Study of Modified Deposition Process for Cu(In,Ga)Se2 Solar Cell Back Contact

As the worldwide demand for renewable energy is increasing, growth of the global share of alternative energy sources would improve overall energy security as well as bring environmental benefits. So far, solar cells - the devices that convert direct sunlight into electricity - are dominated by silicon devices. Another alternative is thin film solar cell, whose main inspiration is to reduce the electricity production cost. Cu(In, Ga)Se2 (CIGS) solar cells are considered to have a great prospective because of reduced material and energy consumption during manufacturing. Many CIGS solar cell manufacturers are already exhibiting GW-scale production capacity. With the development of CIGS applications, it is essential to modify the properties of each of the constituent material to adapt to the new requirements. Molybdenum is the most appropriate material used as the back contact for CIGS solar cells, and is commonly deposited by sputtering onto soda lime glass (SLG). Mo thin films act as the metal contact. The formation of an Ohmic contact at the Mo/CIGS interface is one of the most important properties apart from high conductivity, strong adhesion of the film as well as chemical and mechanical compatibility with the CIGS layer. A suitable amount of sodium is necessary for enhanced solar cell performance. When using soda lime glass (SLG) as a sodium source, the Mo layer acts like a barrier for sodium diffusion and the deposition process provides proper control of sodium supply from the SLG. Structural, thermal, and chemical properties of the Mo film have a direct influence on the growth and nucleation of the CIGS layer as well. In the first part of the thesis, in-situ and ex-situ characterization techniques were used to understand the growth, as well as the morphology and structural properties of the Mo films grown on various substrates, namely Si (100) wafer, soda lime glass and borosilicate glass, at a fixed deposition power and pressure. Real time spectroscopic ellipsometry (RTSE) analysis demonstrated a ...