Contact-free Electrical Characterisation of Novel Materials Using Terahertz Spectroscopy
Terahertz spectroscopy is a powerful, contact-free and non-ionizing technique that allows the accurate measurement of the optoelectronic properties of semiconductor nanomaterials. The detrimental non-radiative pathways, either via surface states or oxidation or defects, that significantly degrades the optoelectronic properties of indium arsenide (InAs), indium arsenide phosphide (InAsP) and tin-based halide perovskites were extensively studied using terahertz spectroscopy.
An ultra-thin InP layer with a ∼2- 3 nm thickness and a higher band gap in comparison, has been utilised to passivate and suppress the detrimental effect of the surface states on the optoelectronic properties of InAs rich nanowires, namely, InAs and InAsP. These surface states dominate and degrade the optoelectronic properties of these nanowires. Using terahertz spectroscopy, a remarkable, long-term improvement in the charge carrier lifetime and charge carrier dynamics was demonstrated while retaining the charge carrier mobility. In addition, a demonstrated three-fold order of magnitude decrease in the surface recombination velocity was achieved by the ultra-thin passivation layer.
A comparative study of how the gradual partial to full substitution of lead with an environmentally friendly alternative, tin, impacts the charge carrier recombination dynamics of metal halide perovskites so as to elucidate the dominating degradation mechanism at each tin content. It was found that Sn-richer films prefer to recombine through a radiative monomolecular channel due to the high background hole density present in these films. In contrast, in the case of equally mixed lead-tin perovskites, where the background hole density had significantly reduced, the non-radiative monomolecular mechanism begins to compete with the radiative monomolecular mechanism, and therefore dominates the low-order monomolecular recombination.
The influence of different anti-solvent treatments on the performance of triple cation mixed lead-tin perovskites to effectively remove undesired Sn4+ dopants was investigated. Amongst chlorobenzene and anisole, toluene was identified to produce the champion optoelectronic properties ranging from grain size, charge carrier mobility, photoconductivity, lower Sn4+ surface elemental composition to power conversion efficiency.
The research works in this thesis contribute to identifying, extensively understanding and mitigating detrimental degradation or non-radiative pathways in novel semiconductors, either via surface states or oxidation or defects. These findings will guide the development and optimisation of these semiconductor materials and allow them to push boundaries in different relevant applications such as solar cells and photodetectors.