Thermal Physics in Electronic and Optoelectronic Materials and Devices by Abhishek Yadav A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Mechanical Engineering) in The University of Michigan 2010 Dissertation Committee: Assistant Professor Kevin P. Pipe, Co-Chair Assistant Professor Max Shtein, Co-Chair Associate Professor Rachel S. Goldman Associate Professor Katsuo Kurabayashi © Abhishek Yadav 2010 All Rights Reserved Acknowledgements First and foremost, I would like to thank my advisors Prof. Kevin P. Pipe and Prof. Max Shtein. Their advice, motivation, and, ideas have been instrumental for this work. Prof. Kevin Pipe was a continuous source of new and innovative ideas. His objectivity and persistence were always helpful when facing challenging research problems. Prof. Max Shtein helped me through infinite discussions we had during our group meetings. His knowledge in organic semiconductors was helpful in bringing my research to quick fruition. His passion towards research and commitment for excellence always inspired me. I am also grateful to my committee members Prof. Rachel S. Goldman and Prof. Katsuo Kurabayashi. Prof. Rachel Goldman provided many critical comments for work on quantum dot superlattices, which led to a better understanding of underlying issues. Being a GSI with Prof. Kurabayashi was helpful in understanding the importance of teaching in academia. This work would not have been possible without help from many colleagues and collaborators including: Paddy Chan, Kwang Hyup An, Huarui Sun, Brendan O’Connor, Yiying Zhao, Steven E. Morris, Yansha Jin, Gunho Kim, Andrea Bianchini, Denis Nothern, Jinjjing Li and Chelsea Haughn. Paddy Chan and Huarui Sun sat next to my desk for most of my stay here, and, shared many meals and ideas about science. Paddy ii Chan introduced me to the field of thermoreflectance, which was the starting point of my work on temperature modulation spectroscopy. Huarui Sun discussed intricate details of my research, and provided many useful ideas. Kwang Hyup An trained me on thermal evaporation system and many other experimental setups. Yansha Jin trained with me on TEM and shared many useful ideas about electron microscopy. Steve Morris helped me with thermal evaporation and purification of organic material. I am also grateful to many friends who made my stay at Ann Arbor fun including: Deepak Kumar, Krispian Lawrence, Kulwinder Singh and Manvendu Bharadwaj. Deepak’s advice on all matters of life was helpful. Finally, this work would not have been possible without the support of my parents Satyanarain and Nisha Yadav and my sister Sugandha Yadav, who have shared my joys throughout past 5+ years. iii Table of Contents Acknowledgements……………………………………………………………………… ii List of Figures…………………………………………………………………………….viii List of Tables………………………………………………………………………….. xv Chapter 1 1 Introduction……………………………………………………………………………….. 1 1.1 Thermal effects on electronic and transport properties of semiconductors ......... 1 1.1.1 How does temperature affect the electronic bandgap? ................................. 1 1.1.2 How does temperature affect the Fermi distribution? ................................... 2 1.1.3 How does temperature affect the scattering of free carriers? ....................... 4 1.1.4 How does temperature affect scattering of phonons? ................................... 5 1.2 Overview of work ................................................................................................. 7 Chapter 2 10 Thermoelectric energy conversion……………………………………………………… 10 2.1 Thermoelectric transport .................................................................................... 10 2.2 Thermoelectric devices ...................................................................................... 11 2.2.1 Thermoelectric power generator ................................................................. 11 where Z is the dimensionless thermoelectric figure of merit defined by: 13 2.2.2 Thermoelectric cooler ................................................................................. 14 iv 2.3 Status of thermoelectric materials ...................................................................... 16 Chapter 3 18 Thermoelectric properties of aligned quantum dot chains……………………………… 18 3.1 Theory of thermoelectric transport ..................................................................... 18 3.2 Nanostructured materials for high efficiency thermoelectric devices ................ 22 3.3 Theory and Calculation Method ......................................................................... 26 3.4 Single Quantum Dot Chains: Size Effects ......................................................... 29 3.5 QD Chain Nanocomposite ................................................................................. 31 3.6 Comparison of QD chains with 3D ordered QD nanocomposites ..................... 36 3.7 Summary and Conclusions ................................................................................. 40 Chapter 4 42 Integrated Thermoelectric Coolers for Mercury Cadmium Telluride Based Infrared Detectors………………………………………………………………………………… 42 4.1 Introduction ........................................................................................................ 42 4.2 Cross-plane Seebeck coefficient and thermal conductivity of small-barrier Hg1- xCdxTe superlattices ...................................................................................................... 43 4.2.1 Measurement of Seebeck coefficient and thermal conductivity of thin films 44 4.2.2 Results and discussion ................................................................................ 49 4.3 Design of integrated thermoelectric cooler ........................................................ 51 4.3.1 Design considerations ................................................................................. 51 4.3.2 Coupled electrothermal modeling ............................................................... 52 4.3.3 Results and discussion ................................................................................ 54 4.4 Summary and conclusions .................................................................................. 56 v Chapter 5 58 Fiber-based flexible thermoelectric power generator………………………………….. 58 5.1 Introduction ........................................................................................................ 58 5.2 Materials and processes for flexible thermoelectric devices .............................. 60 5.3 Single-fiber thermoelectric power generators .................................................... 63 5.3.1 Fabrication method and testing procedure .................................................. 63 5.3.2 Measurement of thermoelectric power generation characteristics .............. 64 5.3.3 Optimization of material parameters for maximizing power generation .... 67 5.4 Multi-fiber thermoelectric power generators: Plain weave geometry ............... 72 5.4.1 Fabrication method and testing procedure .................................................. 73 5.4.2 Results and discussion ................................................................................ 76 5.5 Summary ............................................................................................................ 80 Chapter 6 82 Organic photovoltaics 82 6.1.1 Materials and device structures ................................................................... 83 6.1.2 Working principle of organics photovoltaics .............................................. 85 6.2 Summary ............................................................................................................ 89 Chapter 7 90 Temperature Modulation Spectroscopy for Studying Excitonic Properties of Organic Semiconductors…………………………………………………………………………..90 7.1 Introduction ........................................................................................................ 90 7.2 Determining the nature of excitonic transitions ................................................. 94 7.2.1 Electric field modulation spectroscopy ....................................................... 95 7.2.2 Piezomodulation spectroscopy .................................................................... 97 vi 7.3 Temperature modulation spectroscopy .............................................................. 98 7.3.1 Theory ......................................................................................................... 98 7.3.2 Experimental setup.................................................................................... 100 7.4 Results and Discussion ..................................................................................... 104 7.4.1 Morphology of CuPc films ....................................................................... 104 7.4.2 Optical spectrum and data fitting .............................................................. 107 7.4.3 Temperature modulation spectrum ........................................................... 110 7.5 Summary and Conclusions ............................................................................... 118 Chapter 8 120 Conclusions……………………………………………………………………………. 120 8.1 Summary of present work ................................................................................ 120 8.2 Suggestions for future work ............................................................................. 122 8.2.1 Effect of band bending on thermoelectric power factor