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.

Temperature of solar cells with regard to photoactive and non-photoactive light absorption in concentrator PV modules

A new method has recently been proposed by us for accurate measurement of the solar cell temperature in any operational regime, in particular, at a maximum power point (MPP) of the I-V curve (T-p-n(MPP)). For this, fast switching of a cell from MPP to open circuit (OC) regime is carried out and open circuit voltage V-oc is measured immediately (within about 1 millisecond), so that this value becomes to be an indicator of T-p-n(MPP). In the present work, we have considered a practical case, when a solar cell is heated not only by absorption of light incident upon its surface (called "photoactive" absorption of power), but also by heat transferred from structural elements surrounding the cell and heated by absorption of direct or diffused sunlight ("non-photoactive" absorption of power with respect to a solar cell). This process takes place in any concentrator module with non-ideal concentrators. Low overheating temperature of the p-n junction (or p-n junctions in a multijunction cell) is a cumulative parameter characterizing the quality of a solar module by the factor of heat removal effectiveness and, at the same time, by the factor of low "non-photoactive" losses.

Energy yield determination of concentrator solar cells using laboratory measurements

The annual energy conversion efficiency is calculated for a four junction inverted metamorphic solar cell that has been completely characterized in the laboratory at room temperature using measurements fit to a comprehensive optoelectronic model of the multijunction solar cells. A simple model of the temperature dependence is used redict the performance of the solar cell under varying temperature and spectra characteristic of Golden, CO for an entire year. The annual energy conversion efficiency is calculated by integrating the predicted cell performance over the entire year. The effects of geometric concentration, CPV system thermal characteristics, and luminescent coupling are ighlighted.

Implications of redesigned, high-radiative-efficiency GaInP junctions on III-V multijunction concentrator solar cells

Nonradiative recombination in inverted GaInP junctions is dramatically reduced using a rear-heterojunction design rather than the more traditional thin-emitter homojunction design. When this GaInP junction design is included in inverted multijunction solar cells, the high radiative efficiency translates into both higher subcell voltage and high luminescence coupling to underlying subcells, both of which contribute to improved performance. Subcell voltages within two and four junction devices are measured by electroluminescence and the internal radiative efficiency is quantified as a function of recombination current using optical modeling. The performance of these concentrator multijunction devices is compared with the Shockley–Queisser detailed-balance radiative limit, as well as an internal radiative limit, which considers the effects of the actual optical environment in which a perfect junction may exist.

Extended Triple-Junction Solar Cell 3D Distributed Model: Application to Chromatic Aberration-Related Losses

An extended 3D distributed model based on distributed circuit units for the simulation of triple‐junction solar cells under realistic conditions for the light distribution has been developed. A special emphasis has been put in the capability of the model to accurately account for current mismatch and chromatic aberration effects. This model has been validated, as shown by the good agreement between experimental and simulation results, for different light spot characteristics including spectral mismatch and irradiance non‐uniformities. This model is then used for the prediction of the performance of a triple‐junction solar cell for a light spot corresponding to a real optical architecture in order to illustrate its suitability in assisting concentrator system analysis and design process.

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.

XPS as Characterization Tool for PV: From the Substrate to Complete III-V Multijunction Solar Cells

This contribution aims to illustrate the potential of the X-ray photoelectron spectroscopy (XPS) technique as a tool to analyze different parts of a solar cell (surface state, heterointerfaces, profile composition of ohmic contacts, etc). Here, the analysis is specifically applied to III-V multijunction solar cells used in concentrator systems. The information provided from such XPS analysis has helped to understand the physico-chemical nature of these surfaces and interfaces, and thus has guided the technological process in order to improve the solar cell performance.

Towards 35% Efficient Concentrator Modules Based On LPI Köhler Concentrators With External Confinement Cavities

Multijunction solar cells present a certain reflectivity on its surface that lowers its light absorption. This reflectivity produces a loss in electrical efficiency and thus a loss in global energy production for CPV systems. We present here an optical design for recovering this portion of reflected light, and thus leading to a system efficiency increase. This new design is based on an external confinement cavity, an optical element able to redirect the light reflected by the cell towards its surface again. We have proven the excellent performance of these cavities integrated in CPV modules offering outstanding results: 33.2% module electrical efficiency @Tcell  =  25 °C and relative efficiency and Isc gains of over 6%

Back contacted emitter GaAs solar cells

A new device structure to improve the performance of concentrator GaAs solar cells is described and the first experimental results are reported. The reason for such an improvement relies on a drastic reduction of the shadowing and series resistance losses based on the possibility of back contacting the emitter region of the solar cell. The experimental results obtained with devices of these types, with a simplified structure, fabricated by liquid phase epitaxy, demonstrate the feasibility and correct operation of the proposed back contact of the emitter of the cells.

