Understanding the Space-Charge Layer in SnO 2 for Enhanced Electron Extraction in Hybrid Perovskite Solar Cells.

Tin oxide (SnO 2 ) has been widely used as an n-type metal oxide electron transport layer in perovskite solar cells (PSCs) owing to its superior electrical and optical properties and low-temperature synthesis process. In particular, the interfacial effect between indium tin oxide (ITO) and SnO 2 is an important parameter that controls the charge transport properties and device performance of the PSCs. Therefore, understanding the interfacial effect of ITO/SnO 2 and its role in PSCs is crucial, but it is not studied intensively. Herein, we investigated the space-charge effect at the interface of ITO/SnO 2 using transfer length measurement and conductive atomic force microscopy as a function of SnO 2 thickness. Moreover, optical, morphologic, and device measurements were performed to determine the optimal SnO 2 thickness for PSCs. The space-charge effect was identified in ITO/SnO 2 when the SnO 2 layer was very thin due to electron depletion near the interface. Interestingly, a critical kink point was observed at approximately 10 nm SnO 2 thickness, indicating the electron depletion and weak charge transfer behavior of the device. Thus, a thickness around 20 nm was favorable for the best PSC performance because charge transport behavior in the thin SnO 2 layer was depressed by electron depletion. However, when the thickness of SnO 2 exceeded 50 nm, the device performance deteriorated due to increased series resistance. This study provides a strategy to tune the electron transport layer and boost the charge transport behavior in PSCs, making important contributions to optimizing SnO 2 -based PSCs.