20.6% Efficiency Organic Solar Cells Enabled by Incorporating a Lower Bandgap Guest Nonfullerene Acceptor Without Open-Circuit Voltage Loss.

Simultaneously mitigating both photovoltage and photocurrent losses is crucial for organic solar cells (OSCs) to approach the Shockley-Queisser limit of ideal efficiency. Incorporating a narrower bandgap nonfullerene acceptor (NFA) as a guest component into the host donor:NFA system broadens the absorption spectrum. However, this can also increase the nonradiative decay rate according to the energy-gap law. In this work, ternary OSCs are constructed by combining a narrow bandgap AQx-2F (as host NFA) with a lower bandgap eC9 (as guest NFA), significantly enhancing photocurrent generation without compromising photovoltage. The addition of eC9 acts as a crystallization inducer, extending the crystallization period and increasing the ordered packing distance. This leads to suppressed trap states, elevated dielectric constant, prolonged exciton lifetime, balanced hole/electron transport, and reduced recombination loss. Consequently, the optimized D18:AQx-2F:eC9 ternary OSCs achieve a champion power conversion efficiency (PCE) of 20.6% with a high open-circuit voltage of 0.937 V, a short-circuit current density of 27.2 mA cm-2 and a fill factor of 80.8%, as validated by an independently certified PCE of 20.0%, establishing a new benchmark for bulk heterojunction OSCs. This work demonstrates an effective method to simultaneously mitigate photovoltage and photocurrent losses, paving the way for high-performance OSCs.