Research on intermediate band solar cells and development of experimental techniques for their characterization under concentrated illumination

Abstract This work is a contribution to the research and development of the intermediate band solar cell (IBSC), a high efficiency photovoltaic concept that features the advantages of both low and high bandgap solar cells. The resemblance with a low bandgap solar cell comes from the fact that the IBSC hosts an electronic energy band -the intermediate band (IB)- within the semiconductor bandgap. This IB allows the collection of sub-bandgap energy photons by means of two-step photon absorption processes, from the valence band (VB) to the IB and from there to the conduction band (CB). The exploitation of these low energy photons implies a more efficient use of the solar spectrum. The resemblance of the IBSC with a high bandgap solar cell is related to the preservation of the voltage: the open-circuit voltage (VOC) of an IBSC is not limited by any of the sub-bandgaps (involving the IB), but only by the fundamental bandgap (defined from the VB to the CB). Nevertheless, the presence of the IB allows new paths for electronic recombination and the performance of the IBSC is degraded at 1 sun operation conditions. A theoretical argument is presented regarding the need for the use of concentrated illumination in order to circumvent the degradation of the voltage derived from the increase in the recombi¬nation. This theory is supported by the experimental verification carried out with our novel characterization technique consisting of the acquisition of photogenerated current (IL)-VOC pairs under low temperature and concentrated light. Besides, at this stage of the IBSC research, several new IB materials are being engineered and our novel character¬ization tool can be very useful to provide feedback on their capability to perform as real IBSCs, verifying or disregarding the fulfillment of the “voltage preservation” principle. An analytical model has also been developed to assess the potential of quantum-dot (QD)-IBSCs. It is based on the calculation of band alignment of III-V alloyed heterojunc-tions, the estimation of the confined energy levels in a QD and the calculation of the de¬tailed balance efficiency. Several potentially useful QD materials have been identified, such as InAs/AlxGa1-xAs, InAs/GaxIn1-xP, InAs1-yNy/AlAsxSb1-x or InAs1-zNz/Alx[GayIn1-y]1-xP. Finally, a model for the analysis of the series resistance of a concentrator solar cell has also been developed to design and fabricate IBSCs adapted to 1,000 suns. Resumen Este trabajo contribuye a la investigación y al desarrollo de la célula solar de banda intermedia (IBSC), un concepto fotovoltaico de alta eficiencia que auna las ventajas de una célula solar de bajo y de alto gap. La IBSC se parece a una célula solar de bajo gap (o banda prohibida) en que la IBSC alberga una banda de energía -la banda intermedia (IB)-en el seno de la banda prohibida. Esta IB permite colectar fotones de energía inferior a la banda prohibida por medio de procesos de absorción de fotones en dos pasos, de la banda de valencia (VB) a la IB y de allí a la banda de conducción (CB). El aprovechamiento de estos fotones de baja energía conlleva un empleo más eficiente del espectro solar. La semejanza antre la IBSC y una célula solar de alto gap está relacionada con la preservación del voltaje: la tensión de circuito abierto (Vbc) de una IBSC no está limitada por ninguna de las fracciones en las que la IB divide a la banda prohibida, sino que está únicamente limitada por el ancho de banda fundamental del semiconductor (definido entre VB y CB). No obstante, la presencia de la IB posibilita nuevos caminos de recombinación electrónica, lo cual degrada el rendimiento de la IBSC a 1 sol. Este trabajo argumenta de forma teórica la necesidad de emplear luz concentrada para evitar compensar el aumento de la recom¬binación de la IBSC y evitar la degradación del voltage. Lo anterior se ha verificado experimentalmente por medio de nuestra novedosa técnica de caracterización consistente en la adquisicin de pares de corriente fotogenerada (IL)-VOG en concentración y a baja temperatura. En esta etapa de la investigación, se están desarrollando nuevos materiales de IB y nuestra herramienta de caracterizacin está siendo empleada para realimentar el proceso de fabricación, comprobando si los materiales tienen capacidad para operar como verdaderas IBSCs por medio de la verificación del principio de preservación del voltaje. También se ha desarrollado un modelo analítico para evaluar el potencial de IBSCs de puntos cuánticos. Dicho modelo está basado en el cálculo del alineamiento de bandas de energía en heterouniones de aleaciones de materiales III-V, en la estimación de la energía de los niveles confinados en un QD y en el cálculo de la eficiencia de balance detallado. Este modelo ha permitido identificar varios materiales de QDs potencialmente útiles como InAs/AlxGai_xAs, InAs/GaxIni_xP, InAsi_yNy/AlAsxSbi_x ó InAsi_zNz/Alx[GayIni_y]i_xP. Finalmente, también se ha desarrollado un modelado teórico para el análisis de la resistencia serie de una célula solar de concentración. Gracias a dicho modelo se han diseñado y fabricado IBSCs adaptadas a 1.000 soles.

