Silicotungstate, a Potential Electron Transporting Layer for Low-Temperature Perovskite Solar Cells

Thin films of a heteropolytungstate, lithium silicotungstate (Li₄SiW₁₂O₄₀, termed Li-ST), prepared by a solution process at low temperature, were successfully applied as electron transporting layer (ETL) of planar-type perovskite solar cells (PSCs). Dense and uniform Li-ST films were prepared on FTO glass by depositing a thin Li-ST buffer layer, followed by coating of a main Li-ST layer. The film thickness was controlled by varying the number of coating cycles, consisting of spin-coating and thermal treatment at 150 °C. In particular, by employing 60 nm-thick Li-ST layer obtained by two cycles of coating, the fabricated CH₃NH₃PbI₃ PSC device demonstrates the photovoltaic conversion efficiency (PCE) of 14.26% with J_SC of 22.16 mA cm^-2, V_OC of 0.993 mV and FF of 64.81%. The obtained PCE is significantly higher than that of the PSC employing a TiO₂ layer processed at the same temperature (PCE = 12.27%). Spectroscopic analyses by time-resolved photoluminescence and pulsed light-induced transient measurement of photocurrent indicate that the Li-ST layer collects electrons from CH₃NH₃PbI₃ more efficiently and also exhibits longer electron lifetime than the TiO₂ layer thermally treated at 150 °C. Thus, Li-ST is considered to be a promising ETL material that can be applied for the fabrication of flexible PSC devices.