Enhanced performance of ternary polymer solar cells via property modulation of co-absorbing wide band-gap polymers

A new approach is presented here to maximize the efficiency of ternary blend polymer solar cells (ter-PSCs). The properties of co-absorbing wide band-gap polymers are fine-tuned via the insertion of two different appropriate electron-acceptor units on their backbone in different ratios, and the resulting polymers are utilized as a cobsorbent on ter-PSCs. The copolymerization of benzodithiophene, pyrrolopyrrole-1,3-dione, and difluorobenzothiadiazole derivatives at three different ratios, 1.00:1.00:0.00, 1.00:0.50:0.50, and 1.00:0.25:0.75, produces the polymers P1-P3. The absorption bands of P1-P3 are gradually red-shifted, and the band-gaps are reduced. The crystallinity of the polymers is increased in the order of P1 < P2 < P3. The binary polymer solar cells (bi-PSCs) are fabricated using blends of P1, P2, or P3 with PC70BM and provide comparable power conversion efficiencies (PCEs) of 5.41%, 5.55%, and 4.01%, respectively. However, the inclusion of 20 wt% of each of P1, P2, and P3 on a P4 (=PTB7-Th):PC70BM blend offers PCEs of 9.31%, 9.60%, and 9.85%, respectively, which are higher than that of the bi-PSC made using a blend of P4:PC70BM (PCE = 8.20%). The improved absorption, crystallinity, and carrier mobilities of the ternary blends are the main reasons for their enhanced photovoltaic performance compared with the binary blends.