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Optical and Microstructural Properties of Sputtered Thin Films for Photovoltaic Applications

Adhikari, Dipendra
http://orcid.org/0000-0002-8976-0190
http://rave.ohiolink.edu/etdc/view?acc_num=toledo1573118517150321

Abstract Details

2019, Doctor of Philosophy, University of Toledo, Physics.
Thin film solar cells are promising candidates for generation of low cost and pollution-free energy. The materials used in these devices, mainly the active absorber layer, can be deposited in a variety of industry-friendly ways, so that the cost associated with manufacturing is generally lower than for competing technologies such as crystalline silicon. This dissertation will focus on the fabrication and characterization of nanocrystalline hydrogenated silicon (nc-Si:H) and polycrystalline cadmium telluride (CdTe) thin films by industrially scalable, non-toxic, and comparatively simple magnetron sputtering. The performance of the solar cells incorporating these films as an active absorber layers are discussed. In this work, spectroscopic ellipsometry is used as the primary tool for the characterization of optical and structural properties of thin films and bulk material. As a first case study, the anisotropic optical properties of single crystal strontium lanthanum aluminum oxide (SrLaAlO4) in the form of birefringence and dichroism is obtained from Mueller matrix ellipsometry. SrLaAlO4 exhibit uniaxial anisotropic optical properties and the indirect optical band gap of 2.74 eV. A parametric model consisting of parabolic band critical points (CPs) for electronic transitions and a gap function is used to describe the complex dielectric function spectra in both the ordinary and extra-ordinary directions. The modeling in this case study has applications to both nc-Si:H, an indirect band gap semiconductor, and CdTe which may exhibit microstructural anisotropy depending upon the deposition method. Fabrication and characterization of hydrogenated silicon (Si:H) thin films produced by reactive magnetron sputtering is the second case in this study. RTSE and a virtual interface analysis (VIA) are used to track the growth evolution of sputtered Si:H. From these studies, growth evolution diagrams depicting the nucleation of nanocrystallites from the amorphous phase and nanocrystallite coalescence are developed. Silicon-hydrogen bonding configurations are determined from absorption features extracted from infrared spectroscopic ellipsometry. Ultimately, this work provides a summary of how nanostructure can be manipulated in reactive magnetron sputtered Si:H films, which is necessary for implementation of these materials in photovoltaic (PV) devices. Here we have demonstrated working PV devices produced with RF magnetron sputtered nc-Si:H absorber layers and compare overall device performances to those produced with the more conventional plasma enhanced chemical vapor deposition (PECVD) absorbers. Different absorber layer deposition techniques and atmospheric exposure effects are studied to explain variations in the performance of single junction n-i-p substrate configuration nc-Si:H based solar cells. The cells with nanocrystalline PECVD absorbers and an untextured (planar) back reflector serve as a baseline for comparison with power conversion efficiency near 6%. This efficiency is typical of this device configuration lacking optical enhancement due to scattering of light incident by the back reflector. By comparison, cells with sputtered absorbers achieved efficiencies of about 1%. Comparison of dark / light current-voltage measurements and external quantum efficiency (EQE) indicate that lower performance in devices with sputtered absorbers may be attributed to both low electronic quality within the nc-Si:H absorber and also process incompatibility at the interfaces between the intrinsic layer made by sputtering with the doped layers made by PECVD. The collection probability profile obtained from EQE simulations show substantial carrier concentration losses are identified at the n-/i-interface. The sputtered material itself requires further optimization to reach performance levels comparable to those achievable with PECVD, but this works serves as a baseline for future material and device studies. Finally, the optical, electrical, and microstructural properties of magnetron sputtered CdTe films prepared by glancing angle deposition (GLAD) are studied. From cross-sectional micrographs, increasingly tilted columnar structure occurs with increasing incident oblique angle for as-deposited CdTe films. Films deposited at lower oblique angles closer to normal incidence have mixed (cubic + hexagonal) crystal structure and those prepared at more oblique higher angles have hexagonal wurtzite crystal structure. Post-deposition CdCl2 treated films show enhancement in grain size for samples prepared under all conditions. The optical response in the form of the complex dielectric function (ε = ε1 + iε2) spectra from 0.74 to 5.89 eV for the GLAD thin films are all qualitatively similar to single crystal CdTe. Higher angle deposited samples show columnar structure induced anisotropy in spectra in ε in the transparent spectral range. Application of GLAD CdTe interlayers between CdS and CdTe of the standard CdS/CdTe heterojunction design solar cell shows better performance with 0.9% absolute efficiency increase.
Nikolas Podraza (Committee Chair)
Robert Collins (Committee Member)
Yanfa Yan (Committee Member)
Michael Cushing (Committee Member)
Sylvain Marsillac (Committee Member)
169 p.
Energy; Materials Science; Optics; Physics
Ellipsometry; Optical Anisotropy; Glancing Angle Deposition; Hydrogenated Silicon; Sputtering; CdTe; Solar Cell; Mueller Matrix

Recommended Citations

Citations

  • Adhikari, D. (2019). Optical and Microstructural Properties of Sputtered Thin Films for Photovoltaic Applications [Doctoral dissertation, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1573118517150321

    APA Style (7th edition)

  • Adhikari, Dipendra. Optical and Microstructural Properties of Sputtered Thin Films for Photovoltaic Applications. 2019. University of Toledo, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=toledo1573118517150321.

    MLA Style (8th edition)

  • Adhikari, Dipendra. "Optical and Microstructural Properties of Sputtered Thin Films for Photovoltaic Applications." Doctoral dissertation, University of Toledo, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1573118517150321

    Chicago Manual of Style (17th edition)

toledo1573118517150321
358
© 2019, some rights reserved.Creative Commons License Optical and Microstructural Properties of Sputtered Thin Films for Photovoltaic Applications by Dipendra Adhikari 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.