IMEC, the Leuven, Belgium-based independent research center, has created a single-junction GaAs solar cell on a Ge substrate with a 24.7% conversion efficiency, which the organization said is a record figure. The efficiency was measured and confirmed by the U.S. based National Renewable Energy Laboratory. IMEC hopes to reach an efficiency of 35% or more for this new technology.
The cell, grown epitaxially on the Ge substrate, employs an improved micro-defect distribution. Measuring 0.25 cm2, the cell has an open-circuit voltage of 999 mV, a short-circuit current of 29.7 mA/cm2, and a fill factor of 83.2%. Umicore, a leading materials technology group, produced the Ge substrate using an optimized manufacturing technology aimed at improving the intrinsic germanium crystal quality. The cell was fabricated as part of the ESA-IMAGER project.
Boosting the cell’s efficiency is a step in the development of a hybrid monolithic/mechanically stacked triple-junction solar cell. This type of cell consists of stacks of solar cells made of different semiconductors that are carefully chosen to absorb the solar spectrum as efficiently as possible. Many combinations of materials are available, but IMEC is focusing on stacked cells using III-V materials for the top cells and Ge for the bottom cells.