Controllable synthesis of single crystalline Sn-based oxides and their application in perovskite solar cells

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 SnO₂ and Zn₂SnO₄ nanoparticles (NPs), SnO₂ nanorods, and Zn₂SnO₄ nanocubes. In particular, a ratio of Zn/Sn = 1 results in nanocomposites of single-crystalline SnO₂ nanorods and Zn₂SnO₄ 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)₄²⁻ and Sn(OH)₆²⁻. 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 SnO₂ nanorods and Zn₂SnO₄ nanocubes exhibit superior device performance to TiO₂-based PSCs due to their excellent charge collection ability and optical properties, achieving a power conversion efficiency of ≥17%.