Transient Photophysics of Bismuth Halide Semiconductors for Optoelectronic Applications

Abstract

This thesis describes a series of spectroscopic studies of three different semiconductors based on bismuth halides in order to discover their fundamental photophysical properties. The materials investigated — the double perovskite Cs2AgBiBr6, bismuth iodide, and bismuth oxyiodide — have all been demonstrated in thin film photovoltaic devices fabricated at low temperatures. Propelled by the success of lead halide perovskites, this work forms part of the search for defect tolerant, non-toxic and stable materials for next-generation solar cells. I use transient absorption spectroscopy to show that the charge carrier lifetime in Cs2AgBiBr6 thin films is 1.4 μs. This is significantly longer than previous estimates based on time-resolved photoluminescence measurements, which measure the radiatively decaying carriers, but is a less sensitive probe of the high proportion of carriers which recombine non-radiatively. I propose a radiative recombination mechanism via defects, based on the detection of mid-gap electronic states in the transient absorption spectra. Coherent phonon transients are also measured on ultrafast timescales showing strong electron-phonon coupling in the material, which may contribute to the slow recombination in this material. The recombination dynamics of excitons in the layered material bismuth iodide, BiI3, are investigated using temperature dependent photoluminescence and transient absorption. I show that coupling to interlayer and intralayer phonon modes strongly affects the direct exciton decay pathway through scattering and modulation of the band gap energy. In a single crystal of BiI3, the room temperature exciton lifetime is 72 ns, which indicates this material’s potential for photovoltaics if the defects in thin films can be passivated. I show that the excited states of thin films of layered bismuth oxyiodide, BiOI, have a great degree of energetic disorder, most likely due to the presence of self-trapped excitons and defect states. Transient absorption measurements give an excited state lifetime of 47 ps, which would make efficient photovoltaic performance of BiOI thin film absorbers very unlikely. On the basis of my findings, all three materials show strong coupling between phonons and electronic states, but have excited state lifetimes varying over many orders of magnitude. This should guide the direction of future research towards different applications: the most promising candidate for photovoltaic applications is therefore Cs2AgBiBr6, whereas BiI3 and BiOI have more potential for use in nanostructured devices, such as ultrathin photodetectors, which can exploit their anisotropic nature.