Surface passivation with nitrogen-doped carbon dots for improved perovskite solar cell performance

Undercoordinated lead cations and halide anions on the surface of perovskite layer can form surface trap states and cause electronic disorders which reduce the performance of perovskite solar cells. Nitrogen-doped carbon dots (NCDs) that have rich nitrogen- and oxygen-containing functional groups can effectively interact with the unsaturated metal sites and halide anions on the surface and boundaries of perovskite grains. Herein, low-cost NCDs are utilized as efficient additives to passivate the surface of a solution-processed CH3NH3PbI3 perovskite film, which remarkably reduce charge carrier recombination, as evidenced by the results of time-resolved photoluminescence and electrochemical impedance spectrum measurements. FTIR spectra indicate the formation of hydrogen bonds between the undercoordinated iodine ions on perovskite grains and hydroxyl as well as nitrogenous groups of NCDs. In addition, NCDs additives also help increase interfacial charge transfer from perovskite to electron-transporting layer, leading to an improvement in power conversion efficiency for the solar cell device from 12.12 +/- 0.28% (standard cell fabricated in same conditions) to 15.93 +/- 0.15%.