Photovoltaic performance of the dome-shaped Fresnel-Köhler concentrator

In order to have a cost-effective CPV system, two key issues must be ensured: high concentration factor and high tolerance. The novel concentrator we are presenting, the dome-shaped Fresnel-Köhler, can widely fulfill these two and other essential issues in a CPV module. This concentrator is based on two previous successful CPV designs: the FK concentrator with a flat Fresnel lens and the dome-shaped Fresnel lens system developed by Daido Steel, resulting on a superior concentrator. The concentrator has shown outstanding simulation results, achieving an effective concentration-acceptance product (CAP) value of 0.72, and an optical efficiency of 85% on-axis (no anti-reflective coating has been used). Moreover, Köhler integration provides good irradiance uniformity on the cell surface and low spectral aberration of this irradiance. This ensures an optimal performance of the solar cell, maximizing its efficiency. Besides, the dome-shaped FK shows optimal results for very compact designs, especially in the f/0.7-1.0 range. The dome-shaped Fresnel-Köhler concentrator, natural and enhanced evolution of the flat FK concentrator, is a cost-effective CPV optical design, mainly due to its high tolerances. Daido Steel advanced technique for demolding injected plastic pieces will allow for easy manufacture of the dome-shaped POE of DFK concentrator.

Dome-shaped Fresnel-Köhler: a novel high performance optical concentrator

The dome-shaped Fresnel-Köhler concentrator is a novel optical design for photovoltaic applications. It is based on two previous successful CPV optical designs: the FK concentrator with a flat Fresnel lens and the dome-shaped Fresnel lens system developed by Daido Steel, resulting on a superior concentrator. This optical concentrator will be able to achieve large concentration factors, high tolerance (i.e. acceptance angle) and high optical efficiency, three key issues when dealing with photovoltaic applications. Besides, its irradiance is distributed on the cell surface in a very even way. The concentrator has shown outstanding simulation results, achieving an effective concentration-acceptance product (CAP) value of 0.72, on-axis optical efficiency over 85% and good irradiance uniformity on the cell provided by Köhler integration. Furthermore, due to its high tolerance, we will present the dome-shaped Fresnel-Köhler concentrator as a cost-effective CPV optical design. All this makes this concentrator superior to other conventional competitors in the current market.

NGCPV: A new generation of concentrator photovoltaic cells, modules and systems

This work introduces the lines of research that the NGCPV project is pursuing and some of the first results obtained. Sponsored by the European Commission under the 7th Framework Program and NEDO (Japan) within the first collaborative call launched by both Bodies in the field of energy, NGCPV project aims at approaching the cost of the photovoltaic kWh to competitive prices in the framework of high concentration photovoltaics (CPV) by exploring the development and assessment of concentrator photovoltaic solar cells and modules, novel materials and new solar cell structures as well as methods and procedures to standardize measurement technology for concentrator photovoltaic cells and modules. More specific objectives we are facing are: (1) to manufacture a cell prototype with an efficiency of at least 45% and to undertake an experimental activity, (2) to manufacture a 35% module prototype and elaborate the roadmap towards the achievement of 40%, (3) to develop reliable characterization techniques for III-V materials and quantum structures, (4) to achieve and agreement within 5% in the characterization of CPV cells and modules in a round robin scheme, and (5) to evaluate the potential of new materials, devices technologies and quantum nanostructures to improve the efficiency of solar cells for CPV.

1000x shadow-free mirror-based Köhler concentrator

A new design for a photovoltaic concentrator, the most recent advance based on the Kohler concept, is presented. The system is mirror-based, and with geometry that guaranties a maximum sunlight collection area (without shadows, like those caused by secondary stages or receivers and heat-sinks in other mirror-based systems). Designed for a concentration of 1000x, this off axis system combines both good acceptance angle and good irradiance uniformity on the solar cell. The advanced performance features (concentration-acceptance products ?CAP- about 0.73 and affordable peak and average irradiances) are achieved through the combination of four reflective folds combined with four refractive surfaces, all of them free-form, performing Köhler integration 2 . In Köhler devices, the irradiance uniformity is not achieved through additional optical stages (TIR prisms), thus no complex/expensive elements to manufacture are required. The rim angle and geometry are such that the secondary stage and receivers are hidden below the primary mirrors, so maximum collection is assured. The entire system was designed to allow loose assembly/alignment tolerances (through high acceptance angle) and to be manufactured using already well-developed methods for mass production, with high potential for low cost. The optical surfaces for Köhler integration, although with a quite different optical behavior, have approximately the same dimensions and can be manufactured with the same techniques as the more traditional secondary optical elements used for concentration (typically plastic injection molding or glass molding).