Comparative study on the effects of alkylsilyl and alkylthio side chains on the performance of fullerene and non-fullerene polymer solar cells

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Two novel high gap donor polymers – PBDTTSi-TzBI and PBDTTS-TzBI , based on imide-fused benzotriazole (TzBI) with asymmetric side chains and alkylsilyl (Si) or alkylthio (S) substituted 4,8-di(thien-2-yl)benzo-[1,2- b :4,5- b ′]dithiophene (BDTT) – are successfully synthesized. The effect of the side chain variation on the photophysical, morphological and photovoltaic properties of blends of these polymers with fullerene and non-fullerene acceptors is investigated. The PBDTTSi-TzBI polymer shows a deeper highest occupied molecular orbital energy level, which results in higher open-circuit voltages. Nevertheless, the polymer solar cells fabricated using PBDTTS-TzBI in combination with PC 71 BM afford a higher power conversion efficiency of 7.3% ( vs 4.0% for PBDTTSi-TzBI :PC 71 BM). By using the non-fullerene acceptor ITIC, the absorption of the blends extends to 850 nm and better device efficiencies are achieved, 6.9% and 9.6% for PBDTTSi-TzBI :ITIC and BDTTS-TzBI :ITIC, respectively. The better performance of the PBDTTS-TzBI :ITIC-based devices is attributed to the strong and broad absorption and balanced charge transport, and is among the best performances reported for non-fullerene solar cells based on TzBI-containing polymer donors. • High gap copolymers based on benzodithiophene and imide fused benzotriazole are synthesized. • The polymers are applied in organic solar cells with both fullerene and non-fullerene acceptors. • Side chain variation (alkylthio vs -silyl) is used to optimize the active layer blend properties. • Alkylsilyl substitution leads to higher open-circuit voltages. • Power conversion efficiencies up to 9.6% are achieved under solar illumination.