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A highly efficient fullerene acceptor for polymer solar cells† Dan He,‡ Chuantian Zuo,‡ Shan Chen, Zuo Xiao* and Liming Ding*

Received 18th January 2014, Accepted 28th February 2014 DOI: 10.1039/c4cp00268g www.rsc.org/pccp

C70-based acceptors show great potential in polymer solar cells (PSCs). Two high-LUMO C70 acceptors, the 66p OQMF70 and the 64p bis-OQMF70, based on methano[70]fullerene (C70CH2) were developed. An outstanding power conversion efficiency (PCE) of 6.88% was obtained from OQMF70:P3HT solar cells.

Compared with Ih-symmetric C60, the D5h-symmetric C70 shows a much better light-absorbing capability in the long-wavelength region since its lower molecular symmetry allows more lowenergy transitions to take place.1 Solar cells based on C70 acceptors give a higher photocurrent and power conversion efficiency (PCE) than C60 cells, and it is highly desired to develop new C70 acceptors.2 The 68p-electron phenyl-C71butyric acid methyl ester (PC71BM) is a frequently-used C70 acceptor for polymer solar cells (PSCs).3 But due to its low LUMO energy level, low open circuit voltages (Voc) accompany PC71BM solar cells.4 Shrinking the p conjugation of C70 by multi-addition can effectively lift up fullerene LUMO, thus, enhancing Voc. Recently, several groups have reported efficient 66p-electron C70 acceptors with high LUMO levels.5 Among them, the indene C70 bis-adduct (IC70BA) has shown a remarkable PCE of 7.4% in fullerene:P3HT solar cells.6 Previously, we reported efficient high-LUMO acceptors based on methano[60]fullerenes (C60CH2 and C60(CH2)2).7 The sterically compact CH2 groups can raise fullerene LUMO and simultaneously favour fullerene packing, rendering the solar cells with not only high Voc but also good short-circuit current (Jsc) and fill factor (FF).8 Here we report two high-LUMO acceptors based on methano[70]fullerene (C70CH2), the 66p o-quinodimethane-methano[70]fullerene (OQMF70) and the 64p bis-o-quinodimethane-methano[70]fullerene (bisOQMF70). The optical and electrochemical properties, and photovoltaic performance of OQMF70 and bis-OQMF70 National Center for Nanoscience and Technology, Beijing 100190, China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: Experimental details including synthesis, measurements, and instruments. See DOI: 10.1039/ c4cp00268g ‡ These authors contributed equally to this work.

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Scheme 1

Synthesis of OQMF70 and bis-OQMF70.

were investigated. OQMF70:P3HT solar cells with an inverted structure afforded a decent PCE of 6.88%. C70CH2 was prepared according to the procedure reported by Nakamura et al.9 OQMF70 and bis-OQMF70 were synthesized by adding o-quinodimethane diene to C70CH2 (Scheme 1).10 The two adducts were facilely separated via silica gel column chromatography using CS2 : hexane (1 : 1) as the eluent and were obtained in 52% and 34% yields, respectively. NMR spectra indicate that the two products consist of regioisomers. The integral ratios between aromatic protons (7–8 ppm) and aliphatic protons (2–5 ppm) on 1H NMR are 4 : 6 and 8 : 10 for OQMF70 and bis-OQMF70, respectively, in agreement with the fact that one and two equiv. of diene were added to C70CH2, respectively. High resolution ESI mass spectra showed the expected molecular ion peaks (M+), 958.0754 and 1062.1435, for OQMF70 and bis-OQMF70, respectively. OQMF70 and bisOQMF70 show good solubility, 45 mg mL 1 and 81 mg mL 1 in ODCB, respectively. The absorption spectra of PC71BM, OQMF70, and bisOQMF70 are shown in Fig. 1. Similar to PC71BM, OQMF70 and bis-OQMF70 mainly absorb light in the UV and visible regions (300–700 nm). The absorption onsets for PC71BM, OQMF70, and bis-OQMF70 are 702, 723, and 733 nm, respectively, corresponding to optical bandgaps of 1.77, 1.72, and 1.69 eV, respectively. The electrochemical properties of PC71BM, OQMF70, and bis-OQMF70 were investigated by cyclic voltammetry (Fig. 2). Compared with PC71BM, the first reduction potentials (E1) for OQMF70 and bis-OQMF70 were negatively

