IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 24 (2013) 484010 (8pp) doi:10.1088/0957-4484/24/48/484010

Low temperature, solution-processedalumina for organic solar cells

Jun Peng, Qijun Sun, Zhichun Zhai, Jianyu Yuan, Xiaodong Huang,Zhiming Jin, Kunyang Li, Suidong Wang, Haiqiao Wang and Wanli Ma1

Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, 199 Ren-Ai Road,Suzhou Industrial Park, Suzhou, Jiangsu 215123, People’s Republic of China

E-mail: wlma@suda.edu.cn

Received 3 June 2013, in final form 24 July 2013Published 6 November 2013Online at stacks.iop.org/Nano/24/484010

AbstractIn this work, we have reported for the first time a facile route for developingsolution-processed Al2O3 film at a greatly reduced processing temperature and studied itsapplications in producing inverted polymer solar cells (PSCs). These PSCs using Al2O3 thinfilm as the electron-extraction layer demonstrated improved diode characteristics and achieveda 20% higher power conversion efficiency than devices using the conventional ZnO bufferlayer. Furthermore, the low temperature processing of the Al2O3 film makes it compatiblewith fabrication of flexible organic electronic devices based on plastic substrates.

S Online supplementary data available from stacks.iop.org/Nano/24/484010/mmedia

(Some figures may appear in colour only in the online journal)

1. Introduction

Solution-processible semiconducting materials haveshown great potential in organic photovoltaics (OPVs) [1–3]due to advantages like low manufacturing cost, highproduction throughput, area/shape flexible processes andlight weight. The organic/electrode interface has beenproven to be crucial to device performance [4, 5]. Inpolymer solar cells (PSCs), poly(3,4 ethylenedioxythio-phene):poly(styrenesulfonate) (PEDOT:PSS) has been widelyused as an anode interlayer [6]. However, the acidic andhygroscopic PEDOT causes the degradation of normalstructured devices by slowly etching the indium tin oxidesubstrate (ITO). Hence an inverted solar cell architecturehas been adopted to avoid the use of PEDOT [7–9], inwhich a transparent and chemically stable electron-extractioninterlayer is required to modify the ITO work function.So far, several promising metal oxide materials, such asZnO [10–14], TiOx [15–17] and Cs2CO3 [18, 19] havebeen used in solar cells as such electron-extraction layersbetween the ITO and active layers. Unfortunately, comparedto these extensively investigated materials, Al2O3 has beenrarely studied as a buffer layer used in OPVs [20, 21]

1 Author to whom any correspondence should be addressed.

although it possesses numerous advantages, such as lowcost, non-toxicity, high chemical stability and low interfacestate densities [22], likely due to its difficult fabricationprocess. The traditional deposition techniques for Al2O3

films are atomic layer deposition (ALD) [20, 23], thermalevaporation [21], plasma-enhanced atomic layer deposition(PEALD) [24] and magnetron sputtering [25], which are,however, not suitable for low cost and large scale production.To date, only very few works on solution-processed Al2O3

buffer layers in solar cells have been reported, in whicha high temperature annealing of the Al2O3 films was arequirement [26–30]. For example, in 2003, Palomares et alused a solution-processed Al2O3 blocking layer in dyesensitized solar cells and a high temperature annealing at435 ◦C for 20 min was performed to complete the thermalconversion of the precursor into Al2O3 [26]. In 2012, Ozawaet al developed a new Al2O3 sol–gel processing which againrequired a 1 h thermal annealing at 500 ◦C [29]. Evidently, thehigh temperature process of annealing during the fabricationof the solution-processed Al2O3 films is not compatiblewith the flexible organic electronic devices based on plasticsubstrates. Therefore it will be of great interest to developnew techniques for producing solution-processed Al2O3 filmsunder low temperature.

10957-4484/13/484010+08$33.00 c© 2013 IOP Publishing Ltd Printed in the UK & the USA

Nanotechnology 24 (2013) 484010 J Peng et al

In this work, we have reported a new facile and cost-effective synthesis route for developing solution-processedAl2O3 films as electron-extraction layers in PSCs. Theoptimal device performance can be obtained under lowtemperature (<150 ◦C), which makes Al2O3 a promisingmaterial for use as buffer a layer in flexible organic electronicdevices.

