Metal Oxide Electron-Selective Layers for Inverted Perovskite Solar Cells By Atomic Layer Deposition

Perovskites are promising candidates for low cost, high efficiency light absorbers with a large, tunable band gap. Recent advances in photostability and thermal stability from the introduction of mixed-cation lead mixed-halide perovskites in the form of cesium formamidinium (CsFA) perovskites has lead to significant interest in the development of perovskite-silicon tandem solar cells. Improvements in the deposition of transparent contacts, stability of organics within the device structure and blocking of moisture to the perovskite absorber must be made to realize highly efficient and stable perovskite-silicon devices. The improved thermal stability of the CsFA perovskite has opened up new processing options through atomic layer deposition (ALD) for the conformal coating of metal oxide materials on top of perovskite absorbers. In this work, we explore TiO_x and SnO_x ALD thin films deposited on top of CsFA perovskite devices in an inverted structure as dual-purpose layers for electron-selectivity and degradation protection. We will discuss the effects of ALD processing conditions on TiO_x and SnO_x electronic properties as well as on CsFA perovskite stability. We achieve stable >15% power conversion efficiency with 1.6 eV perovskite devices through maximum power point tracking and demonstrate the effectiveness of ALD metal oxide layers as buffer layers for the sputtering of indium tin oxide contacts for semi-transparent devices with >14% efficiency.