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

Thin films of a heteropolytungstate, lithium silicotungstate (Li<sub>4</sub>SiW<sub>12</sub>O<sub>40</sub>, 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 CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> PSC device demonstrates the photovoltaic conversion efficiency (PCE) of 14.26% with <i>J</i><sub>SC</sub> of 22.16 mA cm<sup>–2</sup>, <i>V</i><sub>OC</sub> of 0.993 mV and FF of 64.81%. The obtained PCE is significantly higher than that of the PSC employing a TiO<sub>2</sub> 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 CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> more efficiently and also exhibits longer electron lifetime than the TiO<sub>2</sub> 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.