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Optical Evaluation and Simulation of Photovoltaic Devices for Thermal Management

Subedi, Indra
http://orcid.org/0000-0003-2300-6010
http://rave.ohiolink.edu/etdc/view?acc_num=toledo155448373019862

Abstract Details

2019, Doctor of Philosophy, University of Toledo, Physics.
Evaluation and understanding of optical properties are essential for the use and design of optoelectronic devices. This dissertation explains the evaluation of optical properties of component layers of the encapsulated photovoltaic (PV) module and uses them in device simulation focusing on thermal management. Sub-bandgap characterizations are not emphasized enough in the PV device design earlier. The examples discussed here range from ordinary glass used to cover solar cells to completed silicon (Si) wafer and thin film cadmium telluride (CdTe) solar cells. This study will focus on two key mechanisms for thermal management: radiation and sub-bandgap reflection. Commercial solar cells have light incident through the glass in solar wavelength range ~250- 2500 nm. This cover glass has an ability to re-radiate heat in the infrared (IR) region, thermal wavelength, from the device to keep the solar cells cool. In contrast to bare semiconductors, glass has a relatively high emissivity aiding in radiative cooling of solar modules. The directional thermal emissivity of solar cell cover glasses with differences in glass composition or manufacture and surface texture are evaluated using specular and specular+diffuse infrared reflectance at a different angle of incidences. Non-textured and textured glasses all exhibit similar emissivity at all angles of incidence regardless of composition and patterning. Both diffuse and specular reflectance must be included for textured glass at any angle of incidence and may be needed for planar glass at a high angle of incidences to determine emissivity accurately. Optical characterization of the semiconductor is important from the perspective of physics and application in devices. There are different features in the optical response related to different physical phenomena such as band to band electronic transitions, vibrational or phonon modes, and free carrier absorption. I have explored optical properties of an epitaxial indium phosphide (InP) film deposited on an iron compensated InP (InP:Fe) wafer at room temperature by ex-situ spectroscopic ellipsometry over a spectral range of 0.038–8.5eV. The complex dielectric function spectra, ε (E) =ε1 (E) + iε2 (E), have been determined by fitting a parametric model to the experimental ellipsometric data. Kramers-Kronig consistent parameterizations have been applied to describe interband transitions and defect-based sub-bandgap absorption in the 0.73–8.5 eV spectral range, and both phonon modes and free carrier properties in the 0.038–0.73 eV range. Spectra in ε from 0.73–8.5 eV shows ten higher energy interband critical point transitions at 1.36, 1.42, 3.14, 3.34, 4.71, 4.97, 5.88, 6.45, 7.88, and 8.22 eV. The direct band gap energy of 1.37 eV and Urbach energy 46 meV are also determined from spectra in ε. A strong optical phonon mode is identified near 305 cm-1. Electronic transport properties, carrier concentration (N) and mobility (μ), calculated from Drude model with N = 1.9 x 1018 cm-3 and μ = 1559 cm2/Vs agree well with direct electrical Hall effect measurement N = 2.2 x 1018 cm-3 and μ = 1590 cm2/Vs. A parameterization of ε from 0.038 to 8.5 eV for the epitaxial InP film is reported. The most commercially successful solar cells are based on silicon (Si) wafer technology, with the fabrication process impacting the infrared optical response of the complete devices. Al is a commonly used material for rear side metallization in commercial Si wafer solar cells. In this dissertation, through-the-silicon (TTS) spectroscopic ellipsometry (SE) is used in a test sample to measure Al+Si interface optical properties like those in Si wafer solar cells. TTS SE is used for evaluation of Al+Si interface optical properties over the 1128-2500 nm wavelength range. For validation, I used the measured interface optical properties in a ray tracing simulation over the 300-2500 nm wavelength range for an encapsulated Si solar cell having random pyramidal texture. Simulated reflectance from a ray tracing model matches well with the measured total reflectance at normal incidence of a commercially available Si module. The Al+Si optical properties presented here enable quantitative assessment of significant irradiance/current flux losses arising from reflection and parasitic absorption in encapsulated Si solar cells. This optical modeling is expanded to passivated emitter and rear contact (PERC) solar cells incorporating scalar scattering theory along with ray tracing. Total reflectance of pyramidical textured cells with and without encapsulation is simulated to determine the appropriate modeling structure. This approach focusing on the encapsulated and bare cell reflectance serve as input for evaluating thermal management strategies for solar cells. For all solar cells, photons with energies below the absorber layer bandgap do not generate electric current. In thin film CdTe PV, it is either absorbed by free-carrier absorption in the transparent conductive oxide (TCO), absorbed by the metallic back contact, or lost by reflection. The parasitic absorption adversely impacts performance by producing heat. I incorporate infrared (IR) optical response in simulations for understanding thermal losses for thin film CdTe. IR-extended quantum efficiency simulations calculate efficiency gains/losses arising from variations in current generated and total reflectance as functions of transparent front contact material in thin film CdTe PV.
Nikolas Podraza (Committee Chair)
Robert Collins (Committee Member)
Yanfa Yan (Committee Member)
Sanjay Khare (Committee Member)
Michael Deceglie (Committee Member)
129 p.
Physics
Optical properties; Thermal management; Spectroscopic ellipsometry; Glass emissivity; Silicon solar cells; PERC; Ray tracing simulation; Indium Phosphide; CdTe

Recommended Citations

Citations

  • Subedi, I. (2019). Optical Evaluation and Simulation of Photovoltaic Devices for Thermal Management [Doctoral dissertation, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=toledo155448373019862

    APA Style (7th edition)

  • Subedi, Indra. Optical Evaluation and Simulation of Photovoltaic Devices for Thermal Management. 2019. University of Toledo, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=toledo155448373019862.

    MLA Style (8th edition)

  • Subedi, Indra. "Optical Evaluation and Simulation of Photovoltaic Devices for Thermal Management." Doctoral dissertation, University of Toledo, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=toledo155448373019862

    Chicago Manual of Style (17th edition)

toledo155448373019862
611
© 2019, some rights reserved.Creative Commons License Optical Evaluation and Simulation of Photovoltaic Devices for Thermal Management by Indra Subedi 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.