Pronounced Backbone Coplanarization by π-Extension in a Sterically Hindered Conjugated Polymer System Leads to Higher Photovoltaic Performance in Non-Fullerene Solar Cells

Achieving both the backbone order and solubility of π-conjugated polymers, which are often in a trade-off relationship, is imperative for maximizing the performance of organic solar cells. Here, we studied three different π-conjugated polymers based on thiazolothiazole (PSTz1 and POTz1) and benzobisthiazole (PNBTz1) that were combined with a benzodithiophene unit in the backbone, where PNBTz1 was newly synthesized. Because of the steric hindrance between the side chains located on neighboring heteroaromatic rings, POTz1 had a much less coplanar backbone than PSTz1 in which such a steric hindrance is absent. However, POTz1 showed higher photovoltaic performance in solar cells that used Y6 as the acceptor material. This was likely due to the significantly higher solubility of POTz1 than PSTz1, resulting in a better morphology. Interestingly, PNBTz1 was found to have markedly higher backbone coplanarity than POTz1, despite having similar steric hindrance between the side chains, most likely owing to the more extended π-electron system, whereas PNBTz1 had good solubility comparable to POTz1. As a result, PNBTz1 exhibited higher photovoltaic performance than POTz1 in the Y6-based cells: specifically, the fill factor was significantly enhanced. Our results indicate that the backbone order and solubility can be achieved by the careful molecular design, which indeed leads to higher photovoltaic performance.

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, two benzodithiophene-thiazolothiazole copolymers, PSTz1 and POTz1, were synthesized, for which the side-chain placement was different. Due to 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 (PC₇₁ BM) and/or non-fullerene (ITIC) as the acceptor materials. Whereas PSTz1 showed higher photovoltaic performance in the PC₇₁ BM-based cell, POTz1 showed higher performance in the ITIC-based cell. Furthermore, in the ternary blend cell, whereas increase in the PC₇₁ BM content improved the photovoltaic performance for the PSTz1 system, it was detrimental to the performance for the POTz1 system. These results could be a good guideline for maximizing the performance of organic photovoltaics.