Halogen-Free p-Conjugated Polymers Based on Thienobenzobisthiazole for Efficient Nonfullerene Organic Solar Cells: Rational Design for Achieving High Backbone Order and High Solubility

In pi-conjugated polymers, a highly ordered backbone structure and solubility are always in a trade-off relationship that must be overcome to realize highly efficient and solution-processable organic photovoltaics (OPVs). Here, it is shown that a pi-conjugated polymer based on a novel thiazole-fused ring, thieno[2',3':5,6]benzo[1,2-d:4,3-d']bisthiazole (TBTz) achieves both high backbone order and high solubility due to the structural feature of TBTz such as the noncovalent interlocking of the thiazole moiety, the rigid and bent-shaped structure, and the fused alkylthiophene ring. Furthermore, based on the electron-deficient nature of these thiazole-fused rings, the polymer exhibits deep HOMO energy levels, which lead to high open-circuit voltages (V(OC)s) in OPV cells, even without halogen substituents that are commonly introduced into high-performance polymers. As a result, when the polymer is combined with a typical nonfullerene acceptor Y6, power conversion efficiencies of reaching 16% and V(OC)s of more than 0.84 V are observed, both of which are among the top values reported so far for halogen-free polymers. This study will serve as an important reference for designing pi-conjugated polymers to achieve highly efficient and solution-processable OPVs.

Automated summary

A pi-conjugated polymer based on a novel thiazole-fused ring, thieno[2',3':5,6]benzo[1,2-d:4,3-d']bisthiazole (TBTz), achieves high backbone order and solubility due to its unique structural features. The polymer exhibits high open-circuit voltages (V(OC)s) in organic photovoltaic cells without halogen substituents, resulting in power conversion efficiencies of up to 16% and V(OC)s of more than 0.84 V when combined with a nonfullerene acceptor Y6. This study provides valuable insights for designing highly efficient and solution-processable pi-conjugated polymers.