Optoelectronic properties of novel semiconductors

Metal-halide perovskites and nanowires are both examples of novel semi- conductors with the potential to further our light-harvesting capabilities. Realising their potential will rest on understanding their fundamental optoelectronic properties.
Metal-halide perovskites have been implemented in efficient photovoltaic devices whose efficiencies are rapidly improving. Once the Shockley-Queisser limit is reached, charge-carrier extraction will be limited only by radiative bimolecular recombination of electrons with holes. Yet, this fundamental process, is still poorly understood in these semiconductors. Here, bimolecular charge-carrier recombination in methylammonium lead triiodide perovskite can be fully explained as the inverse process of ab- sorption. By correctly accounting for contributions to the absorption from excitons and electron-hole continuum states, the van-Roosbroeck-Shockley relation can be utilised to determine bimolecular recombination rate constants from absorption spectra. The sharpening of photon, electron and hole distribution functions is shown to significantly enhances bimolecular charge recombination as the temperature is lowered, mirroring trends in transient spectroscopy. The findings provide vital understanding of band- to-band recombination processes in this hybrid perovskite, which comprise direct, fully radiative transitions between thermalised electrons and holes.
While methylammonium lead triiodide (MAPbI3) has received much attention, formamidinium lead triiodide (FAPbI3) has been less explored. This thesis examines the temperature-dependent phonon absorption, exciton binding energy, and band-gap of FAPbI3 polycrystalline thin films and finds remarkably different behaviour across the β–γ phase transition compared with MAPbI3. While MAPbI3 has shown abrupt changes in the band-gap and exciton binding energy, values for FAPbI3 vary smoothly over a range of 100 – 160 K in accordance with a more gradual transition. In addition this thesis finds that the charge-carrier mobility in FAPbI3 exhibits a clear T−0.5 trend with temperature, in excellent agreement with theoretical predictions for halide perovskites whose electron-phonon interactions are governed by the Frohlich mechanism, but in contrast to the T−1.5 dependence previously observed for MAPbI3. Finally, intra- excitonic transitions were directly observed in FAPbI3 at low temperature, from which a low exciton binding energy is determined of only 5.3 meV at 10 K.
Very low disorder is observed in high quality quantum well tubes (QWT) in GaAs-Al0.4Ga0.6As core-multishell nanowires. Room-temperature photoluminescence spectra were measured from 150 single nanowires enabling a full statistical analysis of both intra- and inter-nanowire disorder. By modelling individual nanowire spectra, each nanowire can be assigned a quantum well tube thickness, a core disorder parameter and a QWT dis- order parameter to each nanowire. A strong correlation was observed be- tween disorder in the GaAs cores and disorder in the GaAs QWTs, which indicates that variations in core morphology effectively propagate to the shell layers. This highlights the importance of high quality core growth prior to shell deposition. Furthermore, variations in QWT thicknesses for different facet directions was found to be a likely cause of intra-wire disorder, highlighting the need for accurate shell growth.
This thesis, along with the work produced during its making, helps to con- tribute to our fundamental understanding of the optoelectronic properties of metal-halide perovskites and III-V nanowires.

Keywords: Optoelectronics
Optics
Physics
Nanowires
Semiconductors
Perovskite
Terahertz
Advisor: Johnston, M