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We present an original study on quaternary GaAsSbN layers and multilayer heterostructures grown by low-temperature liquid-phase epitaxy (LPE) on GaAs substrates aiming to explore their potential for photovoltaic applications. We choose Sn and Mg as suitable dopants for practical use. Our experiments on doping reproducibly show the introduction of controlled concentrations of doping impurities in the range applicable to device fabrication. High-quality n-, p- and nearly compensated GaAsSbN layers covering a large range of carrier concentrations from 1015 to 6 × 1018 cm−3 have been grown at temperatures lower than 600 °C. The successful LPE growth of p-i-n GaAs/GaAsSbN/GaAs heterostructures based on closely compensated i-GaAsSbN is demonstrated for the first time. Temperature-dependent photoluminescence and room temperature surface photovoltage contactless characterization techniques have been used for investigation of the grown structures. These measurements have revealed high optical quality with a low concentration of localized states and red photoresponse limit extended down to 1.2 eV.

Three-directional cine multi-slice left ventricular myocardial velocity mapping (3Dir MVM) is a cardiac magnetic resonance (CMR) technique that allows the assessment of cardiac motion in three orthogonal directions. Accurate and reproducible delineation of the myocardium is crucial for accurate analysis of peak systolic and diastolic myocardial velocities. In addition to the conventionally available magnitude CMR data, 3Dir MVM also provides three orthogonal phase velocity mapping datasets, which are used to generate velocity maps. These velocity maps may also be used to facilitate and improve the myocardial delineation. Based on the success of deep learning in medical image processing, we propose a novel fast and automated framework that improves the standard U-Net-based methods on these CMR multi-channel data (magnitude and phase velocity mapping) by cross-channel fusion with an attention module and the shape information-based post-processing to achieve accurate delineation of both epicardial and endocardial contours. To evaluate the results, we employ the widely used Dice Scores and the quantification of myocardial longitudinal peak velocities. Our proposed network trained with multi-channel data shows superior performance compared to standard U-Net-based networks trained on single-channel data. The obtained results are promising and provide compelling evidence for the design and application of our multi-channel image analysis of the 3Dir MVM CMR data.

Software is a key element on modern research, to the extent that a big part of it will not be possible - or will be much more difficult - without it. There is a strong interest and many initiatives worldwide to ensure that any piece of software - and data - used in research, contributing to papers or in grant applications, is properly accounted for and receives the corresponding credit, including to their creators and contributors. However, there is a flip side: software often implements algorithms and theory described in research papers and books. Sometimes, this use of existing material is referenced in the documentation or in a paper associated to the software, but the software itself remains largely references-free, and the sources it uses, unaccredited for. In this poster we present the Research References Tracking Tool, R2T2, a small tool written in Python that, still in its early stages, aims to fill this gap. Project webpage: https://imperialcollegelondon.github.io/R2T2/ Resources on software citation: https://software.ac.uk/tags/software-citation Resources on software citing research: https://sphinxcontrib-bibtex.readthedocs.io/en/latest/ http://www.doxygen.nl/manual/commands.html#cmdcite

Dilute nitride InGaAsSbN layers grown by low-temperature liquid phase epitaxy are studied in comparison with quaternary InGaAsN layers grown at the same growth conditions to understand the effect of Sb in the alloy. The lattice mismatch to the GaAs substrate is found to be slightly larger for the InGaAsSbN layers, which is explained by the large atomic radius of Sb. A reduction of the band gap energy with respect to InGaAsN is demonstrated by means of photoluminescence (PL), surface photovoltage (SPV) spectroscopy and tight-binding calculations. The band-gap energies determined from PL and ellipsometry measurements are in good agreement, while the SPV spectroscopy and the tight-binding calculations provide lower values. Possible reasons for these discrepancies are discussed. The PL spectra reveal localized electronic states in the band gap near the conduction band edge, which is confirmed by SPV spectroscopy. The analysis of the power dependence of the integrated PL has allowed determining the dominant radiative recombination mechanisms in the layers. The values of the refraction index in a wide spectral region are found to be higher for the Sb containing layers.

Two-dimensional (2D) hybrid perovskites have attracted considerable attention due to their enormous structural and electronical variability, making this class of semiconductors interesting for photovoltaics, light-emitting diodes and lasers. 2D perovskites consist of sheets of bulky organic cations alternately sandwiched by layers of lead halide octahedra. The properties of these materials strongly depend on the thickness of the octahedra layers, defined by the number of octahedra sheets in a layer, n. Consequently, controlling the layer thickness purity (i.e. minimizing the spread in n) is important for any 2D perovskite thin film application. Here, we show that using rationally chosen solvent additives in the precursor solution offers a facile way to control the crystal disorder in 2D perovskites films. Our method leads to significantly reduced variation in n around the target value relative to films obtained by conventional fast-crystallization methods without solvent additives. The improved phase purity in optimized n = 2 and n = 3 films is verified by X-ray diffraction, UV-vis absorption, and photoluminescence measurements. Additionally, we find that 2D perovskite films with n ≥ 2 arising from additive-assisted growth exhibit an unusual crystal orientation with the perovskite interlayers predominantly aligned parallel to the substrate, which we assign to the slow crystallization process induced by the lead-complexing solvent additives. Improved control over the phase purity translates into a better control of the optoelectronic properties of 2D perovskite films. Furthermore, the unusual horizontal crystal orientation of n = 2 and n = 3 films makes this family of tunable organic–inorganic perovskites promising for applications where lateral charge transport is desired, thus enlarging the potential for thin film-based applications of the 2D perovskites.

