Ternary Blend Bulk-Heterojunction Solar Cells Based on Active Layers of PTB7, PC71BM, and PC61BM

Organic bulk-heterojunction (BHJ) solar cells are highly efficient solar cells with additional advantages such as low-cost production, large area, and mechanical flexibility, as they can be fabricated by solution processing without using high vacuum equipment [1]. On the other hand, an extremely high PCE of 7.5% has recently been reported for BHJ solar cells by Murray et al. [2]. The cells were fabricated using a p-type semiconductor, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7), an n-type semiconductor, [6,6]-phenyl-C71-butyric-acid-methyl-ester (PC₇₁BM), and a lithium fluoride (LiF) inserted Al cathode on an indium tin oxide (ITO) coated glass substrate. These promising results inspired many recent studies on BHJ solar cells based on PTB7:PC₇₁BM, and various techniques were proposed to further improve the performance of the BHJ solar cells.

Ternary or quaternary blend solar cells have also been reported to exhibit interesting performances. Lin et al. [3] have demonstrated a significant improvement in PCE by adding the PTB7 polymer to a poly[[4,8-bis[(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-bnodithiophene-2,6-diyl]] (PCD TBT):PC₇₁BM host system in order to form a ternary-blend BHJ solar cell; the addition of 5 wt% of PTB7 resulted in the greatest PCE improvement. Such studies on ternary-blend solar cells showed performance improvements as a result of adding minute amounts of additives to the host active layers. Ohori et al. recently reported significant improvements in the performance of BHJ solar cells based on ternary-blend organic semiconductor materials with smaller amounts of P3HT additives into the PTB7:PC₆₁BM active layers [4]. They suggested that the improvement of their solar cells was due to enhanced carrier transport paths. Xu et al. reported a significant increase in the open-circuit voltage (Voc) of quaternary-blend BHJ solar cells based on P3HT as a donor, and PC₆₁BM, indene-C60 bisadduct (ICBA) and silicon phthalocyanine bis(trihexylsilyl oxide) as acceptors [5]. Cheng et al. fabricated ternary blend BHJ solar cells based on PTB7:ICBA:PC₇₁BM, and found that the solar cells showed the maximum PCE at the ICBA weight fraction of 15% of the total weight of ICBA and PC₇₁BM [6]. They suggested that the PCE improvement was due to the cascade electron transfer through the lowest unoccupied molecular orbital (LUMO).

Notably, ternary blend solar cells based on PTB7, PC₆₁BM, and PC₇₁BM may have similar LUMO cascade structures, but they have not yet been reported. In this paper, we report on BHJ solar cells based on ternary blend solutions of PTB7, PC₇₁BM, and PC₆₁BM.

Bulk-heterojunction solar cells were fabricated using ternary blend dichlorobenzene solutions of poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7):[6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM):[6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM) with different weight ratios between PC₆₁BM and PC₇₁BM. In all the solar cells, the overall weight ratio of polymer to fullerene was maintained at 1:1.5, while the composition of the fullerene component (PC₆₁BM:PC₇₁BM) was varied. The ultraviolet-visible absorption spectra of these ternary blend films showed that the photon absorptions at wavelengths between 300 and 800 nm continuously decreased with the increase of the PC₆₁BM weight fraction in the PC₆₁BM and PC₇₁BM total weight. The measurement results of the solar cell performance showed that the open-circuit voltage notably increased for PC₆₁BM weight fractions between 10% and 90%, while it decreased at 100%. The short-circuit current showed the most significant increase in the PC₆₁BM weight fraction range between 50% and 60%. A power conversion efficiency of 3.4% was achieved when the PC₆₁BM weight fraction was between 50% and 60%. These results may suggest that the transport of the photoexcited electrons between the cathode and the PC₆₁BM/PC₇₁BM nanodomains was enhanced.

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[4]          Y. Ohori, S. Fujii, H. Kataura, and Y. Nishioka, Jpn. J. Appl. Phys., 45 (2015) 04DK09.

[5]          H. Xu, H. Ohkita, H. Benten, and S. Itoh, Jpn. J. Appl. Phys., 53 (2014), 01AB10.

[6]          P. Cheng, Y. Li, and X. Zhan, Energy Environ. Sci., 7 (2014) 2005.