High Open‐Circuit Voltage Organic Solar Cells with 19.2% Efficiency Enabled by Synergistic Side‐Chain Engineering

Abstract Restricted by the energy‐gap law, state‐of‐the‐art organic solar cells (OSCs) exhibit relatively low open‐circuit voltage (V_OC) because of large nonradiative energy losses (ΔE_nonrad). Moreover, the trade‐off between V_OC and external quantum efficiency (EQE) of OSCs is more distinctive; the power conversion efficiencies (PCEs) of OSCs are still <15% with V_OCs of >1.0 V. Herein, the electronic properties and aggregation behaviors of non‐fullerene acceptors (NFAs) are carefully considered and then a new NFA (Z19) is delicately designed by simultaneously introducing alkoxy and phenyl‐substituted alkyl chains to the conjugated backbone. Z19 exhibits a hypochromatic‐shifted absorption spectrum, high‐lying lowest unoccupied molecular orbital energy level and ordered 2D packing mode. The D18:Z19‐based blend film exhibits favorable phase separation with face‐on dominated molecular orientation, facilitating charge transport properties. Consequently, D18:Z19 binary devices afford an exciting PCE of 19.2% with a high V_OC of 1.002 V, surpassing Y6‐2O‐based devices. The former is the highest PCE reported to date for OSCs with V_OCs of >1.0 V. Moreover, the ΔE_nonrad of Z19‐ (0.200 eV) and Y6‐2O‐based (0.155 eV) devices are lower than that of Y6‐based (0.239 eV) devices. Indications are that the design of such NFA, considering the energy‐gap law, could promote a new breakthrough in OSCs.