Indium gallium nitride (InGaN), the same semiconductor that emits LED light, also shows promise in converting solar energy more efficiently into electricity and in capturing heat energy wasted by cars, computers and manufacturing plants.
Nelson Tansu and his group are investigating both applications.
When its ratio of gallium to indium is adjusted, says Tansu, an InGaN-based semiconductor can be engineered to cover almost 95 percent of the sun’s wavelengths, potentially achieving a solar-to-electricity conversion efficiency of 40 to 50 percent. Conventional siliconbased solar cells achieve a conversion efficiency of 11 percent. The greater range of lightwave absorption results in the generation of a larger number of electron-hole pairs, increasing the potential for electricity production.
The key to InGaN’s solar potential, says Tansu, is the material’s large energy bandgap range, which extends from .7 to 3.4 electron volts. By adjusting the compound’s Ga/In ratio, engineers can fine-tune the tandem cell configuration to develop a solar cell device capable of capturing lightwaves spanning ultraviolet to infrared with minimal loss of energy.
“The ability of InGaN to cover almost the entire solar spectrum,” says Tansu, “makes it an excellent material candidate for developing nearly full-spectrum, high-efficiency solar cells.”
Tansu’s group uses Metal Organic Chemical Vapor Deposition (MOCVD) epitaxy to grow single crystals of InGaN with varying indium content on substrates of sapphire and silicon. Their goal is to assemble a tandem solar cell in which individual cells with different bandgaps are stacked in order to maximize the efficiency of solar energy conversion.
“By including several compositions of the material in the same cell,” says Tansu, “we hope to trap photons of varying energies from the solar spectrum at the optimum bandgaps of tandem cells, thus generating electrons and holes with minimum thermalization energy loss. These electrons and holes can then be transported out of devices with optimum power conversion efficiency.”
InGaN and other nitride materials enjoy a potential advantage over existing tandem cell technologies, says Tansu.
“Current tandem cell technology is very expensive because it requires three or four classes of semiconductors, which can necessitate multiple growths in different reactors. The advantage of nitride semiconductor technology is that you can grow the entire tandem cell in a single epitaxy in a single reactor.”
The thermoelectric properties of InGaN semiconductors also enable them to convert thermal energy efficiently into electricity, says Tansu.
“A lot of heat is wasted by industrial equipment, cars, refrigerators, etc. The challenge is to convert that lost heat energy to electrical energy.”
A thermoelectric material converts heat into electricity when variations in its temperature generate voltage differences. This so-called Seebeck effect triggers a flow of electrons. The challenge is to develop a thermoelectric material with a large Seebeck voltage that conducts electricity well without transferring heat.
“We’re investigating InGaN and other nitride semiconducting materials,” says Tansu. “We think they could make viable thermoelectric materials because of their high electric conductivity and poor thermal conductivity.”
The group is focusing on phonons, the discrete nanoparticles of vibrational energy that trigger the transfer of energy within a crystal lattice.
“We’re using nanotechnology to design a structure in which the electron travels rapidly through the semiconductor while the phonon’s propagation is minimized by the phononic bandgap,” says Tansu.
