Triplet exciton formation for non-radiative voltage loss in high-efficiency nonfullerene organic solar cells

Nonfullerene acceptor (NFA) organic solar cells (OSCs) with power conversion efficiency (PCE) reaching up to 18% has shown tremendous potential toward practical applications. However, the non-radiative recombination loss in high-efficiency NFA OSCs is evidently large, and the origin remains unclear. Herein, by combining transient absorption spectroscopy, photovoltaic characterization, and detailed energy loss analysis, we unambiguously show the formation of NFA triplet exciton from free carrier recombination as a main non-radiative recombination and energy loss channel. More importantly, in contrast to the previous energetics point of view, we show side-chain fluorination in high-efficiency NFA OSCs has little effect on charge transfer energy landscapes but efficiently inhibits charge recombination to NFA triplet excitons, leading to higher VOC and PCE. Our findings provide a new view about energy loss in NFA OSCs and pave the way toward OSCs with lower VOC loss and efficiency exceeding 20%.