We report on the electrical and optical properties of ITO and AZO films fabricated directly on silicon substrates under several growth and annealing temperatures. We use broadband spectroscopic ellipsometry measurements (from 300 nm to 20 μm) to obtain a consistent model for the permittivity of each of the films. The results are then used to design an optimized, single layer, high transparency, high conductivity film, suitable as front transparent electrode and low thermal emissivity coating for silicon based solar cells. The best performance is found using the properties of the ITO film grown at 250 °C, with a state of the art resistivity of 0.2 mΩ cm and an optimized thickness of 75 nm which leads to 0.79 average absorptivity in the solar range (300–2000 nm) and 0.21 average emissivity in the thermal range (5–20 μm). The structural characterization of the films using X-Ray diffraction, and the Hall mobility and resistivity measurements of all the films are also provided, complementing and supporting the observed optical properties.

We present the optical and electrical properties of ITO and AZO films fabricated directly on silicon substrates under several growth and annealing temperatures, as well as their potential performance when used as low emissivity coatings in hybrid photovoltaic-thermal systems. We use broadband spectroscopic ellipsometry measurements (from 300 nm to 20 μm) to obtain a consistent model for the permittivity of each of the films. The best performance is found using the properties of the ITO film grown at 250 °C, with a state of the art resistivity of 0.2 mΩ-cm and an optimized thickness of 75 nm which leads to an estimated 50% increase in the extracted power compared to a standard diffused silicon solar cell. The Hall mobility and resistivity measurements of all the films are also provided, complementing and supporting the observed optical properties.

The carrier recombination dynamics of InGaP/InGaAsP quantum wells are reported for the first time. By studying the photoluminescence (PL) and time-resolved PL decay of InGaP/InGaAsP multiple-quantum-well(MQW) heterostructure samples, it is demonstrated that InGaP/InGaAsP MQWs have very low non-radiative recombination rate and high radiative efficiency compared to the control InGaP sample. Along with the analyses of PL emission spectrum and external quantum efficiencies, it suggests that this is due to small confinement potentials in the conduction band but high confinement potentials in the valence band. These results explain several features found in InGaP/InGaAsP MQW solar cells previously., Comment: 6 pages, 5 figures, 2 tables

Four-dimensional (4D) left ventricular myocardial velocity mapping (MVM) is a cardiac magnetic resonance (CMR) technique that allows assessment of cardiac motion in three orthogonal directions. Accurate and reproducible delineation of the myocardium is crucial for accurate analysis of peak systolic and diastolic myocardial velocities. In addition to the conventionally available magnitude CMR data, 4D MVM also acquires three velocity-encoded phase datasets which are used to generate velocity maps. These can be used to facilitate and improve myocardial delineation. Based on the success of deep learning in medical image processing, we propose a novel automated framework that improves the standard U-Net based methods on these CMR multi-channel data (magnitude and phase) by cross-channel fusion with attention module and shape information based post-processing to achieve accurate delineation of both epicardium and endocardium contours. To evaluate the results, we employ the widely used Dice scores and the quantification of myocardial longitudinal peak velocities. Our proposed network trained with multi-channel data shows enhanced performance compared to standard U-Net based networks trained with single-channel data. Based on the results, our method provides compelling evidence for the design and application for the multi-channel image analysis of the 4D MVM CMR data., Comment: 7 pages, 3 figures, accepted by SPIE Medical Imaging 2021

This paper reports on the results of the fifth spectral irradiance measurement intercomparison and the impact these results have on the spread of spectral mismatch calculations in the outdoor characterization of reference solar cell and photovoltaic (PV) devices. Ten laboratories and commercial partners with their own instruments were involved in the comparison. Solar spectral irradiance in clear sky condition was measured with both fast fixed and slow rotating grating spectroradiometers. This paper describes the intercomparison campaign, describes different statistical analysis used on acquired data, reports on the results, and analyzes the impact these results would have on the primary calibration of a c-Si PV reference cell under natural sunlight.

We demonstrate a photoluminescence (PL)-based contactless method to determine the current–voltage ( I-V ) characteristics of the individual subcells in a multijunction solar cell. The method relies upon the reciprocity relation between the absorption and emission properties on a solar cell. Laser light with a suitable energy is used to excite carriers selectively in one junction (1J), and the internal voltages are deduced from the intensity of the resulting luminescence. The I–V curves obtained this way on 1J, 2J, and 6J devices are compared with those obtained using electroluminescence (EL). Good agreement is obtained at high-injection conditions, while discrepancies at low injection are attributed to in-plane carrier transport.

