Visibly transparent conjugated polymers based on non-alternant cyclopenta-fused emeraldicene for polymer solar cells

Abstract

In this study, we synthesized two emeraldicene (EMD)-based conjugated polymers, PBTEMD and PFEMD, through polymerization of 4,7-di(thien-2-yl)benzo[c][1,2,5]thiadiazole and 9,9-bis(2-ethylhexyl)-9H-fluorene, respectively. We then blended these EMD-derived polymers (as electron-donating materials) with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in the active layers of polymer solar cells (PSCs) and investigate their optoelectronic properties and related photovoltaic performance. To best of our knowledge, this study is the first to use EMD derivatives for PSC applications. We compared the molecular structures, absorption behavior, energy levels, thermal properties, and thermal stability of these two polymers to determine their suitability for use in PSCs. The main absorption of PFEMD was in the near-IR spectrum (600–800 nm). We observed a transparency of greater than 80% for the blend film of PFEMD having a thickness of 95 nm; the constructed device exhibited a power conversion efficiency (PCE) of 2.5% and the transparent PFEMD:PC61BM-derived device exhibited a PCE of 1.2% under AM 1.5 G irradiation (100 mW cm−2). We observed a significant improvement in thermal stability for the device incorporating the additive crosslinker TBT-N3; it retained approximately 60% of its initial PCE after accelerated heating (150 °C) for 18 h. In contrast, the PCE of the corresponding normal device decayed to 0.01% of its initial value.