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Final Report Summary - METACELLS (Advanced epitaxy of metamorphic semiconductor structures for multijunction solar cells)

The global objective of METACELLS project is “to develop the epitaxial technique for the growth of metamorphic semiconductor structures that allow realizing the full efficiency potential of multijunction solar cells, paying special attention to the subsequent manufacturability and cost effectiveness of the solar cells developed”. During the first period (outgoing phase) of METACELLS project, the work towards this objective has led to outstanding results, which have advanced the technique and have enabled the attainment of world record efficiencies of photovoltaic conversion with quadruple-junction solar cells.

Lattice-mismatched subcells for 3-junction and 4-junction solar cells have been developed. The focus was put on developing the epitaxy process for the 1eV Ga0.76In0.24As metamorphic junction, which has a large lattice mismatch of ~2% with respect to the GaAs substrate. In summary, high quality ordered GaInP-based CGBs for Ga0.75In0.25As grown on GaAs substrates have been achieved, with threading dislocation densities < 5·105 cm-2. This has enabled the implementation of high quality 1eV Ga0.76In0.24As metamorphic subcells with carrier collection efficiencies nearing 100%, internal luminescence efficiencies of ~ 90% and Eg/q-Voc < 0.35 V (see Figure 1). This result debunks the traditional belief that metamorphic semiconductor structures have necessarily poorer electronic characteristics than latticematched structures, and opens the way to further developments of metamorphic structures for solar cells or any other semiconductor device.

Both triple and quadruplejunction metamorphic solar cells structures have been implemented successfully by tackling challenges such as the development of a high performance Ga0.76In0.24As/GaAs0.75Sb0.25 metamorphic tunnel junction and studying and minimizing the influence of the metamorphic growth on these complex solar cell structures. This work has enabled the implementation of both triple and quadruple-junction metamorphic solar cells structures, achieving world record efficiencies for photovoltaic conversion. The potential for future advancement in the performance of these devices has been analyzed, concluding that the metamorphic multijunction solar cell technology will allow surpassing the 50% photovoltaic conversion efficiency for concentrator terrestrial applications in the medium term. Moreover, metamorphic structures offer also the benefit of an easily tunable bandgap combination to maximize the energy output of concentrator or 1-sun systems at sites with different spectral variations due to different climatic conditions. All in all, this technology will contribute decisively towards the goal of achieving a cost-competitive clean energy source.

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