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Multijunction III-V concentrator cells of several different types have demonstrated solar conversion efficiency over 40% since 2006, and represent the only third-generation photovoltaic technology to enter commercial power generation markets so far. The next stage of solar cell efficiency improvement, from 40% to 50%-efficient production cells, is perhaps the most important yet, since it is in this range that concentrator photovoltaic (CPV) systems can become the lowest cost option for solar electricity, competing with conventional power generation without government subsidies. The impact of 40% and 50% cell efficiency on cost-effective geographic regions for CPV systems is calculated in the continental US, Europe, and North Africa. We take a systematic look at a progression of multijunction cell architectures that will take us up to 50% efficiency, using modeling grounded in well-characterized solar cell materials systems of today's 40% cells, discussing the theoretical, materials science, and manufacturing considerations for the most promising approaches. The effects of varying solar spectrum and current balance on energy production in 4-junction, 5-junction, and 6-junction terrestrial concentrator cells are shown to be noticeable, but are far outweighed by the increased efficiency of these advanced cell designs. Production efficiency distributions of the last five generations of terrestrial concentrator solar cells are discussed. Experimental results are shown for a highly manufacturable, upright metamorphic 3-junction GaInP/GaInAs/Ge solar cell with 41.6% efficiency independently confirmed at 484 suns (48.4 W/cm2) (AM1.5D, ASTM G173-03, 25 °C), the highest demonstrated for a cell of this type requiring a single metal-organic vapor-phase epitaxy growth run. Copyright © 2012 John Wiley & Sons, Ltd.

The performance of multijunction solar cells has been measured over a range of temperatures and illumination intensities. Temperature coefficients have been extracted for three-junction cell designs that are in production and under development. A simple diode model is applied to the three-junction performance as a means to predict performance under operating conditions outside the test range. These data may be useful in guiding the future optimization of concentrator solar cells and systems. Copyright © 2008 John Wiley & Sons, Ltd.

Dual-junction Ga 0.5 In 0.5 P/GaAs solar cells on Ge substrates have rapidly gone from small, high-efficiency laboratory cells, to large-area, high-efficiency cells manufactured at Spectrolab in high volume. Over 500,000 of these dual-junction (DJ) cells with 27-cm 2 area have been produced, with average AM0 load point efficiency of 21.4%. The next step in the evolution of this type of multijunction solar cell has been taken, with the development of triple-junction (TJ) Ga 0.5 In 0.5 P/GaAs/Ge cells. The addition of the germanium third junction, plus several significant improvements in the device structure, have led to a measured efficiency of 27.0% (AM0, 28°C) at Spectrolab on large-area (>30 cm 2 ) TJ cells. The TJ cell is now in production at Spectrolab. Ga 0.5 In 0.5 P/GaAs/Ge cells are viable not only for non-concentrating space applications, but also for terrestrial and space concentrator systems. Efficiencies up to 32.3% at 47 suns under the terrestrial AM1.5D spectrum have been achieved.

This paper proposes the use of a monolithically integrated back scattering layer to improve the efficiency of ultra-thin GaAs single-junction solar cells. The device consists of a double-heterostructure GaAs/InGaP single-junction solar cell integrated with a lattice-matched ZnSe layer etched as the back scattering layer coated with a highly reflective Au film and flip-chip bonded on a carrier substrate. ZnSe surface was roughened to form different textures by wet etching. Without anti-reflective coating, 300×300 µm2 devices with a 300 nm GaAs active region and a reflective ZnSe/Au back scattering layer, demonstrate a short-circuit current density of 17.2 mA/cm2, an open-circuit voltage of 0.915 V, and a power conversion efficiency of 11.8% under 1 sun solar radiation (AM 1.5G, 0.1 W/cm2). External quantum efficiency (EQE) measurements show EQE over 65% at 400 nm and ∼30% at 850 nm.