A quantum wire-on-well (WoW) structure, taking advantage of the layer undulation of an InGaAs/GaAs/GaAsP superlattice grown on a vicinal substrate, was demonstrated to enhance the carrier collection from the confinement levels and extend the carrier lifetime (220 ns) by approximately four times more than a planar reference superlattice. Strained InGaAs/GaAs/GaAsP superlattices were grown on GaAs substrates under exactly the same conditions except for the substrate misorientation (0 and 6 ° off). The growth on a 6 ° off substrate induced significant layer undulation as a result of step bunching and non-uniform precursor incorporation between steps and terraces, whereas the growth on a substrate without miscut resulted in planar layers. The undulation was the most significant for InGaAs layers, forming periodically aligned InGaAs nanowires on planar wells, a WoW structure. As for the photocurrent corresponding to the sub-bandgap range of GaAs, the light absorption by the WoW was extended to longer wavelengths and weakened as compared with the planar superlattice. Almost the same photocurrent was obtained for both the WoW and the planar superlattice. Open-circuit voltage for the WoW was not affected by the longer-wavelength absorption edge, and the same value was obtained for the two structures. Furthermore, the superior carrier collection in the WoW, especially under forward biases, improved fill factor compared with the planer superlattice. Copyright © 2016 John Wiley & Sons, Ltd.

Sub-bandgap sunlight provides a source of heat generation in solar cells that is detrimental to performance, especially in space applications where heat dissipation is limited. In this work we assess the impact that an advanced rear-side contact scheme for multi-junction solar cells has on the cell temperature. Our results show that this scheme reduces the optical power absorption below the bandgap of germanium by 81% compared to a standard, full metallization design. Measurements of the electrical and thermal power fluxes performed in vacuum demonstrate that this lower near-infrared light absorption results in 8% less heat dissipated in the cell with the novel rear-side contact scheme when operating at 25 °C. Modelling of the operating temperature for both cells when fully encapsulated with glass indicates that this effect will also result in a reduction of the operating temperature of 9 °C for the novel design.

Photovoltaics is amongst the most important technologies for renewable energy sources, and plays a key role in the development of a society with a smaller environmental footprint. Key parameters for solar cells are their energy conversion efficiency, their operating lifetime, and the cost of the energy obtained from a photovoltaic system compared to other sources. The optimization of these aspects involves the exploitation of new materials and development of novel solar cell concepts and designs. Both theoretical modeling and characterization of such devices require a comprehensive view including all scales from the atomic to the macroscopic and industrial scale. The different length scales of the electronic and optical degrees of freedoms specifically lead to an intrinsic need for multiscale simulation, which is accentuated in many advanced photovoltaics concepts including nanostructured regions. Therefore, multiscale modeling has found particular interest in the photovoltaics community, as a tool to advance the field beyond its current limits. In this article, we review the field of multiscale techniques applied to photovoltaics, and we discuss opportunities and remaining challenges. © T. Abu Hamed et al., published by EDP Sciences, 2018., European Cooperation in Science and Technology: MP1406, The authors are grateful for the financial support by the COST Action MP1406 “MultiscaleSolar.”

A comprehensive assessment of the nature of the distribution of sub band-gap energy states in bulk GaAsBi is presented using power and temperature dependent photoluminescence spectroscopy. The observation of a characteristic red-blue-red shift in the peak luminescence energy indicates the presence of short-range alloy disorder in the material. A decrease in the carrier localisation energy demonstrates the strong excitation power dependence of localised state behaviour and is attributed to the filling of energy states furthest from the valence band edge. Analysis of the photoluminescence lineshape at low temperature presents strong evidence for a Gaussian distribution of localised states that extends from the valence band edge. Furthermore, a rate model is employed to understand the non-uniform thermal quenching of the photoluminescence and indicates the presence of two Gaussian-like distributions making up the density of localised states. These components are attributed to the presence of microscopic fluctuations in Bi content, due to short-range alloy disorder across the GaAsBi layer, and the formation of Bi related point defects, resulting from low temperature growth.

Bandgap engineering of strain-balanced InGaAs/GaAsP multiple quantum wells (MQWs) allows high-quality materials with an absorption edge beyond GaAs to be epitaxially grown in Ge/GaAs-based multijunction solar cells. We demonstrate MQW solar cells with effective bandgaps ranging from 1.31 eV to as low as 1.15 eV. The bandgap-voltage-offset of MQWs is found to be independent of effective bandgaps and superior to a bulk reference by approximately 0.1 V. This implies the merit of high photovoltage as compared with bulk cells with the same bandgap in addition to their widely bandgap-tunable property. Towards the realization of fully lattice-matched quad-junction devices, we demonstrate a 70-period, 1.15-eV bandgap MQW cell as a promising material in 0.66/1.15/1.51/1.99-eV quad-junction cells, whose practical efficiency has a potential to achieve over 50%. With such a large period number of MQWs, the reverse-biased external quantum efficiency reaches an average of over 60% in the spectral region corresponding to a 1.15-eV subcell; this is achieved with only a-few-percent drop at short-circuit condition. The device presented here reaches the target open-circuit voltage and over 75% of the current density required for realizing a 1.15-eV subcell in a 50%-efficient quad-junction solar cell. We believe that future devices which exploit light-trapping structures and enhanced carrier extraction will be able to reach the desired target. Copyright © 2015 John Wiley & Sons, Ltd.

