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aNational Center for Nanoscience and Tec

OPV.CHINA@yahoo.com; xiaoz@nanoctr.cnbUniversity of Chinese Academy of Sciences,cBruker (Beijing) Scientic Technology Co., L

† Electronic supplementary informationincluding synthesis, measurements10.1039/c3ta11811h

‡ S. Chen and X. Du contributed equally

Cite this: J. Mater. Chem. A, 2013, 1,11170

Received 9th May 2013Accepted 19th July 2013

DOI: 10.1039/c3ta11811h

www.rsc.org/MaterialsA

11170 | J. Mater. Chem. A, 2013, 1, 1

Thermo-cleavable fullerenematerials as buffer layers forefficient polymer solar cells†

Shan Chen,‡ab Xiaoyan Du,‡ab Gang Ye,a Jiamin Cao,ab Hao Sun,c Zuo Xiao*a

and Liming Ding*a

A new class of thermo-cleavable fullerenes, di-tert-butyl methano[60]fullerene-61,61-dicarboxylate

(DBMD), bis(2-methylhexan-2-yl) methano[60]fullerene-61,61-dicarboxylate (BMHMD), and di-tert-butyl

methano[60]fullerene-61,61-dicarboxylate bis-adducts (bis-DBMD), was facilely prepared through one-

step Bingel reaction from C60. At high temperature, the tertiary alkyl ester groups of the three

compounds undergo thermo-cleavage processes to produce insoluble methano[60]fullerene-61-

carboxylic acids (MCAs). BMHMD and bis-DBMD were applied in inverted polymer solar cells as a

cathode buffer layer. Heating BMHMD or bis-DBMD films on ZnO produced MCA1 and MCA2 interfacial

layers, respectively. MCA-modified ZnO possesses a smoother and more hydrophobic surface, and shows

excellent solvent-resistance. The MCA buffer layer dramatically improved the device performance by

enhancing Jsc and FF of the solar cells. The power conversion efficiency (PCE) increased from 3.44%

(control) to 3.79% (MCA1) and 4.10% (MCA2) for a P3HT/PC61BM solar cell, and from 6.95% (control) to

7.13% (MCA1) and 7.57% (MCA2) for a PBDTTT-C/PC71BM solar cell.

Introduction

Cheap production of electricity from solar energy is one of thetop challenges today. For this reason, polymer solar cells (PSCs)have emerged as a promising technology for solar energy har-vesting due to their low cost, light weight, and mechanicalexibility for large-area roll-to-roll fabrication.1 A conventionalbulk-heterojunction (BHJ) PSC generally consists of a trans-parent indium tin oxide (ITO) anode, a hole-collecting poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)buffer layer, a photoactive layer containing a conjugated poly-mer donor (e.g. P3HT) and a fullerene acceptor (e.g. PCBM), anda low work-function metal cathode (e.g. Al). Nowadays, thepower conversion efficiency (PCE) for tandem PSCs has sur-passed 10%, which is approaching the threshold (�15% PCE)for commercialization.2 The rapid advancement of PCE benetsfrom the development of high-performance materials, noveldevice structure and interface engineering technologies.3–5

Apart from PCE, the device stability is a challenging issue. Thepoor stability of conventional PSCs is caused by easy oxidationof the low work-function metal cathode and etching of ITO by

hnology, Beijing 100190, China. E-mail:

Beijing 100049, China

td., Beijing 100081, China

(ESI) available: Experimental details, and instruments. See DOI:

to this work.

1170–11176

acidic PEDOT:PSS.6,7 To solve these problems, making PSCswith an inverted structure has been demonstrated to be aneffective approach. The inverted PSCs employ an air-stable highwork-function metal (e.g. Ag) as anode and an electron-trans-porting metal oxide (e.g. TiO2, ZnO)-coated ITO as cathode.8 As aresult, signicantly improved ambient stability and long life-time can be realized.9

ZnO is one of the most widely used electron transportingmaterials for inverted PSCs due to its high transparency invisible and NIR regions, good electron mobility, and solutionprocessability.10 However, the relatively large energy offsetbetween the conduction band of ZnO and the LUMO of anacceptor, the poor compatibility between ZnO and organiccompounds, as well as the electron traps on a metal oxidesurface, might limit the overall performance of devices.11

