Doctor of Philosophy, University of Toledo, 2022, Physics

CuInSe2 (CIS) and related materials have been studied intensively for applications as the absorber layers of solar cells. CIS absorber layers have favorable electronic and optical properties particularly for applications in the bottom cells of thin film multijunction devices. CIS exhibits a narrow bandgap, strong direct bandgap absorption, and controllable p-type conductivity. In this dissertation research, spectroscopic ellipsometry (SE) has been performed on CIS thin films and solar cells with a goal toward characterizing and optimizing this low bandgap absorber for tandem solar cell applications. CIS for film growth and materials studies was deposited to thicknesses within the range of 500-1000 A by one-stage and two-stage thermal co-evaporation on crystalline silicon (c-Si) wafer substrates. These thin films were characterized by real time SE (RTSE) and by additional analytical techniques. CIS films for incorporation into devices were fabricated using the same deposition system to thicknesses within the range of 1.5-2.0 µm on Mo-coated soda-lime glass (SLG) in the substrate cell configuration. The deposition system and processing that were used for these devices has yielded > 17% efficient CuIn1 xGaxSe2 (CIGS) cells incorporating a standard absorber layer thickness of 2.5 µm and an alloy content of x = [Ga]/{[In + Ga]} = 0.3 [1]. As a narrow bandgap semiconductor absorber of interest for use in the bottom cell of thin film tandem solar cells, CIS can be fabricated by a variety of methods. The highest efficiency solar cells use CIS absorbers prepared by multisource thermal co-evaporation [2]. The initial goal of this dissertation research is the development of a reliable calibration method for CIS multi-source co-evaporation. Five Cu depositions were performed sequentially each on the same c-Si wafer substrate at different Cu source temperatures and measured by RTSE. Five In2Se3 films were deposited and measured similarly. Each layer of the two film stacks w (open full item for complete abstract)

Committee: Robert W. Collins Dr. (Committee Chair); Sanjay V. Khare Dr. (Committee Member); Yanfa Yan Dr. (Committee Member); Nikolas J. Podraza Dr. (Committee Member); Marco Nardone Dr. (Committee Member) Subjects: Physics

Doctor of Philosophy, University of Toledo, 2017, Physics

Spectroscopic ellipsometry (SE) is a powerful tool to characterize multilayered thin films, providing structural parameters and materials optical properties over a wide spectral range. Further analyses of these optical properties can provide additional information of interest on the physical and chemical properties of materials. In-situ real time SE (RTSE) combines high surface sensitivity with fast data acquisition and non-destructive probing, thus lends insights into the dynamics of film growth. In this dissertation, the methods of SE have been applied to investigate the growth and properties of material components used in the CIGS thin film photovoltaic technology. Examples of RTSE data collection and analyses are demonstrated for the growth of selenium (Se), molybdenum diselenide (MoSe2) and copper selenide (Cu2-xSe), used in CIGS technology which can then be applied in complete analysis of three-stage CIGS deposition by co-evaporation. Thin film Mo deposited by sputtering is the most widely used back contact for solar cells using CIGS absorbers. In this study, in-situ and real time characterization have been utilized in order to investigate the growth as well as the structural, optical, and electronic properties of Mo thin films deposited by DC magnetron sputtering at different substrate temperatures. In these studies, the surface roughness on the Mo is observed to decrease with increasing substrate temperature. The growth rate, nucleation behavior, evolution of surface roughness and development of void structures in Mo show strong variations with deposition temperature. In depth analyses of (e1, e2) provide consistent estimates of void fraction, excited carrier mean free path, group speeds of excited carriers and intrinsic stress in the films. Complementary ex-situ characterization of the as deposited Mo films included XRD, resistivity measurements by four-point-probe, SEM, and profilometry. This dissertation describes the research performed on the (In (open full item for complete abstract)

Committee: Robert Collins (Committee Chair); Nikolas J. Podraza (Committee Member); Bo Gao (Committee Member); Jacques G. Amar (Committee Member); Dean M. Giolando (Committee Member) Subjects: Materials Science; Physics; Solid State Physics