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aInstitute of Materials for Electronics

Friedrich-Alexander-University Erlangen-N

Erlangen, Germany. E-mail: Min.Jie@ww.unbBavarian Center for Applied Energy Resear

Erlangen, GermanycEnikolopov Institute of Synthetic Polymer

Sciences, Profsoyuznaya St. 70, Moscow 117dChemistry Department, Moscow State U

119991, RussiaeMacromolecular Chemistry Group (buwmak

(IfP), Bergische Universitat Wuppertal, Gauß

† Electronic supplementary informa10.1039/c3ta12162c

Cite this: J. Mater. Chem. A, 2013, 1,11306

Received 4th June 2013Accepted 19th July 2013

DOI: 10.1039/c3ta12162c

www.rsc.org/MaterialsA

11306 | J. Mater. Chem. A, 2013, 1, 1

A combination of Al-doped ZnO and a conjugatedpolyelectrolyte interlayer for small molecule solution-processed solar cells with an inverted structure†

Jie Min,*a Hong Zhang,a Tobias Stubhan,a Yuriy N. Luponosov,c Mario Kraft,e

Sergei A. Ponomarenko,cd Tayebeh Ameri,a Ullrich Scherfe and Christoph J. Brabecab

We successfully demonstrate a smart strategy to use aluminumdoped ZnO (AZO) and the thiophene-based

conjugated polyelectrolyte P3TMAHT as an interfacial layer in small molecule solution-processed inverted

solar cells. Modification of AZO with a thin P3TMAHT layer increases the photovoltaic properties of the

inverted cell as a result of reduction in the work function of the cathode with well aligned frontier

orbital energy levels for efficient charge transport and reduced surface recombination. The inverted

device achieved �16% performance improvement dominantly by recapturing part of the Voc losses

when going from conventional to the inverted architecture. In addition, the inverted device using the

AZO/P3TMAHT interlayer shows improved device stability in air compared to conventional devices.

Introduction

Bulk heterojunction (BHJ) organic solar cells (OSCs) using smallmolecules have attracted extensive attention due to theirpotential for well-denedmolecular structure, easy purication,easy mass-scale production, high charge carrier mobility, andbetter batch-to-batch reproducibility.1–4 In the last year, thepower conversion efficiency (PCE) of solution-processed smallmolecule (SM) OSCs exceeding 7% was published,5,6 makingthem promising and competitive with polymer-based BHJ solarcells. Though impressive, these devices were prepared andcharacterized as a conventional structure, using low work-function cathodes (e.g. Ca or Al) for collecting the electrons.5–7

Consequently these devices suffer from poor air stability, lowoperational lifetime and thus require proper encapsulation toprevent degradation under oxygen and moisture.8–10 To over-come conventional devices and stability issues, more effortshould be focused on the inverted architecture with stable highwork function metal contacts (e.g. Ag) as anodes. In addition,

and Energy Technology (I-MEET),

uremberg, Martensstraße 7, 91058

i-erlangen.de

ch (ZAE Bayern), Haberstraße 2a, 91058

ic Materials of the Russian Academy of

393, Russia

niversity, Leninskie Gory 1-3, Moscow

ro) and Institute for Polymer Technology

straße 20, 42119 Wuppertal, Germany

tion (ESI) available. See DOI:

1306–11311

the inverted architecture is known to be better compatible forproduction via printing processes.9,11–13

Interface engineering plays a key role in the formation ofcharge selective interfaces and is critical to the whole deviceperformance and stability. However, it was difficult to investi-gate interface engineering in small molecule inverted solar cellsdue to two signicant reasons. On the one hand, highly acidicpoly(3,4-ethyl-enedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) used as the electron blocking layer and hole selective layerat the anode side resulted in decomposition of small moleculesand hindered the selection of an electron transporting layer.12

