Significantly enhanced electron transport of a nonfullerene acceptor in a blend film with a high hole mobility polymer of high molecular weight: thick-film nonfullerene polymer solar cells showing a high fill factor

Abstract

Overcoming fill factor (FF) decay in thick fullerene active layers has been demonstrated with high hole mobility (μ_h) polymers. However, this issue remains a challenge for thick active layers with nonfullerene acceptors. Here we demonstrate a high FF and highly efficient nonfullerene based thick active layer with a high μ_h polymer as the donor. Its relatively balanced hole and electron transport with a μ_h/μ_e ratio of 4.42 in a 320 nm thick blend film is realized by the high molecular weight polymer induced higher electron mobility (μ_e approaching 1 × 10⁻³ cm² (V s)⁻¹) for the blend film. Relative to the pristine IEICO-4F nonfullerene film, 8 times increased μ_e for the blend film corresponds to closer interdigitation of the IEICO-4F lamella and a higher order face-on orientation of the in-plane (200) peak of IEICO-4F molecules, which are very helpful for electron transport. As a result, solar cells with 320 nm thick binary nonfullerene active layers show an outstanding FF of over 70% and a power conversion efficiency of 13.2%, a breakthrough for high μ_h polymers as the donor. Our results suggest that high μ_h polymer donors are promising candidates for nonfullerene based polymer solar cells.