High-efficiency solar photovoltaic cells require suitable high-quality semiconductor materials and careful optical design. This thesis offers a comprehensive overview of the spectroscopic characterization of a novel alloy for use in multi-junction solar cells, silicon-germanium-tin (SiGeSn), and of simulation and theoretical methods suitable for designing efficient solar cells and modelling realistic devices.
The SiGeSn material study uses spectroscopic ellipsometry, temperature and power-dependent photoluminescence, photoreflectance, and reflection and transmission measurements to investigate the band structure of Ge-rich SiGeSn alloys with up to 10% Si and 3% Sn. These epitaxially-grown alloys are shown to be lattice-matched to Ge and GaAs substrates through X-ray diffraction (XRD) measurements, allowing them to be incorporated into commonly-used multi-junction solar cell architectures. A 1 eV direct transition, useful for two, three or four-junction solar cells, is observed for SiGeSn compositions around 8% Si and 2% Sn, although the alloy retains a fundamentally indirect bandgap at these compositions. A novel technique for analyzing photoreflectance data, incorporating both interference effects and critical point features in a self-consistent way, was also developed to fit the SiGeSn photoreflectance data.
Optical modelling of solar cell structures is essential for efficient device design, to maximize absorption in the photovoltaic layers, and for understanding the behaviour of real devices. This thesis demonstrates the use of multiple modelling techniques, including rigorous coupled-wave analysis (RCWA), used to design and model diffraction grating structures for ultra-thin (< 100 nm) GaAs cells, and the transfer matrix method (TMM), used to model multi-junction cells containing SiGeSn and spectrum-splitting multi-junction devices targeting efficiencies over 40%. An integrated modelling method based on angular redistribution matrices calculated through a combination of ray-tracing and the transfer matrix method was used to investigate the infra-red emissivity of silicon heterojunction cells and a perovskite/Si tandem cell. In each case, much original work was done on the code base for these methods, including contributions to open-source solar cell modelling codes.