Therefore, many studies have focused on the development ofnew interfacial materials between ZnO and the active layer toovercome the shortcomings of ZnO.12–14 Among all the interfa-cial materials, fullerene-based interfacial materials (FIMs) haveshown inherent advantages such as their tunable energy levels,good electron mobility, and good compatibility with the activelayer.15 Two types of FIMs have been successfully applied ininverted PSCs. One is the fullerenes functionalized with ananchoring group such as carboxylic acid or phosphonic acidthat can form a self-assembled monolayer (SAM) on ZnO, andthe other is cross-linkable fullerene materials which can form arobust and solvent resistant thin lm on ZnO on heating or UVirradiation.13,14 Though these FIMs can remarkably improve theperformance of inverted PSCs, there are still deciencies for

This journal is ª The Royal Society of Chemistry 2013

Fig. 1 Chemical structures for thermo-cleavable fullerenes.

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these materials. For SAM fullerenes, the drawbacks include therelatively complicated fabrication conditions for forming amonolayer, the insufficient coverage of a SAM on ZnO, and thepossible desorption of a SAM during the wetting process.16 Forcross-linkable fullerenes, tedious syntheses are required due totheir relatively complex molecular structures.14 In this work,we designed a new class of FIMs for inverted PSCs, whichcircumvents their drawbacks. Three thermo-cleavable fullerenederivatives, di-tert-butyl methano[60]fullerene-61,61-dicarb-oxylate (DBMD), bis(2-methylhexan-2-yl) methano[60]fullerene-61,61-dicarboxylate (BMHMD), and di-tert-butyl methano[60]-fullerene-61,61-dicarboxylate bis-adducts (bis-DBMD), wereprepared through a one-step synthesis from C60 (Fig. 1). Thehighly soluble BMHMD and bis-DBMD can form nice lms onZnO by spin-coating and work as precursors for the real FIMs,methano[60]fullerene-61-carboxylic acid (MCA). At hightemperature, the tertiary alkyl ester groups of BMHMD and bis-DBMD can be cleaved and the new fullerene species MCA withcarboxylic acid anchoring groups can be generated in situ. Dueto the chemical bonding between MCA and ZnO and the poor

Scheme 1 Fabrication procedures for inverted solar cells with a thermo-cleav-able fullerene buffer layer.

This journal is ª The Royal Society of Chemistry 2013

solubility of MCA in organic solvents, the MCA buffer layer canwithstand the next solution processing (Scheme 1). The invertedPSCs using thermo-cleavable FIMs demonstrated superiorperformance.

Results and discussion

The thermo-cleavable fullerenes were synthesized by a Bingelreaction from C60. Treating C60 with 1 equivalent of di-tert-butyl2-bromomalonate in the presence of 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) afforded DBMD in 63% yield.17 Similarly, byusing 1 equivalent of bis(2-methylhexan-2-yl) 2-bromomalonateor 2 equivalents of di-tert-butyl 2-bromomalonate, BMHMD orbis-DBMD could be obtained in 45% or 26% yields, respectively.All reactions proceeded smoothly at room temperature. DBMD,bis-DBMD, and BMHMD were then thoroughly characterized byspectroscopic methods (see ESI†). The thermo-cleavagebehavior of these fullerenes was studied by thermogravimetricanalysis (TGA) (Fig. 2). DBMD exhibits a degradation onset at170 �C in N2. The rst-stage weight loss is about 16%, which isidentical to the theoretical weight loss of two isobutenes andone carbon dioxide. This result suggests that DBMD underwentdecomposition at 170 �C and generated a fullerene carboxylicacid, MCA1 (Fig. 2a).18 The generation of MCA1 was furtherconrmed by characterization of the decomposition product ofDBMD. Aer being heated at 180 �C for 10 min, the IR spectrumfor DBMD (Fig. S6a†) shows a broad peak (2700–3300 cm�1) anda sharp peak (1420 cm�1), which can be attributed to thecharacteristic stretching vibration and bending vibration of acarboxylic acid group, respectively. The 1H NMR spectrum ofMCA1 (Fig. S4†) shows a singlet peak at 4.91 ppm and a broadpeak at 10.60 ppm, corresponding to the characteristic signalsof the methine proton and the carboxylic acid proton.19

Compared with DBMD, BMHMD shows a lower degradationonset at 140 �C. The rst-stage weight loss is about 24%, sug-gesting the loss of two alkyl groups and one carbon dioxide andthe generation of MCA1 (Fig. 2b), which was also conrmed byIR and NMR. The rst-stage weight loss for bis-DBMD is about23%, corresponding to the loss of four isobutenes and onecarbon dioxide. MCA2, a fullerene with three carboxylic acidgroups, was generated through this process (Fig. 2c). MCA2 is amixture of regioisomers as indicated by its 1H NMR spectrum(Fig. S5†).

