Over the past 15 years, major advances in Concentrating Photovoltaics (CPV) have been achieved. Ultra-efficient Si solar cells have produced commercial concentration systems which are being fielded today and are competitively priced. Advanced research has primarily focused on significantly more efficient multi-junction solar cells for tomorrow's systems. This effort has produced sophisticated solar cells that significantly improve power production. Additional performance and cost improvements, especially in the optical system area and system integration, must be made before CPV can realize its ultimate commercial potential. Structural integrity and reliability are vital for commercial success. As incremental technical improvements are made in solar cell technologies, evaluation and 'fine-tuning' of optical systems properly matched to the solar cell are becoming increasingly necessary. As we move forward, it is increasingly important to optimize all of the interrelated elements of a CPV system for high performance without sacrificing the marketable cost and structural requirements of the system. Areas such as wavelength absorption of refractive optics need to be carefully matched to the solar cell technology employed. Reflective optics require advanced engineering models to insure uniform flux distribution without excessive losses. In Situ measurement of the 'fine-grain' improvements are difficult as multiple variables such as solar insolation, temperature, wind, altitude, etc. infringe on analytical data. This paper discusses design considerations based on 10 years of field trials of high concentration systems and their relevance for tomorrow's advanced CPV systems.

Highly efficient triple-junction photovoltaic cells are combined with novel tailored imaging and non-imaging concentrator optics, to provide a scalable approach to cost-effective concentrator solar PV systems. The two designs presented are scaleable in that they are arrays of small concentrating elements, which can be tailored in size to suit a range of manufacturing and installation practices. They are both appropriate for commercial rooftop or large, utility scale rollout and have been designed with safety and low cost of energy production as primary targets. This paper is an update on their progress from conception to commercialization.

Over the last 15 years, Solar Systems has developed a dense array receiver PV technology for 500X concentrator reflective dish applications. This concentrator PV technology has been successfully deployed at six different locations in Australia, counting for about 1 MWp of installed peak power. A new Multijunction III-V receiver to replace the current silicon Point-Contact solar cells has recently been developed. The new receiver technology is based on high-efficiency (> 32%) Concentrator Ultra Triple Junction (CUTJ) solar cells from Spectrolab, resulting in system power and energy performance improvement of more than 50% compared to the silicon cells. The 0.235 m² concentrator PV receiver, designed for continuous 500X operation, is composed of 64 dense array modules, and made of series and parallel-connected solar cells, totaling approximately 1,500 cells. The individual dense array modules have been tested under high intensity pulsed light, as well as with concentrated sunlight at the Solar Systems research facility and at the National Renewable Energy Laboratory's High Flux Solar Furnace. The efficiency of the dense array modules ranges from 30% to 36% at 500X (50 W/cm², AM1.5D low AOD, 21°C). The temperature coefficients for power, voltage and current, as well as the influence of Air Mass on the cell responsivity, were measured. The reliability of the dense array multijunction III-V modules has been studied with accelerated aging tests, such as thermal cycling, damp heat and high-temperature soak, and with real-life high-intensity exposure. The first 33-kWp multijunction III-V receiver was recently installed in a Solar Systems dish and tested in real-life 500X concentrated sunlight conditions. Receiver efficiencies of 30.3% and 29.0% were measured at Standard Operating Conditions and Normal Operating Conditions respectively.

In this work, based on the advanced commercial software, the Crosslight APSYS with improved tunnel junction model, two-dimensional (2D) simulation has been performed on the triple-junction (TJ) GaInP/GaAs/Ge solar cell devices. The APSYS simulator solves several interwoven equations including the basic Poisson's equation, and drift-diffusion current equations for electrons and holes. The model of tunnel junction with the equivalent mobility enables an efficient modeling of multi-junction solar cell across the whole solar spectra, where all the spectrum data points are processed by taking into account the effects of multiple layer optical interference and photon generation. Basic physical quantities like band diagrams, optical absorption and generation are demonstrated. The modeled IV characteristics and offset voltage agree well with the published experimental results for TJ GaInP/GaAs/Ge solar cell device. The quantum efficiency spectra have also been computed for the modeled TJ solar cell device. Possible design optimization issues to enhance the quantum efficiency have also been discussed with respect to some applicable features of Crosslight APSYS.

