Thermoelectricity has been on focus for many years, because the world is investing in renewable energy and need energy sources that are environmentally friendly. The aim of thermoelectric technology is to convert heat into electrical energy which can be beneficial to people living in offgrid areas. Since transparent electronics has become an important field, the interest is on transparent n- and p-type thermoelectric materials. In recent years, different transparent thermoelectric n-type semiconductor thin films, such as, for example ZnO, SnO2 or In2O3 have been found and studied extensively, however, the p-type transparent thermoelectric semiconductor counterparts in the low temperature regime have been missing for a while. At the current stage, ntype thermoelectric semiconductors have better efficiency as compared to p-type thermoelectric semiconductors near room temperatures. For this study, p-type CuI thin films were fabricated and optimised based on their structural, morphological, optical, electrical and thermoelectric (power factor) properties. CuI powder was deposited on borosilicate glass substrates using thermal evaporation and electronbeam deposition techniques. The thickness of the films fabricated by both deposition methods was varied in the range of 210 nm to 380 nm. X-ray diffraction (XRD) results revealed that the deposited films for both deposition methods were polycrystalline zincblende structure with a cubic phase. All films are growing along the (111) plane, the crystallite size is increasing with thickness increase for electron-beam deposited films, however, it is decreasing with thickness increase for thermally evaporated thin films. Results obtained for the Raman spectroscopy revealed peaks at 122 cm-1 and 124 cm-1 for films deposited by electron-beam and thermal evaporation deposition techniques respectively. Both Raman peaks belong to the zincblende structure of CuI. The SEM micrographs showed well defined grains for both deposition techniques. The grains have a triangular shape which is expected of the zincblende structure. The grain size was increasing with an increase in thickness for both deposition methods but was higher for thermally evaporated films than thin films deposited by electron-beam method. This is in good agreement with the AFM results which showed higher values of Root Mean Square (RMS) roughness for thermally evaporated films than electron-beam deposited films. xiii All deposited films exhibited a high transparency of 60-90% in the visible range of the spectrum. The bandgaps obtained were lower than the expected value of 3.1 eV as they ranged between 2.85- 3.03 eV. The carrier concentration of the CuI thin films was in the order of 1020 -1021 cm-3 and the carrier concentration increased with an increase in thickness. The values obtained for the Hall coefficient were positive which meant that the grown CuI thin films were p-types. The electrical conductivity at room temperature was seen to increase with an increase in thickness from 59.1 S/cm to 104.4 S/cm and as the thickness increased further there was a slight decrease to 96.3 S/cm for electron-beam deposited thin films. Thermally evaporated CuI films showed a similar behaviour. The electrical conductivity for thermally evaporated CuI thin films increased with thickness increase from 79.0 S/cm to 110.2 S/cm and as the thickness increased further the conductivity value decreased to 105.3 S/cm. The designed home-made Seebeck measurement setup was used to measure the temperature difference between two points in the CuI thin films together with the Seebeck voltage formed due to the temperature difference. As a result, the Seebeck voltage was plotted against the applied temperature difference and the Seebeck coefficient was calculated. The measured Seebeck coefficient was in the range of 164.5 µV/K to 203.8 µV/K for both deposition methods. Finally, the power factor was determined to be ranging between 2.45 to 2.98 µWm-1K -2 for electron-beam deposited films and for thermally evaporated films it ranged between 2.98 and 3.38 µWm-1K -2 .

Thesis (MSc of Physics and Astronomy)---Botswana International University of Science and Technology, 2023