Information on the temperature of a packaged III–V multijunction solar cell mounted on a heat sink, operating under concentrated light is often not readily available. Availability of such information would facilitate the design of different receiver module configurations in a concentrating photovoltaic system (CPV). To this end, a heat transfer model is developed from finite difference techniques to predict the temperature from various parts of a concentrator cell assembly (CCA). The CCA consists of a solar cell mounted on a direct-bonded copper ceramic substrate with bypass diode. Temperatures of the solar cell with applied conformal coating are modeled as well as the temperature difference, ΔT, between the various layers within the CCA. Isotherm contour plots are generated for the cell under different conditions. It is found that the solar cell temperature in the CCA without conformal coating is 32 °C when illuminated at 50 W/cm2 with the CCA back surface temperature at 25 °C. When the CCA is bonded to a surface with thin bondline of a silicone-based thermal adhesive of 2 W/m K under the same intensity and back surface temperature, the cell rises to 37.3 °C. Further, the effects of the thermal adhesive thickness as well as the adhesive thermal conductivity on the solar cell temperature are examined. An effective thermal resistance of the CCA is determined to help in the design of a CPV system. The results from the model are validated against conservation of energy where the heat input from solar radiation on the solar cell is equal to the heat rate by conduction minus the converted electrical power of the cell.

Summary form only given. Power-by-light schemes to provide electrical power to remote opto-electronic modules typically involve the use of multi-segmented photovoltaic (PV) cells or commonly known as laser power converters (LPC) illuminated by an optical fiber to produce output voltages between 2 and 12 volts, with up to several hundred milliwatts of electrical power. Typically these photovoltaic devices are based on a GaAs single junction cell grown on semi-insulating GaAs substrates and they are further segmented and connected in series depending on the output voltage required. Theoretically, these cells should have efficiencies near 60% but currently these cells have efficiencies only in the range of 40 to 45%. Optical losses due to high grid metal coverage and cell isolation trench can reduce efficiency by nearly 10%. In order to eliminate the losses in the multi-segmented cells and to improve the efficiencies further we have designed a vertical multi junction device. The vertical multi-junction approach has several advantages over the multi- segmented approach. The advantages are; a) reduction in optical losses due to trench formation, b) lower metal obscuration losses, due to lower current density in the device, c) reduction of sheet conduction losses, and d) simple process to fabricate the devices. We have successfully designed and fabricated a new vertical multi-junction TV laser power converter with an efficiency of over 53%, a considerably higher efficiency than the regular multi-segmented 2V GaAs laser power converter and this is the highest efficiency reported so far for a photovoltaic device.

Metamorphic III-V semiconductor materials offer access to bandgaps that span key portions of the solar spectrum, enabling new bandgap combinations in multijunction solar cells, and increasing both theoretical and practical efficiency limits for terrestrial concentrator cells. Experimental results are given for the quantum efficiency of metamorphic GaInAs solar cells with bandgap from 1.1 to 1.4 eV, and for metamorphic GaInP with both ordered and disordered group-III sublattices. Variable intensity Jsc vs. Voc measurements are used to compare recombination components due to n = 1 and n = 2 mechanisms in metamorphic and lattice-matched GaInAs, GaInP, and 3-junction solar cells. A record efficiency metamorphic GaInP/ GaInAs/ Ge 3-junction solar cell has been produced with 38.8% efficiency independently confirmed (241 suns, AM1.5D, low-AOD, 25°C), essentially equaling the performance of a lattice-matched 3-junction cell with 39.0% efficiency, the highest efficiency yet demonstrated and verified for a solar photovoltaic conversion device. With the combination of high-quality metamorphic materials that are increasingly less controlled by recombination at dislocations, and the higher efficiency limits afforded by freedom of lattice constant selection, practical terrestrial concentrator cell efficiencies well over 40% are expected in the near future.

