Carlos Algora

A GaAs solar cell with an efficiency of 26.2% at 1000 suns and 25.0% at 2000 suns

A GaAs solar cell without prismatic covers, with the highest efficiency known to the authors in the range of 1000-2000 suns for a single junction, is presented. Low temperature liquid phase epitaxy is used for its growth. In addition to improvements such as the achievement of a good quality material or a low contact resistance, this solar cell exhibits specific enhanced aspects. Among the most noticeable are: (1) an innovative design; (2) a double and gradual emitter layer; (3) a small size: 1 mm/sup 2/, (4) a finger width of the front metal grid of 3 /spl mu/m; and (5) a tailored ARC deposition based on a nondestructive and accurate AlGaAs window layer characterization. As a consequence, an efficiency of 26.2% at 1000 suns and 25.0% at 2000 suns AM1.5D (standard conditions) is achieved thanks mainly to a short-circuit current density at 1000 suns of 26.8 A/cm/sup 2/ (and 53.6 A/cm/sup 2/ at 2000 suns) with a simultaneous series resistance of 3 m/spl Omega//spl middot/cm/sup 2/.

A 3-D model for concentrator solar cells based on distributed circuit units

A three-dimensional (3-D) distributed model for high-concentrator solar cells based on elementary units made up of electrical circuits is presented. The recombination mechanisms are dealt with in detail, paying special attention to the perimeter properties. No ohmic effect is omitted making this a powerful simulation tool for concentrator solar cells. A shunt resistance is also included. The model allows the simulation of the external connections and nonuniform illumination profiles making this model very useful for optimizing future structures and technological processes. The proposed 3-D model is compared with a lumped, two-diode model in the simulation of a GaAs solar cell operating from 1 to 2000 suns. It is found that the 3-D distributed model agrees satisfactorily with the experimental data for all concentrations. The agreement cannot be made simultaneously for both low and high concentrations for the lumped model.

Fiber-based 205-mW (27% efficiency) power-delivery system for an all-fiber network with optoelectronic sensor units

An optical fiber power-delivery system has been developed. An analysis of the spectral response of every component in the system has been carried out. Experimental measurements of the system are presented. We obtained 205 mW of power (5.4 V, 38.3 mA), yielding 27.4% efficiency. As an application, a sensor module is optically powered. This is an electrically isolated system, inasmuch as it also sends the measured data through a fiber. Several other applications are envisaged in the fields of aerospace, avionics, and domotics.

Tunnel Diode Modeling, Including Nonlocal Trap-Assisted Tunneling: A Focus on III–V Multijunction Solar Cell Simulation

Multijunction solar cells (MJCs) based on III-V semiconductors constitute the state-of-the-art approach for high-efficiency solar energy conversion. These devices, consisting of a stack of various solar cells, are interconnected by tunnel diodes. Reliable simulations of the tunnel diode behavior are still a challenge for solar cell applications. In this paper, a complete description of the model implemented in Silvaco ATLAS is shown, demonstrating the importance of local and nonlocal trap-assisted tunneling. We also explain how the measured doping profile and the metalization-induced series resistance influence the behavior of the tunnel diodes. Finally, we detail the different components of the series resistance and show that this can help extract the experimental voltage drop experienced by an MJC due to the tunnel junction. The value of this intrinsic voltage is important for achieving high efficiencies at concentrations near 1000 suns.

The influence of monolithic series connection on the efficiency of GaAs photovoltaic converters for monochromatic illumination

This paper presents a theoretical study of the performance and optimization of monolithically series connected GaAs photovoltaic converters under homogeneous monochromatic illumination. The effects of base resistance, perimeter recombination, and isolation trench optical losses on device efficiency are especially highlighted. All the calculations are made for values of the number of individual photovoltaic converters connected in series, n, of 1 (such is the case of a conventional GaAs photovoltaic converter without monolithic connection) 2, 3, and 6. The results show that the losses in monolithic connection can be minimized by means of an increase in device area together with the inclusion of a highly doped lateral conduction layer, and that monolithic connection does not lead to a greater immunity from high series resistance relative to a conventional photovoltaic converter (as stated in literature). The maximum efficiencies predicted are 60.2, 58.9, 58.5, and 57.5% for n=1, 2, 3, and 6, respectively, for an illumination power density of between 10 and 20 W/cm/sup 2/ and a wavelength of 830 nm. Nevertheless, if a dc-dc converter is considered to boost the voltage of an n=1 GaAs photovoltaic converter (with losses associated to this circuit usually of 20%), its maximum efficiency drops to 48.2%.

