Isomeric Small Molecule Donor with Terminal Branching Position Directly Attached to the Backbone Enables Efficient All‐Small‐Molecule Organic Solar Cells with Excellent Stability

Abstract All‐small‐molecule organic solar cells (ASM‐OSCs) are challenging for their inadequate efficiency and device stability due to their more susceptive morphology. Herein, a family of isomeric small molecule donors (SMDs) is synthesized based on the benzodithiophene–terthiophene core with linear, 1st carbon, and 2nd carbon position branched butyl‐based rhodanine for ASM‐OSCs, respectively. The single crystal of thiophene‐substituted model T‐ s ‐Bu forms a more compact intermolecular packing with herringbone structure than slip‐layered packing‐based T‐ n ‐Bu and T‐ i ‐Bu . SM‐ i ‐Bu and SM‐ s ‐Bu show slightly blue‐shifted absorption and deepened HOMO levels in the neat film compared to SM‐ n ‐Bu . SM‐ s ‐Bu:BO‐4Cl blend films have distinct face‐on packing orientations and suitable fibrous phase separation along with more apparent microcrystals. Finally, SM‐ s ‐Bu : BO‐4Cl ‐based device yields an improved power conversion efficiency of 16.06% compared to 15.12% and 8.22% for SM‐ n ‐Bu : BO‐4Cl and SM‐ i ‐Bu : BO‐4Cl , which is one of the top‐ranked results for BTR‐series SMDs in binary ASM‐OSCs. More importantly, the excellent storage stability with a T 80 lifetime of over 1700 h and decent thermal stability is realized in SM‐ s ‐Bu : BO‐4Cl . This work highlights that the isomeric terminal alkyl with a branching point directly connected to the backbone for SMDs is a promising strategy for improving the crystal packing and film morphology and achieving highly efficient and stable ASM‐OSCs.