Side-chain influences on the properties of benzodithiophene-alt-di(thiophen-2-yl)quinoxaline polymers for fullerene-free organic solar cells

Abstract

Three new benzodithiophene-alt-di(thiophen-2-yl)quinoxaline polymers were prepared with the aim of exploring 5,8-bis(thiophen-2-yl)-2,3-didodecyl-6,7-difluoroquinoxaline (TFQ)-based polymers on fullerene-free organic solar cells (OSCs). The electron-rich benzo[1,2-b:4,5-bʹ]dithiophene unit contains three different substituents, namely 2,6-bis(trimethyltin)-4,8-bis(2-butyloctyloxy)benzo[1,2-b:4,5-bʹ]dithiophene (Distannyl BDTO), 2,6-bis(trimethyltin)-4,8-bis(5-(2-butyloctyl)thiophen-2-yl)benzo[1,2-b:4,5-bʹ]dithiophene (Distannyl BDTT), and 2,6-bis(trimethyltin)-4,8-bis(triisopropylsilylethynyl)-benzo[1,2-b:4,5-ʹ]dithiophene (Distannyl BDTSi). These were polymerized with electron deficient 5,8-bis(5-bromothiophen-2-yl)-2,3-didodecyl-6,7-difluoroquinoxaline (Dibromo TFQ) to afford the polymers P(BDTO-TFQ), P(BDTT-TFQ), and P(BDTSi-TFQ). The estimated band-gaps (Egs) and the highest occupied molecular orbital levels (HOMOs) were 1.88, 1.86, and 1.94 eV, and −5.35, −5.44, and −5.58 eV, respectively. The OSCs made with a blend of polymer and ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone))-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2ʹ,3′-dʹ]-s-indaceno[1,2-b:5,6-bʹ]dithiophene) gave the maximum power-conversion efficiencies (PCEs) of 3.49, 7.06, and 0.75%, respectively. Overall, the substituents attached at the 4- and 8-positions of the BDT unit of the TFQ-based polymers were found to exhibit greatly altered properties, such as their absorption, energy levels, and crystallinity. This resulted in dissimilar photovoltaic performance for the resulting polymers.