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Title: Solution-processed inorganic semiconductors in hybrid photovoltaics : mechanisms of charge separation and effects of interface structure
Author: O'Mahony, Flannan T. F.
ISNI:       0000 0004 5348 7086
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2014
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Nanocrystalline inorganic semiconductors have emerged as attractive alternatives to molecular dyes as photosensitisers for mesoporous metal oxide-based semiconductor-sensitised solar cells (SSSCs). In this configuration, Sb2S3 has shown particular promise, yielding impressive power conversion efficiencies. At the operational heart of these devices, there are a series of interfacial electron transfer reactions that act to separate photogenerated charges. The delicate balance between charge separation and recombination plays an important part in defining device efficiency, but the mechanism by which charge separation occurs in Sb2S3-based systems has been relatively poorly understood. In the first part of this work, transient absorption spectroscopy is used to probe interfacial electron and hole transfer in mesoporous TiO2/Sb2S3-based photovoltaic assemblies. The reported observations point to the importance of the hole transfer reaction not simply as a means of regenerating the sensitiser, but rather as an integral part of the charge separation process. Inspired by these results, a novel device architecture and processing route is developed in which the structural, optical and electrical properties of the sensitised TiO2 film are encompassed in a single component; namely a mesoporous Sb2S3 photoanode. The resulting high surface area is shown to allow for efficient charge transfer to a polymeric hole acceptor, whilst electron transport through the Sb2S3 layer is demonstrated by the fabrication of functioning photovoltaic devices. In order to consider the role of mesostructure in Sb2S3-based solar cells, devices based on a flat Sb2S3/hole transport material (HTM) interface are developed and compared to SSSC analogues. It is found that although these two classes of device are structurally very different, overall performance is remarkably similar. This highlights the impressive performance of the thin-film assembly, but also draws into question the importance of meso-scale morphological control. Through a combination of transient absorption spectroscopy and transient photovoltage measurements, it is determined that structural limitations to interfacial charge separation can be overcome in these systems by the action of an electric field. Understanding the role of mesostructure is a particularly pertinent challenge in the context of organic lead halide perovskite absorbers, with which very high efficiencies have been achieved in a range of structured and non-structured device architectures. Here, it is shown that the presence of a mesostructured electron acceptor to rapidly quench the perovskite excited state enhances the stability of interfacial charge separation and significantly increases innate tolerance to environmental processing conditions. This work highlights a significant advantage of retaining mesoscale morphological control in the preparation of efficient, low cost and stable hybrid photovoltaics.
Supervisor: Haque, Saif Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available