Optical properties and electronic structures of copper indium gallium selenide: Effect of copper deficiency

In this thesis, I describe basic research done to understand the optical properties and electronic structure of polycrystalline thin-film CuIn 1-xGaxSe 2 (CIGS) used for high efficiency photovoltaic (PV) solar cells. The effect of Cu deficiency on CIGS material properties has been intensively studied.; The physics of p-n junctions and their application to photovoltaic solar cells are thoroughly discussed in Chap. 1. Chapter 2 presents a general knowledge of CIGS materials in terms of crystal structure, phase, electronic structure, intrinsic defects, and growth and characterization methods. Chapter 3 covers the physics of spectroscopic ellipsometry in detail, especially for Maxwell's equations, polarization of light waves, Fresnel equations, and the fundamental equation of ellipsometry. The method used to determine the optical constants from ellipsometric parameters is introduced using an easily understandable example, SiO2 on Si.; Chapter 4 present the results of extensive studies of the effect of Cu deficiency on the properties of CuInSe2 (CIS). Phase transitions, optical properties and the electronic structure are discussed for Cu-poor CIS. Probing defect levels in the sub-bandgap region of CIS was successful using spectroscopic ellipsometry and the results are compared with first-principles calculations and data from the literature. In the next chapter, general behaviors of the optical properties of CuIn1-xGa xSe2 are introduced. Similar schemes are used to study the effect of Cu deficiency on CuIn0.8Ga0.2Se 2 and CuIn0.6Ga0.4Se2 as that on CIS. The effect of Cu deficiency on the optical bowing coefficients has been explained.; The maximum efficiency of laboratory thin-film polycrystalline CIGS solar cells exceeds their single-crystal counterparts. This has been explained in terms of Cu deficiency in CIGS. Our studies of the effect of Cu deficiency have provided the first experimental evidence in support of this hypothesis. We find that there is a reduction in the absorption strength in the spectral range of 1-3 eV determined by spectroscopic ellipsometry. This reduction can be explained in terms of the removal of Cu 3d density of states near valence band maximum (VBM) for Cu-poor CIGS. The properties of a distinct surface layer on CIGS are determined using spectroscopic ellipsometry. We found that Cu is more deficient at surface than in the bulk region of the surface using spectroscopic ellipsometry.; These results have important implications for the functioning of polycrystalline photovoltaic devices. The grain boundary (GB) region can be thought as interior surface and is expected to be more Cu deficient than the grain interior (GI) region. In addition, GBs tend to getter defects and impurities during the high-temperature processing used to produce thin-film PX-CIGS. Cu vacancies create a barrier for holes through the reduced DOS at the VBM. Because the GB tends to exclude majority carrier holes, it effectively impedes electron-hole recombination. The combination of gettering defects that cause fast non-radiative recombination with the carrier-separating effects of the GB band structure serve to greatly enhance the efficiency of thin-film PX-CIGS PV devices. Such a GB filter can be a key to the utilization of polycrystalline semiconductors in transport devices.