Carbon Quantum Dots/TiOx Electron Transport Layer Boosts Efficiency of Planar Heterojunction Perovskite Solar Cells to 19%

In planar n-i-p heterojunction perovskite solar cells, the electron transport layer (ETL) plays important roles in charge extraction and determine the morphology of the perovskite film. Here, we report a solution-processed carbon quantum dots (CQDs)/TiO2 composite that has negligible absorption in the visible spectral range, a very attractive feature for perovskite solar cells. Using this novel CQDs/TiO2 ETL in conjunction with a planar n-i-p heterojunction, we achieved an unprecedented efficiency of similar to 19% under standard illumination test conditions. It was found that a CQDs/TiO2 combination increases both the open circuit voltage and short-circuits current density as compared to using TiO2 alone. Various advanced spectroscopic characterizations including ultrafast spectroscopy, ultraviolet photoelectron spectroscopy, and electronic impedance spectroscopy elucidate that the CQ_Ds increases the electronic coupling between the CH3NH3PbI3-xClx and TiO2 ETL interface as well as energy levers that contribute to electron extraction.