Backside Contact Multijunction Solar Cells for Concentrator Applications
The cost of electricity generated by concentrator photovoltaic systems is strongly dependent on the efficiency of the solar cells in the solar concentrator system. The efficiency of these solar cells is limited by the fact that the electrical contacts used to collect current from the solar cell are on top of the cell and partially shadow it. The top-surface electrodes prevent as much as 10% of the collected sunlight from reaching the active area of the solar cell. The problem is further exacerbated as the concentration level is increased. This problem is being addressed by developing a novel method for producing solar cells in which all electrical contacts are made from the backside of the solar cell. The backside contact is combined with an epitaxial liftoff process to facilitate the formation of the contacts and to reduce the manufacturing cost. The cost advantage is realized by multiple reuses of the expensive GaAs substrates, which significantly decreases the raw material consumption. In Phase I, a complete fabrication process was developed for producing backside contact interconnects in epitaxial liftoff-based solar cells. Detailed device simulations of the backside contact solar cell device show excellent performance under high concentration and a relative performance improvement of more than 10% at 500 suns compared with a conventional topside contact solar cell. In the Phase II project, the backside contact fabrication process will be extended to fabricate high-efficiency, triple-junction, inverted-multijunction-metamorphic epitaxial liftoff devices that are optimized for operation at high concentration. A process will be developed for attaching backside contact solar cells to submounts and these submounts will be tested in a solar concentrator test bed. The goal is to achieve a peak efficiency of >44% at 500 suns. The backside contact solar cells will also be subjected to reliability testing. Commercial Applications and Other Benefits: The high efficiency cells to be developed in this project will be used in concentrator photovoltaic systems for utility and commercial electricity generation. The cells should result in a decrease in the cost of solar-generated electricity. Concentrator photovoltaics are projected to account for several gigawatts of generating capacity by 2020. Increased availability of low-cost solar power generation should result in reduced emission of greenhouse gases, increased domestic energy production, and increased national energy security.
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