Tandem Organic Solar Cells with 21.5% Efficiency

Abstract Tandem organic solar cells (OSCs) offer a promising strategy for enhancing light utilization and reducing energy loss, presenting significant potential in achieving high power conversion efficiency (PCE). Herein, a narrow bandgap acceptor, BTA‐4F, featuring a 2‐methyl‐2H‐benzotriazole (BTA) central core, is fabricated, which is designed for the rear sub‐cell of tandem OSCs. Systematic characterizations demonstrate that incorporating strong electron donating groups BTA into central core unit can narrow the bandgap and enhance the electroluminescence external quantum efficiency. These improvements lead to increased current density and reduced voltage loss of single junction OSCs under AM 1.5G illumination and real incident light of the rear sub‐cell. Inspiringly, BTA‐4F‐based single‐junction and tandem OSCs achieve outstanding PCEs of 19.5% and 21.5% (Certified as 21.2%), respectively, which represents the milestone of 21% PCE in the field of OSCs. This study highlights the synergistic benefits of molecular design and implementation of tandem architecture as an effective strategy for enhancing photovoltaic performance of OSCs.