Different from DBMD, both BMHMD and bis-DBMD showgood lm-forming capability. As shown in Scheme 1, a solutionof BMHMD or bis-DBMD in o-dichlorobenzene (ODCB) (4 mgmL�1) was rst spin-coated onto an ITO/ZnO surface. BMHMDand bis-DBMD lms were heated at 160 �C and 180 �C for10 min to provide MCA1 and MCA2 lms, respectively. Atomicforce microscopy (AFM) (Fig. 3) and contact angle measure-ments show that from the ITO/ZnO surface to the ITO/ZnO/MCA1 or ITO/ZnO/MCA2 surface, the root-mean-square (RMS)roughness decreased from 0.96 nm to 0.48 nm or 0.59 nm,respectively, and the water contact angle increased from 32� to72� or 61�, respectively, indicating that MCA-modied ZnOpossesses a smoother and more hydrophobic surface than ZnO.The smaller contact angle of the MCA2 surface compared with

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Fig. 2 TGA curves for (a) DBMD, (b) BMHMD, and (c) bis-DBMD.

Fig. 3 AFM height images for (a) ZnO film, (b) MCA1 film on ZnO, and (c) MCA2film on ZnO (1.0 mm � 1.0 mm).

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that of the MCA1 surface consists with the fact that MCA2possesses more hydrophilic carboxylic acid groups than MCA1.The contact angle change from the precursor lm surface to theMCA surface, i.e. from 80� (BMHMD) to 72� (MCA1) and from86� (bis-DBMD) to 61� (MCA2), reects the thermo-cleavageprocess on the surface. Aer spin-rinsing with ODCB, UV-Visabsorption spectra for MCA1 and MCA2 lms showed nochange, indicating the solvent resistance of these lms (Fig. 4).In sharp contrast, the precursor BMHMD and bis-DBMD lmscan be completely rinsed away with ODCB. The outstandingsolvent resistance property of MCAs originates from the chem-ical bonding between the carboxylic acid group and the ZnO

11172 | J. Mater. Chem. A, 2013, 1, 11170–11176

surface and their extremely low solubility in ODCB. The solu-bility for MCA1 in ODCB is about 0.1 mg mL�1, which is severalhundred times lower than that of soluble fullerenes, such asPC61BM. The solubility for MCA2 in ODCB is much lower,below 0.01 mg mL�1.

LUMO energy levels of MCAs, �3.71 eV for MCA1 and �3.62eV for MCA2, estimated from cyclic voltammetry (CV)measurements of their precursors (BMHMD and bis-DBMD) are

This journal is ª The Royal Society of Chemistry 2013

Fig. 4 UV-Vis absorption spectra for (a) BMHMD andMCA1 films on ZnO and (b)bis-DBMD and MCA2 films on ZnO, before and after being rinsed with ODCB.

Fig. 6 J–V curves for (a) P3HT/PC61BM solar cells and (b) PBDTTT-C/PC71BM solarcells, without and with a MCA1 or MCA2 buffer layer.

Table 1 Performance parameters for devices A–F under AM 1.5G illumination(100 mW cm�2)

DeviceaVoc[V]

Jsc[mA cm�2]

FF[%]

PCEb

[%]Rs[U cm2]

A 0.58 11.3 53 3.44 (3.31) 11.5B 0.59 11.6 55 3.79 (3.68) 8.8C 0.59 12.4 56 4.10 (3.93) 7.3D 0.73 15.6 61 6.95 (6.79) 6.0E 0.72 16.0 62 7.13 (7.02) 5.8F 0.72 16.7 63 7.57 (7.41) 5.4

a Device congurations: A, ITO/ZnO/P3HT:PC61BM (1 : 1,w/w)/MoO3/Ag; B,ITO/ZnO/MCA1/P3HT:PC61BM (1 : 1, w/w)/MoO3/Ag; C, ITO/ZnO/MCA2/P3HT:PC61BM (1 : 1, w/w)/MoO3/Ag; D, ITO/ZnO/PBDTTT-C:PC71BM(1 : 1.5, w/w)/MoO3/Ag; E, ITO/ZnO/MCA1/PBDTTT-C:PC71BM (1 : 1.5,w/w)/MoO3/Ag; F, ITO/ZnO/MCA2/PBDTTT-C:PC71BM (1 : 1.5, w/w)/MoO3/Ag.

b Values in the parentheses are averages of 8–12 devices.

