Control of Optical Properties of Surfaces for Improved Solar Thermophotovoltaic Systems A Ph.D. dissertation Presented to the Faculty of the School of Engineering and Applied Science University of Virginia In Partial Fulfillment of the requirements for the Degree Doctor of Philosophy (Electrical and Computer Engineering) by Craig Ungaro - 2015 Abstract Alternative energy systems are crucial to meeting the expanding energy needs of the modern world. Unfortunately, the wide spectrum of solar radiation greatly limits the efficiency of photovoltaic systems. Solar thermophotovoltaic (STPV) technology overcomes this problem by tailoring the spectrum of incoming light to better match a photovoltaic cell. This has the potential to greatly increase the efficiency of photovoltaic systems and to help meet future energy demands in an environmentally friendly way. The main challenge in designing efficient STPV systems is controlling the optical properties of the system's light absorbing and emitting surfaces. Through the use of nanostructures, the level of optical control needed to make highly efficient STPV systems will be realized. This work presents a study of the effects of various microscale and nanoscale structures on the absorption and emission spectra of surfaces, as well as their direct application in STPV systems. Computer modeling is used to determine the absorption spectra of various nanostructured surfaces and to predict the efficiency gains from using these structures in an STPV system. The ability to economically fabricate structures for large-area systems is also discussed. A design for a full STPV system utilizing nanostructures for improved efficiency is presented, and the losses in such a system are analyzed. The system is then fabricated, and device performance is compared with simulation. A path towards future improvement of STPV systems is discussed. Additionally, a close collaboration was established with the Center for Nanophase Materials (CNM) at Argonne National Laboratory (ANL), leading to several joint research publications and access to modeling computers and fabrication facilities. Approval Sheet This Ph.D. dissertation is submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Electrical and Computer Engineering) Craig Ungaro This Ph.D. dissertation has been read and approved by the Examining Committee: Professor Mool Gupta, Adviser Professor Robert Weikle, Committee Chair Professor Joshua Choi Professor Lloyd Harriot Dr Stephen Gray, Argonne National Labs Accepted for the School of Engineering and Applied Science: Professor James H. Aylor, Dean, School of Engineering and Applied Science - 2015 ii iii Acknowledgments First, I would like to thank my advisor professor Mool C. Gupta for his encouragement, advice, and technical knowledge as I pursue my Ph.D. degree. I would also like to thank him for offering me the wonderful opportunity to be a part of the unique graduate program at NASA Langley and the National Institute of Aerospace. Secondly, Dr. Stephen Gray of Argonne National Labs was an immense help in this research. I appreciate his technical advice on FDTD modeling, as well as his general guidance on my project. I would also like to thank the NASA Langley Professor program and the NSF IUCRC programs for their support of my research project. This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility under Contract No. DE-AC02-06CH1135. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Lastly, I would like to thank my family and friends for their help and support throughout my time as a graduate student. Contents Contents iv List of Tables vi List of Figures vii List of Symbols and Abbreviationsx 1 Introduction1 1.1 Solar Thermophotovoltaic System Overview..................1 1.2 Importance of Solar Thermophotovoltaic Systems...............2 1.2.1 Absorbing Surface Figure of Merit....................4 1.2.2 Emitting Surface Figure of Merit....................6 1.3 Problem Statement................................8 1.4 Objective.....................................9 2 Literature Review 10 2.1 Photovoltaic Cells for Solar Thermophotovoltaic Systems........... 10 2.2 Absorbing Surfaces................................ 12 2.2.1 Bulk Materials.............................. 13 2.2.2 Nanostructures.............................. 14 2.2.3 Composite Materials........................... 15 2.2.4 Comparison of Absorbing Surfaces................... 16 2.3 Emitting Surfaces................................. 17 2.3.1 Bulk Materials.............................. 18 2.3.2 Nanostructures.............................. 19 2.3.3 Multi-band Emitters........................... 