Buried interface management via bifunctional NH4BF4 towards efficient CsPbI2Br solar cells with a Voc over 1.4 V

CsPbI₂Br perovskite solar cells (PSCs) have drawn tremendous attention due to their suitable bandgap, excellent photothermal stability, and great potential as an ideal candidate for top cells in tandem solar cells. However, the abundant defects at the buried interface and perovskite layer induce severe charge recombination, resulting in the open-circuit voltage (V_oc) output and stability much lower than anticipated. Herein, a novel buried interface management strategy is developed to regulate interfacial carrier dynamics and CsPbI₂Br defects by introducing ammonium tetrafluoroborate (NH₄BF₄), thereby resulting in both high CsPbI₂Br crystallization and minimized interfacial energy losses. Specifically, NH₄⁺ ions could preferentially heal hydroxyl groups on the SnO₂ surface and balance energy level alignment between SnO₂ and CsPbI₂Br, enhancing charge transport efficiency, while BF₄⁻ anions as a quasi-halogen regulate crystal growth of CsPbI₂Br, thus reducing perovskite defects. Additionally, it is proved that eliminating hydroxyl groups at the buried interface enhances the iodide migration activation energy of CsPbI₂Br for strengthening the phase stability. As a result, the optimized CsPbI₂Br PSCs realize a remarkable efficiency of 17.09% and an ultrahigh V_oc output of 1.43 V, which is one of the highest values for CsPbI₂Br PSCs.