CORDIS - EU research results

Advanced III-V Materials and Processes Enabling Ultrahigh-efficiency ( 50%) Photovoltaics

Periodic Reporting for period 4 - AMETIST (Advanced III-V Materials and Processes Enabling Ultrahigh-efficiency ( 50%) Photovoltaics)

Reporting period: 2021-07-01 to 2022-12-31

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.
important part of the project was focused on demonstrating the basic building blocks enabling realization of lattice-matched solar cells designs with more than 4 junctions, primarily focusing on MBE processes for low-bandgap GaInNAsSb junctions. The most important technological advance is the synthesis of GaInNAsSb compounds with high N compositions (>6%) and a band-gap as low as 0.7 eV. This is instrumental for demonstrating solar cells with more than 4-junctions. This advance has enabled demonstration of state-of-the-art lattice-matched four-junction (4J) solar cell exhibiting more than 30% efficiency at one sun illumination and projected effeiciency in the range of 46% efficiency under intense illumination conditions.

The most important achievements are:
1) Demonstration of high quality 0.7 eV GaInNAsSb materials incorporating high GaInNAsSb content. This enabled the demonstration of 6 junction solar cells and proved the feasibility to achieve 50% efficiency target.
2) Demonstration of high-performance lattice matched 4J solar cells incorporating two GaInNAsSb sub-cells ; the performance has been proved at 1000 sun CPV illumination condition, albeit the fact that efficiency was limited to about 40% owing to engineering optimization related to fine-tunning of current matching condition, optimized antireflection coatings, optimal grid design, and thermal management.
3) Demonstration of a new method for low-cost fabrication of broadband nanostructured antireflection coatings using scalable and photolithography free process.
4) Demonstration of a novel method for epitaxial liftoff that utilizes a combination of sacrificial and stressor layers for improved controllability.

The results obtaine have provided the basis of an outstanding number of educational and scientific output, largely disseminated as open access reports. This included more than 20 MSc and BSc theses, 5 PhD thesis (out of which two are in completion phase), over 30 scientific journals (several to be published during 2023 based on AMETIST samples), and more than 30 confernce presentations including several invited talks.
The demonstration of high quality 0.7 eV GaInNAsSb materials incorporating high N content, that paved the way for the proof-of-concept demonstration of 6 junction solar cells and predicted efficiency level of 50%, represent the state-of-the art in the field of lattice-matched GaAs/Ge-based solar cells. The experimental validation was based on GaInP/AlGaAs/GaAs/GaInNAsSb/GaInNAsSb/GaInNAsSb multijunction architectures. Simulations outlined that in order to reach 50% efficiency target it is required to use high concentration and more than 2.0 eV top AlGaInP junction. The first 5J and 6J demonstrations were operating close to design points yet the transition to full-scale devices incorporating AlGaInP top junctions could not be completed during the project owing to accumulated delays.

Not only that such cells hold the promise of reaching the highest efficiency ever demonstrated, but the lattice matched configuration enables transition to thin-film architectures with enhanced flexibility. To this end, the innovative lift-off process that was developed based on the incorporation of internal stressor as well as the simple nanostructring method based on alumina and water treatment, have been part of unexpected developments that define the state-of-the-art methodology.
Concept figure describing AMETIST solar cell approach.