2. Experimental details

2.1. Synthesis of the Al2O3 precursor

The Al2O3 precursor was synthesized as follows. First,anhydrous aluminum chloride and acetic acid were mixedtogether at 100 ◦C to react. The reaction mechanism isdescribed by equation (1):

AlCl3 + 3 CH3COOH100 ◦C−→ Al(CH3COO)3↓ + 3HCl↑. (1)

The as-prepared aluminum acetate is a semi-transparent whitecolloid which shows excellent solubility in water and alcoholssuch as isopropyl alcohol, ethanol and 2-methoxy ethanol,while commercially available aluminum acetate is almostinsoluble in these solvents. The alcohol solution of thealuminum acetate colloid can be kept stable for at least afew months in a nitrogen atmosphere. We found that Al2O3fabricated using the 2-methoxy ethanol solution showed thebest performance in PSCs. As a result, it was chosen as thedefault Al2O3 precursor solution. In order to neutralize theexcess acetic acid, stoichiometric ethanolamine was addedinto the precursor solution before film fabrication. TheAl2O3 thin films were obtained by spin-coating the precursorsolution onto UV–O3 pre-treated ITO substrates and this wasfollowed by thermal annealing at various temperatures for10 min. ZnO films were also prepared by using a previouslyreported sol–gel [31] method and used as electron-extractionlayers in reference devices for comparison.

2.2. Preparation of the Al2O3 precursor solution

The Al2O3 precursor solution was prepared under ambientconditions. 2 g of anhydrous aluminum chloride (SinopharmChemical Reagent Co., Ltd, ≥99.5%) was added into 10 mlof acetic acid (Sinopharm Chemical Reagent Co., Ltd,≥99.0%). The solution was stirred at 100 ◦C for 10 minto let the aluminum chloride fully react. The synthesizedaluminum acetate is a translucent white colloid which wasseparated from the solution by centrifugation. For PSCs, theAl2O3 precursor solution was prepared by dissolving 10 mgof freshly prepared aluminum acetate colloid in 1 ml of2-methoxy ethanol and 4 µl of ethanolamine.

2.3. Film characterization

Tapping-mode AFM images were obtained by using a VeecoMulti-Mode V atomic force microscope. SEM images wereobtained by using a FEI Quanta 200 FEG scanning electronmicroscope. The XPS UPS measurement was performed in

an Omicron Nanotechnology system with a base pressureof 2 × 10−10 Torr. An Al Kα radiation source was usedfor the XPS measurement. Transmittances of Al2O3 andZnO films on quartz glass were obtained by using aUV–visible LAKESHORE LAMBDA 750 spectrometer.

2.4. Device fabrication and testing

For inverted polymer solar cells, ITO substrates were cleanedsequentially in an ultrasonic bath of detergent, ultra-purewater, acetone and isopropyl alcohol, and then treatedwith UV–ozone. The Al2O3 precursor solution was thenspin-coated on ITO at 5000 rpm for 40 s and annealed at150 ◦C for 10 min in air. The ZnO precursor solution wasprepared as Chou et al reported [31], and spin-coated at4000 rpm for 40 s, then thermally treated at 150 ◦C for10 min in air. The e-Al2O3 [21] buffer layer (‘e’ indicatingevaporation) was achieved by thermally evaporating anultra-thin aluminum layer (∼2 nm) and then convertingit to Al2O3 through a UV–O3 treatment for 1 min. Asolution of poly(3-hexylthiophene) (10 mg ml−1) and PCBM(8 mg ml−1) in o-dichlorobenzene was spin-coated on thebuffer layer at 700 rpm for 60 s, and then annealed at 120 ◦Cfor 10 min in a glove box. A layer of MoO3 6 nm thickand 100 nm of Al were sequentially deposited on the activelayer through a shadow mask at the rates of 0.5 A s−1 and3 A s−1 respectively under a pressure of 7.0×10−7 mbar (thedevice active area is 7.25 mm2). The current density–voltagecharacteristics of the photovoltaic cells were measured usinga Keithley 2400 (I–V) digital source meter under a simulatedAM 1.5 G solar irradiation at 100 mW cm−2 (Newport, ClassAAA solar simulator, 94023A-U). The light intensity wascalibrated using a certified Oriel Reference Cell (91150V)and verified with an NREL calibrated Hamamatsu S1787-04diode. The external quantum efficiency (EQE) was obtainedby using certified IPCE equipment (Zolix Instruments, Inc.,SolarCellScan100).

