Reducing the Voc Loss of Hole Transport Layer-Free Carbon-Based Perovskite Solar Cells via Dual Interfacial Passivation

The hole transport layer (HTL)-free carbon-based perovskite solar cells (C-PSCs) are promising for commercialization owing to their excellent operational stability and simple fabrication process. However, the power conversion efficiencies (PCE) of C-PSCs are inferior to the metal electrode-based devices due to their open-circuit voltage (V_oc) loss. Herein, time-resolved confocal photoluminescence microscopy reveals that grain boundary defects at the perovskite/carbon interface are very likely to function as nonradiative recombination centers in HTL-free C-PSCs. A versatile additive Li₂CO₃ is used to modify the conformal tin oxide electron transport layer for HTL-free C-PSCs. Li₂CO₃ modification can result in enhanced charge extraction and optimized energy alignment at electron transport layer/perovskite interface, as well as suppressed defects at perovskite top surface due to Li₂CO₃-induced formation of PbI₂ crystallites. Such dual interfacial passivation ultimately leads to significantly improved V_oc up to 1.142 V, which is comparable to the metal electrode-based devices with HTL. Moreover, a record-high PCE of 33.2% is achieved for Li₂CO₃-modified C-PSCs under weak light illumination conditions, demonstrating excellent indoor photovoltaic performance. This work provides a practical approach to fabricate low-cost, highly efficient carbon-based perovskite solar cells.