Enhanced photovoltaic performance of benzodithiophene-alt-bis (thiophen-2-yl)quinoxaline polymers via π-bridge engineering for non-fullerene organic solar cells

A series of novel alternating polymers, namely P(BDTO-TTFQ), P(BDTT-TTFQ), and P(BDTSi-TTFQ), incorporating electron-rich benzo [1,2-b:4,5-b']dithiophene (BDT) derivatives, namely 4,8-bis(2-butyloctyloxy)benzo [1,2-b:4,5-b']dithiophene (BDTO), 4,8-bis(5-(2-butyloctyl)thiophen-2-yl)benzo [1,2-b:4,5-b']dithiophene (BDTT), and 4,8-bis(triisopropylsilylethynyl)-benzo [1,2-b:4,5-']dithiophene (BDTSi), as well as electron-deficient 5,8-bis(5-(4-hexylthiophen-2-yl)thiophen-2-yl)-2,3-didodecyl-6,7-difluoroquinoxaline (TTFQ) units were prepared. The photo-physical, electrochemical, crystallinity, curvature, charge transport, and photovoltaic properties of the TTFQ-based polymers were investigated thoroughly and compared briefly to those of structurally similar 2,3-didodecyl-6,7-difluoro-5,8-di(thiophen-2-yl)quinoxaline (TFQ)-based polymers, namely P (BDTO-TFQ), P(BDTT-TFQ), and P(BDTSi-TFQ), containing BDTO, BDTT, and BDTSi. This study confirmed that the incorporation of additional pi-bridges (3-hxeylthiophene) between the BDT and TFQ units of P(BDTO-TFQ), P(BDTT-TFQ), and P(BDTSi-TFQ) do not significantly alter the properties of the polymers P(BDTO-TFQ) and P(BDTT-TFQ), but do significantly alter the properties of P(BDTSi-TFQ). Consequently, the polymers P (BDTO-TTFQ) and P(BDTT-TTFQ) exhibit comparable power conversion efficiencies (PCEs, 3.99% and 6.69%, respectively) to those of P(BDTO-TFQ) and P(BDTT-TFQ) (3.49% and 7.06%, respectively), but P(BDTSi-TTFQ) exhibits a significantly improved PCE of 6.21% compared to that of P(BDTSi-TFQ) (0.75%).