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PCCP Table 1 Performance for fullerene:P3HT solar cells under AM 1.5G illumination (100 mW cm 2)a

D/A, annealing VOC JSC FF temperature [V] [mA cm 2] [%]

PCEc [%]

1 : 0.4, 1 : 0.6, 1 : 0.8, 1 : 0.6, 1 : 0.6, 1 : 0.6, 1 : 0.6,

150 150 150 110 130 170 150

1C 8C 1C 1C 1C 1C 8Cb

0.85 0.83 0.84 0.81 0.81 0.85 0.81

11.30 11.50 9.86 11.12 11.20 9.52 12.42

66.12 69.22 67.56 67.98 68.59 58.99 68.41

6.35 6.61 5.60 6.12 6.22 4.77 6.88

(6.13 (6.39 (5.38 (6.05 (6.09 (4.66 (6.69

      

0.16) 0.16) 0.19) 0.09) 0.13) 0.21) 0.13)

bis-OQMF70 1 : 0.4, 1 : 0.6, 1 : 0.8, 1 : 0.6, 1 : 0.6, 1 : 0.6, 1 : 0.6,

150 150 150 110 130 170 150

1C 8C 1C 1C 1C 1C 8Cb

0.96 0.95 0.94 0.95 0.94 0.95 0.88

7.82 7.40 4.82 6.08 7.21 6.50 8.91

46.04 58.22 59.46 49.00 55.31 58.93 50.29

3.46 4.09 2.69 2.83 3.75 3.64 3.94

(3.20 (3.86 (2.57 (2.61 (3.52 (3.39 (3.67

      

0.15) 0.15) 0.10) 0.18) 0.12) 0.15) 0.19)

Fullerene

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OQMF70

Fig. 1 Absorption spectra of PC71BM, OQMF70, and bis-OQMF70 in CHCl3 (10 5 mol L 1).

PC71BM

1 : 0.6, 150 1C

0.69 10.03

69.24 4.79 (4.58  0.11)

a

Device structure: ITO/PEDOT:PSS/fullerene:P3HT/Ca/Al. b Device structure: ITO/ZnO/fullerene:P3HT/MoO3/Ag. c Values in the parentheses are averages of 8–9 devices.

Fig. 2

Cyclic voltammograms of PC71BM, OQMF70, and bis-OQMF70.

shifted. The LUMO levels estimated from the empirical equation, LUMO = (E1 + 4.8) eV, are 3.54 eV for OQMF70 and 3.42 eV for bis-OQMF70, respectively, which are 0.13 eV and 0.25 eV higher than that of PC71BM ( 3.67 eV).11 Since Voc is proportional to the energy difference between donor HOMO and acceptor LUMO, OQMF70 and bis-OQMF70 are expected to afford higher Voc than PC71BM in PSCs.12 We first investigated the photovoltaic performance of OQMF70 and bis-OQMF70 in solar cells with a structure of ITO/PEDOT:PSS/fullerene:P3HT/Ca/Al. PC71BM cells were also studied as a reference. The donor/acceptor (D/A) ratio and the annealing temperature were optimized (Table 1). The best D/A ratio was determined to be 1 : 0.6 for both OQMF70 and bisOQMF70 cells. OQMF70 and bis-OQMF70 cells show the highest PCE after being annealed at 150 1C. The best OQMF70 cells afforded a Voc of 0.83 V, a Jsc of 11.50 mA cm 2, a FF of 69.22%, and a PCE of 6.61%, which is 38% higher than that of