3. Results and discussion

3.1. The surface morphology of the Al2O3 layer

Since Al2O3 is highly insulating, the optimal thickness of theAl2O3 film for PSCs has to be kept very low (∼2 nm) in orderto minimize its series resistance and simultaneously modifythe work function of the ITO substrates. The morphologyof the ultra-thin Al2O3 film is therefore critical and it wasexamined by atomic force microscopy (AFM). Figure 1shows the AFM topography of bare ITO (figure 1(a)) andAl2O3-coated ITO annealed at 150 ◦C for 10 min (figure 1(b)).The surface of bare ITO substrate shows discontinuous largegrains of indium tin oxide crystals surrounded by voidswith a roughness of 3.6 ± 0.1 nm. The surface of theAl2O3-coated ITO (figure 1(b)) is much smoother, with adecreased roughness of 2.5 ± 0.2 nm, indicating that thevoids and ITO grains have been filled/covered by an ultra-thinAl2O3 film. The line profiles of the AFM images have been

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Figure 1. Surface morphology of the buffer layers examined by tapping-mode AFM; the image scale is 5 µm× 5µm: (a) bare ITO;(b) Al2O3-coated ITO. SEM topological morphology: (c) and (e), bare ITO on different scales; (d) and (f), Al2O3-deposited ITO ondifferent scales.

investigated and are shown in figure S1 (available at stacks.iop.org/Nano/24/484010/mmedia), which clearly shows thereduced surface roughness after Al2O3 deposition. To confirmthis observation, the film morphology was also investigatedby using scanning electron microscopy (SEM). The resultsare shown in figures 1(c) and (d). It can be observed thatthe surface structure of the ITO grains is less distinct afterthe coating of Al2O3, which coincides well with the previousobservation by using AFM. The coverage of the thin Al2O3film on the ITO grains is more evident when observed undera smaller magnification, as shown in figures 1(e) and (f).Note that some ITO grains and voids are still exposed afterthe deposition of Al2O3, suggesting that the Al2O3 filmis extremely thin. However, such an ultra-thin Al2O3 film(<5 nm) is sufficient for modifying the work function ofITO while introducing minimum series resistance. Thickerand smoother Al2O3 film can be achieved by using a moreconcentrated precursor solution.

3.2. The x-ray and ultraviolet photoelectron spectroscopyanalysis

X-ray photoemission spectroscopy (XPS) was performed toinvestigate the composition of the solution-processed Al2O3film. The XPS sample was prepared according to our devicefabrication conditions. An ultra-thin (<5 nm) Al2O3 film wasdeposited on an ITO substrate and then annealed at 150 ◦C,which is the optimal annealing temperature for our PSCs.As shown in figure 2(a), the Sn 3d, Sn 4d, In 3d and In4d peaks are from the ITO substrate. The Al 2p and O 1speaks were measured and are shown in figures 2(b) and(c) respectively. The C 1s transition is used to correct forcharging, which results in a shift of all other transitions.The calibrated positions of the Al 2p and O 1s peaks areat 74.3 eV and 531.0 eV respectively. These values areconsistent with the characteristic binding energies of Al2O3,indicating the conversion of the precursor into Al2O3 after a

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Figure 2. XPS spectra of solution-processed Al2O3 thin filmdeposited on ITO substrate annealed at 150 ◦C. (a) Full scan spectra.(b) Al 2p core level. (c) O 1s core level.

low temperature thermal annealing at only 150 ◦C. To furtherconfirm the formation of Al2O3, we investigated the ratio ofaluminum to oxygen under different annealing temperatures,

