Thermal‐Driven Phase‐Transition Additive With High Dipole Moment for Multi‐Level Regulation of Active Layer Morphology in Organic Solar Cells

Precise morphological control during both film formation and post‐thermal annealing (TA) remains a formidable challenge for layer‐by‐layer (LbL) processed organic solar cells (OSCs). Conventional solid additives often suffer from functional stagnation during the critical TA step due to their rigid thermal properties. Herein, we develop an innovative “annealing‐activated liquid mediator” strategy for multi‐level morphological control. Such a type of solid additive of BTTz has been designed and engineered by flanking a 5,6‐difluorobenzo[d]thiazole core with thiazole rings at the 4,7 positions. This constructs multiple non‐covalent interactions to optimize the primary aggregation of the D18 donor during deposition. Crucially, a 2‐hexyloctyl alkyl chain precisely tunes the BTTz melting point to 72°C. During TA treatment (80°C–120°C), the melted BTTz acts as a transient fluidic medium, driving the ordered secondary self‐assembly of L8‐BO acceptor into a D18 network. Consequently, the BTTz‐treated devices achieve an outstanding power conversion efficiency (PCE) of 20.03%, representing a ∼10% enhancement over the control one. This dynamic phase‐transition paradigm provides a robust pathway for precise morphology manipulation in high‐performance OSCs.