Investigation of Si/SiO2 and Si/SiOx quantum well structures for applications as energy selective contacts and all-silicon tandem cells

Abstract

In order to satisfy the world’s energy demands while simultaneously preserving the sustainability of the environment, it is inevitable to shift the reliance of fossil fuels to renewable energy sources. Photovoltaic is the fastest growing energy source in the world and the cost of production have reduced significantly over the past decade for it to be considered as a cost-competitive solution. Increasing the cell efficiency and bringing the cost down towards grid parity continues to be the primary motivation for research and development in photovoltaics technology. The third generation photovoltaics involve novel cell designs and concepts that have potentials in achieving very high efficiency and low cost solar cells. These include tandem solar cells, quantum well/dot solar cells, hot carrier solar cells and up-converters. Resonant tunnelling effect in Si/SiO2 quantum well structures could find potential applications in all-Si tandem cells in the form of superlattice and energy selective contacts in hot carrier solar cells in the form of double barrier structure. The fabrication of crystalline Si/SiO2 quantum well to achieve the desired confinement effect is no trivial task. In this thesis, the structures were deposited by RF magnetron reactive sputtering followed by post thermal treatment to crystallize the amorphous silicon layer. The enhancement of crystallization temperature has been observed experimentally for low dimensional Si well in the order of a few nanometres. The crystallinity has also been experimentally demonstrated to be strongly dependent on the annealing temperature rather than the duration. The size of silicon nanocrystals was calculated and compared using different analytical approaches. It was observed that the Si thickness and annealing temperature both plays a role in the size of the nanocrystals. The bandgap enhancement was evident from variation of luminescence energy between 1.3 to 1.8 eV as function of Si well thickness. The origin of this luminescence was studied. The crystallization and photoluminescence properties of Si/SiOx structures (x<2) were also investigated. Finally the feasibility of partially crystalline quantum well for energy selective contact application was discussed.