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aNational Center for Nanoscience and Tec

OPV.CHINA@yahoo.com; xiaoz@nanoctr.cnbUniversity of Chinese Academy of Sciences,

† Electronic supplementary informationincluding synthesis, measurements10.1039/c3ta10231a

‡ S. Chen and G. Ye contributed equally t

Cite this: J. Mater. Chem. A, 2013, 1,5562

Received 16th January 2013Accepted 5th March 2013

DOI: 10.1039/c3ta10231a

www.rsc.org/MaterialsA

5562 | J. Mater. Chem. A, 2013, 1, 55

Efficient and thermally stable polymer solar cells basedon a 54p-electron fullerene acceptor†

Shan Chen,‡ab Gang Ye,‡a Zuo Xiao*a and Liming Ding*a

A 54p-electron fullerene acceptor, bis-thieno-o-quinodimethane-methano[60]fullerene (bis-TOQMF),

featuring a compact –CH2– addend and two thieno-o-quinodimethane addends, has been developed.

Bis-TOQMF possesses good solubility and a high LUMO level of �3.40 eV. Polymer solar cells based on

bis-TOQMF/P3HT show not only high Voc (0.94 V) but also good Jsc (8.09 mA cm�2) and FF (58%). The

highest PCE of 4.56% has been achieved for 54p-electron fullerene solar cells to date. On the contrary, a

54p-electron analogue of bis-TOQMF, tris-thieno-o-quinodimethane-C60 (tris-TOQC), shows low

performance. 1.02% PCE was obtained for tris-TOQC/P3HT solar cells, with high Voc (0.96 V) but much

lower Jsc (2.85 mA cm�2) and FF (35%). Space charge limited current (SCLC) measurements indicate that

the electron mobility of bis-TOQMF is 10 times higher than that of tris-TOQC. Bis-TOQMF/P3HT solar

cells show higher thermal stability than PC61BM/P3HT solar cells.

Introduction

Polymer solar cells (PSCs) have attracted great attention due totheir light weight, mechanical exibility, and solution process-ing.1 In the past few years, remarkable progress has been madein enhancing the power conversion efficiency (PCE) of PSCsowing to the success in materials development. The rstachievement is using low-bandgap polymer donors to replacepoly(3-hexylthiophene) (P3HT) to make a better match of thelight absorption of the cells to solar spectrum, thus to improvethe short circuit current ( Jsc).2 The second one is using higherLUMO level fullerene acceptors to replace [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and [6,6]-phenyl-C71-butyricacid methyl ester (PC71BM) to reduce the energy loss in chargeseparation and to increase the open circuit voltage (Voc).3 Forboth cases, over 7% PCEs have been reported. The secondapproach using high LUMO fullerenes is rather appealingbecause liing fullerene LUMO can be easily realized byshrinking a fullerene p-conjugated system to develop fullerenemulti-adducts.4 In this regard, a number of 56p-electronfullerene bis-adducts have been facilely prepared, whichgenerally show LUMO levels 0.1–0.2 eV higher than those of the58p-electron fullerene mono-adducts.5 Recent researchdemonstrated that fullerene bis-adducts are superior acceptor

hnology, Beijing 100190, China. E-mail:

Beijing 100049, China

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

o this work.

62–5566

materials not only for P3HT-based solar cells but also for someD–A copolymer-based solar cells.5,6

