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Producción Científica, Reducing the formation of cracks during growth of GaInP/GaInAs/Ge 3-junction solar cells on Ge|Si virtual substrates has been attempted by thinning the structure, namely the Ge bottom cell and the GaInAs middle cell. The theoretical analysis performed using realistic device parameters indicates that the GaInAs middle cell can be drastically thinned to 1000 nm while increasing its In content to 8% with an efficiency loss in the 3-junction cell below 3%. The experimental results show that the formation of macroscopic cracks is prevented in thinned GaInAs/Ge 2-junction and GaInP/GaInAs/Ge 3-junction cells. These prototype crack-free multijunction cells demonstrate the concept and were used to rule out any possible component integration issue. The performance metrics are limited by the high threading dislocation density over 2·107cm−2 in the virtual substrates used, but an almost current matched, crack-free, thinned 3-junction solar cell is demonstrated, and the pathway towards solar cells with higher voltages identified., Agencia Estatal de Investigación (project RTI2018-094291-B-I00), Ministerio de Educación, Cultura y Deporte (project FPU-15/03436), Programa Estatal de Promoción del Talento y su Empleabilidad (grant RYC-2014- 15621), Junta de Castilla y León - Fondo Europeo de Desarrollo Regional (project VA283P18)

Qualification standards are on the base of the high endurance and resilience of space solar cells. In these standards, such as the European ECSS-E-ST-20-08C or the American AIAA S-111A counterpart, life tests related with high temperature accelerated tests (among many others) are included. There are several issues that make it difficult to assess the multijunction solar cell life from temperature tests in these standards. For example, in the European standard, an activation energy of 0.7 eV determined many years ago for silicon devices is assumed. On the other hand, the American standard proposes temperature accelerated tests at the temperatures of 50 °C, 80 °C and 110 °C which are clearly low to really accelerate the life tests of solar cells. Accordingly, in this paper we present the resulting reliability figures (reliability function, failure probability and MTTF) derived of an innovative temperature ALT which allows the proper estimation of the activation energy of commercial lattice matched GaInP/Ga(In)As/Ge triple junction solar cells. The main conclusions are: a) an activation energy of 0.97 eV has been estimated. This value results in significantly higher lifetime values for the tested cells. b) the shape parameter obtained from Weibull failure density function, β, is 1.67; c) the tested solar cells perform as robust devices showing high reliability values in the temperature range of 80 °C–130 °C; d) for higher temperatures, especially beyond 150 °C, the reliability decays significantly; f) reliability functions and parameters can be assessed for any operation temperature and failure criterion.

During the MOVPE growth of III-V layers on Ge substrates, Ge atoms can be evaporated or etched from the back of the wafer and reach the growth surface, becoming incorporated into the epilayers. This is the so-called Ge autodoping effect, which we have studied through a set of growth experiments of GaInP and Ga(In)As layers lattice matched to Ge substrates, which have been characterized by Secondary Ion Mass Spectroscopy. The role of V/III ratio and growth rate on Ge autodoping has been studied and a MOVPE reactor pre-conditioning prior to the epitaxial growth of III-V semiconductor layers that mitigates this Ge autodoping has been identified. In addition, the use of 2-in. versus 4-in. Ge substrates has been compared and the use of a Si3N4 backside coating for the Ge substrates has been evaluated.

