Imaging Conductivity in Hybrid Metal Halide Perovskite Thin Films

Whether the argument is increasing energy security, mitigating the effects of global warming, or reducing overall costs, the need for clean, renewable sources of energy is undeniable. As such, energy is one of the main research themes in the EPSRC portfolio. In recent years, hybrid metal halide perovskites have shown extraordinary success as active layers in solar cells. With power conversion efficiencies in excess of 22%, solar cells based on polycrystalline perovskite thin films rival existing silicon technologies.1 Due to the rapid development of the field, much remains to be discovered about the basic chemical and physical properties of perovskite thin films and how these properties determine photovoltaic device efficiency. Specifically, there is a poor understanding of how bulk optoelectronic properties are influenced by the polycrystalline nanostructure of perovskite thin films, as the observance of grain boundary effects on charge-carrier mobility and photoluminescence quantum yield appears contradictory to efficient device performance.2 Using ultrafast spectroscopy and microscopy techniques, the overall goal of this research project will be to create a map of charge movement within perovskite thin films with sub-micron spatial resolution and sub-ps time resolution in order to analyze loss mechanisms due the presence of grain boundaries and non-uniform chemical composition. Additionally, it will survey a range of technologically relevant materials including lead- and tin- based perovskites and mixed-composition perovskites in order to inform the design of perovskite-based solar cells.

The main experimental focus of the project will be on optical-pump/THz-probe spectroscopy (OPTPS), an ultrafast technique that directly probes mobile charge-carriers and allows for simultaneous determination of the charge-carrier mobility and charge-carrier dynamics on a sub-picosecond timescale.3 This technique has proven essential for studying the time-dependent optoelectronic properties of many types of semiconductors including hybrid metal halide perovskites.3,4 In order to better compare bulk and nano-scale properties, the student will construct a microscope for improved spatial resolution below the diffraction limit of THz radiation using newly developing technology. Additionally, she will have the opportunity to utilize other spectroscopy and microscopy instrumentation at the university including the resources of the Warwick Centre for Ultrafast Spectroscopy.

References
(1) Manser, J. S.; Christians, J. A.; Kamat, P. V. Chem Rev 2016, 116, 12956-13008.
(2) Petrus, M. L.; Schlipf, J.; Li, C.; Gujar, T. P. et al. Adv Energy Mater 2017, 7, 1700264.
(3) Lloyd-Hughes, J.; Jeon, T. I. J Infrared Millim Terahertz Waves 2012, 33, 871-925.
(4) Milot, R. L.; Eperon, G. E.; Snaith, H. J.; Johnston, M. B. et al. Adv. Funct. Mater. 2015, 25, 6218-6227.