The success of fullerene bis-adducts stimulates the searchfor fullerene acceptors with even higher LUMO levels, forexample, the 54p-electron derivatives. The LUMO levels of 54p-electron fullerenes like tris-PC61BM, ICTA, OXCTA, and pen-ta(organo)[60]fullerenes are 0.2–0.4 eV higher than those of the58p fullerenes (Fig. 1).5d,7 However, application of these 54pfullerenes in PSCs was unsuccessful. Although up to 0.98 V Vocwas obtained for solar cells based on these acceptors, Jsc and llfactor (FF) were very low, leading to unsatisfying PCEs. The bestPCE achieved for 54p fullerenes to date is 2.63%, obtained fromthe OXCTA/P3HT solar cells.5d The low performance of 54pfullerene derivatives is caused by their low electron mobilities,which may not result from the increased energetic disorderfrom regio-isomerism but from the increased insulating sidegroups (chains) inhibiting the effective packing of fullerenes toform transport pathways for electrons.8 It was predicted thatmulti-adducts of fullerene with small and compact addendscould be efficient acceptors; yet such acceptors have been lessexploited.9 Recently, our group reported two highly efficient 56pacceptors, OQMF and TOQMF, based on methano[60]fullerene(C60CH2).10 The smallest –CH2– group can effectively li thefullerene LUMO without disrupting fullerene packing, leadingto simultaneously improved Voc, Jsc, and FF of the devicescompared with those of PC61BM-based cells. Based on thisdiscovery, we expected that the higher adducts of methano[60]fullerenes, 54p methano[60]fullerenes, might also be efficientmaterials, rendering the solar cells with not only higher Voc butalso good PCEs. Here, we report the synthesis of a 54pmethano-[60]fullerene derivative, bis-thieno-o-quinodimethane-methano-[60]fullerene (bis-TOQMF), based on two-fold Diels–Alder

This journal is ª The Royal Society of Chemistry 2013

Fig. 1 Chemical structures and performance parameters for previously reported54p-electron fullerene derivatives and new acceptors developed in this work.

Table 1 Optical, electrochemical, and thermal properties of bis-TOQMF and tris-TOQC

Acceptorlonset(nm)

Eoptga

(eV)ERed11/2

b

(V)ERed21/2

b

(V)LUMOc

(eV)Td

d

(�C)

bis-TOQMF 687 1.80 �1.40 �1.73 �3.40 207tris-TOQC 684 1.81 �1.47 �1.84 �3.33 221

a Eg ¼ 1240/lonset.b Potential in volts vs. Fc/Fc+. c LUMO energy

levels were calculated using the following equation: LUMO level ¼�(ERed11/2 + 4.8) eV. d The decomposition temperature under N2.

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derivatization of C60CH2, and a 54p fullerene tris-adduct, tris-thieno-o-quinodimethane-C60 (tris-TOQC), for comparison(Fig. 1). Although only one thieno-o-quinodimethane side groupwas replaced by a –CH2– group, bis-TOQMF showed signi-cantly higher performance than tris-TOQC in PSCs. Bis-TOQMFafforded a PCE of 4.56%, which is the highest efficiency everreported for a 54p fullerene acceptor, while 1.02% PCE wasobtained for tris-TOQC under the same conditions. Theoutstanding performance of bis-TOQMF compared with other54p fullerenes resulted from higher Jsc and FF of its devices.Space charge limited current (SCLC) measurements indicatethat the electron mobility of bis-TOQMF is 10 times higher thanthat of tris-TOQC.

Results and discussion

Bis-TOQMF and tris-TOQC were synthesized through additionof the in situ generated thieno-o-quinodimethane diene toC60CH2 and C60, respectively.5i,10 The experimental details canbe found in ESI.† The new fullerene derivatives were charac-terized by 1H NMR, 13C NMR, and high resolution ESI mass