Recently, molybdenum sub-oxides (MoOx) with high workfunctions (WFs), deposited by vacuum deposition, weredemonstrated as promising alternatives to PEDOT:PSS.11–13 Onthe other hand, although various stable low WF metal oxides,such as TiOx and ZnO, as the electron extraction layer in thecathode side have been widely applied for polymeric solar cells(PSCs) to improve the photovoltaic properties of devices,14–18

some metal-oxide-based devices have intrinsic limitationsincluding incomplete ligand removal, unbalanced chargecarrier injection and transport, or inefficient electron–holerecombination, which result in poor control of the electronicsurface properties, high contact resistance or surface recombi-nation.19–21 The aforementioned shortcomings lead to a low llfactor (FF), low open circuit voltage (Voc) and therefore lowpower conversion efficiency (PCE) of the devices. In particular,this effect is observed in small molecule inverted solar cells. Forexample, Heeger and coworkers reported ITO/ZnO/PEIE/(p-DTS(FBTTh2)2):PC70BM/MoO3/Ag inverted solar cells.11 Tooptimize the inverted structure, they introduced poly-ethylenimine, 80% ethoxylated (PEIE), on the top of ZnO tolower the work function of ZnO, and increase the Voc (from

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Fig. 1 (a) The device structure of the inverted N(Ph-2T-DCN-Et)3:PC70BM solarcells. The inset shows the molecular structures of N(Ph-2T-DCN-Et)3 andP3TMAHT. (b) Schematic energy level diagrams for flat band conditions at theAZO/(P3TMAHT)/active layer junction with and without the P3TMAHT layer.

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0.72 to 0.77 V) and PCE of devices (from 6.29% to 7.88%).However, this electron transporting ZnO required a low thick-ness and high-temperature (200 �C for 1 h) thermal annealing tocrystallize the amorphous lm. Therefore, these results drove usto pay close attention to the interface engineering and look fornovel electron transporting layer and interfacial layer

Fig. 2 Surface topographic AFM images (size: 5� 5 mm) of (a) AZO layer, (b) thick P3(d) N(Ph-2T-DCN-Et)3:PC70BM BHJ film.

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materials for various small molecule-based inverted deviceapplications.

An optimized interfacial layer should have air stability, non-toxicity, and the ability to effectively inuence the work functionof adjacent electrodes or transport layers. In addition, it shouldbe able to properly balance charge transport, reduce the inter-facial contact resistance and prevent surface recombina-tion.8,22–25 Recently, conjugated polyelectrolytes (CPEs), whichare conjugated polymers with ionic functionalities, were intro-duced as promising interfacial materials in PSCs.22–26 Forinstance, polyuorene-based CPE interlayers were incorporatedinto a thieno[3,4-b]-thiophene/benzodithiophene copolymer(PTB7)-based PSC and a high PCE of 9.2% was achieved.25 Bazanet al. demonstrated a homopolymer poly[3-(6-trimethylammo-niumhexyl)thiophene] (P3TMAHT) in a poly[N-90 0-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)](PCDTBT) based conventional solar cell with a polythiophene-type electron extracting CPE interlayer (poly[3-(6-trimethy-lammoniumhexyl)thiophene] P3TMAHT) which could improvethe device performance (PCE) from 5.0 to 6.1%.22 Aerwards,Yang et al. incorporated interlayers composed of a blend ofP3TMAHT and sodium dodecylbenzenesulfonate (SDS) as asurfactant into poly(3-hexylthiophene) (P3HT)- or PTB7-based PSCs, with the interlayers adjacent to the Al electrode.23

The P3TMAHT/SDS polyelectrolyte/surfactant complex,details mentioned in ref. 27, was shown to be compatible withvarious material combinations within the active layer, andimproves the device performance and device stability in air.Although already used in polymer-based inverted solar cells,28

TMAHT layer (�30 nm), (c) after deposition of thin P3TMAHTon the AZO layer and

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Fig. 3 (a) J–V characteristics of the conventional and inverted devices based onvarious interfacial layers under the illumination of AM 1.5G, 100 mW cm�2; (b)J–V characteristics of the devices in the dark. (c) EQE spectra of the conventionaldevice and the inverted devices based on various interfacial layers, and absorptionspectra of these blend films of the PEDOT/active layer, AZO/active layer andAZO/P3TMAHT/active layer on the ITO-substrates.

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polythiophene-based CPE interlayers have not been employedso far in small molecule-based inverted solar cells for opti-mizing the device performance.