Luminescent down shifting (LDS) is a light management technique that can be used to exploit more efficiently the short-wavelength photons of the solar spectrum, to which most solar cell technologies exhibit relatively poor response. In practice, LDS is a thermalization process of carriers that takes place outside the solar cell itself and, therefore, has the potential of reducing its operating temperature, and thus increasing its electrical power output in field applications. In this study, we use ray-tracing and an electric-like thermal circuit to calculate the temperature variation of both multicrystalline silicon (mc-Si) and cadmium telluride (CdTe) photovoltaic modules containing LDS layers. We find that the dominant factors for the final operating cell temperature under the modified spectrum are reduced light reflection and parasitic absorption in the encapsulants. LDS, in practice, results in only a small variation compared with standard non-LDS modules, of up to +0.1 °C for the case of mc-Si and -0.6 °C for the CdTe. Although lowering operating cell temperature by means of LDS does not constitute in itself a strong reason for adopting the technology, this study paves the way for using optically active encapsulation layers with this purpose in mind.

1 School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, United Kingdom. 2 PV Lighthouse, Coledale, NSW 2515, Australia 3 ITW Technology Center, Illinois Tool Works Inc., Glenview, IL, 60026, USA 4 ITW Performance Polymers & Fluids, Illinois Tool Works Inc., Boston, MA, 01923, USA 5 School of Materials, Energy, Water and Environmental Sciences, Nelson Mandela African Institute of Science and Technology, Tengeru, Arusha, Tanzania

Luminescent down-shifting (LDS) performance is presented for different production-line cadmium telluride (CdTe) mini-modules manufactured in 2009 (CX1) and 2012 (CX3). The 10 cm × 10 cm mini-modules were laminated with LDS layers that consist of luminescent organic dye-doped ethylene vinyl acetate (EVA) and a fluorinated ethylene propylene (FEP) cover sheet. The external quantum efficiency of the devices was measured before and after the encapsulation. All LDS layers improved the short-circuit current. Lumogen Yellow 083 (Y083) and 170 (Y170) dyes showed the greatest relative improvement in the short-circuit current for the CX1 devices (9.6% and 9.7%, respectively), while Lumogen Yellow 083 showed the largest improvement for the CX3 (5.3%). The different vintages of modules also enabled an investigation of how the enhancement possible with LDS relates to other technological efficiency improvements for CdTe cells and indicates the level of future improvements possible. Finally, initial results of the same EVA-FEP LDS layers that contain the V570, Y083, and mixture of the two dyes are also presented for full-size (120 cm × 60 cm) CX3 production line modules (2012). The full-size module results indicate that scale up of the method is feasible with the Y083 layer, which improves short-circuit current by 4.3%.

The search for higher efficiencies in photovoltaics has led us to develop increasingly complex solar cell architectures that rely on increasingly complex physical processes. The desire to overcome the Shockley-Queisser efficiency limit has caused us to consider stacks of junctions with cascading bandgaps, quantum dots and impurities for the creation of intermediate bands, amongst others. The need to improve in-coupling and absorption in thinner layers requires the development of surface textures and nanoparticles working either in the geometric- or wave-optical regimes, or a combination of both. Quantum wells have been employed to tune bandgaps or to induce angle-selective luminescent emission. More recently, photonic methods have been used to control the black-body emission of solar cells and thus influence their operating temperature. Understanding these processes, requires a detailed knowledge of the paths incident photons take in the device, complex generation and recombination mechanisms (sometimes involving multiple sequential transitions), carrier drift and diffusion, and the paths of emitted photons (either luminescent or incandescent). This sounds like a job for a supercomputer. However, with a few carefully made assumptions, accurate, predictive and insightful models can be developed that allow calculations to be performed on a laptop computer. We will show how these models can help to develop nano-photonic space solar cells that are more radiation hard; to better understand quantum-dot intermediate-band solar cells and improve them using light trapping; and to understand and control black-body emission in silicon solar cells.

The performance of hybrid photovoltaic-thermal systems can be improved using PV cells that are specially designed to generate both electricity and useful heat with maximum efficiency. Present systems, however, use standard PV cells that are only optimized for electrical performance. In this work, we have developed two cell-level components that will improve the thermal efficiency of PV-T collectors, with minimal loss of electrical efficiency. These are a spectrally-selective low-emissivity coating to reduce radiative thermal losses, and a nanotextured rear reflector to improve absorption of the near-infrared part of the solar spectrum for heat generation.

