Cyclohexylethyl‐Engineered Acceptor Enables Uniform Donor–Acceptor Morphology and over 20% Efficient Layer‐by‐Layer Organic Solar Cells

ABSTRACT Layer‐by‐layer (LBL) technique offers a promising platform for optimal vertical phase separation in organic solar cells (OSCs). Nevertheless, the realization of high‐performance LBL‐OSCs is fundamentally limited by solvent‐induced swelling during deposition, a process that promotes uncontrolled donor–acceptor interdiffusion, thereby constraining device performance. To address this challenge, we introduce a molecular design strategy that incorporates a novel third component, BTA‐CyH. Engineered with a rigid cyclohexane unit at the terminus of the N‐side chain in benzotriazole unit, the BTA‐CyH acceptor leverages steric hindrance and self‐assembly to precisely modulate the interpenetration dynamics of the acceptor during LBL film formation. Incorporating 10 wt.% BTA‐CyH into the L8‐BO layer serves a dual function: it enhances donor–acceptor miscibility to promote more D/A (donor/acceptor) interfaces and efficient exciton dissociation, while simultaneously regulating crystallization dynamics by suppressing excessive aggregation and fostering improved molecular ordering and crystallinity. Consequently, the ternary OSCs achieve a remarkable power conversion efficiency (PCE) of 19.73%. With Ph‐4PACz as the hole extraction layer, over 20% efficiency is demonstrated in D18/L8‐BO:BTA‐CyH‐based device. This work highlights rational side‐chain engineering as a powerful and generalizable strategy for manipulating vertical composition profiles and interfacial nanostructures in LBL‐OSCs, paving the way for further advances in sequentially fabricated high‐performance OSCs.