In this work, we constructed efficient small molecule inver-ted solar cells by employing a combination of aluminum dopedZnO (AZO) and a conjugated P3TMAHT as an electron trans-porting layer (see Fig. 1a), and using a blend of star-shapedmolecule tris{4-[500-(1,1-dicyanobut-1-en-2-yl)-2,20-bithiophen-5-yl]phenyl}amine (N(Ph-2T-DCN-Et)3) and 6,6-phenyl-C70-butyric acid methyl ester (PC70BM) as an active layer, as shownin ref. 29. The AZO layer was introduced as an interfacial layerbecause it not only overcomes the conductivity and lm thick-ness restriction of intrinsic metal oxide layers,19 but is alsoprocessed at a relatively low temperature of 140 �C for 10 min.In addition, a P3TMAHT-based CPE interlayer enhanced elec-tron injection and transport at the cathode by lowering the workfunction of AZO and further reduced the inherent incompati-bility between the AZO layer and the organic active layer.

The device structure, the molecular structures of N(Ph-2T-DCN-Et)3 and P3TMAHT, and the energy levels of the compo-nents used in the devices are shown in Fig. 1a and b. Details ofthe methods for the synthesis of N(Ph-2T-DCN-Et)3 andP3TMAHT are described in the literature.29,30 The AZO layer wasprepared by doctor-blading the AZO precursor solution on a pre-cleaned ITO electrode; later conversion of the precursor to AZOvia hydrolysis was achieved by heating the samples to 140 �C for10 min. The work function of AZO characterized by Kelvin probemicroscopy is approximately 4.2 eV, which is close to the lowestunoccupied molecular orbital (LUMO) of PC70BM (4.3 eV) andthus it benets electron transport to ITO. In the following, theP3TMAHT layers were deposited by doctor-blading from 0.1%P3TMAHT (w/v) solutions in methanol. The work function ofAZO reduced to 3.82 eV aer deposition of a thin layer ofP3TMAHT. Indeed, the increased polarity due to the accumu-lation of ions and the formation of a dipole layer at the interfaceof AZO is observed as described in ref. 22 and 25; preparativedetails are described in the Experimental section. Although theabsorption onset wavelength (lonset) of pristine P3TMAHT lmsis �593 nm,23 AZO covered with the very thin P3TMAHT layer ishighly transparent in the visible range as shown in Fig. S1,† andthis will be of benet to light harvesting.

Fig. 2 shows the surface morphologies obtained by atomicforce microscopy (AFM), as measured in an inert nitrogenatmosphere. The AZO lm is composed of nano-sized particleswith a root mean square (RMS) roughness of 3.3 nm, as shownin Fig. 2a. A 30 nm thick P3TMAHT layer showed a relativelysmooth and homogenous surface (RMS roughness ¼ 1.19 nm,Fig. 2b). A thin P3TMAHT layer on AZO is only slightly smootherthan pristine AZO (RMS roughness decreases to 2.82 nm, asshown in Fig. 3c). The RMS roughness of a N(Ph-2T-DCN-Et)3:PC70BM BHJ lm is only 0.31 nm.

The current density versus voltage ( J–V) characteristicsof N(Ph-2T-DCN-Et)3:PC70BM-based conventional (ITO/PEDOT:PSS/N(Ph-2T-DCN-Et)3:PC70BM/Ca/Ag) and inverteddevices (ITO/AZO/(P3TMAHT)/N(Ph-2T-DCN-Et)3:PC70BM/MoO3/Ag) under simulated AM 1.5 and in the dark are shown in Fig. 3aand b, respectively. Device parameters are deduced from the J–V

11308 | J. Mater. Chem. A, 2013, 1, 11306–11311

characteristics in the light and dark (summarized in Table 1).The reference conventional cells based on N(Ph-2T-DCN-Et)3:PC70BM as an active layer have a Voc of 0.939 V, a Jsc of7.79 mA cm�2, a FF of 45.9% and a PCE of 3.36%, while theseries resistance (Rs) is 1.29 U cm2 and the parallel resistance(Rsh) is 1.84 kU cm2. In contrast, the inverted cells based on AZOas an electron transporting layer show a PCE of 3.17% with a Vocof 0.840 V, a Jsc of 8.47 mA cm�2, and a FF of 44.5%. Comparedwith the conventional device, the increased Jsc of the inverted

