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Optoelectronic and Structural Properties of Thin Films for Optoelectronic Devices and Role of Interface Structure on Photovoltaic Device Performance

Jayswal, Niva Kumari
http://rave.ohiolink.edu/etdc/view?acc_num=toledo1691101196761624

Abstract Details

2023, Doctor of Philosophy, University of Toledo, Physics.
Low-cost thin film absorber layer materials with high absorption coefficients (> 105 cm-1 in visible spectral range) and bandgap close to the ideal value for efficient photovoltaic conversion efficiency are leading candidates for thin film photovoltaic (PV) applications. This dissertation discusses the fabrication and optical and microstructural properties of magnetron-sputtered glancing angle deposited CdTe thin film absorber layer material and its application as an interlayer in CdS/CdTe solar cells. In addition, optoelectronic properties of non-toxic and earth-abundant absorber layer material, antimony selenide (Sb2Se3), and optimization of polycrystalline VO2 fabrication from amorphous vanadium oxide (VOx) film along with its optical properties have been discussed. Sb2Se3 is a promising candidate as an absorber layer material in PV applications. I have performed optical property characterization of thin film Sb2Se3 and identified electronic losses when used in a PV device. The indirect bandgap, direct bandgap, and Urbach energy have been determined to be 1.12 eV, 1.17 eV, and 21.1 meV, respectively using photothermal deflection spectroscopy. Optical properties of Sb2Se3 in the form of complex dielectric function (ε = ε1 + iε2) spectra in 0.75 to 4 eV spectral range is determined using spectroscopic ellipsometry. The line shape of ε is obtained using a parametric model which incorporates an Urbach tail, a band edge function, and five critical point oscillators. The optical property spectra in ε and structural parameters in terms of the thickness of solar cell layer components are used as input parameters for external quantum efficiency (EQE) simulation to investigate the electronic and optical losses in Sb2Se3-based solar cells. A carrier collection length of ~ 400 nm and a ~97 % carrier collection probability near the heterojunction in the Sb2Se3 solar cell are identified by comparing experimental and simulated EQE. Next, I describe deposition and characterization of amorphous vanadium oxide (VOx) thin films to optimize polycrystalline VO2 fabrication. Amorphous VOx films are prepared with varying process parameters by reactive radio frequency magnetron sputtering. In-situ real-time spectroscopic ellipsometry (RTSE) measurements are performed to track spectra in ε, thickness and roughness, and oxygen content, x, with depth during growth evolution. VO2 exhibits a reversibly semiconducting-to-metal transition (SMT) at 68°C. Amorphous VOx films with x ~2 are annealed to produce polycrystalline VO2 which is characterized using near-infrared to ultraviolet (NIR - UV) (0.75 - 5.9 eV) spectroscopic ellipsometry. Also, spectroscopic ellipsometry measurements are used to determine temperature-dependent ε spectra and to track the semiconductor-to-metal transition during heating and cooling from room temperature to 70°C. The SMT of VO2 is verified by a substantially increased magnitude of spectra in ε2 at 70°C. A range of oxygen content (x), 1.89 ≤ x ≤ 2.14, of as-deposited amorphous VOx films has been determined which transitions to polycrystalline VO2 after annealing. Finally, I have utilized glancing angle deposition (GLAD) of CdTe and applied this material in CdS/CdTe solar cells as an interlayer to study its effect in PV device performance. The high-temperature GLAD CdTe films are prepared using radio frequency magnetron sputtering at different oblique angles and their optical and microstructural properties have been studied. X-ray diffraction shows that high-temperature GLAD CdTe films exhibit only cubic or mixed-phase (cubic + hexagonal) phases with more preferential crystallite orientations at all glancing angles whereas room temperature GLAD CdTe deposited at higher oblique angles has a metastable hexagonal crystal phase. In CdS/CdTe solar cells, the device performance can be altered by controlling the microstructure of GLAD CdTe interlayer at p/n heterojunction interface. The main focus is to use GLAD CdTe films as an interlayer between the hexagonal wurtzite n-type CdS window layer and the cubic p-type CdTe absorber layer to reduce lattice mismatch and optimize PV device performance. CdS/CdTe PV devices are fabricated by incorporating GLAD CdTe interlayers with varying temperatures, oblique angles, and interlayer thicknesses. An increase in the PV device performance in terms of efficiency, open circuit voltage (Voc), fill factor (FF), and Voc × FF product has been found with a 250 nm hexagonal phase GLAD CdTe interlayer when compared to the baseline device without interlayer.
Nikolas J. Podraza (Committee Chair)
Robert W. Collins (Committee Member)
Yanfa Yan (Committee Member)
Song Cheng (Committee Member)
Terry Bigioni (Committee Member)
130 p.
Physics
Optical Properties; Ellipsometry; Thin Films; Optical and Electronic Loss; Antimony Selenide; Vanadium Oxide; Hysteresis; Glancing Angle Deposition; CdTe; Solar Cell

Recommended Citations

Citations

  • Jayswal, N. K. (2023). Optoelectronic and Structural Properties of Thin Films for Optoelectronic Devices and Role of Interface Structure on Photovoltaic Device Performance [Doctoral dissertation, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1691101196761624

    APA Style (7th edition)

  • Jayswal, Niva. Optoelectronic and Structural Properties of Thin Films for Optoelectronic Devices and Role of Interface Structure on Photovoltaic Device Performance. 2023. University of Toledo, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=toledo1691101196761624.

    MLA Style (8th edition)

  • Jayswal, Niva. "Optoelectronic and Structural Properties of Thin Films for Optoelectronic Devices and Role of Interface Structure on Photovoltaic Device Performance." Doctoral dissertation, University of Toledo, 2023. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1691101196761624

    Chicago Manual of Style (17th edition)

toledo1691101196761624
66
© 2023, some rights reserved.Creative Commons License Optoelectronic and Structural Properties of Thin Films for Optoelectronic Devices and Role of Interface Structure on Photovoltaic Device Performance by Niva Kumari Jayswal is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by University of Toledo and OhioLINK.