University of Denver Digital Commons @ DU Electronic Theses and Dissertations Graduate Studies 1-1-2016 Synthesis and Characterization of Kesterite Cu2ZnSnS4 (CZTS) Thin Films for Solar Cell Application Mohamed M.A. Abusnina University of Denver Follow this and additional works at: https://digitalcommons.du.edu/etd Part of the Electrical and Computer Engineering Commons, and the Materials Science and Engineering Commons Recommended Citation Abusnina, Mohamed M.A., "Synthesis and Characterization of Kesterite Cu2ZnSnS4 (CZTS) Thin Films for Solar Cell Application" (2016). Electronic Theses and Dissertations. 1153. https://digitalcommons.du.edu/etd/1153 This Dissertation is brought to you for free and open access by the Graduate Studies at Digital Commons @ DU. It has been accepted for inclusion in Electronic Theses and Dissertations by an authorized administrator of Digital Commons @ DU. For more information, please contact [email protected],[email protected]. Synthesis and characterization of kesterite Cu2ZnSnS4 (CZTS) thin films for solar cell application __________ A Dissertation Presented to The Faculty of the Daniel Felix Ritchie School of Engineering and Computer Science University of Denver __________ In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy __________ by Mohamed M. A. Abusnina June 2016 Advisor: Mohammad Matin ©Copyright by Mohamed M. A. Abusnina 2016 All Rights Reserved Author: Mohamed M. A. Abusnina Title: Synthesis and characterization of kesterite Cu2ZnSnS4 (CZTS) thin films for solar cell application Advisor: Mohammad Matin Degree Date: June 2016 Abstract The quaternary compound Cu2ZnSnS4 (CZTS) gained considerable attention in the last decade due to its potential as an active-layer semiconductor for low-cost thin-film solar cells. The material is composed of nontoxic and Earth-abundant constituents, has optical properties suitable for photovoltaic application, and can be synthesized using a wide variety of methods. Polycrystalline CZTS was grown in this work using vacuum-based deposition to first deposit metal films (precursors) of Cu, Zn, and Sn. In a subsequent step, the precursors underwent an annealing treatment in sulfur vapor environment (sulfurization) to form CZTS. Using sputtering, a physical vapor deposition (PVD) technique, two different kinds of metal precursors were deposited: a) stacked precursors, in which a stack of metal layers are sequentially deposited on a glass substrate, and b) co-sputtered precursors, in which the three metals are deposited simultaneously. The effect of sulfurization time and temperature was investigated to optimize the process and to study the impact of both factors on the morphological and structural properties of the synthesized compound. CZTS films based on a stacked precursor were prepared and characterized by X-ray fluorescence, energy-dispersive X-ray spectroscopy, electron probe microanalysis, X-ray diffraction, scanning electron microscopy , ultraviolet-visible-near-infrared spectrophotometry, and Auger electron spectroscopy. Films with dense morphology, good crystallinity, and optical energy bandgap close to ii 1.5 eV were obtained using annealing temperatures of 500 °C and 550 °C. Furthermore, the study addressed the issue with Cu2-xS phases segregated on the film surface and suggested a route to avoid their evolution. The secondary-phase dependence on the chemical composition was studied, focusing on suppressing Cu2-xS phases. In addition, we addressed the importance of Sn/Cu ratio as a controlling factor to avoid developing these detrimental phases. An electron backscatter diffraction study confirmed the role of increased Cu content in achieving larger grains. Studying the influence of the stack sequence in the precursor revealed that depositing Zn as the first or second layer leads to better adhesion, and depositing Cu as top layer minimizes the loss of Sn and allows better control of film composition. Solar cells based on CZTS films prepared with the sulfurization of precursors with different stacking orders were produced, and the best device showed power conversion efficiency (PCE) of 4.43%. CZTS films based on co-sputtered precursors showed very promising results in terms of large grains, dense morphology, and smooth surface; however, they suffered from poor adhesion to the Mo layer due to compressive intrinsic stresses. Further investigation is needed to identify the stress origin and improve the contact to the back electrode. The best device based on CZTS films from co-sputtered precursors yielded a PCE of 1.72%. iii Acknowledgements Almost daily for the first year of this project, I asked myself, “Have I made the right decision?,” wondering what really motivated me to work in this research area. The first answer always coming to mind was, “You’re so fortunate to conduct your research at the National Renewable Energy Laboratory (NREL), one of the world’s leading institutions for renewable energy research.” I had come from electrical engineering to a field where people talk about film morphology, crystal structure, and many other terms new to me. So I was feeling overwhelmed, lacking relevant background and experience that I could rely on. At moments, I questioned my ability to complete this dissertation. But this journey has ultimately come to the end with the help and support of certain people to whom I am very thankful. First and foremost, I direct my thanks to my academic advisor, Prof. Mohammad Matin, for accepting me as a Ph.D. student. I also express my gratitude to Dr. Mowafak Al-Jassim for enabling me to conduct my research at NREL. My special thanks go to Dr. Helio Moutinho, who had the greatest impact on my work through valuable explanations and wide-ranging discussions. I am also appreciative to: Jeff Alleman for his ongoing help; Don Gwinner for editing my writing; Bobby To, Maikel van Hest, Jeff Blackburn, and Phil Parilla for training me on different systems; and Clay DeHart and Jeff Carapella for processing my samples into complete devices. Last but not least, I thank my family for believing in me and encouraging me to finish this task. I am thankful to my parents for supporting me in all the steps of my life. My deepest thanks go to my wife, who has always been at my side in good times and bad, and to my children for their understanding and sacrifice when it came to my work. iv Table of Contents Chapter One: Introduction ...............................................................................................1 1.1 Overview .......................................................................................................1 1.2 Aim of the Study ............................................................................................4 1.3 Structure of the Work .....................................................................................5 Chapter Two: Theoretical Background ............................................................................6 2.1 Physics of Photovoltaics .................................................................................6 2.1.1 Light absorption in semiconductors ..................................................6 2.1.2 The p-n junction...............................................................................7 2.1.3 The solar cell ................................................................................. 12 2.1.3.1 Characteristic parameters of a solar cell .................................................. 13 2.1.3.2 Real solar cell (equivalent circuit)........................................................... 15 2.1.3.3 Optical losses in the solar cell ................................................................. 16 2.1.3.4 Recombination ....................................................................................... 17 2.2 Thin-Film Solar Cells ................................................................................... 18 2.2.1 Amorphous silicon ......................................................................... 20 2.2.2 Cadmium telluride (CdTe) ............................................................. 22 2.2.3 Cu(InGa)Se2 (CIGS) ...................................................................... 23 2.3 Cu2ZnSnS4 (CZTS) ...................................................................................... 24 2.3.1 Properties of CZTS ........................................................................ 24 2.3.1.1 Crystal structure ..................................................................................... 24 2.3.1.2 Electronic band structure ........................................................................ 27 2.3.1.3 Intrinsic defects ...................................................................................... 29 2.3.2 Single-phase stability and controlling CZTS chemical composition32 2.3.3 Effect and identification of secondary phases ................................. 35 2.3.4 Fabrication of Cu2ZnSnS4 thin films .............................................. 39 2.3.4.1 Vacuum deposition techniques ............................................................... 40 2.3.4.2 Non-vacuum deposition techniques ........................................................ 42 Chapter Three: Fabrication Approaches and Characterization Techniques ..................... 44 3.1 Fabrication Methods ..................................................................................... 44 3.1.1 Preparing the substrate ..................................................................