This journal is ª The Royal Society of Chemistry 2013

spectra (Fig. S1–S5†). The materials' properties were investi-gated by UV-Vis, cyclic voltammetry (CV), thermogravimetricanalysis (TGA), and differential scanning calorimetry (DSC)methods. The absorption spectra of bis-TOQMF and tris-TOQCin chloroform and lms are shown in Fig. S6.† The opticalbandgaps estimated from the absorption edges in chloroformare 1.80 eV and 1.81 eV for bis-TOQMF and tris-TOQC, respec-tively (Table 1). The lm absorption spectra indicate that bis-TOQMF has higher absorbance than tris-TOQC does in the solidstate. The rst half-wave reduction potentials (ERed11/2 ) for bis-TOQMF and tris-TOQC show signicant negative shiscompared with that of PC61BM (Fig. S7†). The LUMO levels ofbis-TOQMF and tris-TOQC estimated from the empirical equa-tion, LUMO ¼ �(ERed11/2 + 4.8) eV,11 are �3.40 eV and �3.33 eV,respectively, which are 0.28 eV and 0.35 eV higher than that ofPC61BM (�3.68 eV) (Tables 1 and S1†). The LUMO level li-upscaused by p-electron reduction are similar to those of other 54pfullerenes.7 High LUMO levels of bis-TOQMF and tris-TOQC areexpected to provide high Voc for solar cells. The decompositiontemperatures (5% weight loss) for bis-TOQMF and tris-TOQCare 207 �C and 221 �C, respectively (Fig. S8†). DSC curves for bis-TOQMF and tris-TOQC show no phase transition during eitherheating or cooling processes, suggesting their amorphousfeature (Fig. S9†).12 Both bis-TOQMF and tris-TOQC show highsolubility in common organic solvents. For example, the solu-bilities of bis-TOQMF and tris-TOQC in o-dichlorobenzene(ODCB) were determined to be 75 mg mL�1 and 125 mg mL�1,respectively.13

PSCs with a typical structure of ITO/PEDOT:PSS/full-erene:P3HT/Ca/Al were fabricated to evaluate the performanceof bis-TOQMF and tris-TOQC. First, we optimized D/A ratio, lmthickness, and annealing temperature for the devices based ona P3HT/bis-TOQMF blend (Fig. S10–S12†). Then, we evaluatedadditive effects on device performance. Performance parame-ters for these solar cells are listed in Table 2. During the rststage of device optimization, we found that bis-TOQMF solarcells with a D/A ratio of 1 : 0.6 (w/w), an active layer thickness of100 nm, with annealing at 130 �C for 10 min afforded the bestresults, with a high Voc of 0.94 V, a Jsc of 6.91 mA cm�2, a FF of53%, and a PCE of 3.55%. Under the same conditions, however,tris-TOQC solar cells afforded a much lower PCE of 1.12%, witha similar Voc of 0.91 V, but a low Jsc of 3.60mA cm�2 and a low FFof 34%. Solvent additives can dramatically inuence theperformance of the solar cells based on fullerene bis-adducts.3d

We tried to improve the device performance by adding additivessuch as a-chloronaphthalene (a-CN) and 1,8-diiodooctane

J. Mater. Chem. A, 2013, 1, 5562–5566 | 5563

Table 2 Performance of the solar cells based on P3HT and different fullereneacceptors without or with additives under AM 1.5G illumination (97 mW cm�2)

Acceptor Additive Voc (V)Jsc(mA cm�2) FF (%) PCE (%) Rs (U cm2)

bis-TOQMF W/O 0.94 6.91 53 3.55 33.29a-CN 0.94 8.09 58 4.56 20.04DIO 0.92 5.42 48 2.56 51.15

tris-TOQC W/O 0.91 3.60 34 1.12 148.53a-CN 0.96 2.85 35 1.02 118.16DIO 0.94 2.46 35 0.86 146.53

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(DIO). The addition of 3 vol% a-CN into the blend solutiondramatically improved the performance of bis-TOQMF devices.The PCE increased from 3.55% to 4.56%, beneting from theimproved Jsc (8.09 mA cm�2) and FF (58%). In contrast, a DIOadditive has negative effects on device performance, leading tosimultaneously decreased Voc, Jsc, and FF, and a diminishedPCE of 2.56%. The effects of additives on tris-TOQC devices aresmall. For both a-CN and DIO cases, the PCEs show slightdecrease, from 1.12% to 1.02% and 0.86%, respectively. J–Vcurves for bis-TOQMF and tris-TOQC solar cells using a-CN asan additive are shown in Fig. 2. Additives affect device perfor-mance by changing the morphology of the active layer.14 Westudied the additive-induced morphology changes for bis-TOQMF/P3HT and tris-TOQC/P3HT blend lms by atomic forcemicroscopy (AFM) and transmission electron microscopy (TEM)(Fig. S13 and S14†). RMS roughnesses for bis-TOQMF/P3HTblend lms without additives, with a-CN, and with DIO are0.70 nm, 2.06 nm, and 4.24 nm, respectively. TEM imagesindicate that the blend lms without additives and with a-CNshow similar uniform morphology. Large domains with sizes ofa few hundred nanometers exist in the blend lm with DIO.This unfavorable morphology should account for the lowperformance of the solar cells using a DIO additive. Themoderately increased roughness of the lms with a-CN mightresult from the increased crystallinity of P3HT induced bythe additive.3d,15 The increased P3HT crystallinity and noover-aggregation in the lms led to better performance ofbis-TOQMF solar cells with a-CN. AFM and TEM images for tris-

