Abstract
Renewable energy has become the preferred source to replace the ever-depleting conventional energy sources. Photovoltaics, commonly known as solar cells, have been an attractive option in this regard, but their high cost and relatively low efficiency compared to conventional power generation techniques has prevented widespread application. Silicon has been the most widely used photovoltaic material for decades. Thin film solar cells have become an attractive alternative for the conventional bulk crystalline solar cells since they are cheaper to produce. However, the thin film counterpart of crystalline silicon, known as hydrogenated amorphous silicon (a-Si:H), has its drawbacks in photovoltaic applications. The use of excimer lasers for crystallisation of a-Si:H has been widely investigated for microelectronic applications. This thesis is concerned with excimer laser crystallisation of comparatively thick a-Si:H films for photovoltaic applications. Films thicker than 300 nm, which are necessary for adequate light harvesting, undergo partial melting upon excimer laser irradiation up to energy densities of 300 mJcm-2. Partially melted and solidified films result in stratified structures with different grain sized layers with an unconverted amorphous layer at the bottom. These stratified films are employed as light absorbers in different device configurations. Also, the resulting nanocrystalline films are considered to be less susceptible to light degradation due to fractional contributions from the amorphous phase. Surface morphology resulting from excimer laser crystallisation is identified as a low cost light trapping technique. Hydrogenation is found to increase conductivities of excimer laser crystallised silicon, which can be used to further improve the material. For 100 nm thick films an energy window is identified which results is nano-polycrystalline silicon with enhanced band gaps. This phenomenon is proposed to be due to a combination of a critical composition of hydrogen in the films along with confinement effects due to smaller crystallites. Simple Schottky barrier devices and p-i-n structures were utilised to investigate the behaviour of stratified absorber layers. Schottky barrier solar cells show promising photon conversion efficiencies, however, at low photon densities. Optimisation of device fabrication methodology is necessary in order to further improve the p-i-n solar cells which are affected by high series resistances and shunting problems.