Bithiophene Imide‐Based “Chemically Homologous” Active Layer Enables Efficient and Stable All‐Polymer Solar Cells

Comprehensive Summary Precise control over the active layer morphology is critical for achieving high‐performance all‐polymer solar cells (all‐PSCs). In this study, we introduce a fully “chemically homologous” strategy, in which a series of tailored solid additives, polymer donor, and polymer acceptor are constructed from the same bithiophene imide (BTI) building block. Among them, the B3 additive, which shares an identical side chain with the donor, guides the formation of an optimal fibrillar network morphology with enhanced structural order. This optimized microstructure facilitates efficient exciton dissociation and charge transport, yielding a remarkable power conversion efficiency (PCE) of 18.52%—significantly surpassing those of additive‐free (14.54%) and non‐homologous reference devices (<17%). Furthermore, this chemically coherent approach promotes ordered molecular packing, simultaneously suppressing molecular disorder and non‐radiative energy loss, achieving a low energy loss ( E loss ) of 0.509 eV and a high open‐circuit voltage ( V OC ) of 0.943 V. The B3‐processed device also retains outstanding stability, maintaining over 97% of its initial PCE after 1240 h in a nitrogen atmosphere at room temperature. The chemically homologous paradigm demonstrated here provides crucial insights into morphology control and charge transport dynamics in all‐polymer blends, offering a promising pathway toward highly efficient and stable organic photovoltaics.