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Table 1 Performance parameters of N(Ph-2T-DCN-Et)3:PC70BM solar cells with conventional and inverted structures based on various interfacial layers

Device type Voc [V] Jsc [mA cm�2] FF [%] PCE [%] Rs [U cm2] Rsh [kU cm2]

Conventional 0.939 7.79 45.9 3.36 1.29 1.84Inverted-AZO 0.840 8.47 44.5 3.17 1.21 0.85Inverted-AZO/P3TMAHT 0.898 8.85 49.2 3.91 0.96 2.34

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device could be attributed to the enhanced light reection andspatial electric eld redistribution in the device, and thusincrease light collection.18 The slightly decreased FF of theinverted device could result from the decrease in parallelresistance (from 1.84 to 0.85 kU cm2). The surface recombina-tion, work function mismatch or interface barrier might resultin a reduced leakage current density. However, the Voc of theinverted devices decreased signicantly from 0.939 to 0.840 V,and thus resulted in a slightly low PCE of 3.17%. Consequently,we introduced P3TMAHT as an interfacial layer on the AZO layerto reduce the work function of the AZO. The work function ofAZO decreased from 4.2 eV to 3.82 eV aer depositing theP3TMAHT layer, which facilitates electron injection and trans-port by reducing the energy barrier between the transparentcathode and the active layer. This directly resulted in anincreased Voc (from 0.840 to 0.898 V). In addition, comparedwith the AZO based inverted device, the combination of AZOand P3TMAHT layer slightly enhanced Jsc (from 8.47 to 8.85 mAcm�2) and FF (from 44.5% to 49.2%), and thus resulted in ahigher PCE of 3.91%. This best PCE value increased by 16% incomparison with that of the conventional device (3.36%) underthe same experimental conditions. The Rs reduces to 0.96 U cm2

and Rsh increases to 2.34 kU cm2, which explains the better FF(the average FF value is ca. 46%) compared to the conventionaldevice. Considering the improvement of Jsc, Heeger et al. spec-ulated that the increased built-in potential and internal electriceld might facilitate the charge carriers to escape shallow trapsand reduce trap-assisted recombination.11 Kim et al. concludedthat the smoother and more hydrophobic surface of the poly-electrolyte-based buffer layer (in our case the P3TMAHT inter-layer on top of the AZO layer, as shown in Fig. 2c) improves thecompatibility between the organic active layer and AZO and that

Fig. 4 (a) The PCE of conventional and inverted devices stored in air without any estorage time in air.

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the improved contact may induce an enhanced Jsc.8 In addition,the photovoltaic properties of devices with a structure of ITO/P3TMAHT/N(Ph-2T-DCN-Et)3:PC70BM/MoO3/Ag were signi-cantly worse than those in previous trials, as shown in Fig. S2.†It is probably due to the major limitation of using a thin lm ofP3TMAHT which may result in poor electrical contact with theITO.

The external quantum efficiency (EQE) spectra of variousdevices are shown in Fig. 3c. The EQE results are in agreementwith the Jsc of various devices mentioned above. The improvedPCE of the device with the AZO/P3TMAHT layer is also consis-tent with the higher EQE value (>55%) exhibited in the PC70BMabsorption region between 350 and 500 nm. Although theabsorption spectra of blends on AZO and AZO/P3TMAHT layersare similar as shown in Fig. 3c, the higher EQE values of theAZO/P3TMAHT layer based device in the PC70BM absorptionregion prove that AZO/P3TMAHT extracts electrons moreefficiently from PC70BM, and thus successfully reduces therecombination.

