Beatriz Galiana

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.

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.

A comparative study of BSF layers for GaAs-based single-junction or multijunction concentrator solar cells

An effective back surface field is a key structural element for a high-efficiency GaAs concentrator solar cell, either in a multijunction or in a single-junction device. In this paper, several BSF materials are analysed, namely: (1) p++GaAs(Zn), (2) p+Ga0.5In0.5P(Zn) and (3) p++Al0.2Ga0.8As(C). The results of the comparison demonstrate that the best option is C-doped Al0.2Ga0.8As, which exhibits a low series resistance and behaves as an excellent minority carrier mirror; p++GaAs(Zn) shows reduced minority carrier mirror properties resulting from Zn diffusion and p+Ga0.5In0.5P(Zn) is shown to produce important series resistance problems because of an unfavourable heterojunction with GaAs.

III–V multijunction solar cells for ultra-high concentration photovoltaics

In this paper, the benefits of the ultra high concentration (¿ 1000 suns) are shown in terms of cost reduction and efficiency increase. Accordingly, the strategy followed at IES-UPM for more than 15 years is the development of III-V solar cells suitable for operation at 1000 suns or more. Recently, we have developed and manufactured a GaInP/GaAs dual-junction cell which results in an efficiency (measured at the Calibration Laboratory, CalLab, of Fraunhofer Institute in Freiburg) of 32.6% for a concentration range going from 499 to 1026 suns. This efficiency is the world record efficiency for a dual-junction solar cell. Besides, the efficiency is still as high as 31% at 3000 suns. The theoretical optimization of this solar cell (based on an accurate modelling) shows a potential efficiency over 36% at 1000 suns. We have extended this strategy to lattice-matched GaInP/Ga(In)As/Ge triple junction solar cells. First manufactured cells exhibit an efficiency of 31.5% at 1000 suns. The theoretical optimization of this cells show that an efficiency over 43% at 1000 suns is achievable. A roadmap has been established in order to reach this value.

Specific Growth and Characterization Issues in Multi-Junction Solar Cells for Concentrations Above 1000 Suns

This work presents some lines of research currently being investigated in our group that are aimed to contribute to a better performance of multi-junction solar cells at very high concentrations (~1000 suns) and, in addition, to achieve a better characterization of these devices also at high concentrations. The improvement of the performance of the solar cell at high concentrations is addressed in two ways. First, a set of results is presented to ascertain the potential of tellurium as a possible n-type dopant to improve the performance of tunnel junctions. Then, the contribution of the bottom cell BSF layer to the series resistance is analysed. On the other hand, the issue of the linearity of the short circuit current with the irradiance is investigated. This linearity is assessed by measuring the external quantum efficiency of solar cells with bias lights of different intensities

Simulating III–V concentrator solar cells: A comparison of advantages and limitations of lumped analytical models; distributed analytical models and numerical simulation

The simulation of the quantum efficiency and the I-V curves at several concentrations of a high efficiency concentrator dual-junction solar cell is presented using three different approaches: 1) analytic simulation with the one-diode model; 2) analytic simulation with distributed circuit models; and, 3) numerical simulation. The main advantages and limitations of each model are discussed and their performance is compared.

GaInP/GaInAs/Ge triple junction solar cells for ultra high concentration

In this paper we characterize the first functional, lattice matched, GaInP/GaInAs/Ge triple junction solar cells grown and manufactured in our lab with an efficiency conversion of 31.5% at a concentration level of 1000 suns. This is our first approach for transferring the world record double junction solar cell, also developed in our group, into a Ge substrate. First experimental results are presented and the strategy to improve its efficiency is outlined.

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.

MOVPE Technology for the Growth of III-V Semiconductor Structures

Metal-organic vapour phase epitaxy (MOVPE) is the most widely used technology for the growth of III-V compounds in the industry today, and has become the preferred choice for the mass fabrication of a wide range of devices. The I.E.S -U.P.M acquired a research-scale Aixtron MOVPE reactor in 2000 aiming the development of III-V multi-junction concentrator solar cells in a pilot production line. In this paper, a thorough review of the MOVPE technology is presented. Then, the specific configuration, its potentialities and the research being made today at the I.E.S -U.P.M is described. Finally, the achievements and future prospects are explained.

Choices for the epitaxial growth of GalnP/GaAs dual junction concentrator solar cells

In this work we analyse the different possibilities that the GalnP/GaAs double junction solar cell structures grow by MOCVD offers, so as to achieve energy conversion efficiencies higher than 30 %. These possibilities comprise changes in the configuration of layers as well as the variation of the different parameters that affect the epitaxy process, such as temperature, growing time, dopants, partial pressure of gases, etc.