Over the last couple of decades, there has been an intense research on strain balanced semiconductor quantum wells (QW) to increase the efficiency of multi-junction solar (MJ) solar cells grown monolithically on germanium. So far, the most successful application of QWs have required just to tailor a few tens of nanometers the absorption edge of a given subcell in order to reach the optimum spectral position. However, the demand for higher efficiency devices requiring 3, 4 or more junctions, represents a major difference in the challenges QWs must face: tailoring the absorption edge of a host material is not enough, but a complete new device, absorbing light in a different spectral region, must be designed. Among the most important issues to solve is the need for an optically thick structure to absorb enough light while keeping excellent carrier extraction using highly strained materials. Improvement of the growth techniques, smarter device designs - involving superlattices and shifted QWs, for example - or the use of quantum wires rather than QWs, have proven to be very effective steps towards high efficient MJ solar cells based on nanostructures in the last couple of years. But more is to be done to reach the target performances. This work discusses all these challenges, the limitations they represent and the different approaches that are being used to overcome them.

We present a study of melt grown dilute nitride InGaAsN layers by x-ray photoelectron spectroscopy (XPS), Raman and photoluminescence (PL) spectroscopy. The purpose of the study is to determine the degree of atomic ordering in the quaternary alloy during the epitaxial growth at near thermodynamic equilibrium conditions and its influence on band gap formation. Despite the low In concentration (~3%) the XPS data show a strong preference toward In–N bonding configuration in the InGaAsN samples. Raman spectra reveal that most of the N atoms are bonded to In instead of Ga atoms and the formation of N-centred In3Ga1 clusters. PL measurements reveal smaller optical band gap bowing as compared to the theoretical predictions for random alloy and localised tail states near the conduction band minimum.

Multi-quantum wells structures provide some flexibility for adjusting the absorption edge of multi-junction sub-cells. In order to obtain the desirable performance, it is essential to have an accurate model of the MQW properties and of the solar cell as a whole, including the light absorption, carrier extraction and carrier collection mechanisms. In this work, we show that Shockley-Read-Hall (SRH) recombination and insufficient light absorption are the main limiting factors for achieving high currents. The former can be reduced by a smart placement of the QWs inside the structure. By leaving a gap in the MQW stack, where SRH recombination is maximum, an improvement of the current at the maximum power point can be achieved without adding QWs. Increasing their number enhances light absorption but also the thickness of the device and the difficulty for carrier transport across the QW region. In this case, knowing the background doping and the carrier mobilities help to make an optimum solar cell design. In particular, we find than an intentional, low doping might lead to higher currents with short QW stacks than using a longer ones on an intrinsic region.

Quantum wires (QWRs) form naturally when growing strain balanced InGaAs/GaAsP multi-quantum wells (MQW) on GaAs [100] 6° misoriented substrates under the usual growth conditions. The presence of wires instead of wells could have several unexpected consequences for the performance of the MQW solar cells, both positive and negative, that need to be assessed to achieve high conversion efficiencies. In this letter, we study QWR properties from the point of view of their performance as solar cells by means of transmission electron microscopy, time resolved photoluminescence and external quantum efficiency (EQE) using polarised light. We find that these QWRs have longer lifetimes than nominally identical QWs grown on exact [100] GaAs substrates, of up to 1?s, at any level of illumination. We attribute this effect to an asymmetric carrier escape from the nanostructures leading to a strong 1D-photo-charging, keeping electrons confined along the wire and holes in the barriers. In principle, these extended lifetimes could be exploited to enhance carrier collection and reduce dark current losses. Light absorption by these QWRs is 1.6 times weaker than QWs, as revealed by EQE measurements, which emphasises the need for more layers of nanostructures or the use light trapping techniques. Contrary to what we expected, QWR show very low absorption anisotropy, only 3.5%, which was the main drawback a priori of this nanostructure. We attribute this to a reduced lateral confinement inside the wires. These results encourage further study and optimization of QWRs for high efficiency solar cells. © 2014 AIP Publishing LLC.

On the basis of optical characterization experiments and an eight band kp model, we have studied the effect of Sb incorporation on the electronic structure of InAs quantum dots (QDs). We have found that Sb incorporation in InAs QDs shifts the hole wave function to the center of the QD from the edges of the QD where it is otherwise pinned down by the effects of shear stress. The observed changes in the ground-state energy cannot merely be explained by a composition change upon Sb exposure but can be accounted for when the change in lateral size is taken into consideration. The Sb distribution inside the QDs produces distinctive changes in the density of states, particularly, in the separation between excitation shells. We find a 50% increase in the thermal escape activation energy compared with reference InAs quantum dots as well as an increment of the fundamental transition decay time with Sb incorporation. Furthermore, we find that Sb incorporation into quantum dots is strongly nonlinear with coverage, saturating at low doses. This suggests the existence of a solubility limit of the Sb incorporation into the quantum dots during growth. © 2013 American Physical Society., We acknowledge support by MINECO (Projects No. TEC2011-29120-C05-04 and No. ENE2012-37804-C02-02) andCAM(ProjectsNo. S2009ESP-1503 and No. S2009/ENE-1477).

