Ionic Liquid Bridge Assisting Bifacial Defect Passivation for Efficient All-Inorganic Perovskite Cells with High Open-Circuit Voltage

Serious open-circuit voltage (V_oc) loss originating from nonradiative recombination and mismatch energy level at TiO₂/perovskite buried interface dramatically limits the photovoltaic performance of all-inorganic CsPbI_xBr_3-x (x = 1, 2) perovskite solar cells (PSCs) fabricated through low-temperature methods. Here, an ionic liquid (IL) bridge is constructed by introducing 1-butyl-3-methylimidazolium acetate (BMIMAc) IL to treat the TiO₂/perovskite buried interface, bilaterally passivate defects and modulate energy alignment. Therefore, the V_oc of all-inorganic CsPbIBr₂ PSCs modified by BMIMAc (Target-1) significantly increases by 148 mV (from 1.213 to 1.361 V), resulting in the efficiency increasing to 10.30% from 7.87%. Unsealed Target-1 PSCs show outstanding long-term and thermal stability. During the accelerated degradation process (85 °C, RH: 50∼60%), the Target-1 PSCs achieve a champion PCE of 11.94% with a remarkable V_oc of 1.403 V, while the control PSC yields a promising PCE of 10.18% with a V_oc of 1.319 V. In particular, the V_oc of 1.403 V is the highest V_oc reported so far in carbon-electrode-based CsPbIBr₂ PSCs. Moreover, this strategy enables the modified all-inorganic CsPbI₂Br PSCs to achieve a V_oc of 1.295 V and a champion efficiency of 15.20%, which is close to the reported highest PCE of 15.48% for all-inorganic CsPbI₂Br PSCs prepared by a low-temperature process. This study provides a simple BMIMAc IL bridge to assist bifacial defect passivation and elevate the photovoltaic performance of all-inorganic CsPbI_xBr_3-x (x = 1, 2) PSCs.