Optical Transmittance Maximization in Superior Performance Tunnel Junctions for Very High Concentration Applications.

The light transmission through a tunnel junction in a multijunction solar cell depends on the optical properties and thickness of the whole solar cell layers stack, which configure the light absorption, reflection and interference processes taking place inside the semiconductor structure. In this paper the focus is put on the AlGaAs barrier layers of p++AlGaAs/n++GaAs and p++AlGaAs/n++GaInP tunnel junctions inserted into a GaInP/GaAs dualjunction solar cell. The aim is to analyze the effect of the thickness and Al-composition of these barrier layers on the light transmittance of the tunnel junction, using the bottom cell Jsc as the merit figure to appraise it. An intricate relation between this Jsc and the barrier layers parameters, caused by interferential reflectance, was observed. The importance of an appropriate optical design of the semiconductor structure was corroborated by a non-negligible gain in the bottom cell Jsc when choosing the appropriate barrier layers Al-compositions and thicknesses from a range of practical values for which the optical absorption is not the main contributor to the optical losses

Analysis of Chromatic Aberration Effects in Triple-Junction Solar Cells Using Advanced Distributed Models

The consideration of real operating conditions for the design and optimization of a multijunction solar cell receiver-concentrator assembly is indispensable. Such a requirement involves the need for suitable modeling and simulation tools in order to complement the experimental work and circumvent its well-known burdens and restrictions. Three-dimensional distributed models have been demonstrated in the past to be a powerful choice for the analysis of distributed phenomena in single- and dual-junction solar cells, as well as for the design of strategies to minimize the solar cell losses when operating under high concentrations. In this paper, we present the application of these models for the analysis of triple-junction solar cells under real operating conditions. The impact of different chromatic aberration profiles on the short-circuit current of triple-junction solar cells is analyzed in detail using the developed distributed model. Current spreading conditions the impact of a given chromatic aberration profile on the solar cell I-V curve. The focus is put on determining the role of current spreading in the connection between photocurrent profile, subcell voltage and current, and semiconductor layers sheet resistance

Distributed Simulation of Real Tunnel Junction Effects in Multi-Junction Solar Cells

In this paper, we present an improved 3D distributed model that considers real operation regimes in a tunnel junction. This advanced method is able to accurately simulate the high concentrations at which the current in the solar cell surpasses the peak current of the tunnel junction. Simulations of dual-junction solar cells were carried out with different light profiles and including chromatic aberration to show the capabilities of the model. Such simulations show that, under some circumstances, the solar cell short circuit current may be slightly higher than the tunnel junction peak current without showing the characteristic dip in the J-V curve. This behavior is caused by the lateral current spreading towards the dark regions, which occurs through the anode region of the tunnel junction

3-D modeling of perimeter recombination in GaAs diodes and its influence on concentrator solar cells

This paper describes a complete modelling of the perimeter recombination of GaAs diodes which solves most unknowns and suppresses the limitations of previous models. Because of the three dimensional nature of the implemented model, it is able to simulate real devices. GaAs diodes on two epiwafers with different base doping levels, sizes and geometries, namely square and circular are manufactured. The validation of the model is achieved by fitting the experimental measurements of the dark IV curve of the manufactured GaAs diodes. A comprehensive 3-D description of the occurring phenomena affecting the perimeter recombination is supplied with the help of the model. Finally, the model is applied to concentrator GaAs solar cells to assess the impact of their doping level, size and geometry on the perimeter recombination

Capacitance Measurements for Subcell Characterization in Multijunction Solar Cells.

On this paper we present an alternative way to analyze de electronic properties of each subcell from the complete device. By illuminating the cell with light sources which energy is near one of the subcell bandgaps, it is possible to “erase” the presence of such subcell on the CV curve. The main advantages of this technique are that it is not destructive, it can be measured on the complete cell so can be easily implemented as a diagnostic technique for controlling electronic deviations

Status of Ultra-High Concentrator Multijunction Solar Cell Development at IES-UPM.