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close to PC61BM LUMO (�3.67 eV), indicating that MCAs aresuitable for electron accepting and transporting (Fig. S7 andTable S1†).20 Typical P3HT/PC61BM (Fig. 5) solar cells with aninverted structure, ITO/ZnO/P3HT:PC61BM (1 : 1, w/w)/MoO3/Ag(device A), ITO/ZnO/MCA1/P3HT:PC61BM (1 : 1, w/w)/MoO3/Ag(device B), and ITO/ZnO/MCA2/P3HT:PC61BM (1 : 1, w/w)/MoO3/Ag (device C), were fabricated to evaluate the effects ofMCAs on the device performance. The detailed procedures forfabrication of devices B and C are depicted in Scheme 1 and theExperimental section. The optimized thicknesses for MCAinterlayers were about 3 nm. J–V curves for devices A, B, and Care shown in Fig. 6a and the device performance parameters arelisted in Table 1. Under AM 1.5G illumination (100 mW cm�2),the reference device A afforded a PCE of 3.44%, with a Voc of

Fig. 5 Chemical structures for P3HT, PBDTTT-C, PC61BM, and PC71BM.

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0.58 V, a Jsc of 11.3 mA cm�2, and a FF of 53%, while device Bwith MCA1 afforded an increased PCE of 3.79%, with a Voc of0.59 V, a Jsc of 11.6 mA cm�2, and a FF of 55%, and device C withMCA2 afforded an enhanced PCE of 4.10%, with a Voc of 0.59 V,a Jsc of 12.4 mA cm�2, and a FF of 56%. The PCE enhancementof devices B and C withMCAs benets from higher Jsc and FF. Asfullerene derivatives, MCAs act as better acceptors than ZnOand provide extra D/A interfaces for exciton dissociation, thusenhancing Jsc.21 Compared with the ZnO surface, MCAs' surface

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favors the wetting of P3HT/PC61BM blend solution due to theirincreased hydrophobicity. The smoother MCA-modied ZnOsurface favors active layer coating, enhances electrical coupling,lowers contact resistance, and eventually improves Jsc and FF.The series resistances (Rs) of devices B and C are 8.8 U cm2 and7.3 U cm2, respectively, which are smaller than that of device A,11.5 U cm2. To determine whether some unexpectedly dissolvedMCA in the active layer enhances the device performance, wefabricated a P3HT/PC61BM solar cell with some MCA1 addedinto the blend solution (content 0.1 mg mL�1). Compared withdevice A, this cell gave a PCE of 1.78%, with decreased Voc, Jscand FF (see Fig. S8†), suggesting that a little MCA permeatinginto the active layer during the spin-coating process doesnot improve the device performance. This experiment indicatesthat the above inference on the MCA buffer layer's function isreasonable.

To gure out whether MCA buffer layers could affect themorphology of the upper active layer, we investigated themorphological difference between the active layers on ZnO andthose on ZnO/MCA1 or ZnO/MCA2 by AFM (Fig. S9†). The heightimages for P3HT/PC61BM blend lms on ZnO, ZnO/MCA1, andZnO/MCA2 show RMS roughnesses of 3.90 nm, 3.46 nm, and4.86 nm, respectively, while the phase images for these lms arequite similar. The rougher lms on the ZnO/MCA2 surfacesuggest possible stronger interaction between MCA2 and P3HT/PC61BM blend lms. Since the ZnO/MCA2 surface possessesmore carboxylic acid groups, H-bonding between carboxylicacid groups of MCA2 and ester groups of PC61BM could perturbthe phase separation in the P3HT/PC61BM blend from thebeginning of the coating process and eventually lead to amorphological difference in the upper active layer. Such aselective interaction between MCA2 and PC61BMmight accountfor better performance of device C, because it may induce avertical content gradient, with PC61BM enriched at the bottomand P3HT enriched at the top of the active layer, which isbenecial for inverted solar cells.22