Besides the device efficiency improvement, an enhanceddevice stability was observed as expected when the invertedstructure with the AZO/P3TMAHT layer was used. Fig. 4a showsthe efficiency versus storage time in air of the conventional andinverted devices without any encapsulation. The conventionalcells were fabricated using the ITO/PEDOT:PSS/BHJ/Ca/Agstructure as mentioned before.29 Due to the high reactivity of Cawith oxygen and moisture, the conventional device wascompletely degraded and showed no photovoltaic activity aer15 days in air, whereas the inverted device with the AZO/P3TMAHT layer maintained more or less 70% of its initial PCE.The J–V characteristics of the conventional and inverted cells asa function of storage time in ambient air are shown in Fig. S3

ncapsulation. (b) Normalized Jsc, Voc and FF of the inverted device as a function of

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and S4 (ESI).† The changes in normalized Jsc, Voc and FF over theperiod of storage in air are shown in Fig. 4b. Both Voc and FF ofthe conventional device remained almost unchanged during theair exposure test. The decay is mainly found in a drop of Jsc dueto the instability of Ca (as shown in Fig. 4b). In addition, thewater-soluble side groups of P3TMAHT may also be sensitive tomoisture,23 which resulted in a rather slow degradation of Vocand Jsc aer 10 days. The degradation mechanism of smallmolecule inverted solar cells will be investigated in more detailin the near future. Nevertheless, the replacement of theconventional device by an inverted architecture based on AZO/P3TMAHT leads to a substantially improved device lifetime,while the ease of manufacturing via printing processes ismaintained.

In conclusion, we investigated the effect of P3TMAHTmodied AZO interfaces on the performance and stability ofsmall molecule solution-processed inverted solar cells usingN(Ph-2T-DCN-Et)3:PC70BM as the active layer. By modifying AZOwith a thin P3TMAHT-based CPE interlayer, the PCE of theinverted device with the AZO/P3TMAHT layer is increased by16% in comparison with that (3.36%) of the conventional deviceunder the same experimental conditions. The high PCE (3.91%)of the inverted device is beneted from the reduction of thework function of the AZO and ease of extraction of electronsfrom the inherently incompatible hydrophilic metal oxide andhydrophobic active layers. In addition, the inverted device usingthe AZO/P3TMAHT couple improved the device stability in aircompared to conventional devices. Therefore, CPE interlayers(such as P3TMAHT) are an excellent choice for improving theelectron extracting properties in small molecule-based invertedsolar cells.

Experimental sectionSynthesis of Al doped ZnO

Al doped zinc oxide (EEL):Zn(OAc)2$2H2O (20 g), Al(OH)(OAc)2(0.3 g) and Zonyl FSO-100 (0.6 g) were mixed in demineralizedwater (200 mL). The mixture was stirred for 2 h and lteredthrough a 0.45 micrometer lter to remove the insolublematerial.

Inverted architecture devices

The inverted photovoltaic devices were processed and charac-terized in ambient atmosphere. Pre-structured ITO coated glasssubstrates (as obtained from Osram) were subsequently cleanedin acetone and isopropyl alcohol for 10 min each. Aer drying,the substrates were coated with the AZO solution via doctorblading. The AZO precursor was synthesized as optimized andreported earlier based on zinc acetate and aluminum nitrate inethanol.31 Conversion of the precursor to AZO via hydrolysis wasachieved by heating the samples to 140 �C for 10 min. Thethickness of AZO lms is approximately 40 nm. Aerwards thesamples including a P3TMAHT interlayer were immersed in amethanolic P3TMAHT solution with a concentration of ca. 1 mgmL�1 for 24 h (Mn P3TMAHT: 8000 g mol�1, estimated by GPCwith PS calibration). Doctor-bladed P3TMAHT lms were

11310 | J. Mater. Chem. A, 2013, 1, 11306–11311

annealed at 120 �C for 5min in ambient air to form an interlayerof ca. 5 nm. The BHJ active layers (ca. 90–100 nm) were alsodoctor bladed in ambient air from the solution of N(Ph-2T-DCN-Et)3:PC70BM (1 : 2 weight ratio) in dichlorobenzene without anyother process. Finally 10 nm MoOx and 100 nm Ag weredeposited sequentially under 6 � 10�6 Torr by thermal evapo-ration through a shadow mask to form an active area of10.4 mm2.

Conventional architecture devices

Aer drying as mentioned above, the substrates were bladedwith 50 nm PEDOT:PSS (Heraeus, Clevios P VP.Al 4083).Photovoltaic layers consisted of N(Ph-2T-DCN-Et)3:PC70BM(1 : 2 weight ratio) in dichlorobenzene and were bladed on topof the PEDOT:PSS layer. Finally, a calcium/silver top electrode of15/80 nm thickness was evaporated. The typical active area ofthe investigated devices was also 10.4 mm2.