Fig. 2 J–V curves for solar cells based on bis-TOQMF/P3HT and tris-TOQC/P3HTblends containing a-CN.

5564 | J. Mater. Chem. A, 2013, 1, 5562–5566

TOQC/P3HT blend lms with a-CN or DIO show similar featuresas those of bis-TOQMF/P3HT blend lms.

Jsc and FF for bis-TOQMF solar cells reach the highest valuesfor 54p-fullerene-based devices, indicating that bis-TOQMF hassuperior electron mobility compared with other 54p fullerenes.We investigated the charge carrier mobilities of bis-TOQMF/P3HT and tris-TOQC/P3HT blends by the SCLC method. Fig. 3shows J–V curves of the electron-only devices in the dark. Bytting J–V curves in the SCLC mode, electron mobilities for bis-TOQMF and tris-TOQC devices were calculated to be 1.68 �10�5 cm2 V�1 s�1 and 1.52� 10�6 cm2 V�1 s�1, respectively. Theelectron mobility of bis-TOQMF is 10 times higher than that oftris-TOQC, which agrees with our speculation. The only struc-tural difference between bis-TOQMF and tris-TOQC is that onethieno-o-quinodimethane switches to –CH2–. SCLC resultsindicate that the smallest –CH2– group can improve the electronmobility of fullerene multi-adducts probably by favoring theirpacking, suggesting that using small and compact groups tomodify fullerene is a promising approach for creating efficientacceptor materials. Hole mobilities for both devices are similar,�2.0 � 10�4 cm2 V�1 s�1 (Fig. S16†). The unbalanced chargecarrier transport (mh/me ¼ 158, Table S2†) in tris-TOQC solarcells should account for its low FF.5d

Fig. 3 (a) J–V curves for the electron-only devices based on bis-TOQMF/P3HTand tris-TOQC/P3HT blend films in the dark; the thicknesses of the blend films are110 nm and 95 nm, respectively. (b) The corresponding J1/2–V curves.

This journal is ª The Royal Society of Chemistry 2013

Fig. 4 J–V curves for solar cells using (a) bis-TOQMF and (b) PC61BM as acceptorsbefore and after being heated at 130 �C for 10, 20, 35, and 45 h, respectively.

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We studied the performance changes for bis-TOQMF/P3HTcells and PC61BM/P3HT reference cells under heating at 130 �Cfor 45 h in an inert atmosphere. It was observed that bis-TOQMF/P3HT solar cells show higher thermal stability thanPC61BM/P3HT cells (Fig. 4). Aer thermal treatment, the PCE ofPC61BM cells decreased from 4.38% to 0.61%, while that of bis-TOQMF cells showed a relatively small decrease, from 4.47% to3.66% (Fig. S17†). The decrease of PCE for both cases resultedfrom the decrease in Jsc and FF. Optical microscopy indicatesthat the bis-TOQMF/P3HT blend lm remained uniform aerbeing heated at 130 �C for 45 h (Fig. 5a). The multi-addition andthe bulky structure of bis-TOQMF inhibit the molecules toaggregate to form large domains. In sharp contrast, mm-scale

Fig. 5 Optical microscope images for (a) bis-TOQMF/P3HTand (b) PC61BM/P3HTblend films after being heated at 130 �C for 45 h.

