Development of Thin Film Photovoltaics based on CZTS Absorber, Hydrogen Doped In2O3 Window Layers and Modeling the Economic Viability of Utility Scale Power Production in Pakistan /

Abstract

With the capability of producing nearly 600 TW annually, solar energy is one of the renewable energy sources with the potential to meet a large fraction of the world’s increasing energy demand. To make solar technology cost-competitive with carbon-based fuels, cheaper devices are needed to be realized. Unlike crystalline Si solar cells, thin film solar cells have appealed the scientific community due to properties like direct band gap and less material requirements. Prominent members of the thin film technology are CdTe and CIGS PCE 17% and 20%, respectively. However, limited availability of the elements In and Te, and environmental concerns due to Cd, make Cu2ZnSnS4 (CZTS) a better option for photovoltaic devices, since all of its elements are non-toxic and economically feasible. The kesterite CZTS is a quaternary semiconducting compound with a direct band gap of about 1.5 eV and an absorption coefficient of 104 cm-1. It is often employed as a p-type semiconductor in heterojunction PV technology. This thesis is oriented towards low cost and non-toxic synthesis of the p-type absorber CZTS for 2nd generation thin film photovoltaic. CZTS material was successfully developed at a temperature as low as 300 °C. Wet chemistry routes with non-toxic solvents were adopted to develop the CZTS system. Metal chloride, thiourea and a mix of water and ethanol were the starting materials used. Effects of calcination temperature on structure and thermal properties were studied. We also investigated the effects of Zn concentration on the electrical and optical properties of CZTS thin films. Drop casting and spin coating techniques were adopted to deposit CZTS on soda lime glass with uniform coverage. Due to the pronounced requirement for an efficient window layer, we propose hydrogen doped indium oxide (IO:H) as a high mobility and high transparency window layer. IO:H was developed through reactive RF magnetron sputtering. We analyzed the effects of hydrogen and water content in the sputtering atmosphere over the electrical behavior of InO:H layers. To link thin film R & D with the economic viability of the technology, we investigated the role of thin film PV in utility scale PV for Pakistan, using NREL System Advisor Model (SAM). From our study, we evaluated that for a specific capital investment, thin film PV farms of higher capacity compared to c-Si PV could be installed. Yet for the same capacity installed, power output of thin films, due to its high temperature coefficient, is more that of c-Si.