A Dissertation entitled Optical and Microstructural Properties of Sputtered Thin Films for Photovoltaic Applications by Dipendra Adhikari Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics ___________________________________________ Dr. Nikolas J. Podraza, Committee Chair ___________________________________________ Dr. Robert W. Collins, Committee Member ___________________________________________ Dr. Yanfa Yan, Committee Member ___________________________________________ Dr. Michael Cushing, Committee Member ___________________________________________ Dr. Sylvain X. Marsillac, Committee Member ___________________________________________ John Plenefisch, PhD, Dean College of Graduate Studies The University of Toledo December 2019 Copyright 2019 Dipendra Adhikari This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author. An Abstract of Optical and Microstructural Properties of Sputtered Thin Films for Photovoltaic Applications by Dipendra Adhikari Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics The University of Toledo December 2019 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 iii 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 iv 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. v To my Parents and Families vi Acknowledgements I want to express my gratitude to my parents. I am lucky to have you in my life. No matter where I am and what I am working on, you always give me enough freedom, and all the support you can offer. This keeps me working with a warm heart. I am very grateful to my research advisor, Dr. Nikolas Podraza, for his guidance, suggestions, and encouragement throughout my research and his continuous advice and expertise is always appreciated. Without his innovative suggestions and continuous support, none of the works mentioned in this Thesis research would have been possible. I am grateful to my committee members: Dr. Robert Collins, Dr. Yanfa Yan, Dr. Michael Cushing, and Dr. Sylvain Marsillac for being in my committee and providing valuable suggestions. I am thankful to my friends: Prakash Koirala, Maxwell Junda, Kiran Ghimire, Indra Subedi, Prakash Uprety, Niraj Shrestha, Ebin Bastola, and Biwas Subedi for their support and help. I thank the National Science Foundation (NSF) and Office of Naval Research (ONR) for the funding support. Finally, to my wife, Binita, how could I express my acknowledgements to you? I could not imagine how my life would be without your support. You deserve all the honors I received and will get. Without your love, support, motivation, and sacrifice, I could not have done this. Thanks to my son Aayan and daughter Aanvi for their patience and love during this time. vii Table of Contents Abstract ............................................................................................................................. iii Acknowledgements ......................................................................................................... vii Table of Contents ........................................................................................................... viii List of Tables .................................................................................................................... xi List of Figures ................................................................................................................. xiii Chapter 1: Introduction ....................................................................................................1 1.1 Motivation and Background ..............................................................................1 1.2 Dissertation Organization ..................................................................................7 Chapter 2: Spectroscopic Ellipsometry and Instrumentation .....................................10 2.1 Introduction ......................................................................................................10 2.2 Basic Theories of Light Propagation in Materials ...........................................12 2.3 Reflection from a Single Interface ...................................................................15 2.4 Reflection from Multilayer Stack and Matrix Formalism ...............................17 2.5 Effective Medium Theories in Spectroscopic Ellipsometry ............................19 2.6 Global 흈̅-minimization Method for RTSE Data Analysis ...............................21 2.7 Measurement and Instrumentation