Design and Performance Analysis of an Efficient GaAs/AlGaAs Heterojunction Bipolar Transistor Solar Cell A thesis submitted to the Department of Electrical and Electronic Engineering (EEE) of Bangladesh University of Engineering and Technology (BUET) In partial fulfillment of the requirement for the degree of MASTER OF SCIENCE IN ELECTRICAL AND ELECTRONIC ENGINEERING by Sakib Hassan (Roll No.: 1014062235 P) DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING (EEE) BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY (BUET) MAY 2017 i | P a g e ii | P a g e Declaration It is hereby declared that this thesis or any part of it has not been submitted elsewhere for the award of any degree or diploma. Signature of the candidate _____________________________ Sakib Hassan (Roll No.: 1014062235 P) iii | P a g e Acknowledgements First, I would like to express my gratitude towards the Almighty Allah for the strength, courage, and perseverance that He has bestowed throughout my academic life. I am thankful to Him for whatever I have achieved thus far in my educational pursuit. Next, I would like to express gratitude towards my thesis supervisor, Dr. Md. Kawsar Alam, Associate Professor of the Department of Electrical and Electronic Engineering, Bangladesh University of Engineering and Technology, Dhaka for the patient guidance, encouragement, and advice that he has provided throughout the time I have been his student. I consider myself extremely lucky to have such a wise, inspiring, and helpful supervisor who not only answered all the questions and queries and but also provided ample computational resources to aid my research endeavor. I will always be grateful to him for introducing me to the world of research and teaching a systematic approach towards research work. I would also like to express my appreciation to the members of my thesis committee, Dr. Quazi Deen Mohd Khosru, Dr. Md. Ziaur Rahman Khan and Dr. Md. Anwarul Abedin, for their valuable feedback and suggestion on my work. Completing this thesis work would have been all the more difficult were it not for the continuous help and support I received from fellow researcher in ‘Photonics Lab’ - Zabir Ahmed. I would like to take this opportunity to thank my parents for their love, guidance, and influence. I also thank my wife for her continuous support and inspiration. Their belief in me has always been the source of strength in the most trying times. iv | P a g e Abstract In this work, we propose a highly efficient three-terminal heterojunction bipolar transistor solar cell. We have chosen GaAs/AlGaAs material system in this regard for its excellent optoelectronic properties. Rigorous optical and electrical performance analysis through computer simulation has been carried out to characterize the designed device. Usually, the efficiency of the solar cell depends on material composition, absorption of light, carrier transport, recombination etc. In our proposed structure, we have optimized these parameters to enhance the efficiency to a maximum value of 35.6%. We have determined the optimum layer thickness for the designed cell by varying the layer thickness of different regions of the structure and calculating efficiency through optoelectronic simulation. We have also changed the doping of three regions and found out the doping concentration for which the efficiency is maximized. We further investigated the effect of different loss mechanisms (recombination) that occur inside the solar cell and find out the dominant loss mechanism. We have also examined the role of photon recycling effect on the performance of designed solar cell. Finally, we have compared our result with conventional heterojunction solar cell and shown that this novel three-terminal device yields higher efficiency. The analysis and results presented in this thesis would provide a clear guideline for designing HBT based photovoltaic devices. v | P a g e Table of Contents Declaration ……………………………………………………………………………........iii Acknowledgements …………………………………………………………………………iv Abstract………………………………………………………………………………………v Table of Content…………………………………………………………………………… vi List of Figures……………………………………………………………………………..viii List of Tables……………………………………………………………………………...…x Chapter 1 Introduction ..................................................................................................... 1 1.1 Solar Cell: History and Background ....................................................................... 2 1.1.1 First Generation Solar Cell—Wafer Based ...................................................... 5 1.1.2 Second Generation Solar Cells—Thin Film Solar Cells ................................. 5 1.1.3 Third Generation Solar Cells ........................................................................... 7 1.2 Multijunction/Tandem Solar Cell.......................................................................... 11 1.3 Solar Cell Design: Challenges .............................................................................. 13 1.4 Heterojunction Bipolar Transistor Solar Cell........................................................ 15 1.5 Objective of the Thesis .......................................................................................... 16 1.6 Overview of the Thesis ......................................................................................... 17 Chapter 2 Theory and Modeling ..................................................................................... 18 2.1 HBT Solar Cell: Basic Key Concepts ................................................................... 18 2.1.1 Operation of PN Junction Solar Cell ............................................................. 18 2.1.2 HBT Solar Cell (HBTSC) .............................................................................. 20 2.2 Numerical Modeling ............................................................................................. 28 2.2.1 Optical Simulation Model .............................................................................. 29 2.2.2 Electrical Simulation Model .......................................................................... 33 2.3 Benchmarking ....................................................................................................... 36 vi | P a g e Chapter 3 GaAs/AlGaAs HBT Solar Cell: Design and Performance Analysis ........... 38 3.1 Device Design ....................................................................................................... 38 3.1.1 Device Structure............................................................................................. 38 3.1.2 Choice of Material System............................................................................. 39 3.2 Result and Analysis ............................................................................................... 41 3.2.1 Effect of Varying Layer Thickness ................................................................ 42 3.2.2 Effect of Doping ............................................................................................ 44 3.2.3 Effect of Recombination ................................................................................ 46 3.2.4 Effect of SRH recombination: ....................................................................... 48 3.2.5 Effect of photon recycling ............................................................................. 49 3.2.6 Optimized Structure ....................................................................................... 51 3.2.7 Comparison with Conventional Heterojunction Solar Cell Configuration .... 56 3.2.8 Comparison with Other Material Systems ..................................................... 57 Chapter 4 Conclusion and Future Work ........................................................................ 59 4.1 Conclusion ............................................................................................................. 59 4.2 Future work ........................................................................................................... 61 References ........................................................................................................................... 62 vii | P a g e List of Figures Fig. 1.1 Various types of solar cell technologies and trends of development. ...................... 3 Fig. 1.2 Reported timeline of solar cell energy conversion efficiencies since 1976 [16]. ... 4 Fig. 1.3 Cost efficiency of different generation of solar cell technologies [50]. ................. 11 Fig. 1.4 A schematic of an InGaP/GaAs/Ge multijunction/tandem solar cell [51]. ........... 12 Fig. 1.5: Heterojunction bipolar structure to be used as solar cell proposed by Marti and Luque [63]. ............................................................................................................ 15 Fig. 2.1 A PN junction and the corresponding energy band diagram................................. 19 Fig. 2.2 Energy band diagram of a PN junction solar cell along with absorption of photons, electron-hole pair generation, and transportation of excited carrier [64]. ....................................................................................................................... 20 Fig. 2.3 ZnO/CDS/CIGSe/Mo heterojunction solar cell and corresponding energy band diagram. The different bandgap absorbs different energy photons ultimately giving a broadband absorption of the solar spectrum [66]. ................................... 21 Fig. 2.4 Operation of heterojunction bipolar transistor structure as solar cell.