The macroscopic parameters that characterize photovoltaic (PV) performance, including their spatial dependence, especially at high flux, are determined with extensive localized solar measurements on high-efficiency concentrator solar cells. We present two studies that explore (a) the impact of nonuniform flux distribution on PV behavior, (b) how PV parameters vary across the cell surface (of particular interest in many high-concentration optical systems) and (c) the sensitivity of PV parameters to the spatial variation of series resistance R_s that stems from irregular cell metallization. In so doing, we identify current-voltage trends unique to strongly inhomogeneous illumination and to R_s losses at high flux.

The Photovoltaic Cavity Converter (PVCC) under development is a novel approach to convert highly concentrated solar radiation into electricity via a photon entrapment process and subsequent spectral stripping. Equipped with a multi-bandgap, single junction cell system PVCC circumvents most of the present limitations of the four (or more)-junction cell systems with vertical architecture. Our previous studies have shown that the PVCC concept has the potential to reach a collective conversion efficiency of 50% in the near term. Based on our past experiences regarding the cavity geometry and the light injection method we have developed a second generation design for the PVCC that overcomes the limitations of the first generation prototype.

We present results from a p-n junction device physics model for GaInP/GaAs/GaInAsP/GaInAs four junction solar cells. The model employs subcells whose thicknesses have an upper bound of 5μm and lower bound of 200nm, which is just above the fully depleted case for the assumed doping of N_A = 1 x 10¹⁸ cm^-3 and N_D = 1 x 10¹⁷ cm^-3. The physical characteristics of the cell model include: free carrier absorption, temperature and doping effects on carrier mobility, as well as recombination via Shockley-Read-Hall recombination from a single midgap trap level and surface recombination. Upper bounds on cell efficiency set by detailed balance calculations can be approached by letting the parameters approach ideal conditions. However whereas detailed balance calculations always benefit from added subcells, the current matching requirements for series connected p-n multi-junctions indicate a minimum necessary performance from an added subcell to yield a net increase in overall device efficiency. For the four junction cell considered here, optimizing the subcell thickness is an important part of optimizing the efficiency. Series resistance limitations for concentrator applications can be systematically explored for a given set of subcells. The current matching limitation imposed by series connection reduces efficiency relative to independently-connected cells. The overall trend indicates an approximately 5% drop in efficiency for series connected cells relative to identical independently connected cells. The series-connected devices exhibit a high sensitivity to spectral changes and individual subcell performance. If any single subcell within the series-connected device is degraded relative to its optimal design, the entire device is severely hindered. This model allows complex heterostructure solar cell structures to be evaluated by providing device physics-based predictions of performance limitations.

Optical collectors for sunlight concentration on small surfaces have been experimentally studied to evaluate their correspondence with the optical design. Measurement set-up and testing methodologies have been developed and adapted mainly to provide the optical characteristics that are fundamental for the concentration on small surfaces. The collectors are lenses of different type, with various shape and dimensions. They are optically designed to be realised in plastics and to be applied to concentrate the solar light on surfaces with dimensions of the order of 1 cm. Furthermore the optical control compares several samples of the same collector, realised with different production procedures and materials. The tests are aimed to verify the correspondence between the performance of the realised samples and the theoretical features of the designed collector. A specific study has been dedicated to the application to light concentration on PhotoVoltaic cells, whose requirements are a square-shaped image with uniform light distribution and maximum collection efficiency. The described instrumentation and measurement procedures examine total collection efficiency and energy distribution in the collector image plane. A particularly accurate and critical procedure performs the study of the image uniformity, separating the light contributions due the different collector regions. All measurements and in particular the "image contribution test" requires to adapt the testing set-up for each collector shape.

One of the most usual procedures to measure a concentrator optical efficiency is by direct comparison between the photocurrent generated by the compound concentrator/solar cell and photocurrent that single cell would generate under identical radiation conditions. Unfortunately, such procedure can give a good idea of the generator final performance, but can not indicate the real amount of radiation that will impinge over the cell. This apparent contradiction is based on the fact that once the cell is coupled with the concentrator, rays incidence is not perpendicular, but highly oblique, with an angle that can reach 70^o or even greater for high concentration devices. The antireflective coating of the cell does not perform well enough for the whole incidence angle and frequency ranges because low cost is other important requirement for the solar cells. In consequence, the generated photocurrent drops for large incidence angles. In our case, a 70% incidence angle could, in the worst case, mean a 34% loss on generated photocurrent. With the aim of correcting such problem a special device has been designed in the framework of a EU funded project called HAMLET. The concept of the device is to substitute the concentrator receptor by a system formed by an optical collimator that would reduce concentration and incidence angle, and a characterized solar cell. The paper gives the results of this measuring procedure.