This paper discusses the approach taken by Concentrating Technologies (CT), Spectrolab, and Arizona Public Service (APS) to demonstrate a High Concentration Photovoltaic (HCPV) module using Spectrolab's GaInP/GaInAs/Ge triple-junction solar cells. This module is currently connected to an inverter, feeding electricity into the grid at the Solar Test & Research (STAR) facility of APS in Tempe, AZ. Although the module output is small, under 1 kW AC, it is the world's first demonstration of a grid-connected utility module using the same triple-junction solar cell technology that have been used to power satellites and other spacecrafts. This paper also discusses the next steps required to increase module efficiency and enhance its reliability. The economics of the MJ receivers in the context of this module are also discussed.

Results are reported for the effects of chromatic aberration on the electrical performance of triple junction (3J), GalnP/GaAs/Ge concentrator solar cells for indoor measurements. A Fresnel lens was moved away from the solar cell such that the concentration level increased, and then decreased while the lens-to-cell distance continued to increase. From such movement, the FF exhibited a series of minima and maxima in the region where light is apparently focused as determined by the naked eye. These effects are due to chromatic aberrations that affect the FF, and to some extent, the subcell short circuit currents in the 3J stack. Aberrations can drive the 3J device in a top cell-or middle cell-limited mode, and make the top cell FF become more predominant than the middle cell FF, or vice versa with the 3J FF influenced more by either subcell FF. It is shown that the multijunction celt power output is also affected, and the gain in power depends on the size of the solar cell relative to the size of the beam waist, and the focal length of blue light relative to that of red light.

The photovoltaic characterization of triple-junction InGaP2/GaAs/Ge solar cells is presented. Measurements made using a single light source solar simulator are compared with other measurements made using a multi-light source solar simulator that provides a close match to the air mass zero (AM0) solar spectrum. The output spectrum of the solar simulators has been measured, and two methods for calibrating the simulator output intensity haven been employed. The spectral response of the solar cells has been characterized through quantum efficiency measurements. These data are analyzed to determine the effect of the simulator spectrum on the measured photovoltaic response, and in particular, areas where spectral mismatch between the simulator and AM0 can lead to inaccurate performance predictions are highlighted. In addition, the effects of the different calibration techniques on the measured data are studied. Exploiting the capabilities of the multi-source, close matched simulator, the response of each of the three sub-junctions are studied individually, and the interplay between the spectral response of the sub-junctions and the incident spectrum is investigated.

Using the energy bandgap of semiconductors as a design parameter is critically important for achieving the highest efficiency multijunction solar cells. The bandgaps of lattice-matched semiconductors that are most convenient to use are rarely those which would result in the highest theoretical efficiency. For both the space and terrestrial solar spectra, the efficiency of 3-junction GaInP/GaAs/Ge solar cells can be increased by a lower bandgap middle cell, as for GaInAs middle cells, as well as by using higher bandgap top cell materials. Wide-bandgap and indirect-gap materials used in parasitically absorbing layers such as tunnel junctions help to increase transmission of light to the active cell layers beneath. Control of bandgap in such cell structures has been instrumental in achieving solar cell efficiencies of 29.7% under the AMO space spectrum (0.1353 W/cm/sup 2/, 28/spl deg/C) and 34% under the concentrated terrestrial spectrum (AM1.5G, 150-400 suns, 25/spl deg/C), the highest yet achieved for solar cells built on a single substrate.

This paper presents key results of an on-going program to develop terrestrial concentrator modules incorporating high-efficiency multi-junction (MJ) photovoltaic (PV) cell technology. This program evolved from a successful space concentrator module development. Indeed, space mini-concentrators, comprising a stretched-membrane line-focus Fresnel lens and a prism-covered triple-junction solar cell, have been found to perform exceptionally well in the terrestrial environment. In outdoor tests at both ENTECH and NREL, space mini-concentrators have achieved overall module (combined lens and cell) operational solar-to-electric conversion efficiency levels from 25 to 29% under a variety of conditions. The long-term goal of the present program is to incorporate MJ cell technology into existing field-proven modules and two-axis sun-tracking arrays. With twice the efficiency of present silicon-based concentrators, these next-generation MJ concentrators should offer unprecedented economics.

