High Current Density Sn-Based Perovskite Solar Cells via Enhanced Electron Extraction in Nanoporous Electron Transport Layers

Sn-based perovskite solar cells have attracted much attention as the lead-free next generation of perovskite solar cells. However, their efficiency is still low (similar to 12%). One reason is that the carrier diffusion length of Sn-based perovskites is shorter than that of Pb-based perovskites. In this study, we investigated the effects of the electron transport layer (ETL) structure, using a porous niobium oxide (Nb2O5) ETL to create a highly efficient Sn-based solar cell. We quantified the effects of the ETL structure on electron extraction efficiency by time-resolved microwave conductivity (TRMC) measurements. From the TRMC and chemical analyses of the materials, we determined that the porous ETL with smaller pores extracts electrons more effectively from the perovskite layer than the porous ETL with larger pores. To realize this, we shortened the distance that photogenerated carriers must move from the perovskite to the ETL for extraction, resulting in less carrier recombination and higher device performance. The small-pore (pore size: 5 nm) nanoporous niobium oxide cell exhibits the highest level of current density (24.0 mA cm(-2)) and solar cell efficiency (7.0%) among the n-i-p structured Sn-based perovskite solar cells. These results demonstrate the importance of ETL structural design for creating highly efficient Sn-based perovskite solar cells.