estimated from the comparison of the peak areas and thesensitivity factors in their XPS spectra (see figure S2 availableat stacks.iop.org/Nano/24/484010/mmedia). The Al:O ratiosare calculated to be 1:1.98, 1:1.65 and 1:1.49 for film annealedat 120 ◦C, 150 ◦C and 180 ◦C, respectively. We noticed thatthe Al:O ratio at 150 ◦C is slightly higher than the expected1:1.5 for pure Al2O3. The reason for this becomes apparentin figure 3. Although Al 2p core levels in figure 3(a) showa symmetric shape for all samples and were fitted by asingle component, the O 1s peaks in figure 3(b) could bedeconvoluted with roughly three different oxygen species.In addition to the oxygen atoms with a binding energy of531 eV from the Al2O3 matrix, the oxygen species witha higher binding energy at 532.3 eV can be assigned tostrongly adsorbed oxygen, OH–, or H2O during the filmfabrication process [32, 33], which can account for theslightly larger value of the Al:O ratio at 150 ◦C. The oxygenspecies with a lower binding energy at 530.2 eV may beattributed to an AlO(OH) component generated during thehydrolysis. Note that the annealing decreases the amount ofhydrolysis products and increases the amount of Al2O3 in thesolution-processed film, leading to 1:1.5 stoichiometry. Themass loss of the alumina precursor during thermal annealingwas investigated by thermogravimetric analysis (TGA), asshown in figure S3 (available at stacks.iop.org/Nano/24/484010/mmedia). The x-ray diffraction (XRD) pattern ofthe alumina powders is demonstrated in figure S4 (availableat stacks.iop.org/Nano/24/484010/mmedia), with the peaksmatching those of Al2O3. In addition, energy dispersivespectroscopy (EDS) analysis of the alumina powders (baked at150 ◦C) was performed and is shown in figure S5 (available atstacks.iop.org/Nano/24/484010/mmedia). We found that theatomic mass ratio of the alumina is about 1:1.61, which isconsistent with our aforementioned results extracted fromXPS spectra.

The electronic structures of ZnO and the ultra-thin Al2O3buffer layer as a function of the annealing temperature

Figure 3. (a) Al 2p and (b) O 1s XPS spectra of Al2O3 films deposited on Si substrates with different thermal annealing temperatures.

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Figure 4. (a) UPS spectra of ZnO film and Al2O3 precursor films as a function of the annealing temperatures. (b) UPS full spectra,(c) device structure, and (d) energy levels of the P3HT:PCBM based inverted solar cells.

were studied via ultraviolet photoelectron spectroscopy (UPS)(He I emission 21.2 eV). All the UPS measurements wereperformed on buffer layers deposited on ITO substrates.Figures 4(a) and (b) show the UPS spectra of the ZnOfilm and as-deposited Al2O3 thin film at room temperatureand annealed at 100, 150, and 200 ◦C for 10 min. Theas-deposited Al2O3 film shows a photoemission cutoff at16.94 eV, which corresponds to a work function of 4.26 eV.The work function value decreases to 4.02 eV and 3.89 eVupon thermal annealing at 100 ◦C and 150 ◦C respectively.However, after thermal annealing at 200 ◦C, the work functionof the Al2O3-coated ITO goes back to 4.29 eV, which ispossibly due to the morphology change of the Al2O3 filmupon higher temperature annealing. As shown in figure S6(available at stacks.iop.org/Nano/24/484010/mmedia), we canclearly observe at an annealing temperature above 200 ◦Cthe reduced coverage of the Al2O3 film on ITO substrate,likely due to the annealing-driven aggregation of Al2O3clusters. Thus we speculate that the more exposed ITO surfacecauses the change of the film work function towards thevalue of ITO (4.8 eV). For the ZnO-coated ITO, a workfunction of 4.4 eV was determined, which is consistent witha previous report [31]. As the lowest unoccupied molecularorbital (LUMO) of [6,6]-phenyl C61 butyric acid methylester (PCBM) locates between 3.9 and 4.2 eV [34–38], this

implies that the ultra-thin Al2O3 film possesses a suitablework function for the use as an electron-extraction interlayerin our PSCs (see figure 4(d)).

3.3. The performance of the inverted solar cells

To demonstrate the electron-selective effect of the Al2O3buffer layer in PSCs, inverted solar cells were fabricatedwith a structure of ITO/Al2O3/P3HT:PCBM/MoO3/Al (seefigure 4(c)). For comparison, solution-processed ZnO-buffered solar cells were used as control devices in thiswork, since it has been proven that ZnO is very effectivefor use as an electron-extraction layer in PSCs. The J–Vcurves of devices buffered with ZnO, Al2O3 and e-Al2O3(thermal evaporation based) [21] are shown in figure 5(c),while the detailed parameters are summarized in table 1. Thedevices buffered with solution-processed Al2O3 films exhibitthe best power conversion efficiency (PCE) of 3.66% witha short circuit current density (Jsc) of 9.34 mA cm−2, anopen-circuit voltage (Voc) of 0.63 V and a fill factor (FF)of 0.622. The PCE of the e-Al2O3-buffered device is 3.6%.The e-Al2O3 layer is fabricated by thermally evaporatingan ultra-thin aluminum layer (∼2 nm) and then performinga UV–O3 treatment for 1 min to convert the aluminum toAl2O3. The slightly better PCE of Al2O3-buffered devices

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Figure 5. (a) The optical transmittance of Al2O3 (filled circle) and ZnO (filled square). (b) Dark J–V characteristics of devices using ZnOand Al2O3 buffer layers. (c) J–V characteristics of P3HT:PCBM solar cells using Al2O3, ZnO and e-Al2O3 buffer layers. (d) Externalquantum efficiency of the P3HT:PCBM devices using different buffer layers.

