Strategic Halogen Substitution to Enable High‐Performance Small‐Molecule‐Based Tandem Solar Cell with over 15% Efficiency

Abstract Small molecules have been recently highlighted as active materials owing to their facile synthesisis method, well‐defined molecular structure, and highly reproducible performance. In particular, optimizing bulk heterojunction (BHJ) morphologies is important to achieving high performance in solution‐processable small molecule solar cells (SM‐SCs). Herein, a series of benzodithiophene‐based active materials with different halogen atoms substituted at the end‐group, are reported, as well as how these halogen atoms affect the morphology of BHJ architectures through microstructure analyses. Materials with chlorine atoms show a well‐mixed morphology and interpenetrating networks when blended with [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester, facilitating effective charge transportation. This controlled morphology helps attain excellent performance with a power conversion efficiency (PCE) of 10.5% and a highest fill factor of 78.0% without additives. In addition, it can be applied to two‐terminal (2T)‐tandem solar cells, attaining an outstanding PCE of up to 15.1% with complementary absorption in the field of the 2T‐tandem solar cells introducing the SM‐SCs. These results suggest that tailoring interactions with halogen atoms is an effective way to control BHJ architectures, thereby achieving remarkable performance in SM‐SCs.