Diego Alonso-Álvarez

Optical investigation of type II GaSb∕GaAs self-assembled quantum dots

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 ∼500meV, 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.

Carrier recombination effects in strain compensated quantum dot stacks embedded in solar cells

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.

Enhancement of the room temperature luminescence of InAs quantum dots by GaSb capping

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 1.3μm 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 1.3μm emission wavelength.

The Impact of Luminescent Down Shifting on the Performance of CdTe Photovoltaics: Impact of the Module Vintage

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%.

Requirements for a GaAsBi 1 eV sub-cell in a GaAs-based multi-junction solar cell

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.

Luminescent down-shifting for CdTe solar cells: A review of dyes and simulation of performance

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.

InGaAs/GaAsP strain balanced multi-quantum wires grown on misoriented GaAs substrates for high efficiency solar cells

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 μs, 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 lifetim...

Strain Balanced Epitaxial Stacks of Quantum Dots and Quantum Posts

Tesis doctoral inedita. Universidad Autonoma de Madrid, Facultad de Ciencias. (Departamento de Fisica de la Materia Condensada). Fecha de lectura: 11-11-2011

Strain balanced quantum posts

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.

Performance of luminescence down shifting for CdTe solar cells as a function of the incident solar spectrum

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.