Table 1. Photovoltaic performance of the P3HT/PCBM devicesusing various buffer layers.

Buffer layer Jsc (mA cm−2) Voc (V) FF (%) PCE (%)

Al2O3 9.34 0.630 62.2 3.66ZnO 8.03 0.620 61.3 3.05e-Al2O3

a 8.90 0.637 63.5 3.60

a The e-Al2O3 layer was fabricated by thermally evaporating anultra-thin aluminum layer (∼2 nm) and then converting it to Al2O3through a UV–O3 treatment for 1 min.

has demonstrated that the low temperature solution-processedAl2O3 film can be as effective as the thermally evaporatede-Al2O3 film, while the processing procedure for the formeris much simpler. In addition, the PCE of the Al2O3-buffereddevice is about 20% higher than that of the ZnO-buffereddevice (3.05%). The improved PCE can be mainly attributedto the increased Jsc (from 8.03 to 9.34 mA cm−2), whichmight be partially due to the different transmittances ofthe two buffer layers. The transmittances of the Al2O3 andZnO film (both deposited on quartz) were measured andare shown in figure 5(a). A transmittance of above 92%is observed in the range of 300–800 nm for the Al2O3film (the optimal thickness for device performance), whilethe control ZnO film (∼50 nm) shows evidently lowertransmittance over the whole range, especially in the UVregion. The different transmittances suggest that the Al2O3-buffered devices can absorb more incident photons than thecommon ZnO-buffered devices and potentially achieve higher

photocurrent. Moreover, Al2O3 has a larger band gap andis more insulating than ZnO, which makes it more effectivein blocking holes and reducing charge recombination anddevice leakage current. According to the dark J–V curvesshown in figure 5(b), the Al2O3-buffered device exhibitshigher rectification ratio and lower leakage current underreverse bias than the ZnO-buffered one. The improved diodecharacteristics indicate larger parallel resistance and lowerserial resistance for the Al2O3-buffered device. As a resultof all these factors, enhanced device efficiency was obtained.In conclusion, the simple synthesis procedure and superiordevice performance make solution-processed Al2O3 film anexcellent electron-extraction layer in PSCs. To evaluate thephoto-response of devices using different buffer layers andcalibrate the Jsc data, external quantum efficiencies (EQE) ofthe devices were measured and these are shown in figure 5(d).The Jsc calculated by integrating the EQE curve with anAM1.5G reference spectrum is within 5% error compared tothe corresponding Jsc obtained from the J–V curves.

In order to optimize the photovoltaic performance, thedependences of the device PCEs on the concentration ofthe Al2O3 precursor solution and the amount of addedethanolamine were systematically investigated. The resultsare shown in figure 6(a). The best PCE was achieved whenthe precursor solution was prepared by dissolving 10 mg ofaluminum acetate colloid in a mixture of 1 ml of 2-methoxyethanol and 4 µl of ethanolamine. It was found that asmall amount of ethanolamine can help improve deviceperformance, while decreased PCE can be observed with

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Figure 6. (a) The evolution of device PCEs with the concentrationof ethanolamine and Al2O3 precursor in 2-methoxy ethanol. (b) Thevariation of device PCEs with the Al2O3 annealing temperatures.

the addition of more than 4–6 µl of ethanolamine in 1 mlof 2-methoxy ethanol. Since alumina is highly insulating,its thickness should be critical to the device performance.As shown in figure S7 (available at stacks.iop.org/Nano/24/484010/mmedia), we have systematically investigatedthe device performances as a function of the precursorconcentration and spin speed. The best PCE is achievedfor devices buffered with alumina cast from 10 mg ml−1

solution at 5000 rpm. With decreased coating speed andincreased concentration of the alumina precursor solution,the corresponding PCEs decrease dramatically. AFM lineprofiling analysis was carried out to measure the thicknessof the alumina buffer layers fabricated under differentconditions. As shown in figure S8 (available at stacks.iop.org/Nano/24/484010/mmedia), the thicknesses of the aluminabuffer layers cast from 10 mg ml−1 and 20 mg ml−1 solutionat 1000 rpm are about 5.5 nm and 12.5 nm respectively. Withan alumina thickness of 5.5 nm, the device performance isalready significantly poorer than the optimal performance.With an alumina thickness of 12.5 nm, the device PCE isonly 0.22%, which clearly indicates the important effect ofthe Al2O3 thickness on the device performance. The choiceof solvent for the Al2O3 precursor was also investigated.