Graphene has been intensively studied in photovoltaics focusing on emerging solar cells based on thin films, dye-sensitized, quantum dots, nanowires, etc. However, the typical efficiency of these solar cells incorporating graphene are below 16%. Therefore, the photovoltaic potential of graphene has not been already shown. In this work the use of graphene for concentration applications on III-V multijunction solar cells, which indeed are the solar cells with the highest efficiency, is demonstrated. Firstly, a wide optoelectronic characterization of graphene layers is carried out. Then, the graphene layer is incorporated onto triple-junction solar cells, which decreases their series resistance by 35% (relative), leading to an increase in Fill Factor of 4% (absolute) at concentrations of 1,000 suns. Simultaneously, the optical absorption of graphene produces a relative short circuit current density decrease in the range of 0-1.8%. As a result, an absolute efficiency improvement close to 1% at concentrations of 1,000 suns was achieved with respect to triple junction solar cells without graphene. The impact of incorporating one and two graphene monolayers is also evaluated., Comment: 29 pages, 6 figures

Dilute nitrides based on GaAs constitute a family of compounds whose main characteristic is the band-gap tunability, depending on the nitrogen content. In this work we have focussed our attention on the indium free dilute nitrides, i.e. GaNAsSb with a bandgap of around 1.1 eV, to study the effects that doping has on the crystalline structure, electrical and optical properties of the material. For such purpose, p-doped and n-doped GaNAsSb layers were grown by molecular beam epitaxy and characterized using x-ray diffraction, spectroscopic ellipsometry and photoreflectance among other techniques. The GaNAsSb optical properties match the double band-anticrossing model which is the proposed one to explain the dilute nitride band structure. However, the determined optical bandgap value does not follow any trend with doping, neither with concentration nor type. This is related with doping effects on the crystalline quality and layer composition, thus inducing a Sb gradient along layer thickness together with variations in N and Sb concentrations for different doping levels. Besides these structural variations, the complex refraction index, Hall mobility and carrier concentration as a function of temperature have been determined for these GaNAsSb layers.

The energy bandgap of GaInP solar cells can be tuned by modifying the degree of order of the alloy. In this study, we employed Sb to increase the energy bandgap of the GaInP and analyzed its impact on the performance of GaInP solar cells. An effective change in the cutoff wavelength of the external quantum efficiency of GaInP solar cells and an effective increase of 50 mV in the open-circuit voltage of GaInP/Ga(In)As/Ge triple junction solar cells were obtained with the use of Sb. Copyright © 2016 John Wiley & Sons, Ltd.

It was recently shown that the use of graphene as a transparent electrode deposited between the antireflection coating and the front grid of a solar cell can help mitigate resistive losses. Graphene-enhanced solar cells can be manufactured by retrofitting existing off-the-shelf multijunction solar cell designs, making its incorporation into HCPV systems transparent for the module manufacturer. This paper presents a theoretical evaluation of yearly energy yield gains in HCPV systems equipped with graphene-enhanced solar cells as compared with the same system using conventional triple-junction solar cells. These calculations show that the energy production gain when using graphene-enhanced modules may exceed 6% in reasonable locations for CPV (Fes, Madrid, Nicosia) and clearly exceed 7% in a good location such as Phoenix, whereas graphene integration cost would increase the €/Wp merit figure in 2-3%.

This paper addresses the influence of III-V nucleation routines on Ge substrates for the growth of high efficiency multijunction solar cells. Three exemplary nucleation routines with differences in thickness and temperature were evaluated. The resulting open circuit voltage of triple-junction solar cells with these designs is significantly affected (up to 50 mV for the best optimization routine), whereas minimal differences in short circuit current are observed. Electroluminescence measurements show that both the Ge bottom cell and the Ga(In)As middle cell present a VOC gain of 25 mV each. This result indicates that the first stages of the growth not only affect the Ge subcell itself but also to subsequent subcells. This study highlights the impact of the nucleation routine design in the performance of high efficiency multijunction solar cell based on Ge substrates., 7 pages,7 figures

This paper summarizes the state-of-the-art of the lattice matched GaInP/Ga(In)As/Ge triple-junction solar cell developed at the Solar Energy Institute of UPM (IES-UPM). Different research topics tackled over the last years about this structure are described. As result of this work, an efficiency of ~ 40% at $\sim 415\times$ is presented.

