Gradient Regulation of Acceptor Crystal Growth for Synergistically Elongating Exciton Diffusion Length and Mitigating Trap Density toward 20.8% Efficiency Organic Solar Cells

ABSTRACT Controlling morphological evolution during film formation is crucial for high‐efficiency organic solar cells, yet complex intermolecular interactions often hinder controllable solution‐to‐solid growth, resulting in non‐ideal blend microstructures. Here, we report a kinetic‐gradient regulation strategy that enables programmable molecular self‐assembly using a two‐tier volatile solid additive (VSA) system with graded volatility. In this framework, highly volatile Tier‐1 VSAs initiate early‐stage acceptor nucleation and direct molecular packing, while lower‐volatility Tier‐2 VSAs prolong and relay additive–acceptor interactions into later drying stages. Additionally, by systematically tuning the halogen‐bonding propensity and dipole moment of Tier‐2 VSAs, both the strength and duration of additive–acceptor interactions are precisely regulated, enabling fine control over molecular self‐assembly and donor–acceptor interdiffusion. The resulting films exhibit tighter, more ordered packing and well‐regulated phase‐separation length scales, extending exciton diffusion and suppressing trap states, thereby enhancing short‐circuit current and fill factor. Consequently, D18:L8‐BO‐based binary devices achieve a champion efficiency of 20.5% (vs. 19.0% for the control) with improved photostability. This strategy also demonstrates scalability, delivering a 17.4% efficiency in large‐area modules, and generality, enabling 20.8% efficiency in ternary D18:L8‐BO:BTP‐eC9 devices. Overall, this work establishes a kinetic regulation paradigm for precisely directing morphological evolution of bulk‐heterojunction films toward high‐performance organic photovoltaics.