Characterization methods

The current–voltage characteristics of the solar cells weremeasured under AM 1.5G irradiation on an OrielSol1A Solarsimulator (100 mW cm�2). Most of device performancesmentioned in this paper are corrected for the EQE of theparticular device. The EQE was detected with a Cary 500 ScanUV-vis-NIR Spectrophotometer under monochromatic illumi-nation, which was calibrated with a monocrystalline silicondiode. The morphology was investigated by AFM (Veeco ModelD3100, tapping mode). The work function of various layers wasmeasured by using a Kelvin probe (Kelvin Probe TechnologiesInc., SKP5050) in air.

Acknowledgements

The authors gratefully acknowledge the support from theCluster of Excellence “Engineering of Advanced Materials” atthe University of Erlangen-Nuremberg, which is funded by theGerman Research Foundation (DFG) within the framework ofits “Excellence Initiative”. This work has been partially fundedby the DFG Priority Program 1355 “Elementary Processes ofOrganic Photovoltaics” (CJB and US), the Sonderfor-schungsbereich 953 “Synthetic Carbon Allotropes”, the ChinaScholarship Council (CSC), and the Russian Ministry of Scienceand Education (GK no. 11.G34.31.0055). We also thank the SolarTechnologies go Hybrid (SolTech) for nancial support.

References

1 A. W. Hains, Z. Liang, M. A. Woodhouse and B. A. Gregg,Chem. Rev., 2010, 110, 6689–6735.

2 B. Walker, C. Kim and T. Q. Nguyen, Chem. Mater., 2011, 23,470–482.

3 Y. Li, Q. Guo, Z. Li, J. Pei and W. Tian, Energy Environ. Sci.,2010, 3, 1427–1436.

4 (a) J. Roncali, Acc. Chem. Res., 2009, 42, 1719–1730; (b)Y. Z. Lin, Y. F. Li and X. W. Zhan, Chem. Soc. Rev., 2012,41, 4245–4272.

This journal is ª The Royal Society of Chemistry 2013

Paper Journal of Materials Chemistry A

Publ

ishe

d on

22

July

201

3. D

ownl

oade

d by

Uni

vers

itat E

rlan

gen

Nur

nber

g on

15/

06/2

014

10:2

0:48

. View Article Online

5 T. S. van der Poll, J. A. Love, T. Q. Nguyen and G. C. Bazan,Adv. Mater., 2012, 24, 3646–3649.

6 J. Zhou, X. Wan, Y. Liu, Y. Zuo, Z. Li, G. He, G. Long, W. Ni,C. Li, X. Su and Y. Chen, J. Am. Chem. Soc., 2012, 134, 16345–16351.

7 (a) Y. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazanand A. J. Heeger, Nat. Mater., 2011, 11, 44–48; (b) S. L. Shen,P. Jiang, C. He, J. Zhang, P. Shen, Y. Zhang, Y. P. Yi,Z. J. Zhang, Z. B. Li and Y. F. Li, Chem. Mater., 2013, 25,2274–2281.

8 H. Choi, J. S. Park, E. Jeong, G.-H. Kim, B. R. Lee, S. O. Kim,M. H. Song, H. Y. Woo and J. Y. Kim, Adv. Mater., 2011, 23,2759–2763.

