Contrasting Effect of Side-Chain Placement on Photovoltaic Performance of Binary and Ternary Blend Organic Solar Cells in Benzodithiophene-Thiazolothiazole Polymers.

π-Conjugated polymers are important materials for organic photovoltaics. While search for new backbone systems is central to the development of π-conjugated polymers, side chain engineering is also imperative. Here, we synthesized two benzodithiophene-thiazolothiazole copolymers, PSTz1 and POTz1, where the side chain placement is different. Due to the less steric hindrance between the side chains, PSTz1 had a more coplanar backbone than POTz1. This led to significant differences in trend of the performance for the binary and ternary blend cells that used a fullerene (PC 71 BM) and/or a non-fullerene (ITIC) as the acceptor materials. Whereas PSTz1 showed higher photovoltaic performance in the PC 71 BM-based cell, POTz1 showed higher performance in the ITIC-based cell. Furthermore, in the ternary blend cell, whereas increase in the PC 71 BM content improved the photovoltaic performance for the PSTz1 system, it was detrimental to the performance for the POTz1 system. We believe that these results would be a good guideline for maximizing the performance of organic photovoltaics.