Alkyl Chain Tuning of Non-fullerene Electron Acceptors toward 18.2% Efficiency Binary Organic Solar Cells

Tailoring of the chemical structure is an effective method to tune the aggregation and optoelectronic properties of organic photovoltaic materials to boost the performance of organic solar cells (OSCs). Here, four non-fullerene electron acceptor materials, namely, BTP-4F-C8-16, BTP-4F-C7-16, BTP-4F-C6-16, and BTP-4F-C5-16, with different lengths of alkyl chain on the bithiophene units were synthesized, and the impact of chain length on the intermolecular stacking, nanoscale phase separation with polymer donors, optoelectronic properties, and device performance were investigated. Molecular dynamics simulations and experimental exploration show that reducing the chain length from n-octyl (C8) to n-pentyl (C5) can enhance the molecular planarity, shorten the π–π stacking distance, and improve the electron mobility, consequently leading to enhanced structural order, charge mobility, and appropriate phase separation in the blend with PM6, contributing to the achievement of the best power conversion efficiency of 18.20% with a V_OC of 0.844 V, a fill factor of 77.68%, and a J_SC of 27.78 mA cm^–2, which is one of the highest efficiencies of single-junction binary OSCs reported in the literature so far.