Another step has been taken towards making solar power a viable source of electricity in our future. North Carolina State University researchers have created a new technique for improving the overall efficiency of solar panels and solar accumulators. As documented in an article published September 5, 2013 in Applied Physics Letters, entitled, “Effect of GaAs interfacial layer on the performance of high bandgap tunnel junctions for multijunction solar cells,”, Joshua Samberg, Zachary Carlin, Geoff Bradshaw and Jeff Harmon and J.P. Allen all graduate students at North Carolina State University and Dr. Peter Colter, a research assistant professor of electrical engineering, and Dr. John Hauser, an emeritus professor of electrical engineering, have discovered that by inserting a very thin film of gallium arsenide into the connecting junction of stacked solar cells they can eliminate voltage loss without blocking any of the solar energy opening up the potential for vastly more efficient solar cells.
Back in 2002 it was discovered that it might be possible to create a photovoltaic cell sensitive to the full solar spectrum by stacking multiple negatively and positively doped layers to form several current-producing junctions. Multijunction devices or stacked solar cells use a high-bandgap top cell to absorb high-energy photons while allowing the lower-energy photons to pass through. A material with a slightly lower bandgap is then placed below the high-bandgap junction to absorb photons with slightly less energy (longer wavelengths).
The maximum theoretical efficiency that a single-bandgap solar cell can achieve with non-concentrated sunlight is about 33.5%, primarily because of the broad distribution of solar emitted photons. This limiting efficiency, known as the Shockley-Queisser limit, in part arises from the fact that the open-circuit voltage of a solar cell is limited by the bandgap of the absorbing material. Photons that have energies greater than the bandgap are absorbed and the excess energy is lost as heat or not converted if the photon is not perfectly matched to the bandgap energy of the absorbing semiconductor.
Multijunction or stacked devices use a high-bandgap top cell to absorb high-energy photons while allowing the lower-energy photons to pass through. A material with a slightly lower bandgap is then placed below the high-bandgap junction to absorb photons with slightly less energy (longer wavelengths). Typical multijunction cells use two or three absorbing layers, but the pattern of decreasing bandgaps could, in principle, be repeated to create many junctions. The theoretical maximum efficiency increases with the number of junctions, but the junctions loose energy limiting the practical application to three or so layers before the loss of energy becomes too large.
Three-junction devices using elements from the III and V columns of the Periodic table, such as gallium and germanium based semiconductors have reached efficiencies of greater than 44% using concentrated sunlight at 947 suns. This record was verified by the National Renewable Energy Laboratory, NREL. Now, the researchers at North Carolina State University we have created a connecting junction that loses almost no voltage, even when the stacked solar cell is exposed to 70,000 suns of solar energy.
The research at North Carolina State University was underwritten by the U.S. Department of Energy - Energy Efficiency and Renewable Energy SunShot Initiative which invests in multijunction (stacked) solar cell research and solar concentrating lens methods to be used with multijunction solar cells to achieve greater efficiency of solar cells and someday reduce the cost of solar generated power. The goal of the programs is to make solar power cost effective by 2020. The efficiency of the junction, not losing voltage when exposed to the power 70,000 suns of solar energy should be more than sufficient for practical purposes, since concentrating lens research currently underway indicates that they are unlikely to be able to create more than 4,000 or 5,000 suns worth of energy. This discovery at North Carolina State University means that solar cell manufacturers can now create multijunction stacked cells that can handle these high-intensity solar energies without losing voltage at the connecting junctions, thus increasing the layers and potentially improving conversion efficiency. However, the usefulness of this discovery will depend on cost.
In the past, stacked solar cells have primarily been used in space, where there is a premium placed on lightweight power generation, which allows for the use of this relatively high-cost solar technology. For terrestrial electrical generation, the high costs of these semiconductors compared to silicon semiconductor can be offset by using concentrating lenses to increase the power they are exposed to from one sun (no lens) to 4,000-5,000 suns or more. Increasing the amount of light incident on the solar cell, leads to more power production for the multijunction devices. Using concentrating lenses requires also using sun-tracking equipment to optimize the utilization time of the expensive cells, which must be factored into the cost of the system. Due to the land/area requirements of tracking systems and concentrating systems, the cost of the multijunction cells themselves, using multijunction solar devices will remain limited to large commercial or utility applications and are unlikely to be cost effective for a consumer application in my lifetime.
Nonetheless, the North Carolina State University finding is important because the two lines of research being supported by the Department of Energy and the National Science Foundation as part to their SunShot Initiative program is to utilize lenses to concentrate solar energy, to 4,000-5,000 suns and have efficient multijunction stacked cells that can efficiently utilize the concentrated solar power. Existing multijunction solar cells begin loosing voltage if the solar energy is concentrated above 700-1,000 suns, and the more intense the solar energy, the more voltage those junctions lose – thereby reducing the conversion efficiency. Utilizing the North Carolina State University discover of inserting a very thin film of gallium arsenide into the connecting junction of stacked solar cells they can eliminate voltage loss without blocking any of the solar energy and potentially achieve efficiencies beyond the current 43.3% for a multijunction architecture with the gallium arsenide in the connecting junction.