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Concentrating Photovoltaic modules using advanced technologies and cells for highest efficiencies

Periodic Reporting for period 3 - CPVMatch (Concentrating Photovoltaic modules using advanced technologies and cells for highest efficiencies)

Okres sprawozdawczy: 2018-01-01 do 2018-10-31

The project CPVMatch has been bringing the practical performance of high concentrating photovoltaics (HCPV) closer to theoretical limits.

It has been proven that the only path to close the gap between theoretical and practical ultra-high efficiency solar cells is the monolithic multi-junction (MJ) approach. There, each sub solar cell converts a specific part of the sun´s spectrum and thus manages the photons properly. Large area application of MJ cells is - thus solar cell area is reduced by using optical concentrators. This leads to low cost on system level and eventually to low levelised electricity costs.

The project started in May 2015 and ended in October 2018 with an EC contribution of 4.95 M€. The consortium consisted of four research institutions (Fraunhofer ISE, RSE, CEA, Tecnalia), one University (UPM), two industry partners (AZUR Space Solar Power, AIXTRON) and two SMEs (ASSE, Cycleco) and was coordinated by Fraunhofer ISE. In their research the consortium addressed all topics required to manufacture highly efficient CPV modules. This included material issues, manufacturing and equipment aspects and production challenges. University and research institutes were working in close cooperation with industry partners in order to ensure fast industrial exploitation of all results within the whole value chain.

The central objective of CPVMatch was to realise HCPV solar cells and modules with highest efficiencies, with a low environmental impact.
Wafer bonded 4j cells on Ge substrate have been manufactured. Best efficiency realised is 42.6 %. Building blocks that allow for further improvement have been demonstrated including passivated Germanium cells and top cells grown in an upright manner. Putting together these already successfully demonstrated blocks will allow for a efficiencies of 45 % in short term and 46 % is possible in the future. This development was accompanied by intensive modelling which was adapted and further improved.

On the development towards a lattice matched four-junction solar cell that does not require a wafer bonding step important developments on the growth of SiGeSn by MOVPE have been realized. A new MOVPE growth chamber allowing for the growth of both group IV and III-V materials has been installed and qualified. It has been used for the growth of SiGeSn solar devices that for the first time have been grown in the same MOVPE growth chamber utilized for the III-V growth. A SiGeSn/Ge heterojunction solar cell has been manufactured that demonstrated the ability for generating a current density of 16 mA/cm² when implemented in a four-junction device (bandgap 1 eV, filtered by GaAs on top). Besides future implementation in four-junction solar cells the SiGeSn layers show great potential for usage as passivation layers for III-V solar cells.

A new achromatic ‘biFresnel’ lens has been designed and manufactured called achromatic doublet on glass (ADG). The side-by-side comparison between standard SOG (silicone on glass) and ADG lens based mono modules showed almost same efficiency (29.6 % vs. 28.2 % at CSOC), despite the immaturity of the ADG technology compared to SOG. The ADG concept has proven its achromatic behaviour leading to a lower dependence of performance on temperature. Large arrays have been manufactured and modules built.
In order to demonstrate the full potential of achromatic optics a module has been assembled, using four junction solar cells (with 44 – 45 % efficiency) and full glass achromatic lenses with AR coating showing a CSTC efficiency of 41.4%.

A compact, mirror concentrator has been designed and realized. The concept is composed by an off axis paraboloid mirror, as primary optic and an optimized reflective secondary optics as homogenizer. The realized concentration ratio is 980x. Several mono modules and large size modules have been manufactured. So far only 3j cells have been used. Efficiencies of 33.3 % for mono and 30 % for large modules at operating conditions have been achieved (translate to 35% and 32.2% Tcell = 25 °C). Further improvements in the module alignment and replacing the 3j cells with 4j cells pave the way to efficiencies of 40 %.

Measurement tools for the characterisation of CPV optics and cells were developed, introduced and used for the characterisation within the project. In this context CPVMatch partners were continuously supporting the standardisation activities in the IEC technical committee 82 working group 7 responsible for standards in the field of CPV.

A LCA has been performed. The analysis covers the entire life cycle of the product. CO2 emissions between 16 and 18 gCO2eq/kWh were found (standard flat plate PV between 43 and 71 gCO2eq/kWh); further optimisation potential has been identified. These numbers are in the same range as the numbers for wind plants (~13 gCO2/kWh). The analysis of costs is challenging as the technologies are not yet at industrial scale production. However other benefits that CPV is offering have been identified, which at the moment cannot be directly linked to costs as the lower CO2 emissions, the possibility of dual use of the land and potential high local manufacturing content.

Central results and findings of the project were published in scientific papers and presented at conferences. All of the results described above will directly or indirectly be used by partners for current services and products or in the development of new products.
In order to develop the targeted high performing CPV solar cells and modules a significant progress beyond the state of the art was required. Two strategies were adhered to (both for the multi-junction solar cell and module technology). The work on cells and module technologies is accompanied by a profound lifecycle and environmental assessment, the development of adapted characterization methods of new multi-junction cells and HCPV modules and theoretical modelling capabilities.

Within CPVMatch cell the technical performance of multi-junction cells and CPV modules has been increased significantly:
- The project has demonstrated a new wafer bonded four-junction cell concept on Germanium substrate that has already reached 42.6% efficiency and demonstrated building blocks that – if put together – will increase efficiency to 46%.
- SiGeSn as a new 1eV material lattice matched to Ge has been investigated and first solar devices using SiGeSn have been demonstrated showing high current generation which allows implementation in a multi-junction device.
- A world record CPV module with 41.4% efficiency using full glass achromatic lenses has been demonstrated
- A new achromatic lens with industrial feasible manufacturing has been developed and first modules have been assembled. The achromatic behaviour lowering the dependence of the optics on changes of the ambient conditions has been proven.
- Compact mirror modules with high optical performance were developed reaching efficiencies of 35% using triple junction cells.
- New characterisation tools and methods were developed and introduced – the standardisation process in the IEC TC82 WG7 was supported
- The life cycle assessment demonstrated low carbon footprints for CPV (16-18 CO2eq/kWh) that are in the range of wind power plants. Significant cost reductions are to be expected.

All these developments go beyond the state of the art at the beginning of CPVMatch and underlie that CPV is one of the most promising solar energy technologies.
Indoor characterization test bench METHOD. © P. Avavian/CEA
Array of Novel Achromatic Doublet on Glass Fresnel lenses. © IES-UPM
Wafer with multi-junction concentrator solar cells. © Fraunhofer ISE
New compact mirror-based concentrator module. © ASSE
World record CPV module - 41.4% efficiency at CSTC © Fraunhofer ISE/A.Wekkeli
Metalorganic vapour phase epitaxy (MOVPE) reactor. © AIXTRON