title: Optical properties of earth-abundant semiconductors for renewable energy
Creators: name: Birkett, M
Publisher: name: University of Liverpool Repository
Year: 2016

description: The research described primarily addresses the experimental determination of optical properties in emerging photovoltaic (PV) materials. Work proceeds with two specific aims: to consolidate and clarify experimental practice on the determination of optical properties in polycrystalline systems, then to apply any findings in investigations of the relatively unstudied semiconductors copper nitride and copper antimony sulphide, which fulfil many of the requirements for next generation PV materials: earth-abundance, scalability, bipolar doping, near-optimal band gaps, strong absorption, and beneficial transport properties. While the literature already offers extensive theoretical treatments of optical phenomena and the propagation of light, somewhat less-discussed is the task of practical determination of optical properties via the inversion of measured spectra. In the most general cases inversion may be non-trivial even for simple systems: no global solutions may exist. Furthermore, emerging thin-film photovoltaic technologies may utilise material which, whilst commercially attractive, may not be suited for study by reflection/transmission spectroscopy, while researchers often choose rather elementary spectral reduction approaches where more desirable alternatives exist. After reviewing various models, methods and issues, a self-consistent code is described which determines absorption spectra with improved accuracy. Practical work on copper nitride Cu3N and copper antimony sulphide CuSbS2 comprises experimental and first-principles investigations. Optical studies via FTIR and spectroscopic ellipsometry establish absorption and refractive index spectra for both materials; in each case, strong absorption (exceeding 6e4/cm) is found just beyond the absorption onset. Direct band gaps, average phonon energies and Bose-Einstein electron-phonon interaction strengths are determined by fitting the temperature-dependence of the absorption edge. Atypically small temperature-dependence of the direct gap is found in Cu3N (along with an optically-active TO-phonon mode), whilst in CuSbS2 a possible excitonic state is seen at low-temperature just above the absorption onset; a symmetry analysis suggests distinctly enhanced absorption for this state: further work with oriented single crystals is proposed. Structural investigations by x-ray diffraction and Rietveld or Pawley refinement find Cu3N and CuSbS2 geometries broadly consistent with prior findings. Credible thermal expansion is finally established in Cu3N between 4.2 and 280 K by temperature-dependent XRD; very little expansion is seen below 100 K: further synchrotron work is proposed. A quasi-harmonic model estimates the Cu3N zero Kelvin lattice parameter, Debye temperature and average Gruneisen parameter. Density-functional theory calculations on Cu3N and CuSbS2 suggest band structures and symmetries, band gap evolution, selection rules for optical dipole transitions, valence-band density of states (supported via x-ray photoelectron spectroscopy), and evaluate potential structural distortions: such as perovskite rigid-unit modes.