20 2.3.4 Comparison of Emitting Surfaces.................... 21 2.4 Combined Solar Thermophotovoltaic Systems................. 22 3 STPV System Fundamentals and Modeling 24 3.1 STPV System Model............................... 25 3.1.1 The Ray Tracing Method........................ 29 3.2 Surface Emissivity................................ 31 3.2.1 The Transfer Matrix Method (TMM).................. 31 3.2.2 The Finite Difference Time Domain (FDTD) Method......... 33 3.3 Conclusion..................................... 40 4 Modeled Solar Thermophotovoltaic System Design Results 42 4.1 Ideal Solar Thermophotovoltaic System..................... 42 4.1.1 Practical Considerations......................... 44 4.2 Practical System Design............................. 46 iv Contents v 4.2.1 Solar Concentrators............................ 46 4.2.2 System Temperature........................... 47 4.2.3 Emitter to Absorber Area Ratio..................... 47 4.2.4 Absorbing Surfaces............................ 49 4.2.5 Emitting Surfaces............................. 54 4.2.6 Photovoltaic Cells............................. 58 4.2.7 Complete System............................. 59 5 Experimental 65 5.1 Photovoltaic Cells................................. 65 5.2 Absorbing Surface................................. 66 5.2.1 Pseudo-random Nanocone Arrays.................... 66 5.2.2 Glancing Angle Deposition........................ 68 5.3 Emitting Surface................................. 70 5.3.1 Periodic Nanohole Arrays........................ 70 5.3.2 Dielectric/Metal Stacks.......................... 71 5.4 Solar Concentration and Simulation....................... 72 5.4.1 Solar Concentrator System........................ 72 5.4.2 Solar Simulation............................. 74 5.5 STPV System................................... 75 5.5.1 Emitter/Absorber Area Ratio (Aratio).................. 76 5.5.2 Temperature Measurements....................... 77 6 System Characterization and Results 79 6.1 High-efficiency System.............................. 79 6.1.1 High Efficiency System Improvements.................. 83 6.2 Solar System................................... 85 6.2.1 Solar System Improvements....................... 85 7 Conclusions and Future Work 87 7.1 Conclusions.................................... 87 7.2 Future Work.................................... 89 Bibliography 91 Appendices 103 A List of Publications 103 B Published Works 104 List of Tables 2.1 Efficiency of selected absorbing surfaces..................... 16 2.2 Efficiency of selected emitting surfaces...................... 22 4.1 Ebg,Voc, FF, and EQE of PV cells....................... 59 4.2 List of losses in a simulated STPV system.................... 61 4.3 List of default values for system simulations, and the sensitivity of these values. 62 4.4 Optimized efficiency of different system types.................. 62 2 5.1 Measured Voc,Isc, FF, and Pmax of 1.48 cm GaSb PV cells purchased from JX Crystals..................................... 65 5.2 Average reflectivity of microtextured tungsten substrates at 405 nm, 532 nm, 633 nm, 790 nm, 980 nm, and 1064 nm wavelengths............... 67 6.1 Simulated sources of PV cell heating....................... 81 6.2 List of values for experimental system...................... 82 6.3 List of losses in a simulation of the experimental STPV system........ 82 vi List of Figures 1.1 Diagram of flat and cylindrical STPV systems..................2 1.2 Ideal absorption spectra for an absorbing surface................4 1.3 Ideal spectra for an emitting surface for an STPV system...........7 2.1 The optimal temperature for an STPV system at different PV cell bandgaps. 11 3.1 The flow of power through an STPV system................... 26 3.2 The path of an emitted ray through a simple STPV system.......... 30 3.3 A nano-cone array is broken down into a sum of layers of varying eff ..... 32 3.4 Components of the Drude plus four Lorentzian operators plus r for tungsten. 35 3.5 A comparison between simulation and data found in literature for the ab- sorbance of flat tungsten.............................. 36 3.6 FDTD simulation setup.............................. 37 3.7 Change in integrated reflectance due to simulation time............ 39 3.8 Surface plot of nano-cones with randomized placement, height, and radius.. 40 4.1 The maximum efficiency of an ideal STPV system at various temperatures.. 44 4.2 Graphs of emitter-absorber area ratio and system efficiency vs. emitter band- width........................................ 45 4.3 Relative efficiency vs. Aratio for planar STPV system.............