Rainer Krause

New Efficiency Frontiers with Wafer-Bonded Multi-Junction Solar Cells.

Triple-junction (3J) solar cells will soon be history. The next generation of multi-junction (MJ) devices are now reaching efficiencies far beyond the record levels of 3J cells on Germanium. In this paper we present results of a 4J wafer-bonded solar cell with bandgaps 1.88 / 1.45 // 1.10 / 0.73 eV measured with an improved efficiency of 46.5% at 324x by Fraunhofer ISE. Design for cost has, from the outset, been a priority with the development of engineered substrates to replace costly and low yielding InP substrates, a product building on Soitec’s proprietary SmartCut technology. Wafer bonding enables the electro-mechanical combination of lattice and thermal expansion mismatched materials with electrically conductive, transparent bonds, enabling concentrator solar cells to be built from high quality 2J GaInP/GaAs absorbers lattice-matched to GaAs bonded to high quality 2J GaInPAs/GaInAs absorbers lattice matched to InP that operate well at high concentration.

Four-Junction Wafer-Bonded Concentrator Solar Cells

The highest solar cell conversion efficiencies are achieved with four-junction devices under concentrated sunlight illumination. Different cell architectures are under development, all targeting an ideal bandgap combination close to 1.9, 1.4, 1.0, and 0.7 eV. Wafer bonding is used in this work to combine materials with a significant lattice mismatch. Three cell architectures are presented using the same two top junctions of GaInP/GaAs but different infrared absorbers based on Germanium, GaSb, or GaInAs on InP. The modeled efficiency potential at 500 suns is in the range of 49-54% for all three devices, but the highest efficiency is expected for the InP-based cell. An efficiency of 46% at 508 suns was already measured by AIST in Japan for a GaInP/GaAs//GaInAsP/GaInAs solar cell and represents the highest independently confirmed efficiency today. Solar cells on Ge and GaSb are in the development phase at Fraunhofer ISE, and the first demonstration of functional devices is presented in this paper.

New module design with 4-junction solar cells for high efficiencies

CPV systems using 3-junction solar cells have proven good performance and reliability. As the performance of 3-junction cells is saturating [1], the logical next step is the introduction of 4-junction (4-J) solar cells into CPV modules. The current world record for cell efficiency under concentrated light of 46.0% was achieved using a wafer bonded Soitec 4-J cell (SmartCell™) [1-4]. This paper discusses a new module design, optimized for harvesting the full potential provided by this high cell efficiency. Prototype modules have been built in order to explore the design space. A module efficiency of 38.9% under concentrator standard test conditions has been achieved.

Development of high efficiency wafer bonded 4-junction solar cells for concentrator photovoltaic applications

The next generation of multi-junction concentrator solar cells will have to reach higher efficiencies than todays devices. At the same time these solar cells must be reliable in the field, be manufacturable with good yield and at sufficiently low cost. Inevitably the request of higher efficiency requires four or even more junction devices. A four-junction solar cell combination of GaInP/GaAs//GaInAsP/GaInAs with bandgap energies of 1.9, 1.4, 1.1, 0.7 eV is developed in a close collaboration between the Fraunhofer ISE, Soitec, CEA-LETI and HZB. This 4-junction cell hits close to the optimum of theoretical efficiency contour plots and has the potential to reach efficiencies up to 50 % under concentration. Challenges are associated with lattice-mismatch between GaAs and InP which is overcome by direct wafer-bonding. The high cost of the InP is addressed by the use of engineered substrates which only require a 500 nm thin mono-crystalline InP layer instead of several hundred μm. Excellent solar cell results up to 44.7 % efficiency have been obtained under concentration for devices manufactured on InP bulk substrates. The high cell efficiency is also supported by out-door characterization of one cell below a Fresnel lens with 16 cm2 aperture area. 38.5 % conversion efficiency has been measured for this mono-module in Freiburg under real operating conditions without any corrections.

Wafer bonded 4-junction GaInP/GaAs//GaInAsP/GaInAs concentrator solar cells

Multiple-junction solar cells made from III-V compound semiconductors are delivering the highest solar-electric conversion efficiencies. Increasing the number of junctions offers the potential to reach higher efficiencies. Direct wafer bonding offers a unique opportunity to combine lattice mismatched materials through a permanent, electrically conductive and optically transparent interface. In addition, the use of Smart Cut ™ technology, associated with its material recycling capabilities allows from a cost perspective the use of expensive bulk material such as InP. Combination of both technologies opens new opportunities to deliver cost effective high efficiency solar cells. In this respect, we have been able to demonstrate a record efficiency of 44,7% with a wafer bonded 4-junction GaInP/GaAs//GaInAsP/GaInAs concentrator solar cell with bandgap energies of 1.88/1.44//1.11/0.70 eV respectively. The bandgaps are chosen to be close to optimal for conversion under concentrated sunlight [1]. This paper prese...

CPV module design optimization for advanced multi-junction solar cell concepts

A network model for multi-junction solar cells has been combined with ray tracing and finite element simulations of a Fresnel lens in order to interpret experimentally derived measurement results. This combined model reveals a good agreement between simulation and measurement for advanced four-junction solar cells under a Fresnel lens when the cell-to-lens distance was varied. Thus, the effect of fill factor drop caused by distributed series resistance losses due to chromatic aberration is well described by this model. Eventually, this model is used to calculate I-V characteristics of a four-junction cell, as well as of a upright metamorphic and lattice-matched triple-junction solar cell under the illumination profile of a Fresnel lens. A significant fill factor drop at distinct cell-to-lens distances was found for all three investigated solar cell types. In this work we discuss how this fill factor drop can be avoided. It is shown that already a halving of the sheet resistance within one of the lateral c...

SOITEC’s 4 junction solar cell module one year on sun

During the last years SOITEC has co-developed a 4-junction solar cell (SmartCell™) with world record conversion efficiency. To verify suitability for concentrator application in SOITEC’s Concentrix™ CPV module, early long-term outdoor performance monitoring was initiated. To date, some module prototypes have been already exposed for more than one year in desert environment; no performance degradation is observable. Accelerated ageing testing executed on the cells also indicates comparable reliability performance to standard triple junction concentrator solar cells.