We present an experimental and theoretical study of GaInAs(Sb)N layers with thickness around 2 mu m, grown by liquid-phase epitaxy (LPE) on n-type GaAs substrates. The samples are studied by surface photovoltage (SPV) spectroscopy and by photoluminescence spectroscopy. A theoretical model for the band structure of Sb-containing dilute nitrides is developed within the semi-empirical tight-binding approach in the sp(3)d(5)s*s(N) parameterisation and is used to calculate the electronic structure for different alloy compositions. The SPV spectra measured at room temperature clearly show a red shift of the absorption edge with respect to the absorption of the GaAs substrate. The shifts are in agreement with theoretical calculations results obtained for In, Sb and N concentrations corresponding to the experimentally determined ones. Photoluminescence measurements performed at 300K and 2 K show a smaller red shift of the emission energy with respect to GaAs as compared to the SPV results. The differences are explained by a tail of slow defect states below the conduction band edge, which are probed by SPV, but are less active in the PL experiment. Funding Agencies|research fund of the Sofia University [114/2016]; COST action 1406 MutiscaleSolar

Multiple-quantum-wells (MQW) in multijunction solar cells allow the absorption edges of the subcells to be tuned individually to maximize the energy yields. In this paper, we present the properties a quantum well material for InGaP quantum well solar cells and the performances of the dual-junction dual-MQW devices. We demonstrate that this dual-junction device can achieve 28% efficiency at 100 suns, which can be further improved by reducing the series resistance in the device.

The luminescent materials that have been used for luminescent down-shifting (LDS) range from organic molecules and organometallic complexes to quantum dots (QDs), each of them with certain strengths and weaknesses. In this work, we introduce five figures of merit to quantitatively evaluate how the properties of these materials compare with the desirable properties of an optimum LDS layer, focusing our attention in the cadmium sulfide/cadmium telluride (CdS/CdTe) PV technology. In order to elaborate a quantitative ranking with the candidate materials, we have used a ray-tracing simulation software to calculate the expected enhancement in a CdTe module conversion efficiency. Finally, we briefly discuss the issues that directly affect the industrial application of these dyes for LDS, such as the amount of material required, its cost and its stability under the sunlight.

In this work we address the performance of luminescence down shifting (LDS) layers in combination with cadmium sulfide/cadmium telluride (CdS/CdTe) solar cells as a function of the solar spectrum irradiance and power distribution, as would be the case in a real outdoor situation. To this purpose we have simulated the module efficiency when a CdS/CdTe mini-module is illuminated with a solar spectrum characteristic of different hours of the day and for five distinct days. Our results indicate that the LDS layer improves the conversion efficiency of the module in all scenarios (between 6 and 10%), where the improvement is most prominent at dawn and dusk (more than 20% on cloudy/summer days). The reason for this variation lies in the power distribution of the spectra, having a greater contribution of short-wavelength light in the morning or late in the afternoon. Under such blue-rich spectra the LDS layers operate very efficiently. Furthermore, we find that the relative efficiency improvement induced by an LDS layer has a roughly linear dependence with the average photon energy of the solar spectrum.

The 3D distribution of self-assembled stacked quantum dots (QDs) is a key parameter to obtain the highest performance in a variety of optoelectronic devices. In this work, we have measured this distribution in 3D using a combined procedure of needle-shaped specimen preparation and electron tomography. We show that conventional 2D measurements of the distribution of QDs are not reliable, and only 3D analysis allows an accurate correlation between the growth design and the structural characteristics., This work was supported by the Spanish MINECO (projects TEC2011-29120-C05-03 and Consolider Ingenio 2010 CSD2009-00013) and the Junta de Andalucía (PAI research group TEP-946 INNANOMAT). TEM measurements were carried out at DME-SCCYT-UCA.

For the first time a luminescent down-shifting (LDS) layer has been applied to a full size PV module to improve its short-wavelength response. An average 4.3% relative increase in the short-circuit current density (J SC ) was recorded for the Calyxo cadmium telluride (CdTe) modules after the best LDS layer was applied. The LDS layers consisted of a 0.5 mm thick ethylene vinyl acetate (EVA) host doped with Lumogen F Violet 570 and Yellow 083 dyes with a 0.127 mm thick top encapsulation sheet of fluorinated ethylene propylene (FEP). The same LDS layer was also applied to mini-modules (10cm × 10 cm) cut from production line modules, sold as CX3, which resulted in a J SC increase of 7%. LDS layers containing each of the dyes alone were also applied to both full size and mini-modules.

This research was sponsored by the Office of Basic Energy Sciences, Materials Sciences and Engineering Division, U.S. Department of Energy (SJP, MV), by the Spanish MCI (projects CONSOLIDER INGENIO 2010 CSD2009-00013 andTEC2008-06756-C03-02/TEC,) and the Junta de Andalucía (PAI research’s groups TEP-120 and TIC-145; project P08-TEP-03516).

