In this thesis we develop a new first-principles method for the calculation of optical absorption spectra and band structures in semiconductors and insulators at finite tem- peratures. The theoretical framework of our methodology originates back to 1950s in two pivotal research papers by F. Williams and M. Lax. Here, we expand the scope of the pioneering works by Williams and Lax, and we present a new theory of phonon- assisted optical absorption and temperature-dependent band structures. We demon- strate that our technique is highly efficient and simple to the point that a single calcu- lation is sufficient to capture temperature-dependent absorption coefficients including the effect of quantum zero-point motion. We report calculations of optical absorption spectra and of direct and indirect band gaps of Si, C, GaAs and MAPbI<sub>3</sub> . We obtain good agreement with experiment and with previous calculations. The approach pro- posed in this thesis is highly versatile, and can straightforwardly be combined with improved descriptions of the dielectric function by including electron-hole effects via the Bethe-Salpeter and GW equations. In this thesis we also investigate the underly- ing mechanisms leading to the "inverse Varshni effect" in materials that belong to the family of metal halide perovskites. We show, using the example of MAPbI 3 , that the vibrational modes with metal-halide-metal bending or rocking character are the major cause of the band gap opening with increasing temperature. To this aim we present an approach for elucidating the physics underpinning the changes of the band gap with vibrational motion. Our methodologies developed in this thesis are simple to imple- ment in any electronic structure package as a post-processing step, having the potential to find broad applications in the ab-initio community. We anticipate that our work will open the way to predictive calculations, as well as will contribute to the better under- standing of the optoelectronic properties of solids at finite temperatures.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:736035 |
| Date | January 2017 |
| Creators | Zacharias, Marios |
| Contributors | Giustino, Feliciano |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://ora.ox.ac.uk/objects/uuid:4fad686d-c675-44e6-85c7-2725e6598ca5 |
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