Kinetic Management of Vertical Phase Separation via Thermal Annealed Donor‐Assisted Sequential Deposition Enables 20.81% Efficiency in Organic Solar Cells

Sequential deposition (SD) process has emerged as a key method for precisely controlling the active layer morphology, which is crucial for enhancing the photovoltaic performance of organic solar cells (OSCs). However, conventional SD methods often face challenges such as interfacial erosion and disordered crystallization, which limit further efficiency improvements. Herein, we report a novel thermally annealed donor‐assisted sequential deposition (TD‐SD) strategy aimed at modulating the film‐formation kinetics during the SD process. By pre‐annealing the donor layer, an ordered donor molecular packing is formed. Moreover, the thermal effect provided by the donor accelerated the aggregation kinetics of the acceptor, which helps mitigate the erosion of the donor by the acceptor. Therefore, a more pronounced vertical gradient distribution is formed. Such an optimized morphology promotes exciton dissociation and charge transport. Consequently, the devices based on TD‐SD strategy exhibit superior photovoltaic performance, with the champion device achieving a power conversion efficiency (PCE) of 20.35%, up from 18.65% for the D18‐Cl/L8‐BO device. Notably, the strategy also enabled a high PCE of 20.81% (certified as 20.25%) in the D18‐Cl/L8‐BO:BTP‐eC9 system, placing it among the top‐performing OSCs. This study offers a practical route for precisely controlling the morphology of OSCs processed via SD processing.