Simultaneous Li-Doping and Formation of SnO₂-Based Composites with TiO₂: Applications for Perovskite Solar Cells

Tin oxide (SnO₂) has been recognized as one of the beneficial components in the electron transport layer (ETL) of lead-halide perovskite solar cells (PSCs) due to its high electron mobility. The SnO₂-based thin film serves for electron extraction and transport in the device, induced by light absorption at the perovskite layer. The focus of this paper is on the heat treatment of a nanoaggregate layer of single-nanometer-scale SnO₂ particles in combination with another metal-dopant precursor to develop a new process for ETL in PSCs. The combined precursor solution of Li chloride and titanium(IV) isopropoxide (TTIP) was deposited onto the SnO₂ layer. We varied the heat treatment conditions of the spin-coated films comprising double layers, i.e., an Li/TTIP precursor layer and SnO₂ nanoparticle layer, to understand the effects of nanoparticle interconnection via sintering and the mixing ratio of the Li-dopant on the photovoltaic performance. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) measurements of the sintered nanoparticles suggested that an Li-doped solid solution of SnO₂ with a small amount of TiO₂ nanoparticles formed via heating. Interestingly, the bandgap of the Li-doped ETL samples was estimated to be 3.45 eV, indicating a narrower bandgap as compared to that of pure SnO₂. This observation also supported the formation of an SnO₂/TiO₂ solid solution in the ETL. The utilization of such a nanoparticulate SnO₂ film in combination with an Li/TTIP precursor could offer a new approach as an alternative to conventional SnO₂ electron transport layers for optimizing the performance of lead-halide perovskite solar cells.