3 páginas, 4 figuras, Quantum posts are assembled by epitaxial growth of closely spaced quantum dot layers, modulating the composition of a semiconductor alloy, typically InGaAs. In contrast with most self-assembled nanostructures, the height of quantum posts can be controlled with nanometer precision, up to a maximum value limited by the accumulated stress due to the lattice mismatch. Here, we present a strain compensation technique based on the controlled incorporation of phosphorous, which substantially increases the maximum attainable quantum post height. The luminescence from the resulting nanostructures presents giant linear polarization anisotropy., We acknowledge the financial support by MICINN (Grant Nos./ TEC2008-06756-C03-01/03, ENE2009-14481-C02-02, CSD2006-0004, CSD2006-0019, TEC2008-06756-C03-02/TEC, and CSD2009-00013), CAM (Grant No. S2009ESP-1503 and S2009/ENE-1477), CSIC (Grant No. PIF 200950I154), and J. And. (PAI Research Group under Grant No. TEP-120 and P08-TEP-03516).

Using the mechano-optical stress sensor technique, we observe a counter-intuitive reduction of the compressive stress when InAs is deposited on GaAs (001) during the growth of quantum posts. Through modelling of the strain fields, we find that such anomalous behaviour can be related to the strain-driven detachment of In atoms from the crystal and their surface diffusion towards the self-assembled nanostructures., We acknowledge the financial support by Spanish MINECO through Grant Nos. ENE2012-37804-C02-02 and TEC2011-29120-C05-04, and by Spanish CAM through Grant Nos. S2009/ESP-1503 and S2009/ENE-1477.

The self assembly of quantum dots by heteroepitaxy of lattice-mismatched semiconductors is based on elastic energy relaxation, which spontaneously occurs at the growth front when the largest atoms in the crystal cluster together. Because a larger covalent radius is related to weaker bonds, and this is in turn fundamentally related to smaller bandgaps, the formation of quantum dots leads to a confinement potential for electrons and/or holes. This effect has applications ranging from ultralow threshold diode lasers to highly efficient solar cells and usually requires the stacking of multiple quantum dots. The number of layers is limited by the stress accumulated during growth due to the larger covalent radius of the atoms that constitute the quantum dots. This accumulated stress can be relieved by introducing in the epitaxial layers compensating atomic species with a smaller covalent radius, enabling a reduction of spacer layer thickness. In the limit of sub-nanometer spacer thickness, the quantum dots, which have a tendency to line up vertically, fuse into a quantum post. The current efforts to optimize the properties of strain balanced quantum dot stacks and quantum posts are reviewed. The stacking of self-assembled epitaxial quantum dots with sub-nanometer spacer layers leads to the formation of quasi-1D nanostructures or quantum posts (QP). Using a matrix material with a small lattice parameter for strain compensation, the length of the QPs can be increased upwards of 100 nm. The resulting anisotropic strain field favors the luminescence emitted with dominant transverse magnetic component in the [1-10] direction. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim., MICINN (TEC2008-06756-C03-01/03, ENE2009-14481-C02-02, CSD2006-0004, CSD2006-0019, TEC2008-06756-C03- 02/TEC and CSD2009-00013), CAM (S2009ESP-1503,4 S2009/ENE-1477), CSIC (PIF 200950I154) and J. And. (PAI research group TEP-120 and P08-TEP-03516).

Trabajo presentado al "Quantum Dot", celebrado en Nottingham (UK) del 26 al 30 de Abril de 2010., InAs/GaAs quantum dots exposed to Sb after growth exhibit spectral changes. We study in the present paper an idealized nanostructure consisting of a homogeneous distribution of the quaternary GaInAsSb surrounded by a barrier of GaAs. We nd that the valence band o set is a critical parameter in modelling its electronic structure. Depending on this value, we predict a transition from type-I to type-II band alignment at a di erent Sb concentration. The addition of Sb to reduce the transition energy while keeping a type-I alignment is only of bene t at low Sb concentration, This work was supported by the Spanish MICINN (projects TEC2008-06756-C03-01/02/TEC, CONSOLIDER INGENIO 2010 CSD2006-0019 and CSD2009-00013), the Junta de Andalucía (PAI research groups TEP-120 and TIC-145; project P08-TEP-03516) and Comunidad Autónoma de Madrid S2009ESP-1503.

Trabajo presentado al 35th PVSC, celebrado en Honolulu (US) del 20 al 25 de Junio de 2010., In this work we present strain balanced InAs quantum post of exceptional length in the context of photovoltaics. We discuss the general properties of these nanostructures and their impact in the practical implementation of an intermediate band solar cell. We have studied the photocurrent generated by strain balanced quantum posts embedded in a GaAs single crystal, and compared our results with quantum dot based devices. The incorporation of phosphorous in the matrix to partially compensate the accumulated stress enables a significant increase of the quantum post maximum length. The relative importance of tunneling and thermal escape processes is found to depend strongly on the geometry of the nanostructures., We acknowledge support by MICINN (NANOGEFFES ENE2009-14481-C02-02, TEC2008-06756-C03-02/TEC, Consolider GENESIS MEC CSD2006-0004, Consolider CSD2009-00013), CAM (Numancia 2 S2009/ENE-1477), CSIC (PIF 200950I154) and Junta de Andalucía (PAI research group TEP-120, project P08-TEP-03516).

