Controllable Exciton Diffusion Length and Ultrafast Charge Generation in Ternary Organic Solar Cells

Charge generation, a critical process in the operation of organic solar cell (OSC), requires thorough investigation in an ultrafast perspective. This work demonstrates that the utilization of alloy model for the non-fullerene acceptor (NFA) component can regulate the crystallization properties of active layer films, which in turn affects exciton diffusion and hole transfer (HT), ultimately influencing the charge generation process. By incorporating BTP-eC7 as a third component, without expanding absorption range or changing molecular energy levels but regulating the ultrafast exciton diffusion and HT processes, the power conversion efficiency (PCE) of the optimized PM6:BTP-eC9:BTP-eC7 based ternary OSC is improved from 17.30% to 17.83%, primarily due to the enhancement of short-circuit current density (JSC). Additionally, the introduction of BTP-eC7 also reduces the trap state density in the photoactive layer which helps to reduce the loss of JSC. This study introduces a novel approach for employing ternary alloy models by incorporating dual acceptors with similar structures, and elucidates the underlying mechanism of charge generation and JSC in ternary OSCs. This work demonstrates the feasibility of regulating exciton diffusion and hole transfer to promote efficient charge generation, and achieving high-performance OPV devices by increasing short-circuit current density. Fast and efficient charge generation requires an increased exciton diffusion length and faster hole transfer. In addition, the reduced trap state in the ternary system is also beneficial for reducing recombination of charges. This work reveals the co-mechanism of charge generation and trap state on JSC. image