Ultrasonic spray deposition of TiO2 electron transport layers for reproducible and high efficiency hybrid perovskite solar cells

The fabrication of high efficiency perovskite solar cells at larger scales will rely on the optimized deposition conditions of every layer using scalable methodologies. Most current perovskite devices that employ the archetypal TiO2 hole blocking layer utilise a simple air-brush approach. This approach is not scalable as it results in significant layer inhomogeneity across larger devices areas. To overcome this inherent limitation, in this work we use ultrasonic spray deposition as an alternative approach for the TiO2 deposition. Focusing on identical reaction chemistries as for air-brush, namely bis(isopropoxide)-bis(acetylacetonate) titanium (IV) based solutions, we find that under optimized conditions smooth TiO2 layers can be readily deposited over scalable areas on fluorine doped tin oxide. Using these layers at electron transport layers within diodes and perovskite solar cell devices, we develop effective benchmarks that predict the effectiveness of the underlying charge transport layer. This enables for the standardisation of the electron transport properties within any batch of solar cells, thus providing a readily accessible pathway towards enhancing reproducibility of fabricated devices. Under these optimized conditions, methylammonium lead halide photovoltaic devices readily possessing efficiencies of >16% are achieved. Importantly, the mean batch efficiency of devices fabricated using the ultrasonic spray deposition method is significantly improved and the standard deviation is drastically narrowed. The deposition of an additional meso-porous layer is found to lead to further improvements for both of these parameters.