Modeling of the resistive losses due to the bus-bar and external connections in III-V high-concentrator solar cells

A model for the analysis of the resistive losses due to current flow through the bus-bar and external connections, mainly focused on III-V high-concentrator solar cells, is presented. Initially, a formulation that takes two-dimensional current flow into account in the bus-bar is proposed for calculating the bus-bar equivalent resistance. Next, a simplification assuming a one-dimensional (1-D) current flow is considered and a fully analytical model is obtained. Then, both models are compared and the applicability range of the 1-D approximation is established. The model is then applied to the analysis of the inverted square grid with several configurations for the external connections. Finally, the potential of the model is illustrated with a set of simulations carried out using a 1000/spl times/ concentrator GaAs solar cell and a number of conclusions of practical and technological interest are extracted.

Analysis of wirebonding techniques for contacting high concentrator solar cells

The results of the wirebonding technique application to make connections on high concentrator solar cells are presented in this paper. The most usual wirebonding techniques in current microelectronic industry have been analyzed (i.e., aluminum wedge bonding and thermosonic gold ball bonding). In the first part of the paper the influence of wirebonding processes on solar cell performance is discussed theoretically. The theoretical approach is followed by a comprehensive experimental analysis, which focuses on: a) the influence of mechanical damage on the device I-V curve; b) bondability analysis; c) the influence of wirebonding damage on device lifetime; d) analysis of the electrical resistance of the connection. Finally some conclusions of practical interest are drawn and the main results of the work are summarized.

Microplasma breakdown in high-concentration III-V solar cells

III-V high-concentration solar cells have demonstrated a significant degree of technological maturity. However, before their implantation on an industrial scale, these devices need to go through many tests in order to prove their reliability. While carrying out these tests in reverse bias, microplasma breakdown was found in these devices. Therefore, this letter presents the microplasma breakdown, never reported before, for III-V solar cells. It gives an explanation to such an anomalous reverse I-V curve and analyzes its future influence in the device degradation.

Preliminary temperature accelerated life test (ALT) on lattice mismatched triple-junction concentrator solar cells-on-carriers

A temperature accelerated life test on concentrator lattice mismatched Ga0.37In0.63P/Ga0.83In0.17As/Ge triple-junction solar cells-on-carrier is being carried out. The solar cells have been tested at three different temperatures: 125, 145 and 165°C and the nominal photo-current condition (500X) is emulated by injecting current in darkness. The final objective of these tests is to evaluate the reliability, warranty period, and failure mechanism of these solar cells in a moderate period of time. Up to now only the test at 165°C has finished. Therefore, we cannot provide complete reliability information, but we have carried out preliminary data and failure analysis with the current results.

Evaluation of the reliability of commercial concentrator triple-junction solar cells by means of accelerated life tests (ALT)

A temperature accelerated life test on commercial concentrator lattice-matched GaInP/GaInAs/Ge triple-junction solar cells has been carried out. The solar cells have been tested at three different temperatures: 119, 126 and 164 °C and the nominal photo-current condition (820 X) has been emulated by injecting current in darkness. All the solar cells have presented catastrophic failures. The failure distributions at the three tested temperatures have been fitted to an Arrhenius-Weibull model. An Arrhenius activation energy of 1.58 eV was determined from the fit. The main reliability functions and parameters (reliability function, instantaneous failure rate, mean time to failure, warranty time) of these solar cells at the nominal working temperature (80 °C) have been obtained. The warranty time obtained for a failure population of 5 % has been 69 years. Thus, a long-term warranty could be offered for these particular solar cells working at 820 X, 8 hours per day at 80 °C.