H. Cotal

III-V multijunction solar cells for concentrating photovoltaics

Concerns about the changing environment and fossil fuel depletion have prompted much controversy and scrutiny. One way to address these issues is to use concentrating photovoltaics (CPV) as an alternate source for energy production. Multijunction solar cells built from III–V semiconductors are being evaluated globally in CPV systems designed to supplement electricity generation for utility companies. The high efficiency of III–V multijunction concentrator cells, with demonstrated efficiency over 40% since 2006, strongly reduces the cost of CPV systems, and makes III–V multijunction cells the technology of choice for most concentrator systems today. In designing multijunction cells, consideration must be given to the epitaxial growth of structures so that the lattice parameter between material systems is compatible for enhancing device performance. Low resistance metal contacts are crucial for attaining high performance. Optimization of the front metal grid pattern is required to maximize light absorption and minimize I2R losses in the gridlines and the semiconductor sheet. Understanding how a multijunction device works is important for the design of next-generation high efficiency solar cells, which need to operate in the 45%–50% range for a CPV system to make better economical sense. However, the survivability of solar cells in the field is of chief concern, and accelerated tests must be conducted to assess the reliability of devices during operation in CPV systems. These topics are the focus of this review.

High-efficiency space and terrestrial multijunction solar cells through bandgap control in cell structures

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.

Next-generation, high-efficiency III-V multijunction solar cells

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 todays 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).

Triple-junction solar cell efficiencies above 32%: the promise and challenges of their application in high-conceniration-ratio PV systems

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

Temperature Dependence of the IV Parameters from Triple Junction GaInP/InGaAs/Ge Concentrator Solar Cells

0.3/W or less at concentration levels between 300 to 500 suns.

The effects of chromatic aberration on the performance of GaInP/GaAs/Ge concentrator solar cells from Fresnel optics

Results are reported for the temperature dependence of the IV parameters as a function of concentration level for triple junction (3J), GaInP/GaAs/Ge concentrator solar cells measured outdoors. Each IV parameter was measured in the range from 1 to 950 suns. The temperature coefficient of Voc, dVoc/dT, showed an abrupt decrease at low concentration but then decreased slowly at mid concentration for which a steady value was reached at higher concentration. In this latter case, the generation of minority carriers from high injection conditions overcomes the effects of the dark saturation current. The characteristics of the temperature coefficient of Jsc, dJsc/dT, however, are primarily dependent on high injection conditions coupled by the influence of the subcell that limits the current in the 3J cell stack. It is suggested the subcell that limits the device current is also influenced by chromatic aberration effects from the Fresnel lens. Values for dVoc/dT and dJsc/dT as a function of the concentration ratio are presented

Heat transfer modeling of concentrator multijunction solar cell assemblies using finite difference techniques

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.

Metamorphic and Lattice-Matched Solar Cells Under Concentration

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.

First demonstration of multi-junction receivers in a grid-connected concentrator module

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, 25degC), 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

Multijunction Solar Cells for Dense-Array Concentrators

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 Spectrolabs 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 worlds 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.