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PC71BM cells (4.79%). The superior performance of OQMF70 cells benefits from high Voc and Jsc. Though possessing a high Voc of 0.95 V, bis-OQMF70 cells gave a relatively low PCE of 4.09%, which is due to the low Jsc (7.40 mA cm 2) and FF (58.22%). The reduced mobility of bis-OQMF70 accounts for its low PCE (vide infra). The increasing tendency in Voc from 68p PC71BM to 66p OQMF70 to 64p bis-OQMF70 consists with that in LUMO levels of these acceptors. The performances of OQMF70 and bis-OQMF70 in inverted solar cells with a structure of ITO/ZnO/fullerene:P3HT/MoO3/Ag were studied. Compared with the conventional cells, the inverted OQMF70 cells showed higher Jsc (12.42 mA cm 2) and slightly reduced Voc (0.81 V) and FF (68.41%). The 6.88% PCE is among the highest PCE reported for inverted fullerene: P3HT solar cells.13 In contrast, the inverted bis-OQMF70 cells showed a reduced PCE of 3.94% compared with the conventional cells. Remarkable decreases in Voc and FF were observed, which might be due to the large energy gap between the LUMO level of bis-OQMF70 ( 3.42 eV) and the conduction band of ZnO ( 4.40 eV). The significant reduction in Rsh from 1182.7 O cm2 (conventional) to 633.1 O cm2 (inverted) could be correlated with the decreases in Voc and FF. Space charge limited current (SCLC) measurements indicate that OQMF70 possesses a good electron mobility of 2.0  10 4 cm2 V 1 s 1, which is lower than that of PC71BM (2.6  10 4 cm2 V 1 s 1) but much higher than that of bis-OQMF70 (6.9  10 5 cm2 V 1 s 1) (see ESI†). External quantum efficiency (EQE) spectra for OQMF70 and bis-OQMF70 cells were compared (Fig. S5, ESI†). OQMF70 cells showed higher EQE than bis-OQMF70 cells in the 400–740 nm region, corresponding with the higher Jsc of OQMF70 cells. Atomic force microscopy (AFM) indicates that the OQMF70:P3HT blend film is smoother than the bis-OQMF70:P3HT film (Fig. 3). The root-mean-square (RMS) roughnesses for OQMF70:P3HT

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In summary, two new acceptors, OQMF70 and bis-OQMF70, based on methano[70]fullerene (C70CH2) were synthesized and applied in PSCs. The 66p OQMF70 possesses a suitable LUMO level and good electron mobility, demonstrating an outstanding PCE of 6.88% in inverted fullerene:P3HT solar cells. Blending OQMF70 with some high-performance lowbandgap polymers to obtain highly efficient solar cells is currently ongoing.

Acknowledgements This work was supported by the ‘‘100 Talents Program’’ of Chinese Academy of Sciences and the National Natural Science Foundation of China (21374025, 21372053, and 21102028).

Notes and references

Fig. 3 AFM height images (left) and phase images (right) OQMF70:P3HT (top) and bis-OQMF70:P3HT (bottom) blend films.

for

Fig. 4 XRD profiles for OQMF70:P3HT and bis-OQMF70:P3HT blend films.

and bis-OQMF70:P3HT films are 0.7 and 1.9 nm, respectively. In the phase image for the OQMF70:P3HT film, we observe very clear nano-structures like fine knitting fabrics, suggesting that OQMF70 might possess better miscibility with P3HT than bisOQMF70. In X-ray diffraction (XRD) profiles, OQMF70:P3HT and bis-OQMF70:P3HT films showed an intense (100) reflection peak at 2y = 5.41, corresponding to an interlayer d-spacing of 16.4 Å, and two weak (200) and (300) reflection peaks at 2y = 10.8 and 16.31, respectively (Fig. 4).14 The higher intensity of the (100) reflection peak of the OQMF70:P3HT film than that of the bis-OQMF70:P3HT film indicates a higher crystallinity of P3HT in the former. More D/A interfaces and higher P3HT crystallinity in the active layer might account for higher Jsc and FF for OQMF70 cells.

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A highly efficient fullerene acceptor for polymer solar cells.

C70-based acceptors show great potential in polymer solar cells (PSCs). Two high-LUMO C70 acceptors, the 66π OQMF70 and the 64π bis-OQMF70, based on m...
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