Ga/sub 0.5/In/sub 0.5/P/GaAs/Ge solar cells have been fabricated at Spectrolab under the Multijunction Solar Cell Manufacturing Technology (Mantech) program, sponsored by the US Air Force and NASA. The cells were irradiated with increasing 1 MeV electron fluences, and the degradation of their PV parameters was characterized using light I-V and external QE. Analysis of the PV parameters of the GaInP top subcell showed little degradation, and was not a limitation for triple junction (3J) cell performance. Furthermore, the radiation degradation of the Ge subcell PV parameters was almost negligible. The GaAs subcell I/sub sc/, however, did limit the device performance as is traditionally documented. The final-to-initial maximum power ratio (P/P/sub 0/) of 3J cells was near 0.833 at a fluence of 1/spl times/10/sup 15/ e/sup -//cm/sup 2/, and matches Spectrolab's presently established value of 0.83 for standard production of space-qualified dual-junction cells.

Results from Spectrolab-grown Ga/sub 0.5/In/sub 0.5/P/GaAs/Ge structures optimized for the AM1.5D spectrum are described along with progress toward developing next generation multijunction solar cells for high concentration ratios (X). The epitaxially-grown layers were processed into triple junction cells both at Spectrolab and NREL, and I-V tested vs. X. Cells were tested with efficiencies as high as 32.4% near 372 suns. The FF limited the performance with increasing X as a result of the increased role of the series resistance. The V/sub oc/ vs. X showed its log-linear dependence on I/sub sc/ over 1000 suns. Based on cell improvements for space applications, multijunction cells appear to be ideal candidates for high efficiency, cost effective, PV concentrator systems. Future development of new 1 eV materials for space cells, and further reduction in Ge wafer costs, promises to achieve cells with efficiencies >40% that cost $0.3/W or less at concentration levels between 300 to 500 suns.

Next-generation solar cell approaches such as AlGaInP/GaAs/GaInNAs/Ge 4-junction cells, lattice-mismatched GaInP/GaInAs/Ge, concentrator cells, and improved 3-junction device structures hold the promise of greater efficiency than even today's highly successful multijunction cells. Wide-bandgap tunnel junctions, improved heterointerfaces, and other device structure improvements have resulted in several record-efficiency GaInP/GaAs/Ge cell results. Triple-junction (3J) cells grown in this work have demonstrated 29.3% efficiency for space (AMO, 1 sun). Space concentrator 3J cells have efficiency up to 30.0% at low concentration (AMO, 7.6 suns), and terrestrial concentrator cells grown at Spectrolab and processed at NREL have reached 32.3% (AM1.5D, 440 suns).

The Naval Research Laboratory has presented numerous reports on the radiation hardening of the two-terminal InP/Ga/sub 0.47/In/sub 0.53/As tandem solar cells for space use. To date, most work has been done on n/sup +/p devices. However, in order to facilitate the heteroepitaxial growth of these tandems on substrates such as Si or Ge, the polarity of the subcells may need to be reversed. With this in mind, spectral response measurements on irradiated homoepitaxial p/sup +/n InP solar cells were carried out for several fluences of 1 MeV electrons. The quantum efficiency data degraded at high wavelengths but no significant degradation was observed at low wavelengths. An important material parameter in solar cell devices is the minority carrier diffusion length (L) for which L/sub P/ was measured in the present study. This parameter was measured as a function of electron irradiation using the steady-state short-circuit current method. The degradation of L/sub P/ was plotted as a function of electron fluence and the data were fit to the standard semi-empirical model which relates /spl Delta/L to the damage constant, K/sub L/. The fit yielded values for K/sub L/ and the unirradiated minority carrier diffusion length, L/sub 0/.