High mobility wide bandgap polymer donors based on benzobisoxazole for polymer solar cells

Abstract

The performance of polymer solar cells (PSCs) has rapidly developed, benefitting from the advancements in crescent-shaped small molecule acceptors (SMAs). The polymer donors that match with SMAs are mostly aimed at generating more excitons, and therefore their optical bandgaps ($E_g^{\text{opt}}$) are typically moderate. However, from the perspective of efficient and sufficient energy utilization, the development of wide bandgap polymer donors with maximum absorption peaks closer to 500 nm is still of great significance, as it allows for the utilization of the strongest region of solar radiation energy. Here, we designed and synthesized two polymer donors based on benzobisoxazole with different molecular backbones, namely PBBO-1 and PBBO-2. They have wide $E_g^{\text{opt}}$ of 2.01 eV and 2.15 eV, respectively. Additionally, without halogen groups, they exhibit deep HOMO energy levels of −5.43 eV and −5.56 eV, respectively. Due to their good planarity, both exhibit a high hole mobility of ∼7 × 10⁻⁴ cm² V⁻¹ s⁻¹ after blending with Y6. As a result, PBBO-1:Y6, which has a more ideal morphology and overall better photovoltaic conversion dynamics, achieved a PCE of 9.90 %, surpassing the PCE of 8.99 % for PBBO-2:Y6. This work provides a new strategy for designing high mobility wide bandgap polymer donors.