Easily Accessible Low Band Gap Polymer for Efficient Nonfullerene Polymer Solar Cells with a Low E_loss of 0.55 eV

Polymer solar cells (PSCs) have been attracting attention as a promising photovoltaic technology, and power conversion efficiencies (PCEs) of over 14% have been demonstrated. However, the state-of-the-art conjugated polymers for PSCs are based on the complex molecular structure that requires several steps for synthesis and purification, which impedes low materials cost for the PSCs. Thus, it is a key challenge to develop high-performance conjugated polymers with easily accessible building blocks for applications in practical PSCs. Here, a novel low band gap polymer (PFBTBOX) which comprises readily available building blocks was designed and synthesized to be used as an electron donor for PSCs. Affirmative optoelectronic properties of PFBTBOX for photovoltaic applications resulted in high PCEs up to 10.20 and 8.11% with nonfullerene and fullerene acceptors, respectively. Moreover, PFBTBOX exhibited minimal energy loss as low as 0.55 eV with a nonfullerene acceptor, indicating that PFBTBOX is a promising electron donor polymer for practical PSCs with high efficiency exceeding 10%.