Compound semiconductor solar cells are providing the highest photovoltaic conversion efficiency, yet their performance lacks far behind the theoretical potential. This is a position we have challenged in AMETIST by engineering advanced III-V optoelectronics materials and heterostructures for better utilization of the solar spectrum. To this end, AMETIST work plan was based on three vectors of excellence in: i) material science and epitaxial processes, ii) advanced solar cells exploiting nanophotonics concepts, and iii) new device fabrication technologies.
Novel heterostructures (e.g. GaInNAsSb, GaNAsBi), providing absorption in a broad spectral range from 0.7 eV to 1.4 eV, have been
developed and monolithically integrated in tandem cells with up to 4, 5 and 6 junctions. Novel methods for light-trapping and spectral control of solar radiation have been developed to further enhance the absorption. To ensure a path for practical long-term impact, the project has validated the use of state-of-the-art molecular-beam-epitaxy (MBE) processes for fabrication of economically viable high efficiency solar cells.
The project provides the unquestionable proof of concept for the essential building blocks enabling the development of realistic solar cell architectures incorporating up to six lattice-matched junctions, enabling conversion efficiency in the range of 50% under concentrated photovoltaic (CPV) conditions. The work plan and the results obtained are instrumental for the development of more efficient space photovoltaic systems, with enhanced functionality, such as superior power-to-weigth ratio and mechanical flexibility. In general, lattice-matched multijunction solar cells offer several benefits over their competing multijunction technologies, including practicality and cost-effectiveness. The designs demonstrated require significantly less material and have a simpler fabrication process compared to established metamorphic solar cell technology. This is because lattice-matched cells do not require thick metamorphic buffer layers, resulting in lower overall thickness and improved quality of the multijunction stack. Also, substrate removal is not needed, which simplifies the processing of these cells compared to established inverted metamorphic solar cell technology. Additionally, fully lattice-matched multijunction designs can use similar tunnel junctions for all interconnects between subcells, further simplifying the structure. Compared to wafer-bonded or mechanically stacked multijunction cells, the monolithic approach is simpler and cheaper as it eliminates the need for handling multiple substrates.
Lattice-matched architecture also offers benefits in terms of increased functionality, especially for space solar cells. The key building blocks, i.e. the low-bandgap GaInNASb solar cells, can be grown on Ge substrate widely used in space applications. Lattice-matched III−V cells are also well-suited for thin film processing due to minimized internal strain, allowing fabrication of flexible, lightweight multijunction structures with high power-to-weight ratios, essential for space applications.
