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In this work, we propose and realize three different design strategies to implement an optical cavity in GaAs thin film solar cells in order to confine its internal luminescence and hence to exploit photon recycling. The strategies are based on the definition of a highly reflective and very conductive back side, whereas front side light extraction is limited by total internal reflection. We show characterization results on the internal reflectivity of the back reflector and on the contact resistance at the rear side, important quantities for a good functioning of the final solar cell. First, a back side using only metal was optimized with a pure Ag layer leading to an internal reflectivity of 95.2% and a contact resistance of 1.0 × 10−4 O for a 1 cm2 device. With a metal‐dielectric stack at the back side and electrical contacts made by metals via point‐contacts, a second approach led to averaged internal reflectivity of 98.0% and contact resistance of 1.8 × 10−4 O for a 1 cm2 device. A third strategy in which a transparent conductive oxide in combination with a metal layer was used did not show the expected results in optical and electrical properties. We fabricated and characterized solar cells with the most promising back sides. When comparing with an ordinary reference GaAs solar cell, external radiative efficiency increased by factors of 150% and 90% for the thin film solar cells with pure Ag and with the metal‐dielectric stack at the back side, allowing enhancements of 19 and 13 mV in VOC, respectively.

III-V multi-junction solar cells are promising devices for photovoltaic applications under very high concentration levels of sunlight. Shadowing losses of the front side metallisation and ohmic resistance losses in the metal grid limit the practical cell size typically to around 1 cm2 at 1000 suns. The use of a full back-contact architecture, similar to the metal-wrap-through (MWT) technology known in silicon photovoltaics, can help to overcome this limitation. Furthermore, positioning both the positive and negative contact pads on the rear side of concentrator solar cells opens the possibility for efficient packaging solutions and the realisation of dense array receivers with low metal shadowing. The MWT technology addresses conventional concentrating photovoltaics as well as combined conventional concentrating photovoltaic-thermal applications and offers specific advantages for large-area devices at high intensities. This work presents the first experimental results for MWT architectures applied to III-V tandem solar cells and discusses specific challenges. An efficiency of 28.3% at 176 suns and 27.2% at 800 suns has been measured for the best MWT Ga0.51In0.49P/Al0.03Ga0.97As tandem solar cells.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 1367-1371, III-V multi-junction solar cells are promising devices for photovoltaic applications under very high concentration levels of sunlight. Shadowing losses of the front side metallization and ohmic resistance losses in the metal grid limit the practical cell size typically to around 1 cm2 at 800 suns. The use of a full back-contact architecture, similar to the Metal Wrap-Trough (MWT) technology known in silicon photovoltaics, can help to overcome this limitation. Furthermore, positioning both the positive and negative contact pads on the rear side of concentrator solar cells opens the possibility for efficient packaging solutions and the realization of dense array receivers with low metal shadowing. The MWT technology addresses conventional concentrating photovoltaics as well as combined PV-thermal applications and offers specific advantages for large area devices at high intensities. This work presents first experimental results for MWT architectures applied to III-V tandem solar cells and discusses specific challenges. An efficiency of 28.3 % at 176 suns and 27.2% at 800 suns has been measured for the best MWT Ga0.51In0.49P/Al0.03Ga0.97As tandem solar cells.

29th European Photovoltaic Solar Energy Conference and Exhibition; 1991-1995, The highest conversion efficiencies are reached by multi-junction III-V solar cells. Wafer-bonding offers new possibilities to advance beyond the industrial standard triple-junction cells and its efficiency limitations. Here we present for the first time results of two novel 4-junction device concepts based on direct wafer-bonding with a realistic efficiency potential of 50% under concentrated sunlight. In the first concept, an inverted grown GaInP/GaAs tandem cell is bonded to a metamorphic GaInAs/Ge tandem cell. Frist bonded 4-junction solar cell on Ge shows an efficiency of 34.5% under one sun AM1.5d. In the latter concept a GaSb cell will be bonded to an inverted metamorphic triple-junction solar cell. The main challenge in this approach is a conductive direct wafer bond between the two cell stacks. Here, bond characteristics between GaSb and Ga0.29In0.71P and another between GaSb and Ga0.79In0.21As are shown for the first time.

