Advanced Semiconductor Materials for Breakthrough Photovoltaic Applications

The project addressed the need for improved multijunction solar cells as identified within the Solar America Initiative program. The basic Ge/InGaAs/InGaP triple-junction structure that has led to record commercial efficiencies remains unoptimized due to excess current in the germanium component. Furthermore, its deployment cannot be scaled up to terawatt-level applications due to bottlenecks related to germanium’s cost and abundance. The purpose of the program was to explore new strategies developed at Arizona State University to deposit germanium films on much cheaper silicon substrates, largely eliminating the germanium bottleneck, and at the same time to develop new materials that should lead to an improvement in multijunction efficiencies. This included the ternary alloy SiGeSn, which can be inserted as a fourth junction in a Ge/SiGeSn/InGaAs/InGaP structure to compensate for the excess current in the bottom cell. Moreover, the possibility of depositing materials containing Sn on Si substrates created an opportunity for replacing the bottom Ge cell with a GeSn alloy, which, combined with new III-V alloys for the top cells, should enable 4-junction structures with perfectly optimized band gaps. The successes of the program, to be described below, has led to the developments of new strategies for the growth of high-quality germanium films on Si substrates and to a widespread recognition that SiGeSn is likely to play a significant role in future generations of high-efficiency devices, as demonstrated by new research and intellectual property efforts by major US industrial players