Smoothing and coverage improvement of SnO2 electron transporting layer by NH4F treatment: Enhanced fill factor and efficiency of perovskite solar cells

Abstract

Surface modification of SnO2electron transporting layer (ETL) plays a critical role in the performance of SnO2-based planar perovskite solar cells (PSC). Here, we show how long-time NH4F-based water bath treatment of SnO2 layer makes smoothing and morphological improvements and enhances the device performance. Recently it was shown that short-time NH4F treatment (2 sec)with spin coatingmethod reduces interface traps by improving surface chemistry. Here we observethe smoothing of SnO2films as a result of long-time NH4F treatment, which could be a resultof a slightetching-deposition process. Absorption and resistivity measurements indicate that SnO2 etching process is involved in morphology healing. This has led to considerably better FTO coverage and improved hole blocking, as evidenced by cyclic voltammetry measurements. Additionally, we observe the presence of F on the surface (X-ray photoelectron spectroscopy) and possibly inside the layer, as there ishigher carrier density in NH4F treated films. Improvements in thePSC device performance are accompanied byhigher recombination resistance, and better electron capturing properties, as evidenced by electrochemical impedance spectroscopy and photoluminescence quenching.