Low-Temperature Plasma-Assisted Atomic-Layer-Deposited SnO₂ as an Electron Transport Layer in Planar Perovskite Solar Cells

In this work, we present an extensive characterization of plasma-assisted atomic-layer-deposited SnO₂ layers, with the aim of identifying key material properties of SnO₂ to serve as an efficient electron transport layer in perovskite solar cells (PSCs). Electrically resistive SnO₂ films are fabricated at 50 °C, while a SnO₂ film with a low electrical resistivity of 1.8 × 10^-3 ω cm, a carrier density of 9.6 × 10¹⁹ cm^-3, and a high mobility of 36.0 cm²/V s is deposited at 200 °C. Ultraviolet photoelectron spectroscopy indicates a conduction band offset of ∼0.69 eV at the 50 °C SnO₂/Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_2.7Br_0.3) interface. In contrast, a negligible conduction band offset is found between the 200 °C SnO₂ and the perovskite. Surprisingly, comparable initial power conversion efficiencies (PCEs) of 17.5 and 17.8% are demonstrated for the champion cells using 15 nm thick SnO₂ deposited at 50 and 200 °C, respectively. The latter gains in fill factor but loses in open-circuit voltage. Markedly, PSCs using the 200 °C compact SnO₂ retain their initial performance at the maximum power point over 16 h under continuous one-sun illumination in inert atmosphere. Instead, the cell with the 50 °C SnO₂ shows a decrease in PCE of approximately 50%.