Rational Manipulation of Fluorination Sites Enables 19.58% Efficiency Binary Organic Solar Cells with Optimized Energy Levels and Improved Charge Transfer

Abstract

Recent advancements in organic solar cells (OSCs) have been driven by Y‐series acceptors and their derivatives. Investigations on the types/quantities of central halogen atoms and terminal halogen atoms sites have achieved significant progress. However, the influence of central halogen atomic sites on the photovoltaic performance of acceptors remain understudied. In this work, two asymmetric acceptors (CH‐o2F and CH‐m2F) with distinct central‐core fluorination substitution sites are rationally designed based on the structurally symmetric CH‐6F, employing density functional theory calculations and molecular dynamics simulations. The theoretical insights revealing better‐matched energy levels and the potential for improved molecular stacking motivated further experimental investigations. The upshift of lowest unoccupied molecular orbital and blue shift of ultraviolet absorption, leading to a higher open‐circuit voltage. The femtosecond transient absorption spectroscopy test shows that the faster exciton diffusion and dissociation in PM6:CH‐m2F film. Their good miscibility, enhanced charge mobility, and reduced recombination further contributed to increased fill factor and short‐circuit current density. The synergistic effect of energy level modulation and molecular stacking optimization achieved a boosted power conversion efficiency of 19.58% in binary OSCs. This study highlights the critical role of precise fluorination site manipulation in tuning optoelectronic properties and provides new opportunities for high efficiency OSCs materials.