Germanium solar cells are used as bottom subcells in many multijunction solar cell designs. The question remains whether the thermal load originated by the growth of the upper layers of the multijunction solar cell structure affects the Ge subcell performance. Here, we report and analyze the performance degradation of the Ge subcell due to such thermal load in lattice-matched GaInP/Ga(In)As/Ge triple-junction solar cells. Specifically, we have detected a quantum efficiency loss in the wavelength region corresponding to the emitter layer (which accounts for up to 20% loss in equivalent JSC) and up to 55 mV loss in VOC of the Ge subcell as compared with analogous devices grown as single-junction Ge solar cells on the same type of substrates. We prove experimentally that there is no direct correlation between the loss in VOC and the doping level of the base. Our simulations show that both the JSC and VOC losses are consistent with a degradation of the minority carrier properties at the emitter, in particular at the initial nanometers of the emitter next to the emitter/window heterointerface. In addition, we also rule out the gradual emitter profile shape as the origin of the degradation observed. Our findings underscore the potential to obtain higher efficiencies in Ge-based multijunction solar cells if strategies to mitigate the impact of the thermal load are taken into consideration. This work has been supported by the European Commission through the LONGESST project (FP7 grant agreement no 607153), by the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement no 656208, by the Spanish Ministerio de Economía y Competitividad through the projects TEC2014-54260-C3-1-P and TEC2015-66722-R, and from the Madrid local government under contract S2013/MAE-2780 (MADRID-PV). I García is funded by the Spanish “Programa Estatal de Promoción del Talento y su Empleabilidad” through a Ramón y Cajal grant. The authors would also like to thank Jesus Bautista for his continuous support and NREL for the processing and facilities for the measurement of the solar cells. This paper reflects only the author's view, and the funding agency is not responsible for any use that may be made of the information it contains.

The refractive index and extinction coefficient of chemical vapour deposition grown graphene are determined by ellipsometry analysis. Graphene films were grown on copper substrates and transferred as both monolayers and bilayers onto SiO2/Si substrates by using standard manufacturing procedures. The chemical nature and thickness of residual debris formed after the transfer process were elucidated using photoelectron spectroscopy. The real layered structure so deduced has been used instead of the nominal one as the input in the ellipsometry analysis of monolayer and bilayer graphene, transferred onto both native and thermal silicon oxide. The effect of these contamination layers on the optical properties of the stacked structure is noticeable both in the visible and the ultraviolet spectral regions, thus masking the graphene optical response. Finally, the use of heat treatment under a nitrogen atmosphere of the graphene-based stacked structures, as a method to reduce the water content of the sample, and its effect on the optical response of both graphene and the residual debris layer are presented. The Lorentz-Drude model proposed for the optical response of graphene fits fairly well the experimental ellipsometric data for all the analysed graphene-based stacked structures.

Tunnel junctions are key for developing multijunction solar cells (MJSC) for ultra-high concentration applications. We have developed a highly conductive, high bandgap p + + -AlGaAs/n + + -GaInP tunnel junction with a peak tunneling current density for as-grown and thermal annealed devices of 996 A/cm 2 and 235 A/cm 2, respectively. The J–V characteristics of the tunnel junction after thermal annealing, together with its behavior at MJSCs typical operation temperatures, indicate that this tunnel junction is a suitable candidate for ultra-high concentrator MJSC designs. The benefits of the optical transparency are also assessed for a lattice-matched GaInP/GaInAs/Ge triple junction solar cell, yielding a current density increase in the middle cell of 0.506 mA/cm 2 with respect to previous designs.

The optical properties of p-type, n-type and nominally undoped (AlxGa1−x)yIn1−yP layers have been determined in a wide spectral range. The layers under study have been chosen with compositions and dopant concentrations which make them interesting for their use in III-V multijunction solar cells. The layers have been measured by variable angle spectroscopic ellipsometry and, irrespective of composition and doping, their optical response has been modelled using the same model dielectric function consisting of two asymmetric Tauc-Lorentz oscillators and a 3D-M0 Adachi term. The results show that transition energy values change with layer composition, whilst for layers of the same material (i.e. GaInP or AlInP), the band-gap transition energy E0 shows a strong dependence on the order parameter. The refractive indexes and extinction coefficients deduced from the ellipsometric data have been used to fit reflectance measurements for the same layers and an excellent agreement has been achieved, thus validating the model dielectric function proposed for this kind of materials.

