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

Periodic Reporting for period 2 - AMETIST (Advanced III-V Materials and Processes Enabling Ultrahigh-efficiency ( 50%) Photovoltaics)

Reporting period: 2018-07-01 to 2019-12-31

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 challenge in AMETIST by engineering advanced III-V optoelectronics materials and heterostructures for better utilization of the solar spectrum, enabling efficiencies approaching practical limits. The work is strongly motivated by the global need for renewable energy sources. To this end, AMETIST framework is 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, will be synthesized and monolithically integrated in tandem cells with up to 8-junctions (see figure 1). Nanophotonic methods for light-trapping, spectral and spatial control of solar radiation will be developed to further enhance the absorption. To ensure a high long-term impact, the project will validate the use of state-of-the-art molecular-beam-epitaxy pro-cesses (MBE) for fabrication of economically viable ultra-high efficiency solar cells.

The ultimate efficiency target is to reach a level of 55%. This would enable to generate renewable/ecological/sustainable energy at a levelized production cost below ~7 ¢/kWh, comparable or cheaper than fossil fuels. The work will also bring a new breath of developments for more efficient space photovoltaic systems. AMETIST will leverage the leading position of the applicant in topical technology areas relevant for the project (i.e. epitaxy of III-N/Bi-V alloys and key achieve-ments concerning GaInNAsSb-based tandem solar cells). Thus it renders a unique opportunity to capitalize on the group expertize and position Europe at the fore-front in the global competition for demonstrating more efficient and economically viable photovoltaic technologies.
The first half 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 reported in 2019, exhibiting 29% efficiency at one sun illumination and 40% efficiency at 200 suns intensity.

During the project we have published 25 journal articles and presented a high number of conference articles. We have also completed 2 bachelor and 3 master theses have been finalized, while 4 PhD theses are in progress.
Progress beyound the state of the art:
1. The first demonstration of highly efficient lattice matched 4-junctions (GaInP/GaAs/GaInNAsSb/GaInNAsSb and AlGaAs/ GaAs/GaInNAsSb/GaInNAsSb) solar cells.

2. Optimized epitaxy for growth of high-quality and GaInNAsSb solar cell materials with bandgaps down to 0.7 eV.

3. Development of processing steps (etching & passivation) optimized for GAInNAsSb-based solar cells with mesa architecture.

The main results expected by the end of the project:
1. Demonstration of 50% efficiency at 1000x terrestrial solar concentration;
2. Demonstration of >35% efficiency for space illumination;
3. Establish a complete chain of processes for fabrication of dilute-nitride multijunction solar cells with novel architecture (including thin-film solar cells).
Concept figure describing AMETIST solar cell approach.