A lattice-matched, triple-junction solar cell proposed by an international team of scientists has the potential to break the 50 percent conversion efficiency mark, a goal in multijunction photovoltaic development. Produced by scientists in the Electronics Technology and Science Div. of the US Naval Research Laboratory (NRL), in collaboration with researchers at Imperial College London and MicroLink Devices Inc. of Niles, Ill., the solar cell could achieve this efficiency with novel semiconductor materials and with band structure engineering via strain-balanced quantum walls. Multijunction solar cells are those in which each junction is tuned to different solar spectrum wavelength bands to enhance efficiency. High bandgap semiconductor material is used to absorb the short-wavelength radiation, with longer-wavelength parts transmitted to subsequent semiconductors. A schematic diagram of a multijunction solar cell formed from materials lattice-matched to InP and achieving the bandgaps for maximum efficiency. Courtesy of the US Naval Research Laboratory. Theoretically, an infinite-junction cell could achieve maximum power conversion percentage of almost 87 percent. The challenge now for scientists is to develop a semiconductor material system that can attain a wide range of bandgaps and that could be grown with high crystalline quality. The NRL design can achieve direct bandgaps from 0.7 to 1.8 eV with materials that are all lattice-matched to an indium phosphide (InP) substrate. “Having all lattice-matched materials with this wide range of bandgaps is the key to breaking the current world record,” Walters said. “It is well-known that materials lattice-matched to indium phosphide can achieve bandgaps of about 1.4 eV and below, but no ternary alloy semiconductors exist with a higher direct bandgap.” The key innovation enabling this new road to high efficiency is the identification of InAlAsSb quarternary alloys as a high bandgap material layer that can be grown lattice-matched to InP. Drawing from the experience with Sb-based compounds for detector and laser applications, the NRL team modeled the band structure of. The NRL scientists, working with MicroLink Devices and Rochester Institute of Technology in New York, will now execute a three-year materials and device development program to realize this photovoltaic technology under a US Department of Energy Advanced Research Projects Agency-Energy project. For more information, visit: www.nrl.navy.mil
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