Brookite TiO2 as a low-temperature solution-processed mesoporous layer for hybrid perovskite solar cells

As solution-processable and low-cost semiconductors, organolead halide perovskites are attracting enormous attention for application as promising photovoltaic absorbers capable of a high-power conversion efficiency over 20%. A mesoporous layer of titanium oxide, which requires sintering at high temperature (400-500 degrees C), serves as an efficient electron collector as well as a scaffold for crystal nucleation. To enable the rapid low-cost manufacture and construction of lightweight flexible solar cells built on plastic films, a sinter-free electron collection layer (mesoporous and compact layer) is required. In this study, a highly crystalline layer of brookite (orthorhombic TiO2) was prepared by a sinter-free solution process as an efficient mesoporous electron collector. Strong inter-particle necking of the brookite nanoparticles by a dehydration-condensation reaction enabled the formation of a highly uniform mesoporous layer at low temperature (130-150 degrees C). In comparison with an anatase TiO2 meso-structure prepared by high temperature (500 degrees C) sintering, the brookite electron collector exhibits a photovoltaic performance with a greater fill factor and 100 mV-higher open-circuit voltage.