Synergetic modification on buried and upper surfaces of perovskites with nitrogen-doped carbon quantum dots for efficient and stable solar cells

Tin oxide (SnO2), which is used as an electron transport layer in perovskite solar cells (PSCs), however, has nonnegligible defects. The surface of SnO2 films contains hydroxyl groups (–OH), which increases film defects and affects the quality of the upper perovskite. The energy level mismatch is also found in SnO2, which leads to severe nonradiative recombination and unsatisfactory photovoltaic performance. Meanwhile, excessive PbI2 residues on the perovskite surface serve as catalytic decomposition sites, significantly undermining device stability. In this study, nitrogen-doped carbon quantum dots (N-CQDs) were synthesized by using human hair as a raw material and were employed as an additive for interface between SnO2 and perovskites to improve interfacial charge transport. In addition, these N-CQDs were also used as a surface modifier on top of the perovskite layer to reduce residual PbI2. These results significantly increase the power conversion efficiency (PCE) of the related devices from 19.22% to 21.49%. The CQDs-modified device also shows significantly enhanced operational stability that when exposed to 30% humidity, the PCE of the optimized device retains 86% of its initial value after 1200 h. This study emphasizes the great potential of N-CQDs as a multifunctional interfacial modifier for fabrication of efficient and stable PSCs.