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

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 T₈₀ 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.