This PDF file contains the front matter associated with SPIE Proceedings Volume 6649, including the Title Page, Copyright information, Table of Contents, Plenary Papers, and the Conference Committee listing.

Concentrator systems are emerging as a low-cost, high-volume option for solar-generated electricity due to the very high utilization of the solar cell, leading to a much lower $/Watt cost of a photovoltaic system. Parallel to this is the onset of alternative solar cell technologies, such as the very high efficiency multi-junction solar cells developed at NREL and Spectrolab over the last two decades. The relatively high cost of these types of solar cells has relegated their use to non-terrestrial applications. However, recent advancements in both multi-junction concentrator cell efficiency and their stability under high flux densities has made their large-scale terrestrial deployment significantly more viable. Amonix has designed, developed and fabricated modules using the high efficiency multi-junction cells from Spectrolab. One of these modules has been deployed at the University of Nevada, Las Vegas. The module has been in continuous operation beginning May 2006. The efficiency has been measured periodically and has shown a range from 26.1% to 28.5%. The latest measurement, made on February 20^th showed an efficiency of 28.0 % at 956 DNI and an ambient temperature of 13 °C. This excellent stability of the multi-junction module's performance promises to pave the way for future installations of this advanced technology. One short-term example of this is a new Amonix-designed module capable of 30% efficiency and 300 Watts per module. This module's performance, along with more testing of the long-term performance of the initial design will be presented at the time of the conference.

The commercial rooftop environment poses difficult challenges for concentrating photovoltaic (CPV) systems. Rooftop CPV must not only meet low cost and high energy production targets common to ground mounted systems but also must solve safety, wind loading, and area usage requirements in ways that are compatible with the rooftop environment. To meet these requirements we have developed a low-profile carousel-mounted array of Fresnel concentrators using triple junction solar cells. In this paper we describe the key features of the opto-mechanical and thermal design for manufacturability and reliability. These features include the concentration level, the mechanical drive scheme, the configuration of the lens with secondary optical element, and passive cooling. Also described are elements of the optical component testing and assembly methods. We present exemplary results of environmental testing and measurements of electro-optical performance.

The performance of the XR solar concentrator, using a high efficiency multi-junction solar cell developed recently by Spectrolab, is presented. The XR concentrator is an ultra-compact Nonimaging optical design composed of a primary mirror and a secondary lens, which can perform close to the thermodynamic limit of concentration (maximum acceptance angle for a given geometrical concentration). The expected acceptance angle of the concentrator is about ±2 deg for a geometrical concentration of 800x (a Fresnel lens and secondary system typically has ±0.6 deg of acceptance for 300x of geometrical concentration). This concentrator is optimized to improve the irradiance distribution on the solar cell keeping it under the maximum values the cell can accept. The XR concentrator has high manufacturing tolerance to errors and can be produced using low cost manufacturing techniques. The XR is designed with the Simultaneous Multiple Surface (SMS) design method of Nonimaging Optics. Its application to high-concentration photovoltaics is now being developed in a consortium led by The Boeing Company, which has recently been awarded a project by the US DOE in the framework of the Solar America Initiative.

Quantum Dot Solar Concentrators (QDSCs) are static, non-imaging concentrators which do not require expensive solar tracking and concentrate both direct and diffuse light. Optical efficiencies (η_opt) and concentration ratios (C) of a single plate QDSC were calculated by Monte-Carlo ray-trace modelling. Consideration of reflection, refraction, quantum dot (QD) photon emission and absorption and light attenuation in the device matrix were included in the analysis. In this paper, the effect of placing plane and diffuse reflectors at the rear surface was analyzed. Mirrors with a structured surface (saw-tooth shaped) were also modelled and the effect of each reflector type on C was determined, for direct and diffuse incident light. The diffuse and structured reflectors perform better than the plane reflector under direct light, but there is no significant difference under diffuse light. A spectrally selective reflector, placed at the top surface, reflects light emitted by QDs inside the escape cone back into the concentrator. For a particular set of model parameters, the model results show an increase in C of 13% due to the inclusion of a spectrally selective reflector.

Optimizing a concentrator system which uses multijunction solar cells is challenging because: (a) the conditions are variable, so the solar cells rarely operate under optimal conditions and (b) the conditions are not controlled, so any design problems are difficult to characterize. Any change in the spectral content of direct-beam sunlight as it passes through the concentrator optics is of particular interest, as it can reduce the performance of multijunction cells and is difficult to characterize. Here we show how the fill factor can be used to detect and diagnose this sort of a "spectral skewing" by the concentrator optics during outdoor operation. The work presented here is for GaInP₂/GaAs tandem cells, but the conclusions are equally valid for GaInP₂/GaAs/Ge triple-junction cells.

We present an experimental characterization study of high-efficiency ultra-small multi-junction concentrator solar cells. Particular emphasis is placed on the sensitivity of photovoltaic performance to the intensity and distribution of the concentrated sunlight delivered to the cell. This type of information is important in the design and optimization of the most recent generations of high-concentration photovoltaic systems. Miniaturizing the solar cell can give rise to an increase in the concentration value at which cell efficiency peaks, facilitates passive heat rejection and permits the use of all-glass optics. However, few measurements have been published for ultra-small cells. We report and analyze extensive measurements, up to approximately 5000 suns, on the 1.0 mm² active region within the busbars of such commercial concentrator solar cells.

A Photovoltaic (PV) cell design "package" that has the capability to triple power output (compared to traditional flat-mounted, Si-based PV panels) in half the square-footage at as much as 50% less cost (again, compared to traditional flat-mounted, Si-based PV panels) has been designed, built, prototyped, and tested.