We report on the effect of Sb on the microstructure of GaInP layers grown by metal organic vapor phase epitaxy (MOVPE). These layers exhibit a CuPtB single variant ordering due to the intentional misorientation of the substrate (Ge(001) substrates with 6°misorientation towards the nearest [111] axis). The use of Sb as a surfactant during the GaInP growth does not modify the type of ordering, but it is found that the order parameter (η) decreases with increasing Sb flux. Dark field microscopy reveals a variation of the angle of the antiphase boundaries (APBs) with Sb amount. The microstructure is assessed through high angle annular dark field (HAADF) experiments and image simulation revealing Z-contrast loss in APBs due to the superposition of ordered domains. © the Owner Societies 2017.

Reflectance Anisotropy Spectroscopy (RAS) was employed to determine the optimal specific molar flow of Sb needed to grow GaInP with a given order parameter by MOVPE. The RAS signature of GaInP surfaces exposed to different Sb/P molar flow ratios were recorded, and the RAS peak at 3.02 eV provided a feature that was sensitive to the amount of Sb on the surface. The range of Sb/P ratios over which Sb acts as a surfactant was determined using the RA intensity of this peak, and different GaInP layers were grown using different Sb/P ratios. The order parameter of the resulting layers was measured by PL at 20 K. This procedure may be extensible to the calibration of surfactant-mediated growth of other materials exhibiting characteristic RAS signatures.

GaInP is a material commonly employed for the top subcells of different multijunction solar cells architectures. In this study, the performance of GaInP top cells has been improved by increasing the energy band gap with the use of Sb as a surfactant during the MOVPE growth of the structures. The optimization of the appropriate Sb molar flow was done by Reflectance Anisotropy Spectroscopy. Different characterization techniques have been employed to assess the effect of Sb on the morphology, microstructure and optoelectronic properties of the resulting GaInP grown with different Sb/P ratios. Finally, the performance of several GaInP subcells with different order parameters has been assessed.

Axially defined GaInP single nanowire (NW) p-i-n junctions are demonstrated, with photocurrent response and yellow-green electroluminescence near the indirect bandgap crossover point at 2.18 eV (569 nm). We use DEZn and H2S as p- and n-type dopants, and find that they both affect the material composition and the crystal structure. The photovoltaic efficiency is comparable to single NW devices from binary III-V materials. These results demonstrate the potential of GaInP nanowires as a high-bandgap material for multijunction solar cells and light-emitting devices in the visible regime. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729929]

A quantitative temperature accelerated life test on sixty GaInP/GaInAs/Ge triple-junction commercial concentrator solar cells is being carried out. The final objective of this experiment is to evaluate the reliability, warranty period, and failure mechanism of high concentration solar cells in a moderate period of time. The acceleration of the degradation is realized by subjecting the solar cells at temperatures markedly higher than the nominal working temperature under a concentrator, while the photo-current nominal conditions are emulated by injecting current in darkness. Three experiments at three different temperatures are necessary in order to obtain the acceleration factor which relates the time at the stress level with the time at nominal working conditions. However, up to now only the test at the highest temperature has finished. Therefore, we can not provide complete reliability information but we have analyzed the life data and the failure mode of the solar cells inside the climatic chamber at the highest temperature. The failures have been all of them catastrophic. In fact, the solar cells have turned into short circuits. We have fitted the failure distribution to a two parameters Weibull function. The failures are wear-out type. We have observed that the busbar and the surrounding fingers are completely deteriorated.