The Physics of Solar Energy Conversion

Research on advanced energy conversion devices such as solar cells has intensified in the last two decades. A broad landscape of candidate materials and devices were discovered and systematically studied for effective solar energy conversion and utilization. New concepts have emerged forming a rather powerful picture embracing the mechanisms and limitation to efficiencies of different types of devices. The Physics of Solar Energy Conversion introduces the main physico-chemical principles that govern the operation of energy devices for energy conversion and storage, with a detailed view of the principles of solar energy conversion using advanced materials. Key Features include: Highlights recent rapid advances with the discovery of perovskite solar cells and their development. Analyzes the properties of organic solar cells, lithium ion batteries, light emitting diodes and the semiconductor materials for hydrogen production by water splitting. Embraces concepts from nanostructured and highly disordered materials to lead halide perovskite solar cells Takes a broad perspective and comprehensively addresses the fundamentals so that the reader can apply these and assess future developments and technologies in the field. Introduces basic techniques and methods for understanding the materials and interfaces that compose operative energy devices such as solar cells and solar fuel converters.

Contents Preface........................................................................................................................................................................xv Acknowledgments....................................................................................................................................................xvii Author.......................................................................................................................................................................xix Chapter 1 Introduction to Energy Devices...............................................................................................................1 References...............................................................................................................................................9 PART I Equilibrium Concepts and Kinetics Chapter 2 Electrostatic and Thermodynamic Potentials of Electrons in Materials...............................................13 2.1 Electrostatic Potential..................................................................................................................13 2.2 Energies of Free Electrons and Holes.......................................................................................... 14 2.3 Potential Energy of the Electrons in the Semiconductor............................................................. 17 2.4 The Vacuum Level....................................................................................................................... 17 2.5 The Fermi Level and the Work Function....................................................................................20 2.6 The Chemical Potential of Electrons........................................................................................... 21 2.7 Potential Step of a Dipole Layer or a Double Layer....................................................................23 2.8 Origin of Surface Dipoles............................................................................................................24 2.9 The Volta Potential......................................................................................................................25 2.10 Equalization of Fermi Levels of Two Electronic Conductors in Contact....................................27 2.11 Equilibration of Metal Junctions and the Contact Potential Difference......................................28 2.12 Equilibrium across the Semiconductor Junction.........................................................................29 General References............................................................................................................................... 31 References............................................................................................................................................. 31 Chapter 3 Voltage, Capacitors, and Batteries.........................................................................................................33 3.1 The Voltage in the Device...........................................................................................................33 3.2 Anode and Cathode.....................................................................................................................34 3.3 Applied Voltage and Potential Difference...................................................................................35 3.4 The Capacitor..............................................................................................................................37 3.5 Measurement of the Capacitance.................................................................................................38 3.6 Energy Storage in the Capacitor..................................................................................................40 3.7 Electrochemical Systems: Structure of the Metal/Solution Interface..........................................40 3.8 Electrode Potential and Reference Electrodes.............................................................................42 3.9 Redox Potential in Electrochemical Cells...................................................................................44 3.10 Electrochemical and Physical Scales of Electron Energy in Material Systems..........................45 3.11 Changes of Electrolyte Levels with pH.......................................................................................46 3.12 Principles of Electrochemical Batteries.......................................................................................47 3.13 Capacity and Energy Content......................................................................................................50 3.14 Practical Electrochemical Batteries............................................................................................. 51 3.14.1 Zinc-Silver Battery.......................................................................................................... 51 3.14.2 Sodium-Sulfur Battery....................................................................................................52 3.15 Li-Ion Battery.............................................................................................................................. 53 General References...............................................................................................................................57 References.............................................................................................................................................57 Chapter 4 Work Functions and Injection Barriers.................................................................................................59 4.1 Injection to Vacuum in Thermionic Emission.............................................................................59 4.2 Richardson–Dushman Equation..................................................................................................60 4.3 Kelvin Probe Method.................................................................................................................. 61 4.4 Photoelectron Emission Spectroscopy.........................................................................................63 4.5 Injection Barriers.........................................................................................................................66 4.6 Pinning of the Fermi Level and Charge-Neutrality Level...........................................................69 General References...............................................................................................................................73 References.............................................................................................................................................73 Chapter 5 Thermal Distribution of Electrons, Holes, and Ions in Solids............................................................... 75 5.1 Equilibration of the Electrochemical Potential of Electrons....................................................... 75 5.2 Configurational Entropy of Weakly Interacting Particles...........................................................76 5.3 Equilibrium Occupancy of Conduction Band and Valence Band States.....................................76 5.4 Equilibrium Fermi Level and the Carrier Number in Semiconductors.......................................79 5.5 Transparent Conducting Oxides.................................................................................................. 81 5.6 Hot Electrons...............................................................................................................................82 5.7 Screening.....................................................................................................................................84 5.8 The Rectifier at Forward and Reverse Voltage............................................................................85 5

Juan Bisquert is a professor of applied physics at the Universitat Jaume I de Castello and the funding director of the Institute of Advanced Materials at UJI. He earned an MSc in physics in 1985 and a PhD from the Universitat de Valencia in 1992. The research work is in perovskite solar cells, semiconductor optoelectronics, mixed ionicelectronic conductors, and solar fuel converters based on visible light and semiconductors for water splitting and CO2 reduction. His most well-known work is about the mechanisms governing the operation of nanostructured and solution-processed thin film solar cells. He has developed insights in the electronic processes in hybrid organic–inorganic solar cells, combining the novel theory of semiconductor nanostructures, photoelectrochemistry, and systematic experimental demonstration. His contributions produced a broad range of concepts and characterization methods to analyze the operation of photovoltaic and optoelectronic devices. He is a senior editor of the Journal of Physical Chemistry Letters. He has been distinguished several times in the list of ISI Highly Cited Researchers. Bisquert created nanoGe Conferences and is the president of the Fundacio Scito. He wrote a novel of speculative fiction, The Canamel Conjecture.