Recombination reduction on lead halide perovskite solar cells based on low temperature synthesized hierarchical TiO2 nanorods

Abstract

Intensive research on the electron transport material (ETM) has been pursued to improve the efficiency of perovskite solar cells (PSCs) and decrease their cost. More importantly, the role of the ETM layer is not yet fully understood, and research on new device architectures is still needed. Here, we report the use of three-dimensional (3D) TiO₂ with a hierarchical architecture based on rutile nanorods (NR) as photoanode material for PSCs. The proposed hierarchical nanorod (HNR) films were synthesized by a two-step low temperature (180 °C) hydrothermal method, and consist of TiO₂ nanorod trunks with optimal lengths of 540 nm and TiO₂ nanobranches with lengths of 45 nm. Different device configurations were fabricated with TiO₂ structures (compact layer, NR and HNR) and CH₃NH₃PbI₃, using different synthetic routes, as the active material. PSCs based on HNR-CH₃NH₃PbI₃ achieved the highest power conversion efficiency compared to PSCs with other TiO₂ structures. This result can be ascribed mainly to lower charge recombination as determined by impedance spectroscopy. Furthermore, we have observed that the CH₃NH₃PbI₃ perovskite deposited by the two-step route shows higher efficiency, surface coverage and infiltration within the structure of 3D HNR than the one-step CH₃NH₃PbI_3−xCl_x perovskite.