Different solvents, such as deionized water and isopropylalcohol, were used during the device fabrication. Theresults (table S1 available at stacks.iop.org/Nano/24/484010/mmedia) show that the devices using 2-methoxy ethanolsolution demonstrate the best performance. Note that arelatively high PCE approaching 3% was achieved for devicesusing distilled water solution, which can potentially makethe processing more environmental friendly. On the basis ofthe aforementioned UPS measurement, we concluded that theannealing temperature is critical to the work function of theAl2O3 film. Therefore, further study of the annealing effecton the device performance was carried out. As shown infigure 6(b), with elevated annealing temperature, the deviceperformance is gradually improved. It is worth noting thateven devices based on as-prepared Al2O3 film can achieve adecent PCE of 3.2% under room temperature. The best PCEis achieved for devices annealed at an optimal temperatureof 150 ◦C. At higher temperatures, device performancedrops rapidly due to the deteriorated morphology of theAl2O3 buffer layer (figure S6 available at stacks.iop.org/Nano/24/484010/mmedia) as mentioned earlier. During themeasurement we found that the Al2O3 films have a lightsoaking effect, which is often observed in PSCs bufferedwith solution-processed metal oxides such as ZnO [39] andTiO2 [40, 41]. The performance of the devices will graduallyimprove and reach a steady state after 15–30 min illuminationunder a solar simulator (see figure S9 available at stacks.iop.org/Nano/24/484010/mmedia).

4. Conclusions

In conclusion, we have reported for the first time a facileroute for developing solution-processed Al2O3 films at atemperature lower than 150 ◦C and studied their applicationin inverted PSCs. The PSCs using a Al2O3 thin film asthe electron-extraction layer demonstrated improved diodecharacteristics and achieved a 20% higher PCE than thedevices using the conventional ZnO buffer layer. Furthermore,this facile method of depositing solution-processed Al2O3film under low temperature could be applicable to other thinfilm electronic devices such as organic light emitting diodes,field effect transistors, memories and inverters.

Acknowledgments

This work was supported by the National High TechnologyResearch and Development Program of China (863 Program)(grant No. 2011AA050520), the National Natural ScienceFoundation of China (grant No. 61176054), and thePriority Academic Program Development of Jiangsu HigherEducation Institutions. This was also a project supported bythe Fund for Innovative Research Teams of Jiangsu HigherEducation Institutions.

References

[1] Li G, Zhu R and Yang Y 2012 Nature Photon. 6 153[2] Brabec C J, Gowrisanker S, Halls J J M, Laird D, Jia S and

Williams S P 2010 Adv. Mater. 22 3839

7

Nanotechnology 24 (2013) 484010 J Peng et al

[3] Thompson B C and Frechet J M J 2008 Angew. Chem. Int. Edn47 58

[4] Yip H L and Jen A K Y 2012 Energy Environ. Sci. 5 5994[5] Chen L M, Xu Z, Hong Z and Yang Y 2010 J. Mater. Chem.

20 2575[6] Carbonera R, Po C, Bernardi A and Camaioni N 2011 Energy

Environ. Sci. 4 285[7] Li G, Chu C W, Shrotriya V, Huang J and Yang Y 2006 Appl.

Phys. Lett. 88 253503[8] White M, Olson D, Shaheen S, Kopidakis N and Ginley D

2006 Appl. Phys. Lett. 89 143517[9] Tan Z a, Zhang W, Zhang Z, Qian D, Huang Y, Hou J and

Li Y 2012 Adv. Mater. 24 1476[10] Huang J, Yin Z and Zheng Q 2010 Energy Environ. Sci. 4 3861[11] Hau S K, Yip H L, Zou J and Jen A K Y 2009 Org. Electron.

