Photothermal Deflection Spectroscopy of Amorphous, Nanostructured and Nanocomposite Thin Films
Doctor of Philosophy
Dr. Giovanni Fanchini
The energy needs of the modern world are growing day by day, while sources of non-renewable fossil fuels are limited, so there is a need to efficiently use the existing resources and explore renewable energy sources. In order to harvest, store and efficiently utilize renewable energy, we need to explore new materials and improve the performance of existing ones. Among others, hydrogenated amorphous silicon (a-Si:H) with high optical absorption in the visible range of electromagnetic spectrum, is a low cost material for solar cells. But the efficiency of such solar cells is comparatively low because of intrinsic defects associated with its material structure and its degradation under illumination. Also the optical transparency and electrical conductivity of the window electrode are important factors that affect solar cell performance. Transparent and conducting carbon-based films (TCCF) have great potential to be used as electrodes in optoelectronics due to their transparency and high electrical conductivity. TCCF are not yet as competitive with indium-tin oxide (ITO) as transparent electrical conductors. In order to improve the efficiency of such materials, one needs to understand and curtail the defects for better cell performance. This study is an experimental investigation of the optical and thermal properties of solar-grade materials and nanocomposites using photothermal deflection spectroscopy (PDS). PDS is a non-contact experimental technique based on the mirage effect. An automated PDS setup was assembled that is capable of measuring weak optical absorptions and thermal properties of thin film samples. A complementary setup, the 3-omega method, for thermal conductivity measurement was also built and used to compare the results obtained by the two methods. However, our primary focus was on the PDS setup as a non-contact, non-destructive and sensitive technique. Also the role of convection heat transfer in PDS in the presence of highly thermally conducting nanoparticles in photothermal fluid is investigated. The defects formation in a-Si:H thin films under light soaking was investigated and a model is proposed for self-repair of defects in thin films. Also optical, electrical and thermal properties of a set of graphene/graphene-like platelet thin films were investigated. A relationship between the electrical and thermal conductivities of these samples was established that could be applied to a large class of graphene-based thin films. The trade-off between electrical and thermal properties, along with transparency, will help the design of applications where electrical conductance, thermal management and transparency are required.
Ahmed, Muhammad Shafiq, "Photothermal Deflection Spectroscopy of Amorphous, Nanostructured and Nanocomposite Thin Films" (2013). Electronic Thesis and Dissertation Repository. 1678.