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Title: Transient optoelectronic characterisation and simulation of perovskite solar cells
Author: Calado, Philip
ISNI:       0000 0004 7658 1577
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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Lead halide perovskites are a class of solution-processable semiconductor materials showing great potential for photovoltaic applications. While perovskite solar cell (PSC) efficiencies have escalated rapidly to beyond 22% in recent years, the materials suffer from a number of chemical instabilities and the processes underlying the optoelectronic response of devices are not well understood. This thesis investigates the device physics of PSCs using novel transient optoelectronic measurements combined with device simulation. A one-dimensional numerical drift-diffusion model capable of solving for electrons, holes and a single ionic charge carrier was developed to simulate perovskite devices. The inclusion of a high density of mobile ionic species in the absorber layer is found to have important consequences on both device performance and the interpretation of established measurements. Transient optoelectronic measurements are presented showing that mobile ions are present in architectures of PSC that do not exhibit current-voltage hysteresis. Simulations of p-i-n structured devices indicate that a combination of mobile ions and field-dependent interfacial recombination rates are critical to reproducing hysteresis. Transient ideality factor measurements are used to identify the dominant recombination mechanisms in PSCs. Changes in the perceived ideality factor are correlated to localised recombination, the charge carrier population overlap, and ion density profiles in simulated devices. Simulations are used to assess the validity of a zero-dimensional model applied to small perturbation transient photovoltage (TPV) measurements on perovskite devices. Two analytical models are proposed to explain the different regimes of behaviour in devices with high rates of interfacial recombination. The TPV decay in perovskites is identified as being predominantly a measure of the transport properties of the absorber layer. Together these findings demonstrate the combined power of experimental measurements and simulation to improve our physical understanding of new semiconductor technologies.
Supervisor: Barnes, Piers ; Nelson, Jenny Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral