOC — open circuit voltage; JSC — short circuit currentdensity; FF — fill factor; PCE — power conversion efficiency) measured underillumination with 100 mW cm−2.

Active layer VOC/mV JSC/μA cm−2 FF PCE/%

Pristine P3EBT 260 18 0.29 1.3 10−3

PbS-P3EBT – – – –

ZnS-P3EBT 610 8 0.22 b10−3

CdS-P3EBT 510 460 0.28 0.06

Fig. 9. AFM-height images (tapping mode) of a CdS-P3EBT nanocomposite layer (leftimage) and a ZnS-P3EBT nanocomposite (right image).

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throughout the layer, with edges towering up to approx. 700 nm. Thesecraters are formed during the thermal conversion step caused by theevaporation of volatile decomposition products of thiourea and thecounterions of the metal salts, i.e. acetate. However, as shown in theTEM-investigations before, a dense nanoscaled network of hexagonalCdS crystals is created and despite of the rough morphology of thesurface first promising photovoltaic activity has been achieved. In thecase of ZnS-nanocomposites (right image) we found a similar roughsurface morphology, whereas the height-differences are lower as in theCdS-P3EBT composite. The optimization of the morphology is consid-ered to be a key parameter for the improvement of the performance ofthe solar cells. This might be achieved by optimization of the heatingrate to influence the decomposition rate and thus the evolution ofvolatile species. Further improvements could be achieved by applyingadditional interfacial layers between the active layer and the electrodes,which should improve the electrical and optical properties of the device,but could also act as smoothing layer in this particular case.

4. Conclusion

In this contribution,we introduced a novel in-situ synthesis route fornanoscaled inorganic–organic semiconductor composite layers byconverting metal salts and thiourea directly in the polymer–matrixinto inorganic metal sulfides — CdS, PbS, and ZnS — by a moderatethermal annealing step. In case of CdS- and ZnS-P3EBT, compositeswithnanoscale phase separation are obtained, whereas PbS-P3EBT compo-sites exhibit large PbS crystals up to 1 μm. First experiments of CdS-P3EBT and ZnS-P3EBT nanocomposite layers in photovoltaic devicesprove that this facile process leads to materials showing photovoltaicactivity. A simple demonstrator device containing a CdS-P3EBT activelayer exhibited a PCE of 0.06%. Improving surface roughness byoptimization of processing parameters (e.g. precursor chemicals,heating step) and utilization ofmore complex device geometries shouldincrease the performance of the solar cells. However, the simplicity ofthe process and the fact that simple precursors are used qualify thismethod as an interesting alternative to obtain inorganic–organicnanocomposites for hybrid photovoltaics.

Acknowledgements

The authors thank Franz Stelzer, Robert Saf and Dieter Meissner forhelpful discussions. Financial support by the Austrian Research

Promotion Agency (FFG), the Christian Doppler Research Association(CDG), the Austrian Federal Ministry of Economy, Family and Youth(BMWFJ) and the ISOVOLTAIC GmbH is gratefully acknowledged.

References

[1] S.H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J.S. Moon, D. Moses, M. Leclerc, K.Lee, A.J. Heeger, Nat. Photon. 3 (2009) 297.

[2] J. Hou, H. Chen, S. Zhang, R.I. Chen, Y. Yang, Y. Wu, G. Li, J. Am. Chem. Soc. 131(2009) 15586.

[3] Konarka Press release, retrieved on 2010–02 from http://www.konarka.com/index.php/site/pressreleasedetail/konarka_solar_panels_power_neubers_energy_sun_bags.

[4] G. Dennler, M.C. Scharber, C.J. Brabec, Adv. Mater. 21 (2009) 1323.[5] B.C. Thompson, J.M.J. Frechet, Angew. Chem. Int. Ed. 47 (2008) 58.[6] M.C. Scharber, D. Muehlbacher, M. Koppe, P. Denk, C. Waldauf, A.J. Heeger, C.J.

Brabec, Adv. Mater. 18 (2006) 789.[7] T. Kirchartz, K. Taretto, U. Rau, J. Phys. Chem. C 113 (2009) 17958.[8] B.A. Gregg, S.G. Chen, R.A. Cormier, Chem. Mater. 16 (2004) 4586.[9] A. Pivrikas, N.S. Sariciftci, G. Ju, Prog. Photovolt. 15 (2008) 677.

[10] J. Jo, S. Na, S. Kim, T. Lee, Y. Chung, S. Kang, D. Vak, D. Kim, Adv. Funct. Mater. 19(2009) 2398.