After the successful implementation of a record performing dual-junction solar cell at ultra high concentration, in this paper we present the optimization of key aspects in the transition to a triple-junction device, namely the hetero nucleation of III-V structures onto germanium substrates. This optimization is based on in-situ RAS measurements during the MOVPE growth of the triple-junction solar cell structure and subsequent AFM analysis. The correlation between RAS and AFM allows detecting which RAS features correlate with good morphology and low RMS roughness. TEM analysis confirms that the quality of the triple-junction structures grown is good, revealing no trace of antiphase disorder, and showing flat, sharp and clear interfaces. Triple-junction solar cells manufactured on these structures have shown a peak efficiency of 36.2% at 700X, maintaining an efficiency over 35% from 300 to 1200 suns

Study of non-uniform light profiles on high concentration III-V solar cells using quasi-3D distributed models

The quasi-3D models based on distributed circuit units are a powerful tool to analyse the performance of a solar cell from the point of view of its electrical behavior. Quite accurate models have been developed in the past that reproduce the experimental data of single-junction solar cells very closely. These models help in the determination of the origin of the peculiarities of the dark and one sun or high concentration experimental I-V curves. They also allow the design of the front grid, the analysis of the impact of the electrical parameters of the solar cell on the performance of the final device, etc. In this work, these models are used to study the effect of non-uniform profiles, generated by the concentrator optics, on the performance of a concentrator solar cell. The design of the front grid is then optimized to minimize the losses introduced by the light distribution, even taking into account the effect of the tracking system misalignment. As an introductory application example of multijunction solar cells analysis with this kind of modeling, the effect of the chromatic aberration on a double junction solar cell is presented

Optimizing Bottom Subcells for III-V-on-Si MultiJunction Solar Cells

Dual-junction solar cells formed by a GaAsP or GaInP top cell and a silicon bottom cell seem to be attractive candidates to materialize the long sought-for integration of III-V materials on silicon for photovoltaic applications. Such integration would offer a cost breakthrough for photovoltaic technology, unifying the low cost of silicon and the efficiency potential of III-V multijunction solar cells. In this study, we analyze several factors influencing the performance of the bottom subcell of this dual-junction, namely, 1) the formation of the emitter as a result of the phosphorus diffusion that takes place during the prenucleation temperature ramp and during the growth of the III-V layers; 2) the degradation in surface morphology during diffusion; and 3) the quality needed for the passivation provided by the GaP layer on the emitter

Analysis of the behavior of multijunction solar cells under high irradiance Gaussian light profiles showing chromatic aberration with emphasis on tunnel junction performance

In this work, we explain the behavior of multijunction solar cells under non-uniform (spatially and in spectral content) light profiles in general and in particular when Gaussian light profiles cause a photo-generated current density, which exceeds locally the peak current density of the tunnel junction. We have analyzed the implications on the tunnel junction's limitation, that is, in the loss of efficiency due to the appearance of a dip in the I–V curve. For that, we have carried out simulations with our three-dimensional distributed model for multijunction solar cells, which contemplates a full description of the tunnel junction and also takes into account the lateral resistances in the tunnel junction. The main findings are that the current density photo-generated spreads out through the lateral resistances of the device, mainly through the tunnel junction layers and the back contact. Therefore, under non-uniform light profiles these resistances are determinant not only to avoid the tunnel junction's limitation but also for mitigating losses in the fill factor. Therefore, taking into account these lateral resistances could be the key for jointly optimizing the concentrator photovoltaic system (concentrator optics, front grid layout and semiconductor structure

Te doping of GaAs using metalorganic vapor phase epitaxy: volatile vs. non volatile

The incorporation of Te into the crystal lattice, when it is used as an n-type dopant for GaAs grown by metalorganic vapor phase epitaxy, is studied. For this purpose, several growth temperatures, total pressures, growth rates, and substrate misorientations have been analyzed, from which it is concluded that depending on the substrate misorientation and total pressure used, the Te behaves like a volatile dopant or a nonvolatile dopant as result of the enhancement or minimization of its adsorption onto the growth surface