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

Abstract 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.