Siloxane‐Tethered Halogenated Thiophene Additives Enable Excellent Fibrillar Morphology and Air Processing for High‐Performing Polymer Solar Cells

Abstract For organic solar cells (OSCs), morphological optimization and high‐performance air‐processing of the photoactive layer are important concerns. Herein, three siloxane‐tethered halogenated thiophene additives Cl‐Th‐SiO, Br‐Th‐SiO, and I‐Th‐SiO, with chlorine, bromine, and iodine substitutions on the thiophene, are designed and synthesized, respectively. With the three additives, fibrillar active layer networks based on polymer donor D18 and nonfullerene acceptor L8‐BO can be achieved. Among the three additives, the chlorinated Cl‐Th‐SiO possesses the largest dipole moment and the strongest interactions to D18 and L8‐BO, leading to the closet π – π stacking for both the D18 and L8‐BO. The fibrillar morphology is beneficial for enhancing exciton dissociation and charge transport as well as reducing trap‐state density, finally achieving remarkably improved photovoltaic performances for the three additives processed OSCs. Among the additives, Cl‐Th‐SiO is the most powerful, which affords the N 2 ‐processed D18:L8‐BO binary OSCs with excellent PCEs of 20.64% (certified 20.11%). When casting the active layers in air with 60% and 90% relative humidity, high PCEs of 20.45% and 20.19%, respectively, can be achieved, representing the most powerful air‐processed active layers. Our results suggest that the additive design in this work is promising toward ambient‐processed high‐performing OSCs.