Encouraged by the good function of the thermo-cleavablefullerene buffer layers in P3HT/PC61BM solar cells, we tried toapply the buffer layers to enhance the performance of solar cellsbased on an outstanding low bandgap polymer PBDTTT-C andPC71BM (Fig. 5). Solar cells with congurations of ITO/ZnO/PBDTTT-C:PC71BM (1 : 1.5, w/w)/MoO3/Ag (device D), ITO/ZnO/MCA1/PBDTTT-C:PC71BM (1 : 1.5, w/w)/MoO3/Ag (device E), andITO/ZnO/MCA2/PBDTTT-C:PC71BM (1 : 1.5, w/w)/MoO3/Ag(device F) were studied. Similar to P3HT/PC61BM cells, devices Eand F with MCAs showed better performance compared withthe reference device D (Fig. 6b and Table 1). Device F with MCA2showed a PCE of 7.57% (Voc 0.72 V, Jsc 16.7 mA cm�2, and FF63%). The PCE enhancement in two types of solar cells by usingbis-DBMD indicates that it is a promising interfacial materialfor application in inverted organic solar cells.

Conclusions

A new class of thermo-cleavable fullerene materials was devel-oped to be used as buffer layers for improving the performanceof inverted PSCs. The thermo-cleavage concept was realized by

11174 | J. Mater. Chem. A, 2013, 1, 11170–11176

designing and synthesizing tertiary alkyl ester group-modiedfullerene precursors, DBMD, BMHMD, and bis-DBMD, whichcan undergo a thermo-cleavage to produce insoluble fullereneswith carboxylic acid groups (MCAs) in situ. MCA buffer layersenhance the performance of inverted solar cells by improvingexciton dissociation and electron transport and decreasingseries resistance. MCA2 with three carboxylic acid groupsgenerated from bis-DBMD exhibited better function than MCA1with one carboxylic acid group from BMHMD. Using MCA2 asthe buffer layer, 19% and 9% enhancement in PCE were ach-ieved for P3HT/PC61BM and PBDTTT-C/PC71BM solar cells,respectively. These results indicate the potential of thermo-cleavable fullerene derivative bis-DBMD in making highly effi-cient inverted solar cells.

ExperimentalSynthesis of DBMD

C60 (1000 mg, 1.39 mmol) and CHBr(COOtBu)2 (500 mg, 1.69mmol) were dissolved in toluene (340 mL), then 1,8-dia-zabicyclo[5.4.0]undec-7-ene (DBU) (250 mL, 1.67 mmol) wasadded to the solution. The mixture was degassed with Ar threetimes and stirred at room temperature for 2 h. Then, toluenewas removed by a rotary evaporator. The residue was puried bysilica gel column chromatography with CS2/toluene (1/1) as theeluent and afforded DBMD (800 mg, yield: 63%). 1H NMR(CDCl3, 400 MHz), d (ppm): 1.70 (s, 18H, CH3);

13C NMR (CDCl3/CS2, 100 MHz), d (ppm): 28.14, 54.16, 72.24, 84.70, 138.92,140.88, 141.94, 142.22, 142.96, 142.99, 143.07, 143.87, 144.49,144.66, 144.77, 145.13, 145.21, 145.35, 162.47. ESI-HRMS:C71H18O4 (M

+) calc. 934.12051, found 934.12107.

Synthesis of BMHMD

C60 (200 mg, 0.28 mmol) and bis(2-methylhexan-2-yl) 2-bromo-malonate (104 mg, 0.28 mmol) were dissolved in toluene(70 mL), then DBU (50 mL, 0.33 mmol) was added to the solu-tion. The mixture was degassed with Ar three times and stirredat room temperature for 3 h. Then, toluene was removed by arotary evaporator. The residue was puried by silica gel columnchromatography with CS2/toluene (1/1) as the eluent andafforded BMHMD (128 mg, yield: 45%). 1H NMR (CDCl3, 400MHz), d (ppm): 1.99–1.90 (m, 4H, CH2), 1.67 (s, 12H, CH3), 1.34–1.48 (m, 8H, CH2), 0.92 (t, J¼ 7.2 Hz, 6H, CH3);

13C NMR (CDCl3,100 MHz), d (ppm): 14.29, 23.17, 26.20, 26.40, 40.88, 54.68,72.49, 87.41, 138.97, 141.02, 142.06, 142.35, 143.10, 143.21,143.99, 144.62, 144.80, 144.90, 145.26, 145.33, 145.47, 145.93,162.39. ESI-HRMS: C77H34O4N (M + NH4

+) calc. 1036.24878,found 1036.24598.

