Advanced Material Technology and Future Of III-V Multijunction Solar Cells

Silicon has been the material of choice for photovoltaic designers because it is inexpensive and relatively well understood—and, of course, because it has properties that make it appropriate for photovoltaic applications. Recent advances in photovoltaic technology, however, have made other materials in combination attractive for the design of solar cells. These new cells exceed the inherent limitations of silicon as a solar cell material. New designs layer semiconductor materials with differing band gap energies to result in higher conversion efficiencies. Although silicon can be used as one of these layers, alloys combining Group III elements with Group V elements are enticing choices because of the wide range of band gap energies they offer designers. These multijunction cells may someday achieve efficiencies in excess of 50%. NREL has developed a multijunction cell design that has achieved 34% efficiency under concentrated light. Multi-junction solar cells created from III-V semiconductor materials exhibit high efficiencies matched by no other existing photovoltaic technology. Multi-junction solar cells are composed of 3 layers of material that have different band gaps. The top layer has the largest band gap while the bottom layer has the smallest band gap. This design allows less energetic photons to pass through the upper layer(s) and be absorbed by a lower layer, which increases the overall efficiency of the solar cell. One important design consideration is that the photocurrent generated in each layer must be the same since the layers are in series. In addition, the band gaps of each layer should differ by approximately equal energies so that the spectrum of incident radiation is most effectively absorbed. Although multi-junction solar cells are very efficient, they are also very expensive. Due to their high cost, multi-junction solar cells are primarily used in systems in outer space and as collector cells where a large amount of sunlight is reflected onto the cell. The use of multi-junction solar cells made of III-V semiconductor materials appears to be restricted to limited applications while single crystalline silicon semiconductors have a wider application due to the lesser cost. This paper will focus on the present and future design, practical and theoretical efficiency, and applications of III-V multi-junction solar cells.