Carbon Quantum Dots/TiO_<i>x</i> Electron Transport Layer Boosts Efficiency of Planar Heterojunction Perovskite Solar Cells to 19%

Abstract

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)/TiO 2 composite that has negligible absorption in the visible spectral range, a very attractive feature for perovskite solar cells. Using this novel CQDs/TiO 2 ETL in conjunction with a planar n-i-p heterojunction, we achieved an unprecedented efficiency of ∼19% under standard illumination test conditions. It was found that a CQDs/TiO 2 combination increases both the open circuit voltage and short-circuits current density as compared to using TiO 2 alone. Various advanced spectroscopic characterizations including ultrafast spectroscopy, ultraviolet photoelectron spectroscopy, and electronic impedance spectroscopy elucidate that the CQDs increases the electronic coupling between the CH 3 NH 3 PbI 3– x Cl x and TiO 2 ETL interface as well as energy levers that contribute to electron extraction.