9 T. Stubhan, H. Oh, L. Pinna, J. Krantz, I. Litzov andC. J. Brabec, Org. Electron., 2011, 12, 1539–1543.

10 S. K. Hau, H.-L. Yip, N. S. Baek, J. Zou, K. O'Mallery andA. K.-Y. Jen, Appl. Phys. Lett., 2008, 92, 253301.

11 A. K. K. Kyaw, D. H. Wang, V. Gupta, J. Zhang, S. Chand,G. C. Bazan andA. J.Heeger, Adv.Mater., 2013, 25, 2397–2402.

12 T. Yamamoto, J. Hatano, T. Nakagawa, S. Yamaguchi andY. Matsuo, Appl. Phys. Lett., 2013, 102, 013305.

13 H.-Y. Lin, W.-C. Huang, H.-H. Chou, C.-Y. Hsu, J. T. Lin andH. W. Lin, Chem. Commun., 2012, 48, 8913–8915.

14 H. Oh, J. Krantz, I. Litzov, T. Stubhan, L. Pinna andC. J. Brabec, Sol. Energy Mater. Sol. Cells, 2011, 95, 2194.

15 Z. Q. Liang, Q. F. Zhang, Q. Wiranwetchayan, J. T. Xi,Z. Yang, K. Park, C. D. Li and G. Z. Cao, Adv. Funct. Mater.,2012, 22, 2194–2201.

16 R. Søndergaard, M. Helgesen, M. Jørgensen and F. C. Krebs,Adv. Energy Mater., 2011, 1, 68.

17 Z. Tan, W. Zhang, Z. Zhang, D. Qian, Y. Huang, J. Hou andY. Li, Adv. Mater., 2012, 24, 1476.

18 J. Y. Kim, S. H. Kim, H.-H. Lee, K. Lee, W. L. Ma, X. Gong andA. J. Heeger, Adv. Mater., 2006, 18, 572.

19 T. Stubhan,M. Salinas, A. Ebel, F.C.Krebs, A.Hirsch,M.Halikand C. J. Brabec, Adv. Energy Mater., 2012, 2, 532–535.

This journal is ª The Royal Society of Chemistry 2013

20 H. Ma, H. L. Yip, F. Huang and A. K. Y. Jen, Adv. Funct.Mater., 2010, 20, 1371.

21 H. Choi, H. Cho, S. Song, H. Suh, S. Park and J. Y. Kim, Phys.Chem. Chem. Phys., 2010, 12, 15309.

22 J. H. Seo, A. Gutacker, Y. Sun, H. Wu, F. Huang, Y. Cao,U. Scherf, A. J. Heeger and G. C. Bazan, J. Am. Chem. Soc.,2011, 133, 8416.

23 Y. M. Chang, R. Zhu, E. Richard, C. C. Chen, G. Li andY. Yang, Adv. Funct. Mater., 2012, 22, 3284.

24 T. B. Yang, M. Wang, C. H. Duan, X. W. Hu, L. Huang,J. B. Peng, F. Huang and X. Gong, Energy Environ. Sci.,2012, 5, 8208–8214.

25 Z. C. He, C. M. Zhong, S. J. Su, M. Xu, H. B. Wu and Y. Cao,Nat. Photonics., 2012, 6, 591–595.

26 Z. He, C. Zhong, X. Huang, W.-Y. Wong, H. Wu, L. Chen,S. Su and Y. Cao, Adv. Mater., 2011, 23, 4636.

27 (a) M. Knaapila, R. C. Evans, V. M. Garamus, L. Almasy,N. K. Szekely, A. Gutacker, U. Scherf and H. D. Burrows,Langmuir, 2010, 26, 15634–15643; (b) R. C. Evans,M. Knaapila, N. Willis-Fox, M. Kra, A. Terry,H. D. Burrows and U. Scherf, Langmuir, 2012, 28, 12348–12356.

28 K. Zilberberg, A. Behrendt, M. Kra, U. Scherf and T. Riedl,Org. Electron., 2013, 14, 951–957.

29 J. Min, Y. N. Luponosov, T. Ameri, A. Elschner,S. M. Peregudova, D. Baran, T. Heumuller, N. Li,F. Machui, S. Ponomarenko and C. J. Brabec, Org.Electron., 2013, 14, 219–229.

30 U. Scherf, A. Gutacker and N. Koenen, Acc. Chem. Res., 2008,41, 1086–1097.

31 (a) T. Stubhan, I. Litzov, N. Li, M. Salinas, M. Steidl, G. Sauer,K. Forberich, G. J. Matt, M. Halik and C. J. Brabec, J. Mater.Chem. A, 2013, 1, 6004–6009; (b) T. Stubhan, J. Krantz, N. Li,F. Guo, I. Litzov, M. Steidl, M. Rihter, G. J. Matt andC. J. Brabec, Sol. Energy Mater. Sol. Cells, 2012, 107, 248–251.

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