High‐Efficiency Layer‐by‐Layer All‐Polymer Solar Cell Enabled by Bottom‐Layer Optimization

All‐polymer solar cells (all‐PSCs) have attracted extensive attention for their advantages in long‐term thermal‐ and photostability. However, the power conversion efficiencies (PCEs) of all‐PSCs still lag behind organic solar cells based on small‐molecular acceptors. The long‐chain entanglement between polymers brings complex morphological problems, which contribute to lower fill factor (FF). Herein, an effective approach is proposed to build the ideal morphology and pseudo‐p–i–n vertical component distribution in all‐polymer active layer by independently casting donor and acceptor films with different solvents. Through the solvent engineering, the layer‐by‐layer device with o ‐xylene and carbon disulfide (O‐XY:CS 2 )‐processed D18 donor layer achieves a PCE of 17.53%, much higher than commonly used solvents (chloroform and chlorobenzene) processed donor layers with PCEs of 16.49 and 16.04%, respectively. In‐depth investigation reveals that outstanding performance of O‐XY:CS 2 ‐processed donor layer device is attributed to mitigated bimolecular recombination, more balanced mobility, and reduced trap density of states, which contribute to the enhancement of short‐current density and FF. Moreover, favorable morphology network also brings prolonged lifetime under continuous light soaking. This work presents a simple and effective way to obtain ideal active layer morphology and construct favorable vertical component distribution through optimizing donor morphology in all‐PSCs.