Superior aggregation, morphology and photovoltaic performance enabled by fine tuning of fused electron-deficient units in polymer donors

Abstract

Copolymerization of an electron-rich donor (D) unit with an electron-deficient acceptor (A) unit to construct efficient D-π-A-π type donors is an effective strategy for organic solar cell applications. The electron-deficient unit fusion, endows extended π-conjugation plane and insures excellent photoelectronic property, has great advantages to build A moiety and gradually receives considerable attention. In this work, we adopt benzo[2,1-b:3,4-b’]dithiophene and benzopyrazine (BP), benzothiadiazole (BT) and benzoselenadiazole (BS) to cleverly construct a series of fused A units with different electron-deficient ability, and further synthesize three polymer donors PBDP-BP, PBDP-BT, and PBDP-BS, respectively. The relationships between structure and performance were systematically investigated. PBDP-BT shows a moderate aggregation behavior in both solution and film, and the highest hole mobility among the three polymers. After blending with Y6, the PBDP-BT:Y6-based film has the strongest absorption, favorable compatibility, superior crystallinity, and uniform phase separation morphology compared with PBDP-BP or PBDP-BS based blend films. Thus, the device based on PBDP-BT:Y6 has the highest and balanced charge mobility, suppressive recombination, reduced energy loss and achieves an outstanding PCE of 15.14%, which is superior to PBDP-BP:Y6 (8.55%) and PBDP-BS:Y6 (6.85%). These results provide learnable guidelines for future fused electron-deficient unit-based donor design for photovoltaic application.