28th European Photovoltaic Solar Energy Conference and Exhibition; 1-6, III-V multi-junction solar cells have become standard in space and in terrestrial concentrator systems. The current landmark for III-V multi-junction solar cells is the Ga0.50In0.50P/Ga0.99In0.01As/Ge triple-junction, which only contains lattice-matched layers. This MOVPE-grown device is available from different suppliers in large quantities. Due to the mature development status of this device new concepts are being investigated to increase the efficiencies further. A toolbox of new ideas and methods is available such as metamorphic and/or inverted growth, wafer bonding, different substrates, nanostructures, MBE growth and new materials. These have been used and combined intensively in recent years to develop and realize new structures of III-V multi-junction solar cells. This paper gives an overview about the technology perspectives and research trends in III-V multi-junction solar cells. Eventually, a new record efficiency value of 44.7% @ 297xAM1.5d for a 4-junction wafer-bonded solar cell is presented.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 964-968, III-V multi-junction solar cells grown on Ge substrates have achieved the highest solar cell conversion efficiency of 41.6 %. However, this device manufacturing is still costly and a major share of the cost can be attributed to the cost of the Ge substrate. Therefore, a replacement of Ge by Si would be beneficial. However, elaborate challenges arise from the 4.1% difference in lattice constant for Si and GaAs and due to the non-polar to polar interface leading to threading dislocations and anti-phase domains, respectively. To meet the required high crystalline quality for III-V multi-junction solar cells this paper presents approaches for the Si-to-GaAs transition evaluated on an industrial 300 mm CRIUS Closed Coupled Showerhead MOVPE reactor. Anti-phase domain free GaP nucleation layers with only 0.36% lattice mismatch at room temperature were grown on Si substrate. On top of these layers step-graded Ga1-xInxP buffer structures mediate the lattice constant to GaAs although present process parameters have not yet enabled confinement of threading dislocations inside the buffer structure. A promising alternative route is the growth of an all Ge buffer layer directly on Si. Here we report on the results of the used twostep growth process.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 18-23, The standard Ga0.5In0.5P/Ga0.99In0.01As/Ge triple-junction (3J) solar cell is reaching its practical average efficiency limit of 30% for AM0 and 40% under concentration (xAM1.5d) as can be observed for AZUR’s latest products for space application (3G30) and CPV (4C40). In addition, AZUR SPACE has developed dense array modules (ADAM) for dish concentrator application. Regarding modern cell architectures, recent progress by Fraunhofer ISE and AZUR SPACE is reported, including improved GaInNAs subcells for lattice-matched 4J and 6J solar cells, 3J MQW cells reaching 30% AM0 efficiency, continuous work on upright metamorphic 3J cells as well as processing of inverted cell structures. The paper gives a brief overview on these and other new concepts such as inverted metamorphic 3J and 4J cells and multi-junction solar cells based on semiconductor bonding.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 130-134, This paper presents a spectral beam splitting arrangement providing the basis for a four-junction photovoltaic receiver with virtually ideal band gap combination. A light trapping assembly in form of a 45° parallelepiped applying two spectrally selective beam splitters and three solar cells realizes an approach to reach very high solar-electrical conversion efficiency. The spectral beam splitters developed and used in this arrangement show nearly ideal broadband characteristics to split the solar spectrum into three parts. The solar cell selection includes III-V semiconductors and silicon. They were all fabricated at Fraunhofer ISE and assembled in the receiver setup. The experimental setup demonstrated an outdoor efficiency of more than 34 %.

In vitro influence of sodium selenite on cytochrome P-450-dependent formation of active oxygen species on lipid peroxidation (LPO) in rat liver microsomes was studied. Sodium selenite (10(-6)-10(-3) M) did not influence rates of NADPH/Fe-induced formation of active oxygen species (O2.-, OH., H2O2) and NAPDH-dependent LPO. Only at 10(-3) M selenite caused significant decrease in production of hydrogen peroxide in microsomes. Data obtained suggest that sodium selenite at physiological concentrations does not influence formation of active oxygen species by cytochrome P-450 and the rate of enzymatic LPO in rat liver microsomes.