Perovskite Bifunctional Device with Improved Electroluminescent and Photovoltaic Performance through Interfacial Energy‐Band Engineering

: Currently, photovoltaic/electroluminescent (PV/EL) perovskite bifunctional devices (PBDs) exhibit poor performance due to defects and interfacial misalignment of the energy band. Interfacial energy-band engineering between the perovskite and hole-transport layer (HTL) is introduced to reduce energy loss, through adding corrosion-free 3,3'-(2,7-dibromo-9H-fluorene-9,9-diyl) bis(n,n-dimethylpropan-1-amine) (FN-Br) into a HTL free of lithium salt. This strategy can turn the n-type surface of perovskite into p-type and thus correct the misalignment to form a well-defined N-I-P heterojunction. The tailored PBD achieves a high PV efficiency of up to 21.54% (certified 20.24%) and 4.3% EL external quantum efficiency. Free of destructive additives, the unencapsulated devices maintain >92% of their initial PV performance for 500 h at maximum power point under standard air mass 1.5G illumination. This strategy can serve as a general guideline to enhance PV and EL performance of perovskite devices while ensuring excellent stability.