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Characterization and Optimization of CuInSe2 Solar Cells Applicable for Tandem Devices

Sapkota, Dhurba Raj
http://orcid.org/0000-0001-6809-3440
http://rave.ohiolink.edu/etdc/view?acc_num=toledo1671215831442438

Abstract Details

2022, Doctor of Philosophy, University of Toledo, 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 Å 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 was subjected to multi-time RTSE analysis to determine the effective thickness deposition rate given by (film volume)/[(area)(time)]. Such a calibration enables selection of Cu and In evaporation source temperatures for deposition of a CIS film at a specified rate and with a specified Cu stoichiometry y. In the CIS materials studies, RTSE analyses of CIS thin films deposited at different substrate temperatures were performed, and the structural evolution of each film was studied. In addition, two different two-stage processes were explored for thin film CIS, and RTSE measurements were performed to characterize these CIS deposition processes as well. Native oxide coated c-Si was used for the deposition of CIS thin films in these materials studies. For both two-stage processes, the temperature of the substrate was fixed at 570C, which is the maximum possible when SLG substrates are used. In the first two-stage process, In2Se3 is deposited first, and then the film is exposed to Cu and Se flux according to In2Se3 + (2Cu + Se)  2(CuInSe2). This process is analogous to the second stage of conventional three-stage CIGS deposition [3]. In the second two-stage process, Cu2xSe is deposited first, and then the film is exposed to In and Se flux according to Cu2Se + (2In + 3Se)  2(CuInSe2). This is analogous to the third stage of three-stage CIGS deposition [3]. The structural as well as optical properties of the In2Se3, Cu2xSe, and CIS are established by analyzing the RTSE data obtained during the depositions. For both processes, RTSE is applied to characterize the CIS film at the time when the stoichiometric point is reached during the second stage. In additional CIS materials studies, a mapping capability for CIS Cu stoichiometry y = [Cu]/[In] over the deposition area was established based on a complex dielectric function spectra  = 1 − i2 parametrically dependent on y with a bandgap critical point Eg decreasing linearly from 1.030 eV for y = 0.7 to 1.016 eV for y = 1.1. In addition to these spectra for CIS, a complete set of spectra in the real and imaginary parts (1, 2) of the complex dielectric function were measured and parameterized for the CIS solar cell components. The combined results enabled analysis of SE data for the solar cell in terms of accurate structural and optical models useful for device simulations. The CIS solar cell devices in this study were fabricated by applying different deposition parameters and approaches for thermal co-evaporation of the absorber. Absorber deposition parameters applied in this study include deposition rate and substrate temperature, and different approaches include one-stage and two-stage thermal co-evaporation procedures. Completed CIS thin film solar cells were fabricated from the SLG/Mo/CIS structures by over-depositing layers of CdS/ZnO/ZnO:Al. A highest efficiency CIS solar cell of 11.15% is obtained in a one-stage absorber procedure using a deposition rate of 3.3 Å/s and a higher substrate temperature of 620ºC, made possible by special SLG. More advanced procedures for CIS deposition and processing are proposed as future work including three-stage CIS designed analogously to the three-stage CIGS deposition process, post deposition treatment with alkali halides, and targeted Ga additions to the CIS absorber. The completed solar cells were studied using ex-situ SE methods. The resulting layer thicknesses and dielectric functions obtained from these SE analyses were used to simulate the external quantum efficiency (EQE) spectra of the devices assuming complete collection of electrons and holes generated within their active layers. Electronic losses were identified by comparison with the measured EQE. In summary, this dissertation research has established the foundations for the development of a 1.0 eV bandgap CIS material suitable for incorporation as a bottom cell absorber of multijunction solar cells.
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)
418 p.
Physics
CIS devices, RTSE, In-Situ SE, Optimization,

Recommended Citations

Citations

  • Sapkota, D. R. (2022). Characterization and Optimization of CuInSe2 Solar Cells Applicable for Tandem Devices [Doctoral dissertation, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1671215831442438

    APA Style (7th edition)

  • Sapkota, Dhurba Raj. Characterization and Optimization of CuInSe2 Solar Cells Applicable for Tandem Devices. 2022. University of Toledo, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=toledo1671215831442438.

    MLA Style (8th edition)

  • Sapkota, Dhurba Raj. "Characterization and Optimization of CuInSe2 Solar Cells Applicable for Tandem Devices." Doctoral dissertation, University of Toledo, 2022. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1671215831442438

    Chicago Manual of Style (17th edition)

toledo1671215831442438
118
© 2022, some rights reserved.Creative Commons License Characterization and Optimization of CuInSe2 Solar Cells Applicable for Tandem Devices by Dhurba Raj Sapkota is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by University of Toledo and OhioLINK.