Synthesis of bis-DBMD

C60 (500 mg, 0.69 mmol) and CHBr(COOtBu)2 (615 mg,2.08 mmol) were dissolved in toluene (170 mL), then DBU(311 mL, 2.08 mmol) was added to the solution. The mixture wasdegassed with Ar three times and stirred at room temperaturefor 2 h. Then, toluene was removed by a rotary evaporator. Theresidue was puried by silica gel column chromatography with

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toluene as the eluent and afforded bis-DBMD (204 mg, yield:26%). 1H NMR (CDCl3, 400 MHz), d (ppm): 1.60–1.71 (CH3);

13CNMR (CDCl3, 100 MHz), d (ppm): 28.05, 28.10, 28.14, 52.02,53.18, 53.54, 55.47, 70.98, 71.34, 71.65, 71.89, 72.19, 72.35,84.29, 84.41, 84.43, 84.45, 84.48, 84.57, 138.26, 138.42, 138.48,138.56, 138.77, 138.95, 140.32, 140.63, 140.84, 141.44, 141.58,141.61, 141.64, 141.69, 141.83, 141.90, 141.99, 142.04, 142.09,142.22, 142.40, 142.58, 142.61, 142.80, 142.92, 143.05, 143.10,143.19, 143.23, 143.33, 143.44, 143.65, 143.73, 143.78, 143.86,143.97, 144.06, 144.10, 144.33, 144.53, 144.56, 144.64, 144.79,144.89, 145.00, 145.06, 145.15, 145.22, 145.27, 145.34, 145.43,145.51, 145.66, 145.85, 146.00, 146.24, 146.30, 146.32, 146.34,146.38, 146.41, 146.47, 146.56, 146.78, 146.83, 147.08, 147.14,147.34, 147.98, 162.37, 162.40, 162.43, 162.48, 162.50, 162.54.ESI-HRMS: C82H36O8 (M

+) calc. 1148.24102, found 1148.23989.

Device fabrication

A patterned ITO glass with a sheet resistance of 15 U sq�1 wasultrasonically cleaned using detergent, distilled water, acetone,and isopropanol sequentially and then subjected to UV–ozonetreatment. The ZnO precursor was prepared according to theliterature.10b The ZnO precursor solution was spin-coated(4000 rpm for 30 s) onto an ITO-glass substrate. The lms wereannealed at 200 �C for 30 min in air. The ZnO lm thickness isabout 30 nm. A precursor (BMHMD or bis-DBMD) solution (4mg mL�1) in ODCB was spin-coated (1200 rpm for 60 s) ontoZnO. A BMHMD or bis-DBMD lm was heated at 160 �C and 180�C for 10 min to turn into a MCA1 or MCA2 lm, respectively. AP3HT/PC61BM blend in ODCB (1 : 1 w/w, 34 mgmL�1) was spin-coated (800 rpm for 30 s) onto MCA. Then, the wet blend lmswere put into glass Petri dishes to undergo the solvent anneal-ing process.23 The thicknesses of the active layers (�180 nm)were measured by a KLA Tencor D-120 prolometer. Finally,MoO3 (�7 nm) and Ag (�100 nm) were thermally evaporatedunder a shadow mask (pressure: ca. 10�4 Pa). The effective areaof the devices is 4 mm2. J–V curves were measured on acomputerized Keithley 2420 SourceMeter. The measurementswere done in air under 100 mW cm�2 irradiation (calibratedwith a NREL certied standard silicon cell (4 cm2)) from axenon-lamp-based solar simulator (Newport Oriel Solar Simu-lator, Model 91159A). For PBDTTT-C/PC71BM cells, the fabri-cation process is similar except active layer fabrication: aPBDTTT-C/PC71BM blend in ODCB (1 : 1.5 w/w, 15 mg mL�1)with 3 vol% 1,8-diiodooctane (DIO) was spin-coated (800 rpmfor 60 s) onto MCAs.

Acknowledgements

This work was supported by the “100 Talents Program” ofChinese Academy of Sciences and National Natural ScienceFoundation of China (21102028).

Notes and references

1 (a) G. Li, R. Zhu and Y. Yang, Nat. Photonics, 2012, 6, 153; (b)Y. Li, Acc. Chem. Res., 2012, 45, 723; (c) H. J. Son, B. Carsten,I. H. Jung and L. Yu, Energy Environ. Sci., 2012, 5, 8158.

This journal is ª The Royal Society of Chemistry 2013

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This journal is ª The Royal Society of Chemistry 2013