Charge Injection and Electrical Response in Low-Temperature SnO2-Based Efficient Perovskite Solar Cells.

Defining low-temperature engineering protocols for efficient planar perovskite solar cell (PSC) preparation is important for fabrication simplification and low-cost production. In the present work, we have defined a low-temperature (123 °C) protocol for the preparation from a solution of SnO2 layers which are efficient for an application as an electron transporting layer (ETL) in PSCs. Thin, conformal, and transparent layers have been obtained. The related PSCs have shown best devices with a power conversion efficiency of 18.22% and low-hysteresis J- V curves (a hysteresis index of 6.7%). Charge injection has been thoroughly studied by photoluminescence decay measurements. The decay curves followed a biexponential function. The injection of holes into the spiro-OMeTAD layer was found very fast and is a no-limiting step. On the other side, the charge injection into the oxide ETLs depends on its structure and on the oxide. The time constant for the low-temperature SnO2 layers is close to that of the mesoporous benchmark layers with a fast (surface) and a slow (bulk) component at 11 and 129 ns with relative contributions calculated at 13% and 87%, respectively. The phenomena occurring at a longer time scale have been investigated by impedance spectroscopy. The SnO2 cell spectra showed no intermediate-frequency inductive loop. The very low frequency part of the spectra was characterized by the beginning of an arc of a circle at the origin of a very large resistance over a large applied potential range. This resistance, along with an intermediate-frequency resistance, has been assigned to a recombination resistance and explains the very large  Voc achievable with SnO2 PSCs. The existence of a capacitance at the intermediate frequency with a noticeable low value at about 0.2 mF·cm-2 is linked with the low hysteresis of the devices.