Controllable synthesis of single crystalline Sn-based oxides and their application in perovskite solar cellsElectronic supplementary information (ESI) available: SEM image of Zn doped SnO2 synthesized using NaOH as the OH− supplier instead of N2H4 under the low Zn/Sn molar ratio conditions (Fig. S1), plane-view SEM images of tin-based nanostructure films on fused silica substrates (Fig. S2), plane-view SEM images of perovskite/ETL films on fused silica substrates (Fig. S3), cross-sectional SEM i

We synthesized single-crystalline Sn-based oxides for use as electron-transporting layers (ETLs) in perovskite solar cells (PSCs). The control of the Zn-to-Sn cation ratio (Zn/Sn = 0-2) in a fixed concentration of hydrazine solution leads to the formation of various types of Sn-based oxides, i.e. , spherical SnO 2 and Zn 2 SnO 4 nanoparticles (NPs), SnO 2 nanorods, and Zn 2 SnO 4 nanocubes. In particular, a ratio of Zn/Sn = 1 results in nanocomposites of single-crystalline SnO 2 nanorods and Zn 2 SnO 4 nanocubes. This is related to the concentration of free hydrazine unreacted with Zn and Sn ions in the reaction solution, because the resulting OH − concentration affects the growth rate of intermediate phases such as ZnSn(OH) 6 , Zn(OH) 4 2− and Sn(OH) 6 2− . Additionally, we propose plausible pathways for the formation of Sn-based oxides in hydrazine solution. The Sn-based oxides are applied as ETLs and annealed at a low temperature below 150 °C in PSCs. The PSCs fabricated by using the nanocomposite ETLs consisting of single-crystalline SnO 2 nanorods and Zn 2 SnO 4 nanocubes exhibit superior device performance to TiO 2 -based PSCs due to their excellent charge collection ability and optical properties, achieving a power conversion efficiency of ≥17%. We synthesized single-crystalline Sn-based oxides for use as electron-transporting layers (ETLs) in perovskite solar cells (PSCs).