InP/Si engineered substrates formed by wafer bonding and layer transfer have the potential to significantly reduce the cost and weight of III-V compound semiconductor solar cells. InP/Si substrates were prepared by He implantation of InP prior to bonding to a thermally oxidized Si substrate and annealing to exfoliate an InP thin film. Following thinning of the transferred InP film to remove surface damage caused by the implantation and exfoliation process, InGaAs solar cells lattice-matched to bulk InP were grown on these substrates using metal-organic chemical vapor deposition. The photovoltaic current-voltage characteristics of the InGaAs cells fabricated on the wafer-bonded InP/Si substrates were comparable to those synthesized on commercially available epi-ready InP substrates, and had a ~20% higher short-circuit current which we attribute to the high reflectivity of the InP/SiO₂/Si bonding interface. This work provides an initial demonstration of wafer-bonded InP/Si substrates as an alternative to bulk InP substrates for solar cell applications.

A Quantum Dot Solar Concentrator (QDSC) is based on the Luminescent Solar Concentrator (LSC), a concept first introduced in the 1960s. LSCs consist of a flat plate of polymer material doped with a luminescent dye. A percentage of incident insolation, absorbed and re-emitted by the dye molecules is trapped inside the plate by total internal reflection. Reflective material situated on three of the edges and the back surface increases the trapping efficiency of the plate. Through successive reflection events light is concentrated onto a photovoltaic (PV) cell positioned on the fourth edge of the plate. Degradation of luminescent dyes prevented LSCs from being fully developed. A QDSC replaces luminescent dyes with semiconductor nanocrystals known as quantum dots (QDs). Passivation of QD cores with shells of higher band gap material is expected to provide increased stability. QDs offer further advantages such as broad absorption spectra to utilize more of the solar spectrum and size tunability that allows spectral matching of the QDs emission to the peak efficiency of PV cells. Small-scale QDSCs have been fabricated using QDs bought commercially. The QDs have an emission wavelength of 600nm, close to the peak efficiency of a typical silicon PV cell. The systems were electrically characterized using a 4 cm monocrystalline PV cell optically matched to the QDSC edge with silicon oil. To investigate the effect of shape and size on concentrator efficiency, four different sized quadratic, two triangular and three circular QDSCs of various diameters were fabricated.

The Levelized Cost of Energy (LCOE) takes into account more than just the cost of power output. It encompasses product longevity, performance degradation and the costs associated with delivering energy to the grid tie point. Concentrator optical design is one of the key components to minimizing the LCOE, by affecting conversion efficiency, acceptance angle and the amount of energy concentrated on the receiver. Optical systems for concentrators, even those at high concentrations ( >350X) can be designed by straightforward techniques, and will operate under most circumstances. Adding requirements for generous acceptance angles, non-destructive off-axis operation, safety and high efficiency however, complicate the design. Furthermore, the demands of high volume manufacturing, efficient logistics, minimal field commissioning time and low cost lead to quite complicated, system level design trade-offs. The technology which we will discuss features an array of reflective optics, scaled to be fabricated by techniques used in the automotive industry. The design couples a two-element imaging system to a non-imaging total internal reflection tertiary in a very compact design, with generous tolerance margins. Several optical units are mounted in a housing, which protects the optics and assists with dissipating waste heat. This paper outlines the key elements in the design of SolFocus concentrator optics, and discusses tradeoffs and experience with various design approaches.

A Quantum Well GaAs/In_0.10GaAs single junction solar cell, with p-i-n structure, has been fabricated by metal-organic vapor-phase epitaxy (MOVPE). In this letter, we report on the study of a MQW solar cell structure with different thickness of i-layers and pairs of QWs, which was used to extend the absorption region and reduce recombination losses. The efficiency of varied design was discussed accompanying the carrier capture, carrier escape and radiative recombinations in QWs. The optimized design parameters of the solar cell structures were determined. The GaAs/InGaAs QW solar cell is proposed to extend the long-wavelength absorption, as a candidate for the next-generation high-efficiency multi-junction solar cell.

Concentration PV systems are emerging recently and primary lenses have been developed for concentrating solar incident. Although the authors already produced 500X dome-shaped Fresnel lens, its production process was complicated because of the shape. The objective of this study is to investigate new design method for non-imaging Fresnel with flat upper surface so that the production can be easier. The design of prisms is formulated by means of non-linear optimization to have maximum acceptance half angle with edge-ray principle. It is shown that designing more than 500X flat Fresnel lens is possible. The study also presents optimal condition of lens size requiring energy payback year to satisfy from the viewpoint of life cycle energy production and consumption.

In this work we present the basic considerations of the solar concentrator design, operation and automatization. This concentrator is located at Temixco, Morelos, Mexico, where the geographic and climatic conditions are ideal for its operation because it accounts with the greatest constant illumination in Mexico. We have obtained up to 1000°C of temperature concentration with the corresponding setup (with an opening diameter plate of 332 cm). In order to optimize the operation of this concentrator we use a control circuit designed to track the apparent sun position, considering the variables corresponding to the specific place. The implementation of the remote control modules based on RF is necessary because of the computer, which controls all movements of the motors, must be isolated of the environment, making a suitable and practical arrange.

Two types of solar concentrators for use with standard silicon photovoltaic cells are compared. The first is a spectral shifting luminescent concentrator that absorbs light in one spectral band and re-emits light at longer wavelengths where the absorption of standard silicon photovoltaic cells is more efficient. The second type is a holographic planar concentrator that selects the most useful bands of the solar spectrum and concentrates them onto the surface of the photovoltaic cell. Both types of concentrators take advantage of total internal reflected light, do not require tracking, and can operate with both direct and diffuse sunlight. The holographic planar concentrator provides a simpler and more cost effective solution with existing materials and construction methods.