Effect of Ester Side Chains on Photovoltaic Performance in Thiophene-Thiazolothiazole Copolymers

Thiazolothiazole-based pi-conjugated polymers are promising semiconducting materials in organic photovoltaics (OPVs). In this study, we report on a series of thiophene-thiazolothiazole based polymers having ester side chains, PTzBTE and PTzBTEE. We first show a new synthetic methodology for their common monomer having ester group, in which the total yield was significantly improved by a factor of ten compared to the previous methodology. This resulted in polymer samples with high molecular weights. We then show that OPV cells using PTzBTE, in combination with PC61BM, gave quite high efficiencies of as high as 8.9% despite its limited absorption range. This value is significantly higher than that for PTzBT having only alkyl groups as the side chain and that obtained for a low-molecular weight PTzBTEE sample reported previously. On the other hand, although PTzBTEE had a well-ordered structure similar to PTzBTE, it showed lower OPV performances. Through investigations of electronic properties and structural order and morphology in thin films, structureproperty-device performance relationships are carefully discussed. In addition, the OPV cells fabricated with a non-halogenated solvent showed efficiencies comparable to the cells fabricated by a conventional halogenated solvent.