Abstracts Category : Other

Optical Physics of Cu(In,Ga)Se2 Solar Cells and Their LayerComponents

by Abedl Rahman Ibdah

Polycrystalline Cu(In1-xGax)Se2 (CIGS) thin filmtechnology has emerged as a promising candidate for low cost andhigh performance solar modules. The efficiency of CIGS solar cellsis strongly influenced by several key factors. Among these factorsinclude Ga composition and its profile in the absorber layer,copper content in this layer, and the solar cell multilayerstructure. As a result, tools for the characterization of thin filmCIGS solar cells and their layer components are becomingincreasingly essential in research and manufacturing. Spectroscopicellipsometry is a non-invasive technique that can serve as anaccurate probe of component layer optical properties and multilayerstructures, and can be applied as a diagnostic tool for real-time,in-line, and off-line monitoring and analysis in small area solarcell fabrication as well as in large area photovoltaicsmanufacturing. Implementation of spectroscopic ellipsometryprovides unique insights into the properties of complete solar cellmultilayer structures and their layer components. These insightscan improve our understanding of solar cell structures, overcomechallenges associated with solar cell fabrication, and assist inprocess monitoring and control on a production line.In thisdissertation research, Cu(In,Ga)Se2 films with different Cucontents have been prepared by the one stage co-evaporationprocess. These films have been studied by real time spectroscopicellipsometry (RTSE) during deposition, and by in-situ SE at thedeposition temperature as well as at room temperature to extractthe dielectric functions (e1, e2) of the thin film materials.Analytical expressions for the room temperature dielectricfunctions were developed, and the free parameters that describethese analytical functions were in turn expressed as functions ofthe Cu content. As a result of this parameterization, thedielectric function spectra (e1, e2) can be predicted for anydesired composition within the range of the samples investigated.This capability was applied for mapping the structural andcompositional variations of CIGS thin films deposited over a 10 cm 10 cm substrate area.In another application presented in thisdissertation, a non-invasive method utilizing ex-situ spectroscopicellipsometry analysis has been developed and applied to determinenon-destructively the Ga compositional profile in CIGS absorbers.The method employs parameterized dielectric function spectra (e1,e2) of CIGS versus Ga content to probe the compositional variationwith depth into the absorber.In addition, a methodology forprediction of the external quantum efficiency (QE) includingoptical gains and losses for a CIGS solar cell has been developed.The methodology utilizes ex-situ spectroscopic ellipsometryanalysis of a complete solar cell, with no free parameters, todeduce the multilayer solar cell structure non-invasively andsimulate optical light absorption in each of the layer components.In the case of high efficiency CIGS solar cells, with minimalelectronic losses, QE spectraAdvisors/Committee Members: Collins, Robert (Advisor).