A 2,5-difluoro benzene-based low cost and efficient polymer donor for non-fullerene solar cells

Although the latest progress in the power conversion efficiency (PCE) of non-fullerene polymer solar cells (NFPSCs) has proved their potential as next-generation solar technologies, the development of efficient wide-bandgap (WBG) donor polymers having a low-cost and easily scalable molecular design is still lagging in terms of number and diversity. In this contribution, we report a new WBG polymer donor, PBDT-2FBnT, based on alternative benzodithiophene as the donor unit and 2,5-difluoro benzene (2FBn) as the acceptor unit, which was prepared by a facile three-step process with an overall yield over 80% using cheap raw materials. Benefitting from the incorporation of an electron-deficient 2FBn unit, PBDT-2FBnT demonstrated lower frontier energy levels with an optical bandgap of 2.12 eV. Additionally, the combination of 2FBn with an adjacent two thiophene bridge in the polymer backbone significantly minimized the steric hindrance via non-covalent F-S and C-H-F inter/intramolecular interactions, thereby promoting highly coplanar geometry for effective molecular packing and charge transport. Consequently, optimized blends of PBDT-2FBnT with an ITIC acceptor delivered complementary panchromatic absorption, well-aligned energy levels, higher charge carrier mobilities, and well-distributed nano-fibrillar morphology, thereby leading to a remarkable efficiency of 9.5% with a good trade-off between corresponding photovoltaic parameters. Thus, the high photovoltaic performance of PBDT-2FBnT together with simple preparation can provide a promising way for the future development of low-cost PSCs, and we can foresee further PCE improvements of the PBDT-2FBnT structure via synergistic variation of donor and acceptor material.