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Title: Vapour-deposited hybrid metal-halide perovskite thin films for photovoltaic applications
Author: Patel, Jay B.
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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Hybrid metal-halide perovskite semiconductors have recently shown to be excellent photoabsorbers for photovoltaic devices. There are a variety of techniques that can be used to fabricate hybrid metal-halide thin films, from facile low temperature solution processing methods to vapour deposition methods. Physical vapour deposition, in particular thermal evaporation, is one such technique, which is easily up-scalable and widely used in industry. Additionally, vapour deposition is a solvent-free technique that yields smooth, uniform and pin-hole free thin films. This thesis reports an investigation of hybrid metal-halide perovskite, CH3NH3PbI3 (MAPbI3) thin films, and their subsequent implementation in photovoltaic devices. The initial investigation follows the formation dynamics of the MAPbI3 thin films from sequentially depositing the precursors, PbI2 and CH3NH3I (MAI), whilst keeping the substrates at 273 K. Upon heating the substrates to room temperature, the MAI diffused into the PbI2. The diffusion is tracked using Fourier transform infrared spectroscopy (FTIR) and x-ray crystal diffraction analysis. The results show that exposing the films to moist air facilitates the diffusion and aids in creating a crystalline perovskite thin film. Dual-source vapour deposition is a technique whereupon both the precursors are evaporated at the same time, yielding a perovskite thin film immediately. An investigation on how different substrates influence the perovskite thin film, was carried out. It is found that perovskite growth on TiO2 induces regions of amorphous perovskite at the interface. The presence of amorphous perovskite explains why photovoltaic devices which have a TiO2/MAPbI3 interface show current-voltage hysteresis. However, incorporating a layer of continuous fullerene thin film on the substrate leads to the formation of crystalline perovskite at the interface, and the corresponding photovoltaic devices show no hysteresis. MAPbI3 photovoltaic devices were carefully investigated at low-temperatures (15 K - 350 K) using Fourier transform photocurrent spectroscopy (FTPS). The photocurrent spectra of MAPbI3 devices showed that the excess PbI2 usually found in MAPbI3 thin films contributes to the photocurrent. Furthermore, a lower limit of the exciton binding energy of 9.1 meV was extracted for MAPbI3 by comparing the low temperature absorption spectra with the photocurrent spectra. Finally, organic bulk heterojunction photovoltaic devices were investigated. The effect of UV light on the devices for 18 hours was carefully tracked using FTPS. Initially, the results show a decrease in external quantum efficiency (EQE) at high photon energies. A closer look at the EQE spectra reveal a change in the sub-band gap absorption, indicative of a change in the micromorphology of the bulk heterojunction. These findings indicate the need to find non-fullerene acceptors.
Supervisor: Herz, Laura Sponsor: Merck Group ; Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Fourier transform infrared spectroscopy ; Solar energy--Research ; Thin-film Fabrication