HeKMod4 – Highly Efficient Concentrator Module with GaSb-Based Four-Junction Solar Cell

Duration: September 2014 - February 2019
Contracting Authority/ Sponsors: Federal Ministry for Economic Affairs and Energy (BMWI)
Project Focus:
GaSb-based four-junction solar cell under the sun simulator to measure its IV-characteristic.
© Fraunhofer ISE
GaSb-based four-junction solar cell under the sun simulator to measure its IV-characteristic.
External quantum efficiency and current densities under the AM1.5d spectrum of a bonded 4-junction solar cell with a GaSb bottom cell.
© Fraunhofer ISE
External quantum efficiency and current densities under the AM1.5d spectrum of a bonded 4-junction solar cell with a GaSb bottom cell. The cell currently exhibits an efficiency of 39.54 % under concentration.
Partial image of the new external quantum efficiency measurement set-up: Micromirror array under illumination with the color spectrum generated by a refraction grating.
© Fraunhofer ISE
Partial image of the new external quantum efficiency measurement set-up: Micromirror array under illumination with the color spectrum generated by a refraction grating.

In high-concentrating photovoltaics (HCPV), the sunlight is focused with a factor of 300 to 1000 using optics onto a small solar cell with an area of few square millimeters. As the cell area is very small, multi-junction solar cells can also be used. These use the sunlight very efficiently across the entire solar spectrum. In the concentrator system, all components, such as solar cell, optics, electrical wiring, thermal design, and production technologies, must be adjusted carefully for the module in the "HeKMod4" project. We use our long-term experiences in these topics to develop a concentrator module with a target efficiency of up to 39%. The basis is a novel four-junction solar cell with a Gallium Antimonide (GaSb) bottom cell. New characterization methods are developed for component and system assessment.

In the project "HeKMod4", an innovative concentrator module with a target efficiency of 36% to 39% is developed. To realize the best possible module design, we investigate all components individually to optimize the complete system. The component development focuses on solar cells with four pn junctions, optics and module design.

For this purpose, a novel four-junction solar cell with a Gallium Antimonide (GaSb) bottom cell is developed in the project. It has a particularly high efficiency potential of approx. 50%. For this purpose, two epitaxy processes on GaAs and GaSb substrates, as well as a wafer bonding technology must be developed. Four-junction solar cells were developed and optimized which currently achieve efficiencies of 40% under concentrated illumination. Solar cells with a GaSb and a GaInAsSb bottom cell were also produced. The latter material allows the absorption to be extended further into the infrared wavelength range. Currently, te largest losses are caused by a suboptimal current adaptation of the partial cells as well as a relatively high shadowing by the front contact fingers. But also the interaction between the materials and up to 40 individual layers must be further optimized.

For the characterization of these solar cells with more than three pn junctions, new characterization methods and calibration routines are established. Within the project, a new measuring set-up was developed for determining the external quantum efficiency (EQE) of multi-junction cells. This test-rig is based on the frequency multiplex procedure, where the EQE can be determined instantaneously over a wide wavelength range opposite to conventional, sequential measuring methods. The core element of the measuring tool is a mirror array that is used for frequency modulation of the monochromatic light. First EQE measurements on multi-junction solar cells were successfully completed and showed excellent agreement compared to measurements on the same solar cell at the calibration laboratory of Fraunhofer ISE.

The prototype module concept developed in the project is characterized by small solar cells and optics. The heat can be distributed effectively by the small local heat input and high optical efficiencies can be achieved by the mirror optics.

In the project, different module concepts with passively cooled mirror optics were first theoretically investigated. Extensive simulations were run to determine the heat distribution in the different concepts. Finally, the concepts were compared and assessed with respect to optical efficiency, manufacturability, and solar cell temperature. A module design with a very small solar cell (solar cell diameter 460µm and mirror optical edge length 12.8mm) was selected for the construction of prototypes. As a result, the thermal input is distributed very well and a design with shadowing losses of only 3% due to the heat spreader and conductor path is possible. Another advantage is that very low module heights can be realized enabling a space-saving module transport.

After the conceptual elaboration of the module concept, the individual components such as mirror optics, solar cells, conductor path on glass were produced and examined. First measurements on optics and solar cells were carried out (see Figure 5). A very good acceptance angle of >±0.8° expected from simulations was confirmed with a very precise diamond-turned optics and a triple-junction solar cell.

© Fraunhofer ISE
Prototype CPV module with 14 mirrors and solar cells.
© Fraunhofer ISE
Prototype CPV module with 14 mirrors and solar cells.