Exploring the impact of electron transport layer thickness and morphology on perovskite infiltration and photoresponse in HTM-free self-powered photodetector

Methylammonium lead Iodide (MAPbI3) is emerging as the frontrunner for high-performance photo-detection due to its superior optoelectronic characteristics and ease of processing. Titanium dioxide (TiO2) is the commonly employed electron transport layer in conventional perovskite-based photodetectors. However, the fundamental understanding of the role of TiO2 morphology and its thickness on the photodetector performance is yet to be explored explicitly. This work systematically investigates the impact of morphology (mesoporous versus compact TiO2 layer) and thickness of the TiO2 layer (50 nm - 3000 nm) on the MAPbI3 photodetector performance. The devices have been fabricated in a hole transport material (HTM)-free architecture with carbon as the top contact. The heterojunction between MAPbI3 and TiO2 renders the device with self-powered capability. The influence of the perovskite sensitisation in the TiO2 electrodes, carrier recombination characteristics and charge-carrier transfer resistances have been studied using photoluminescence and impedance spectroscopy. The TiO2 layer morphology and thickness affected the MAPbI3 infiltration and film formation kinetics. Impedance and photo-luminescence spectroscopies reveal that TiO2 layers, with thickness above 460 nm, negatively affected the device performance. An optimum thickness of 460 nm-mesoporous TiO2 layer exhibited a superior responsivity and rise time of 2.9 A/W and 21 ms, respectively. The poor performance at higher thicknesses is due to the increased interfacial charge transfer resistance and charge carrier recombination. This work is the first report of extensive optimisation and study on the impact of TiO2 film morphology and thickness on the MAPbI3 photodetectors. The present work will be a key for modelling and designing high-performance perovskite photodetectors.