Trabajo presentado en el 33rd IEEE Photovoltaic Specialists Conference, celebrado en San Diego, CA (Estados Unidos), del 11 al 16 de mayo de 2008, In this work we report the stacking of 10 and 50 InAs quantum dots layers using 2 monolayers of GaP for stress compensation and a stack period of 18 nm on GaAs (001) substrates. Very good structural and optical quality is found in both samples. Vertical alignment of the dots is observed by transmission electron microscopy suggesting the existence of residual stress around them. Photocurrent measurements show light absorption up to 1.2 µm in the nanostructures together with a reduction in the blue response of the device. As a result of the phosphorus incorporation in the barriers, a very high thermal activation energy (431 meV) has also been obtained for the quantum dot emission., The authors gratefully acknowledge financial support by the Spanish MEC and CAM through projects 200560M089, S-05050/ENE-0310, TEC-2005-05781-C03- 01 and -02, Consolider-Ingenio 2010 CSD2006-0004, the Junta de Andalucia (project TEP383, group TEP120) and by the European Commission through the SANDIE Network of Excellence (NMP4-CT-2004-500101).

3 páginas, 3 figuras., In this work we report the stacking of 50 InAs/GaAs quantum dot layers with a GaAs spacer thickness of 18 nm using GaP monolayers for strain compensation. We find a good structural and optical quality of the fabricated samples including a planar growth front across the whole structure, a reduction in the quantum dot size inhomogeneity, and an enhanced thermal stability of the emission. The optimized quantum dot stack has been embedded in a solar cell structure and we discuss the benefits and disadvantages of this approach for high efficiency photovoltaic applications., The authors gratefully acknowledge financial support by the Spanish MEC and CAM through Projects 200560M089, S-05050/ENE-0310, TEC-2005-05781-C03-01, and -02, Consolider-Ingenio 2010 CSD2006-0004, the Junta de Andalucia (Project TEP383, Group TEP120), and by the European Commission through the SANDIE Network of Excellence (NMP4-CT-2004-500101).

We have studied the emission and absorption properties of type II GaSb/GaAs quantum dots embedded in a p-i-n photodiode. The excitation power evolution provides clear signatures of the spatially separated confinement of electrons and holes in these nanostructures. We have estimated the confinement potential for the holes to be ~500 meV, leading to an intense room temperature emission assisted by recapture processes from the wetting layer. Photocurrent measurements show strong absorption in the wetting layer and in the quantum dots at room temperature which are important for photodetection applications based in this system, The authors gratefully acknowledge financial support by the Spanish MEC and CAM through projects 200560M089, S-05050/ENE-0310, TEC-2005-05781-C03-01, Consolider-Ingenio 2010 CSD2006-0004, and by the European Commission through the SANDIE Network of Excellence (NMP4-CT-2004-500101).

The authors have studied the use of antimony for the optimization of the InAs/GaAs(001) self-assembled quantum dot (QD) luminescence characteristics in the 1.3 µm spectral region. The best results have been obtained by capping InAs QDs with 2 ML of GaSb grown on top of a 3 ML GaAs barrier separating the InAs and the GaSb layers. This results in an order of magnitude enhancement of the room temperature luminescence intensity at 1.3 µm emission wavelength., Support by CAM (Project Nos. 200560M089, S-505/ENE-310, S-505/ESP/000200), by MEC (Project Nos. TEC-2005-05781-C03-01, NAN2004-09109-C04-01, Consolider-Ingenio 2010 CSD2006-00019, RyC2003) and by the European Commission through the SANDIE Network of Excellence (NMP4-CT-2004-500101)is acknowledged. This work would have not been possible without the expert help of F. Briones and D. Fuster.

Multi-junction solar cells achieve high efficiency by stacking sub-cells of different bandgaps (typically GaInP/GaAs/Ge) resulting in efficiencies in excess of 40%. The efficiency can be improved by introducing a 1 eV absorber into the stack, either replacing Ge in a triple-junction configuration or on top of Ge in a quad-junction configuration. GaAs0.94Bi0.06 yields a direct-gap at 1 eV with only 0.7% strain on GaAs and the feasibility of the material has been demonstrated from GaAsBi photodetector devices. The relatively high absorption coefficient of GaAsBi suggests sufficient current can be generated to match the sub-cell photocurrent from the other sub-cells of a standard multi-junction solar cell. However, minority carrier transport and background doping levels place constraints on both p/n and p-i-n diode configurations. In the possible case of short minority carrier diffusion lengths we recommend the use of a p-i-n diode, and predict the material parameters that are necessary to achieve high efficiencies in a GaInP/GaAs/GaAsBi/Ge quad-junction cell.