This journal is ª The Royal Society of Chemistry 2013

PC61BM crystals appeared in PC61BM/P3HT blend lms aerthe thermal treatment (Fig. 5b and S18†). Similar phenomenawere observed previously.5f,9c,16

Conclusions

A novel 54p-electron fullerene acceptor, bis-TOQMF, featuring ahigh LUMO level and a compact –CH2– addend has beendeveloped and applied in PSCs. Unlike other reported 54pfullerene acceptors, bis-TOQMF afforded solar cells with notonly high Voc (0.94 V) but also good Jsc and FF, leading to thehighest PCE (4.56%) for PSCs based on 54p fullerene and P3HT.The high performance of bis-TOQMF originates from its highelectron mobility, which is probably due to the good packing ofthe molecules favored by the smallest –CH2– group. Our resultssuggest that using small and compact addends to modifyfullerene is a promising approach to obtain highly efficientacceptor materials. More efforts on developing high perfor-mance fullerene acceptors are currently ongoing in our lab.

ExperimentalSolar cell fabrication and characterization

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 given UV–ozone treat-ment. A 30 nm thick poly(3,4-ethylenedioxythiophene)-poly-styrene sulfonic acid (PEDOT:PSS, Clevios� P VP Al 4083) layerwas formed on ITO substrates by spin coating an aqueousdispersion (4000 rpm for 30 s) onto the ITO glass. PEDOT:PSScoated substrates were dried at 140 �C for 10 min. A P3HT/fullerene blend in ODCB (24 mg mL�1) without or with 3 vol%additives (a-CN, DIO) was spin-coated (1200 rpm for 60 s) onto aPEDOT:PSS layer. Then, the lms were annealed at 130 �C for10 min. The thicknesses of the active layers (90–100 nm) weremeasured by KLA Tencor D-120 prolometer. Finally, Ca(�10 nm) and Al (�100 nm) were thermally evaporated under ashadow mask (pressure: ca. 10�4 Pa). The effective area for thedevices is 4 mm2. J–V curves were measured on a computerizedKeithley 2420 SourceMeter. Device characterization was done inair under 97 mW cm�2 irradiation (calibrated with a NRELcertied standard silicon cell (4 cm2)) from a xenon-lamp-basedsolar simulator (Newport Oriel Solar Simulator, Model 91159A).AFM was carried out on a Dimension 3100 microscope (Veeco).TEM was performed on a FEI Tecnai G2 F20 electron micro-scope operated at 200 kV. Optical microscopy images wereobtained by using a DM4000 microscope (Leica).

Electron mobility measurement

The structure of electron-only devices is Al/active layer/Ca/Al.17

Al (�80 nm) was rst evaporated onto a glass substrate. A P3HT/fullerene blend in ODCB with 3 vol% a-CN was spin-coated ontoAl lms. The lms were annealed at 130 �C for 10 min. Finally,Ca (�5 nm) and Al (�100 nm) were thermally evaporated undera shadow mask (pressure: ca. 10�4 Pa). J–V curves weremeasured on a computerized Keithley 2420 SourceMeter in thedark.

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Hole mobility measurement

The structure of hole-only devices is ITO/PEDOT:PSS/activelayer/MoO3/Al.18 A 30 nm thick PEDOT:PSS layer was formed onITO substrates by spin coating an aqueous dispersion (4000rpm for 30 s) onto an ITO glass. PEDOT:PSS coated substrateswere dried at 140 �C for 10 min. A P3HT/fullerene blend inODCB with 3 vol% a-CN was spin-coated onto a PEDOT:PSSlayer. The lms were annealed at 130 �C for 10 min. Finally,MoO3 (�6 nm) and Al (�100 nm) were successively evaporatedonto the active layer under a shadow mask (pressure: ca. 10�4

Pa). J–V curves were measured on a computerized Keithley 2420SourceMeter in the dark.

Acknowledgements

This work was supported by the “100 Talents Program” ofChinese Academy of Sciences and National Natural ScienceFoundation of China (21102028). Funding from Ministry ofScience and Technology of China is greatly appreciated(2010DFB63530).

Notes and references

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