Elucidating the role of TiCl4 post-treatment on percolation of TiO2 electron transport layer in perovskite solar cells

Abstract

The ideal electron transport layer of a high performance perovskite solar cell should have good optical transparency, high electron mobility, and an energy level alignment well-matched with the perovskite material. In this work, we investigate the role of TiCl₄ post-treatment of the mesoporous TiO₂ electron transport layer by varying the concentration of TiCl₄ and characterizing optical and electrical properties, charge carrier dynamics, and photovoltaic performance of mesoscopic CH₃NH₃PbI₃ solar cells. It is found that the TiCl₄ treatment provides an additional interconnection between the TiO₂ particles, leading to better percolation as evident from high resolution cross-section images and chemical maps. This enhances effective electron mobility in the material as well as significantly reduces average sub-bandgap absorption due to defects and electronic disorder determined by photothermal deflection spectroscopy. Moreover, improvement of interfacial contact due to a smoother surface contributes to more efficient charge extraction and suppressed charge recombination and reduced hysteresis. As a result, the optimized device based on TiCl₄ post-treated mesoporous TiO₂ achieved the highest conversion efficiency of 17.4 % compared with 14.1 % for the device with pristine mesoporous TiO₂.