Role of the electron transport layer in dictating the nanoscale heterogeneity in all-inorganic perovskite absorbers – correlating the optoelectronic and crystallographic properties

Abstract

Within the field of perovskite photovoltaics, there has been heavy focus on either improving the conductivity/mobility of the charge transport layers [electron transport (ETL) or hole transport layers (HTL)], or tuning their energy alignment with the perovskite absorber for optimising the device efficiency, with little attention paid to the impact of the underlying charge transport layer on the structural and optoelectronic properties of the perovskite overlayer. For example, in the n–i–p device architecture, the ETL provides a key surface upon which the perovskite film grows. In this work, electron backscatter diffraction (EBSD) and cathodoluminescence (CL) spectroscopy are used to show a direct correlation between optical emission and structural properties of all-inorganic CsPbI₂Br perovskite absorber thin films with a selection of inorganic underlying ETLs, giving insights into the vital role of the ETL. Comparisons are drawn between the effect of three commonly used electron transport layers (zinc oxide, titanium dioxide and tin oxide) on the optical emission and crystallographic properties of the CsPbI₂Br perovskite thin films processed at two different annealing temperatures. Among the ETLs, zinc oxide is found to promote perovskite films with enhanced grain size and preferred growth along the [100] orientation, and relatively uniform light emission for the high temperature processed layer, showing its strong potential as a low-cost electron transport layer for the development of perovskite solar cells. Titanium dioxide is found to result in a high level of heterogeneity in the light emission when the perovskite is processed at low temperature, while tin oxide is found not to promote large grain growth. The observed variations are understood in terms of the differences in thermal expansion coefficient of the perovskite as compared to those of the ETLs as well as the leading strain in the lattice. The results from the study show the importance of considering perovskite growth effects when selecting an underlayer.