Mechanism and Applications of Large and Persistent Photoconductivity in Cadmium Sulfide by Han Yin B.Eng. Materials Science and Engineering, Peking University (2014) S.M. Mechanical Engineering, Massachusetts Institute of Technology (2016) Submitted to the Department of Mechanical Engineering in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Mechanical Engineering at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY February 2020 ©2020 Massachusetts Institute of Technology. All rightsreserved. Signature f A u oj.: Signatureredacted Han Yin Department of Mechanical Engineering Signature redacted January 10, 2020 Certified by: Rafael Jaramillo Assistant Professor of Materials Science and Engineering Thesis Supervisor Certified by: Signature redacted Nicholas Xuanlai Fang Profes ' aianieering Chair pted hv- Signature redacted~ AceA o CNSTT1STTT OFTECHNOLOGY Nicolas adjiconstantinou Professdr of Mechanical Engineering ;U FEB 0 5 2020 Chair, Departmental Committee on Graduate Students 0 LIBRARIES [This page is intentionally left blank.] 2 Mechanism and Applications of Large and Persistent Photoconductivity in Cadmium Sulfide by Han Yin Submitted to the Department of Mechanical Engineering on January 10, 2020 in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Mechanical Engineering Abstract Photoconductivity is the phenomenon where electrical conductivity changes as a result of photoexcitation of new charge carriers. In some semiconductors, photoconductivity is accompanied with enormous conductivity change and long decay time after photoexcitation is ceased. This effect is called large and persistent photoconductivity (LPPC). LPPC is due to the trapping of photo-generated minority carriers at crystal defects. Theory has suggested that anion vacancies in II-VI semiconductors are responsible for LPPC due to negative-U behavior, whereby two minority carriers become kinetically trapped by lattice relaxation following photo-excitation. By performing a detailed analysis of photoconductivity in CdS, we provide experimental support for this negative-U model. We also show that, by controlling sulfur deficiency in CdS, we can vary the photoconductivity of CdS films over nine orders of magnitude, and vary the LPPC characteristic decay time from seconds to 104 seconds. Sulfur vacancies are deep donors at equilibrium in the dark, but convert to shallow donors in a metastable state under photoexcitation. We demonstrate two-terminal all-electrical thin film resistive switching devices that exploit this defect-level switching (DLS) mechanism as a new way to control conductivity. We introduce a hole injection layer to inject holes into the deep donor levels in CdS and switch CdS into a "photoconductive" state. The device is in low resistance state as fabricated, and shows repeatable resistance switching behavior under electrical bias with no electro-forming. Results from mechanism study rule out switching mechanisms based on mass transport and support our DLS hypothesis. LPPC is pronounced in n-type carrier-selective contact (CSC) materials in thin film solar cells, but its effect is rarely recognized. We numerically model the effect of LPPC in CSC by switching defect levels between deep and shallow donor states. CSC photoconductivity can substantially affect solar cell performance. For instance, the power conversion efficiency of both CIGS and CdTe solar cells can be improved by over 4% (absolute) depending on the photoconductivity of the CdS CSC. The primary underlying cause is the influence of CSC shallow donor density on the junction depletion region. Optimizing CSC photoconductivity may be effective in solar cell engineering across multiple platforms. Thesis Supervisor: Rafael Jaramillo Title: Assistant Professor of Materials Science and Engineering 3 [This page is intentionally left blank.] 4 Acknowledgments First and foremost, I would like to express my sincere gratitude to Prof. Rafael Jaramillo for his kind guidance in the past five years. Raf is a remarkable instructor and knowledgeable researcher who led me into the field of semiconductor physics and worked together with me through various projects. There are always obstacles during research, but Raf constantly embraces an optimistic mind. His attitude inspired me whenever I was depressed by bad research results, and we together made this thesis possible. I am obliged to Prof. Nick Fang and Prof. Jeehwan Kim for their willingness to be in my thesis committee and their insightful suggestions during committee meetings which led to great improvements of my thesis. It has also been great time working and discussing technical problems with our illustrious group members. Our discussions can often lead to new ideas and innovative ways to solve research problems. I also want to thank my collaborators for their constructive suggestions and assistance in performing measurements which could be done at MIT. I am grateful to my parents for their constant support and love through my life. They always encourage me to pursue my goals and really appreciate this freedom during my growth. Finally, I want to thank my friends for their company during the years at MIT. Many thanks to Huifeng Du, who is also a PhD student at the Mechanical Engineering department at MIT, for his friendship since undergraduate. I also want to thank my partner Siyu for meeting me at the best time, for her accompany and care, and for being the person to share my feelings with. 5 [This page is intentionally left blank.] 6 Table of Contents Chapter 1. Introduction ..................................................................................................... 13 1.1 Photoconductivity in semiconductors ................................................................. 13 1.1.1 Photoconductivity process .......................................................................... 13 1.1.2 Recombination processes............................................................................. 17 1.1.3 Recombination models................................................................................. 19 1.2 Cadmium sulfide as a photoconductor............................................................... 21 1.2.1 Thin film deposition techniques.................................................................. 22 1.2.2 Optoelectronic applications ......................................................................... 24 1.3 Persistent photo-effects in semiconductors........................................................ 25 1.3.1 Observation of persistent photoconductivity ............................................... 25 1.3.2 Theory of persistent photoconductivity ...................................................... 26 1.4 M otivation and main findings for this study....................................................... 29 Chapter 2. Mechanism of Large and Persistent Photoconductivity in CdS.................. 33 2.1 Introduction ................................... 33 2.2 Experimental Section.......................................................................................... 35 2.2.1 Synthesis of CdS thin films.......................................................................... 35 2.2.2 Structural and morphology characterizations .............................................. 36 2.2.3 Optical measurements................................................................................. 37 2.2.4 Compositional measurements ...................................................................... 38 2.2.5 Electrical measurements ............................................................................. 38 2.3 Results..................................................................................................................... 40 2.3.1 CdS structure and morphology ................................................................... 40 2.3.2 Basic optical and electrical measurements ................................................. 43 2.3.3 The relationship between photoconductivity and synthesis parameters.......... 44 2.3.4 M odeling photoconductivity ....................................................................... 52 2.4 Discussion............................................................................................................... 63 2.4.1 Modeling large and persistent photoconductivity in CdS............................ 63 2.4.2 Energy level diagram ................................................................................... 64 2.5 Conclusion .............................................................................................................. 66 2.6 Appendix................................................................................................................. 70 2.6.1 Conditions for transient maxim um photoconductivity ................................ 70 2.6.2 Analyzing numerical simulations of the standard model............................. 72 7 Chapter 3. Two-Terminal Resistive Switches Based on Defect-Level Switching in CdS74 3.1 Introduction............................................................................................................. 74 3.1.1 Two-term inal electronic devices................................................................. 74 3.1.2 Point defects in sem iconductors................................................................. 75 3.1.3 Defect-level switching device concept ....................................................... 76 3.2 Experim ental m ethods .....................................................................................