Influence of triisopropyl(prop-1-ynyl)silane side chain on the properties of pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione-based polymeric donor for organic solar cells

A new alternating copolymer, P(BDTSi-PPD), comprising 4,8-bis(triisopropylsilylethynyl)-benzo[1,2-b:4,5-']dithiophene (BDTSi) and 4,6-bis(thiophen-2-yl)-2,5-dio-ctylpyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione (PPD) units was prepared. The photophysical, electrochemical, and photovoltaic properties of P(BDTSi-PPD), as well as the backbone curvature, charge transport, and crystallinity, were studied and briefly compared with those of structurally similar high-energy-converting wide-bandgap copolymers, namely P(BDTO-PPD), containing 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene (BDTO) and PPD units. Polymer P(BDTSi-PPD) displayed a slightly broader absorption band, deeper highest occupied molecular orbital (HOMO), and lower pi-pi stacking ability compared to those of P(BDTO-PPD). Nevertheless, the poor power conversion efficiency of organic solar cells (OSCs) employing a binary blend of P(BDTSi-PPD):PC70BM was inferior (PCE -1.08%) to that of devices made from the P(BDTO-PPD):PC70BM blend (PCE -5.42%). However, the inclusion of 3 vol% P(BDTSi-PPD) in the PTB7-Th:PC70BM blend significantly enhanced the PCE of the resulting ternary OSCs, where the PCE increased from 7.86% to 8.51%. In contrast, the PCE of the PTB7-Th:PC70BM blend increased to 8.34% with the addition of 10 vol% P(BDTO-PPD) to the primary blend. Overall, P(BDTSi-PPD) works as an additive material in OSCs, whereas P(BDTO-PPD) acts mainly as a co-absorber. This study demonstrates that replacing 2-ethylhexyloxy (EHO) with a triisopropyl(prop-1-ynyl)silane (TIPS) substituent on the backbone of benzo[1,2-b:4,5-b']dithiophene (BDT) units of PPD-based polymers results in significant attenuation of the photovoltaic properties.