Crystallization Regulation and Defect Passivation for Efficient Inverted Wide‐Bandgap Perovskite Solar Cells with over 21% Efficiency

Wide‐bandgap (WBG) perovskite solar cells (PSCs) have drawn great attention owing to their promising potential for constructing efficient tandem solar cells. However, the rapid crystallization results in poor film properties and easy formation of defects, thereby greatly restricting the acquisition of a small open‐circuit voltage (VOC) deficit due to the severe nonradiative recombination. Herein, it introduced the triethanolamine borate (TB) to effectively slow down the rapid crystallization for preparing highly crystalline and uniform WBG perovskite films with reduced defects. The strong intermolecular interaction (e.g., coordination and hydrogen bond) between TB and perovskite can suppress the halide vacancy formation and inhibit phase segregation for improving long‐term stability. The devices based on a 1.65 eV perovskite absorber achieved a high efficiency of 21.55% with a VOC of 1.24 V, demonstrating the VOC deficit is as low as 0.41 V, which is one of the lowest reports. By combining a semitransparent WBG subcell with a narrow‐bandgap tin‐based PSC, the four‐terminal tandem solar cell delivers a high efficiency of 26.48%. It reveals the degradation pathways of a wide range of state‐of‐the‐art nonfullerene acceptors from molecular to aggregation level. The structural confinement and molecular ordering are responsible for molecular conformational stability under illumination. The origin of increased nonradiative decay under illumination is predominantly in the aggregated states with strong intermolecular interactions while the intramolecular exciton dynamics are stable.