Self-boosting non-hydrolytic synthesis of Cl-passivated SnO2 nanocrystals for universal electron transport material of next-generation solar cells

Recently, tin oxide (SnO2) has attracted great attention as a promising electron transport layer (ETL) material for perovskite solar cells (PSCs) due to its high electron mobility, low-temperature fabrication and superior photocatalytic stability. However, there remains room for further utilization of the full potential of this material and improvement in power conversion efficiency (PCE) of devices, including establishment of controllable synthetic process and efficient surface modification. In this study, we first report on the novel tert-butyl alcohol (t-BuOH)-mediated non-hydrolytic synthesis of SnO2 nanocrystals (NCs) with effective pre-Cl-passivation without additional treatment. In this reaction, the by-products, tert-butyl chloride and water, are readily hydrolyzed to t-BuOH and chloride ions, which can be continuously reused and boost the reaction. Consequently, Cl effectively passivates the surface of SnO2 NCs as well as facilitates the dispersion stability, leading to reduced trap density and enhanced charge extraction characteristics of the SnO2 ETL. The device fabricated using the t-BuOH-mediated SnO2 NCs exhibits improved device efficiency, excellent reproducibility, reduced hysteresis, and longer device stability compared to the device using SnO2 NCs synthesized by conventional non-hydrolytic process. For the regular type (n-i-p) PSCs, the device achieves a PCE of 20.2% and maintains 97% of initial efficiency after 70 days under ambient conditions. In addition, unnecessary high temperature annealing and additional treatment of the non-hydrolytically synthesized SnO2 NCs enable their direct deposition on the perovskite layer, thereby leading to inverted type (p-i-n) PSCs with the best PCE of 17.0%.