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Photoconductivity and Minority Carrier Lifetime in Tin Sulfide and Gallium Arsenide Semiconductors for Photovoltaics

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Photoconductivity and Minority Carrier Lifetime in Tin Sulfide and Gallium Arsenide Semiconductors for Photovoltaics Photoconductivity and Minority Carrier Lifetime in Tin Sulfide and Gallium Arsenide Semiconductors for Photovoltaics by Frances Daggett Lenahan Submitted to the Department of Materials Science and Engineering in Partial Fulfillment of the Requirements for the Degree of Bachelor of Science at the Massachusetts Institute of Technology June 2016 © 2016 Frances Lenahan All rights reserved The author hereby grants to MIT permission to reproduce and to distribute publicly and electronic copies of this thesis document in whole or in part in any medium now known or hereafter created Signature of Author………………………………………………………………………………… Department of Materials Science and Engineering April 29, 2016 Certified by………………………………………………………………………………………… Tonio Buonassisi Associate Professor of Mechanical Engineering Thesis Supervisor Accepted by……...………………………………………………………………………………… Geoffrey S. D. Beach Professor of Materials Science and Engineering Chairman, Undergraduate Thesis Committee Read by……………………………………………………………………..……………………… Rafael Jaramillo Toyota Career Development Assistant Professor of Materials Science & Engineering Thesis Reader 2 Photoconductivity and Minority Carrier Lifetime in Semiconductors for Photovoltaics by Frances Daggett Lenahan Submitted to the Department of Materials Science and Engineering on April 29th, 2015, in partial fulfillment of the requirements for the degree of Bachelor of Science in Materials Science and Engineering Abstract The growth and maintenance of the modern technological world requires immediate solutions in the field of clean, renewable energy. One prominent solution is the rapid advancement of solar cell technologies due to the wide availability of solar energy and the growing versatility of harnessing it. As efficiencies for these devices creep upwards, it becomes increasingly more important to find the greatest inhibiting factor. Through a solar cell simulator program (SCAPS), improvements in the minority-carrier lifetime of cell materials show not only significant improvements in cell efficiencies, but also an un-masking of improvements by other properties, which are inhibited when the lifetime is too short. This work aims to calculate the mobility- lifetime products (µτ) of gallium arsenide (GaAs) and annealed and un-annealed tin sulfide (SnS) with respective p-doping carrier concentrations of 1018 cm-3, 1016 cm-3, and 1015 cm-3 through photoconductivity measurements. Films are 1 µm thick and have a four-bar and two-bar contact configuration to model carrier conductivity as a sheet. For calculations, two methods of modeling charge carrier generation are considered; a uniform generation throughout the film and a depth and wavelength-dependent generation. This work found values on the order of 10-1 cm2 V-1, 10-4 cm2 V-1, and 10-5 cm2 V-1, for GaAs, annealed SnS, and un-annealed SnS, respectively, for both methods of calculation. The simplified approach considering a uniform generation yielded lower results than the depth and wavelength dependent calculations by about a factor of two. All values were three to four orders of magnitude higher than those found in the literature. For this reason, it is believed that the majority-carrier is dominating measurements due to an inhibited minority-carrier lifetime. Thesis Supervisor: Tonio Buonassisi Title: Associate Professor of Mechanical Engineering 3 4 Acknowledgements I would like to acknowledge a number of persons and communities that have helped me not only get through MIT and write this thesis, but develop as a person and a scientist. Firstly, I would like to acknowledge the unique MIT community for always offering the means and opportunities to pursue every little passion and the DMSE and SUMS community for both introducing Materials Science and bolstering a love for the work and each other. To Dr.Jagadeesh Moodera for the opportunity of my first research experience and invaluable teachings on curiosity and motivation as the foundations of research. To Professor Craig Carter for the carefree optimism. I would often find myself learning only to meet a deadline, but Professor Carter helped me remember that the best thing about learning is just the new knowledge we obtain. To the PV Lab Community for always being supportive and collaborative. I immediately felt like a part of the team, which truly contributed to every positive outlook I had about the research. In particular, to Professor Tonio Buonassisi for setting one of the greatest examples of leadership I’ve ever seen. I am so grateful to have been an undergraduate researcher in this lab. Much gratitude to doctoral candidate Rupak Chakraborty and fellow senior Luisa Barrera for all their work on the coding for calculating the mobility-lifetime product under non-uniform generation and an extra special thank you to doctoral candidate Riley Brandt for his endless guidance, wisdom, and patience throughout my time on the team. Finally, I am forever grateful for the support and love of my parents. Everything you do is for the betterment of your children. I can only hope to one day follow your example. 5 6 Contents 1. Introduction .......................................................................................................................12 1.1 Motivation ....................................................................................................................................12 2. Background Physics ......................................................................................................14 2.1 Energy Bands ..............................................................................................................................14 2.2 Doping ...........................................................................................................................................17 2.3 Recombination and Traps ........................................................................................................18 2.4 Basics of a Solar Cell ................................................................................................................20 3. Charge Carrier Lifetime and Photoconductivity ............................................24 3.1 Introduction to Lifetime ............................................................................................................24 3.2 Influence of Lifetime .................................................................................................................24 3.3 Introduction to Photoconductivity .........................................................................................27 3.4 Determining the Mobility-Lifetime Product .......................................................................27 3.4.1 Simplified Approach ..........................................................................................................28 3.4.2 Considering Non-uniform Generation ..........................................................................29 3.5 Methods of Experimental Derivation ....................................................................................31 3.5.1 Snake Configuration ..........................................................................................................32 3.5.2 Van der Pauw Configuration ...........................................................................................33 3.5.2 Bar Configuration ...............................................................................................................34 4. Experimental Procedures ............................................................................................35 7 4.1 Cyrostat Tool ...............................................................................................................................35 4.2 Sample Fabrication ....................................................................................................................36 4.3 Mounting Samples .....................................................................................................................37 4.4 Data Collection and Processing ..............................................................................................38 4.5 Solar Cell Capacitance Simulator (SCAPS) .......................................................................39 5. Results ...................................................................................................................................43 5.1 Test Sample Gallium Arsenide (GaAs) ................................................................................43 5.2 Tin Monosulfide (SnS) .............................................................................................................45 5.2.1 Tests for Sub-band-gap Light ..........................................................................................45 5.2.2 Noise Floor ...........................................................................................................................46 5.2.3 Effects of annealing ...........................................................................................................46 5.2.4 Depth and Wavelength Dependent Generation ..........................................................47 5.2.5 Low Illumination Measurements ...................................................................................49 5.2.6 Seebeck Measurements .....................................................................................................50 6. Conclusions .........................................................................................................................51
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