[11] B.R. Saunders, M.L. Turner, Adv. Colloid Interface Sci. 138 (2008) 1.[12] M.D. McGehee, MRS Bull. 34 (2009) 95.[13] E. Holder, N. Tessler, A.L. Rogach, J. Mater. Chem. 18 (2008) 1064.[14] D.V. Talapin, J. Lee, M.V. Kovalenko, E.V. Shevchenko, Chem. Rev. 110 (2010) 389.[15] B. Sun, H.J. Snaith, A.S. Dhoot, S. Westenhoff, N.C. Greenham, J. Appl. Phys. 97

(2005) 014914.[16] B. Sun, N.C. Greenham, Phys. Chem. Chem. Phys. 8 (2006) 3557.[17] J. Albero, E. Martinez-Ferrero, J. Ajuria, C. Waldauf, R. Pacios, E. Palomares, Phys.

Chem. Chem. Phys. 11 (2009) 9644.[18] N.C. Greenham, X. Peng, A.P. Alivisatos, Phys. Rev. B 54 (1996) 17628.[19] W.J.E. Beek, M.M. Wienk, R.A.J. Janssen, Adv. Mater. 16 (2004) 1009.[20] P. Ravirajan, A.M. Peiro, M.K. Nazeeruddin, M. Graetzel, D.D.C. Bradley, J.R.

Durrant, J. Nelson, J. Phys. Chem. B 110 (2006) 7635.[21] D.C. Olson, S.E. Shaheen, M.S. White, W.J. Mitchell, Adv. Funct. Mater. 17 (2007)

264.[22] S. Kumar, T. Nann, J. Mater. Res. 19 (2004) 1990.[23] I. Gur, N.A. Fromer, A.P. Alivisatos, J. Phys. Chem. B 110 (2006) 25543.[24] A. Anctil, B. Landi, J. Worman, R. Raffaelle, MRS Spring Meeting, San Francisco,

USA, 20078, April 9–13, 1013.[25] M. Piber, T. Rath, T. Griesser, G. Trimmel, F. Stelzer, D. Meissner, Conference

Record of the 2006 IEEE 4th World Conference on Photovoltaic EnergyConversion, Hawaii, USA, May 7–12, 2006, 1, 2006, p. 247.

[26] E. Arici, H. Hoppe, F. Schaeffler, D. Meissner, M.A. Malik, N.S. Sariciftci, Thin SolidFilms 451–452 (2004) 612.

[27] S.A. McDonald, G. Konstantatos, S. Zhang, P. Car, E.J.D. Klem, L. Levina, E.H. Sargent,Nat. Mater. 4 (2005) 138.

[28] S. Guenes, K.P. Fritz, H. Neugebauer, N.S. Sariciftci, S. Kumar, G.D. Scholes, Sol.Energy Mater. Sol. Cells 91 (2007) 420.

[29] X. Jiang, R.D. Schaller, S.B. Lee, J.M. Pietryga, V.I. Klimov, A.A. Zakhidov, J. Mater.Res. 22 (2007) 2204.

[30] J. Boucle, P. Ravirajan, J. Nelson, J. Mater. Chem. 14 (2007) 3141.[31] T.-W. Zeng, H.-H. Lo, C.-H. Chang, Y.-Y. Lin, C.-W. Chen,W.-F. Su, Sol. EnergyMater.

Sol. Cells 93 (2009) 952.[32] S. Dayal, N. Kopidakis, D.C. Olson, D.S. Ginley, G. Rumbles, Nano Lett. 10 (2010)

239.[33] P.A. van Hal, M.M. Wienk, J.M. Kroon, W.J.H. Verhees, L.H. Slooff, W.J.H. van

Gennip, P. Jonkheijm, R.A.J. Janssen, Adv. Mater. 15 (2003) 118.[34] W.J.E. Beek, M.M.Wienk, M. Kemerink, X. Yang, R.A.J. Janssen, J. Phys. Chem. B 109

(2005) 9505.[35] S.D. Oosterhout, M.M. Wienk, S.S. van Bavel, R. Thiedmann, L.J.A. Koster, J. Gilot, J.

Loos, V. Schmidt, R.A.J. Janssen, Nat. Mater. 8 (2009) 818.[36] Y. Dong, J. Lu, F. Yan, Q. Xu, High Perform. Polym. 21 (2009) 48.[37] V. Vidya, S. Ambily, S.N. Narang, S. Major, S.S. Talwar, Colloids Surf. A 198–200

(2002) 383.[38] C. Jing, X. Xu, X. Zhang, Z. Liu, J. Chu, J. Phys. D 42 (2009) 075402.[39] H. Liao, S. Chen, D. Liu, Macromolecules 42 (2009) 6558.[40] J. Emerson-Reynolds, J. chem. Soc. Trans. 45 (1884) 162.[41] P. O'Brien, J. McAleese, J. Mater. Chem. 8 (1998) 2309.