10 1401[12] Hsieh C H, Cheng Y J, Li P J, Chen C H, Dobosc M,

Liang R M and Hsu C S 2010 J. Am. Chem. Soc. 132 4887[13] Bulliard X, Ihn S G, Yun S, Kim Y, Choi D, Choi J Y, Kim M,

Park J H, Choi W and Cho K 2010 Adv. Funct. Mater.20 4381

[14] Sun Y, Seo J H, Takacs C J, Seifter J and Heeger A J 2011Adv. Mater. 23 1679

[15] Kim J Y, Kim S H, Lee H H, Lee K, Ma W, Gong X andHeeger A J 2006 Adv. Mater. 18 572

[16] Kuwabara T, Nakayama T, Uozumi K, Yamaguchi T andTakahashi K 2008 Sol. Energy Mater. Sol. Cells 92 1476

[17] Waldauf C, Morana M, Denk P, Schilinsky P, Coakley K,Choulis S A and Brabec C J 2006 Appl. Phys. Lett.89 233517

[18] Liao H H, Chen L M, Xu Z, Li G and Yang Y 2008 Appl.Phys. Lett. 92 173303

[19] Lee Y I, Youn J H, Ryu M S, Kim J, Moon H T and Jang J2011 Sol. Energy Mater. Sol. Cells 95 3276

[20] Zhou Y, Cheun H, Potscavage W J Jr, Fuentes-Hernandez C,Kim S J and Kippelen B 2010 J. Mater. Chem. 20 6189

[21] Zhang H and Ouyang J 2010 Appl. Phys. Lett. 97 063509[22] Vitanov P, Harizanova A and Ivanova T 2009 Thin Solid Films

517 6327

[23] Delft J van, Garcia-Alonso D and Kessels W 2012 Semicond.Sci. Technol. 27 074002

[24] Koo J B, Lim J W, Kim S H, Yun S J, Ku C H and Lee J H2007 Thin Solid Films 515 3132

[25] Lee J, Kim J H and Im S 2004 J. Appl. Phys. 95 3733[26] Palomares E, Clifford J N, Haque S A, Lutz T and Durrant J R

2003 J. Am. Chem. Soc. 125 475[27] Vitanov P, Agostinelli G, Harizanova A, Ivanova T,

Vukadinovic M, Quang N L and Beaucarne G 2006 Sol.Energy Mater. Sol. Cells 90 2489

[28] Palomares E, Clifford J N, Haque S A, Lutz T and Durrant J R2002 Chem. Commun. 1464

[29] Ozawa H, Okuyama Y and Arakawa H 2012 Dalton Trans.41 5137

[30] Zhang X, Sutanto T, Taguchi T, Tokuhiro K, Meng Q,Rao T N, Fujishima A, Watanabe H, Nakamori T andUragami M 2003 Sol. Energy Mater. Sol. Cells 80 315

[31] Chou C H, Kwan W L, Hong Z, Chen L M and Yang Y 2011Adv. Mater. 23 1282

[32] No S Y, Eom D, Hwang C S and Kim H J 2006J. Electrochem. Soc. 153 F87

[33] Yang C S, Kim J S, Choi J W, Kwon M W, Kim Y J, Choi J Gand Kim G T 2000 J. Indust. Eng. Chem. 6 149

[34] Lenes M, Wetzela G J A H, Kooistra F B, Veenstra S C,Hummelen J C and Blom P W M 2008 Adv. Mater. 20 2116

[35] He Y, Chen H Y, Hou J and Li Y 2010 J. Am. Chem. Soc.132 1377

[36] Liu C, Xiao S, Shu X, Li Y, Xu L, Liu T, Yu Y, Zhang L,Liu H and Li Y 2012 ACS Appl. Mater. Interfaces 4 1065

[37] He Y and Li Y 2011 Phys. Chem. Chem. Phys. 13 1970[38] Morvillo P and Bobeico E 2008 Sol. Energy Mater. Sol. Cells

92 1192[39] Kuwabara T, Kawahara Y, Yamaguchi T and Takahachi K

2009 ACS Appl. Mater. Interfaces 1 2107[40] Steim R, Choulis S A, Schilinsky P and Brabec C J 2008 Appl.

Phys. Lett. 92 093303[41] Sista S, Park M H, Hong Z, Wu Y, Hou J, Kwan W L, Li G

and Yang Y 2010 Adv. Mater. 22 380

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