West Chester University
A better understanding of the physics of the solid state is essential to more developing more sustainable energy sources and better use of energy once it is generated.Two examples of this are understanding correlated electron systems and relaxation dynamics of charge carriers in semiconductors. An example of a strongly correlated electron system is a superconductor. Or even more dramatically, high temperature superconductors. Semiconductors are at the heart of every computer chip. However, they are also the materials from which solar cells and solid-state lighting are made.
Superconductors are materials that conduct electricity without resistance. In normal wires, resistance leads to loss of a certain fraction of the electrical energy as heat. Currently, about 10% of electricity transmitted through our long-distance electrical grid is lost this way. This represents an enormous amount of energy. Effectively, over 100 power plants have been built just to heat the wires of the electrical grid or stated another way, the equivalent of the generation capacity of over 100 power plants is wasted due to resistance losses.
Superconducting cables could dramatically reduce this loss. Superconducting devices will also be one component in the smart grid.The smart grid will possess the following characteristics and performance features:
A smart grid will be able to integrate distributed sources of power from conventional power plants as well as solar, wind and other sources.
Implementing the smart grid will require advances in physics as well as electrical engineering. For instance, we still have no basic theory to explain how high temperature superconductors work.
When a photon of sufficient energy is absorbed in a semiconductor, it can excite an electron from the valence band to the conduction band. A hole (the absence of an electron, which can be thought of as a positive quasiparticle) is left behind in the valence band. How the electron and hole relax is very important for energy applications.
If the excited electron and hole can be separated and moved out of the semiconductor, electrical current is generated. This is the basis of a solar cell, also called a photovoltaic cell. Making solar cells that more efficiently convert light from the sun into electricity is an extremely active area of research. Take a look at our page on Energy Frontiers for more information on solar energy. Here we'll just mention that increases solar cell efficiency is impacted by several physical phenomena. First, the light has to be absorbed not reflected. Making better antireflection layers can significantly increase efficiency but it has to be done in an inexpensive way. Therefore, we need to understand better how light interactes with a surface and enters a material. A perfect anti-reflection coating would have zero reflectivity for all wavelengths of interest for all angles of incidence.
Sometimes, we want the electron and hole to find each other again. Radiative recombination occurs when and electron falls out of the conduction band and fills a hole in the valence band with the simultaneous emission of light. This is what happens in the semiconductor lasers that are used in bar code scanners. It can also be harnessed to create solid state lighting devices. Solid state lighting can be as much as 70% efficient. Compare that to the 5% efficiency of a regular incandescent light bulb. Properly constructed, they could also have a lifetime that would be longer than that of most houses. The potential energy savings of switching all lighting to solid state lighting are enormous. Currently about 22% of all electricity is used for lighting. Were all conventional bulbs replaced by solid state lighting, this would drop to 2%.
Getting the most out of light emitting semiconductor devices - not only those in solid state lighting but also in optical communications networks - will required bettter understanding of other advanced topics such as photonic crystals. In general, a better understanding of matter on the nanoscale (and even smalller) and with ultrafast time resolution is a goal of physics. As such, great advances in pulsed laser technology, microscopy and spectroscopy still await more fundemental understanding of physics.