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8th World Conference on Photovoltaic Energy Conversion; 396-398, Combining epitaxial lift-off with a dielectric-metal back mirror can boosts III-V solar cells efficiencies by enhanced photon recycling without sacrificing the costly growth wafer. In this work, GaAs and GaInP/GaInAs solar cells are produced with a patterned MgF2/Ag mirror. The sub-bandgap reflectance increases by 2.1% and 2.6%, respectively, compared to the devices with a full Ag back mirror. We investigate the effect of increased sub-bandgap reflection on the temperature-induced open-circuit voltage degradation during operation in low-pressure environments.

37th European Photovoltaic Solar Energy Conference and Exhibition; 618-620, In this study, photon recycling mechanisms in single junction thin-film GaAs solar cells are investigated. An approach of theoretical modeling supported by experimentally obtained results is used in order to correlate the reflectance of the cell's rear layers, the photon recycling probability and the solar cell performance. Solar cells with different top and bottom metallization configurations are produced and their performance is optically and electrically analyzed. It is shown that the photon recycling probability increases with the rear mirror reflectance and solar cell thickness, which results in the increase of the devices open circuit voltage. We identify the front grid coverage, which is usually disregarded in rear mirror focused studies, to be strongly reducing the photon recycling probability of the devices. Furthermore, based on preliminary calculations, the internal radiative efficiency of the solar cells seems to prevent further increase of the devices' open circuit voltage as a result of improvements to the rear mirror of the cells. In order to achieve a significant benefit of the increased photon recycling probability to the solar cell performance, the limiting mechanisms hindering the cells internal radiative efficiency need to be identified and properly addressed.

36th European Photovoltaic Solar Energy Conference and Exhibition; 1543-1545, The effect of particle irradiation on the performance of GaAs solar cells with a range of architectures is studied. Solar cells with shallow and deep junction design processed on the native wafer as well as into a thin-film were irradiated by 1-MeV electrons with fluence up to 1x1015 e-/cm2 and by 1-MeV protons with fluence up to 1x1013 H+/cm2. The degradation of the cell performance due to irradiation was studied both experimentally and theoretically, with good agreement between modeled and measured data. We identify a degradation of the hetero-interfaces quality due to irradiation exposure, characterized by an increase of the interface recombination velocity. The solar cell performance degradation greatly depends on the junction depth and the thickness of the active layers, and therefore, a shallow junction solar cell processed into a thin-film geometry is found to be the most radiation resilient structure for space applications. The presence of a rear reflector allows the design of thinner devices that show potential for increased beginning-of-life performance and improved radiation hardness.

Graded refractive index layers reduce the reflection and increase the coupling of light into a substrate by optical impedance matching at the interfaces. Due to the optical impedance matching, reflections at the interfaces are not possible for a broad wavelength range, rendering this type of anti-reflection coating a promising candidate for III/V multi-junction solar cells. Graded refractive index layers can be modeled using a transfer-matrix method for isotropic layered media. We derive the transfer-matrix method and we show calculations of the reflection from and the transmission into an AlInP layer coated with different anti-reflection coatings. We describe a new type of anti-reflection coating based on tapered semiconductor nanowires and we show reflection and transmission measurements of those kind of anti-reflection coatings on top of different substrates.

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31st European Photovoltaic Solar Energy Conference and Exhibition; 1422-1425, Thin-film III-V solar cells provide excellent characteristics for application in space solar panels. However, the currently applied copper stabilization foil might diffuse into the active solar cell material, potentially inducing detrimental damage to the solar cell. In four different accelerated life time tests it was found that copper diffusion indeed has a potentially detrimental effect on device performance. A reduction in Jsc is observed for deep junction cells, due to a minority carrier life time reduction. More noticeably Voc is affected in shallow junction cells, indicating enhanced non-radiative recombination due to introduction of Cu trap levels in the depletion region. Contact layers with high carrier densities are found to be unfavourable, as high dopant concentrations negatively influence the material quality of the layer. Additionally it was found that the Au contact/mirror functions as a barrier to Cu diffusion but that it will break down eventually. A dependence on the Cu thickness was also observed, but further experiments are required to explain this dependence. Although an optimized device design can inhibit Cu diffusion, it is still advisable to either implement a suitable diffusion barrier or find an alternative metal for the handling and support foil.

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Epitaxial lift-off is used to create thin-film III–V solar cells without sacrificing the GaAs wafer. It is based on selective etching of an AlAs release layer between the wafer and the cell structure using an HF solution. The wafer can be reused for subsequent deposition runs thereby reducing the cost of the cells. The thin-film cell can be transferred to any new carrier, e.g. glass, plastic, silicon, or metal foil. Although epitaxial lift-off was first demonstrated in 1978, it took until the 1990s to make significant progress in understanding the process and devising new ways to increase the etch rate. The first single-junction epitaxial lift-off cells were made in 1996. Thin-film cells offer new cell applications based on their flexibility, low weight, and possibility to deposit the cell structure in reverse order. Today the world record for single-junction cells is held by a thin-film GaAs cell, who’s performance is partly based on the increased photonrecycling factor in cells with a back contact acting as a mirror. Also state of the art tandem and inverted metamorphic thin-film cells have been demonstrated.

The epitaxial lift-off (ELO) process is utilized to produce thin-film III-V devices, while the substrate (GaAs wafer) on which the III-V structure was grown can be reused. However, so far the direct reuse of these GaAs wafers is inhibited by the remnants on the wafer surface that cannot be removed in a straightforward fashion utilizing general cleaning methods. Therefore, etching of GaAs wafers in hydrofluoric acid was investigated by microscopic techniques, profilometry and X-ray photoelectron spectroscopy. It was found that immediately after etching the wafer surface is covered by a brown layer of elemental arsenic. The thickness and uniformity of this layer depend on both illumination during etching and the HF concentration. During storage of the etched wafer the As layer is replaced by As2O3 particles. It is shown that oxide particles form only when the wafer is exposed to light in the presence of air. A model that explains the As formation and the subsequent particle formation is given.

We report a novel graded refractive index antireflection coating for III/V quadruple solar cells based on bottom-up grown tapered GaP nanowires. We have calculated the photocurrent density of an InGaP-GaAs-InGaAsP-InGaAs solar cell with a MgF2/ZnS double layer antireflection coating and with a graded refractive index coating. The photocurrent density can be increased by 5.9 % when the solar cell is coated with a graded refractive index layer with a thickness of 1\mu m. We propose to realize such a graded refractive index layer by growing tapered GaP nanowires on III/V solar cells. For a first demonstration of the feasibility of the growth of tapered nanowires on III/V solar cells, we have grown tapered GaP nanowires on AlInP/GaAs substrates. We show experimentally that the reflection from the nanowire coated substrate is reduced and that the transmission into the substrate is increased for a broad spectral and angular range., Comment: 5 Figures

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Contains fulltext : 84345.pdf (Publisher’s version ) (Open Access) 6th International Conference on Concentrating Photovoltaic, ICPV-6