In this paper we report on research activities at the Fraunhofer Institute for Solar Energy Systems (ISE) and Concentrix Solar in the area of secondary optics for FLATCON ® modules. This concentrator photovoltaic (CPV) technology is based on Fresnel-lenses as primary optics, passive heat spreaders and triple-junction III-V solar cells. In the first part of the paper, a field performance analysis is presented for Concentrix CPVsystems recently installed in Spain. Subsequently, the performance of the first FLATCON ® modules with reflective and refractive secondaries are evaluated (FLATCON ® II) in indoor and outdoor measurements. As a result of this development, the first module with automated assembly process of the secondary optics could be manufactured. The highest outdoor efficiency measured for this kind of module is 29.1 %, which is the highest module efficiency achieved at the Fraunhofer ISE so far.

Progressing beyond 3-junction inverted-metamorphic multijunction solar cells grown on GaAs substrates, to 4-junction devices, requires the development of high quality metamorphic 0.7 eV GalnAs solar cells. Once accomplished, the integration of this subcell into a full, monolithic, series connected, 4J-IMM structure demands the development of a metamorphic tunnel junction lattice matched to the leV GalnAs subcell. Moreover, the 0.7 eV junction adds about 2 hours of growth time to the structure, implying a heavier annealing of the subcells and tunnel junctions grown first. The final 4J structure is above 20 | _im thick, with about half of this thickness used by the metamorphic buffers required to change the lattice constant throughout the structure. Thinning of these buffers would help reduce the total thickness of the 4 J structure to decrease its growth cost and the annealing time. These three topics: development of a metamorphic tunnel junction for the 4th junction, analysis of the annealing, and thinning of the structure, are tackled in this work. The results presented show the successful implementation of an antimonide-based tunnel junction for the 4th junction and of pathways to mitigate the impact of annealing and reduce the thickness of the metamorphic buffers.

In this work, we investigated the impact of intensive heat loads on concentrator solar cells assemblies. As test samples, we employed lattice-matched and lattice-mismatched triple-junction solar cells made of GaInP/GaInAs/Ge. The thermal loads were induced by different manners. We used drying cabinets, external power supplies, and a combination of both to achieve maximum test temperatures of 180 °C. For the performance analysis, we utilized flash light solar simulators and an electroluminescence (EL) imaging tool. Our experiments revealed a significant difference depending on the applied manner of heating. The highest impact was observed for the pure heat treatment in drying cabinets. This was particularly visible in the spatial EL images, but also in the IV curves. In contrast, running the concentrator solar cells as forward-biased diodes using an external current supply of 2000 mA, which corresponds to 2000 suns, did not lead to any significant changes in EL and IV curves. However, deformation of the front metallization was observed. In conclusion, pure heat treatment can be considered as a cost-efficient alternative to pinpoint weak points in solar cell receivers

This paper presents the new module generation (CX-75-III), which was developed in an all-embracing cost optimization. The single primary lens was refined for optical efficiency and its size increased to 5.7 x 5.7 cm², thus dramatically reducing the part count. Thermal simulations were used in order to dimension the heat spreader and ensure the reliability due to a low cell temperature. Furthermore, recent developments for cost optimization and high module efficiencies are presented. Results on prototype modules with a new heat spreader design, a new primary lens and also with refractive secondary optical elements are shown. The use of reflective secondaries has lead to an STC efficiency of 30.6%. Furthermore, different anti-reflection coatings have been examined in order to assess their impact on performance and reliability..

In this work the impact of accelerated aging on the spectral transmission and the mechanical robustness of silicone elastomers for concentrator photovoltaic applications was investigated. Therefore, specific test samples were manufactured. The samples were annealed at 150 degrees C to assure a complete cross-linking These samples were exposed to humidity freeze, to a pressure cooker test, and to UV light. To investigate optical materials under UVA intensity up to 10 W/cm(2) a test setup was developed. Thus, a UV dosage of 10000 kWh/m(2) was applied to the silicone samples after thermal treatment. The mean transmission was used as a measure to identify changes in the spectral behavior and was, therefore, compared after the stress tests with the initial value. No total failures but rather degradation was observed, mainly in the range of ultraviolet and visible light. In addition, the shear strengths for the silicone elastomers were compared before and after stress