Geoffrey S. Kinsey

40% efficient metamorphic GaInP∕GaInAs∕Ge multijunction solar cells

An efficiency of 40.7% was measured and independently confirmed for a metamorphic three-junction GaInP∕GaInAs∕Ge cell under the standard spectrum for terrestrial concentrator solar cells at 240 suns (24.0W∕cm2, AM1.5D, low aerosol optical depth, 25°C). This is the initial demonstration of a solar cell with over 40% efficiency, and is the highest solar conversion efficiency yet achieved for any type of photovoltaic device. Lattice-matched concentrator cells have now reached 40.1% efficiency. Electron-hole recombination mechanisms are analyzed in metamorphic GaxIn1−xAs and GaxIn1−xP materials, and fundamental power losses are quantified to identify paths to still higher efficiencies.

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-saturation-current charge-compensated InGaAs-InP uni-traveling-carrier photodiode

Charge compensation is utilized in an InGaAs-InP uni-traveling-carrier photodiode to mitigate the space-charge effect. A 20-/spl mu/m-diameter photodiode achieved a bandwidth of 25 GHz and large-signal 1-dB compression current greater than 90 mA; the output power at 20 GHz was 20 dBm. A smaller /spl sim/100-/spl mu/m/sup 2/ photodiode exhibited a bandwidth of 50 GHz and large-signal 1-dB compression current greater than 50 mA. The maximum RF output power at 40 GHz was 17 dBm.

Waveguide avalanche photodiode operating at 1.55 μm with a gain-bandwidth product of 320 GHz

An In/sub 0.52/Al/sub 0.48/As-In/sub 0.53/Ga/sub 0.47/As waveguide avalanche photodiode with a record gain-bandwidth product of over 320 GHz has been demonstrated. A bandwidth of 28 GHz was achieved at low gains with low excess noise and a quantum efficiency of 16% at 1.55 /spl mu/m.

Advances in High-Efficiency III-V Multijunction Solar Cells

The high efficiency of multijunction concentrator cells has the potential to revolutionize the cost structure of photovoltaic electricity generation. Advances in the design of metamorphic subcells to reduce carrier recombination and increase voltage, wide-band-gap tunnel junctions capable of operating at high concentration, metamorphic buffers to transition from the substrate lattice constant to that of the epitaxial subcells, concentrator cell AR coating and grid design, and integration into 3-junction cells with current-matched subcells under the terrestrial spectrum have resulted in new heights in solar cell performance. A metamorphic Ga 0 .44 In 0 .56 P / Ga 0.92 In 0.08 As/ Ge 3-junction solar cell from this research has reached a record 40.7% efficiency at 240 suns, under the standard reporting spectrum for terrestrial concentrator cells (AM1.5 direct, low-AOD, 24.0 W/cm 2 , 25 ∘ C ), and experimental lattice-matched 3-junction cells have now also achieved over 40% efficiency, with 40.1% measured at 135 suns. This metamorphic 3-junction device is the first solar cell to reach over 40% in efficiency, and has the highest solar conversion efficiency for any type of photovoltaic cell developed to date. Solar cells with more junctions offer the potential for still higher efficiencies to be reached. Four-junction cells limited by radiative recombination can reach over 58% in principle, and practical 4-junction cell efficiencies over 46% are possible with the right combination of band gaps, taking into account series resistance and gridline shadowing. Many of the optimum band gaps for maximum energy conversion can be accessed with metamorphic semiconductor materials. The lower current in cells with 4 or more junctions, resulting in lower I 2 R resistive power loss, is a particularly significant advantage in concentrator PV systems. Prototype 4-junction terrestrial concentrator cells have been grown by metal-organic vapor-phase epitaxy, with preliminary measured efficiency of 35.7% under the AM1.5 direct terrestrial solar spectrum at 256 suns.

Performance and Reliability of Multijunction III-V Modules for Concentrator Dish and Central Receiver Applications

Over the last 15 years, Solar Systems have 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 more than 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 Laboratorys 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, 21C). 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

Status of C3MJ+ and C4MJ Production Concentrator Solar Cells at Spectrolab

Multijunction solar cells based on III-V semiconductors, having recently demonstrated 43.5%, remain the worlds most efficient solar cells, and the preferred technology in point-focus and dense-array concentrator photovoltaic (CPV) system architectures. The year 2011 proved to be a pivotal year for CPV technology, with multiple power plant installations in the megawatt to tens of megawatt scale. Spectrolab is working closely with CPV system manufacturers to provide a reliable and well-characterized cell technology, in volumes commensurate with this increasing demand. The evolutionary C3MJ+ and the C4MJ cell technologies are the latest in a sequence of CPV solar cell designs, with conversion efficiencies approaching or greater than 40%. Both technologies have completed detailed characterization and qualification programs, including accelerated laboratory and (for C4MJ) on-sun reliability testing, and have entered into high volume production at Spectrolabs manufacturing facility in Sylmar, CA. The metamorphic C4MJ technology affords new opportunities to optimize cell designs, taking into consideration both the spectral optical transmittance of a particular CPV system and the installation sites average solar resource over a typical meteorological year.

Origin of dark counts in In0.53Ga0.47As∕In0.52Al0.48As avalanche photodiodes operated in Geiger mode

A dark count rate in InP-based single photon counting avalanche photodiodes is a limiting factor to their efficacy. The temperature dependence of the dark count rate was studied to understand its origin in In0.53Ga0.47As∕In0.52Al0.48As separate-absorption-charge-multiplication avalanche photodiodes. The dark count rate was observed to be a very weak function of temperature in the range from 77Kto300K. Various mechanisms for dark count generation were considered. Simulations of band-to-band tunneling in the In0.52Al0.48As multiplication layer were found to agree well with the experimental temperature dependence of dark count rate at various excess biases. To reduce tunneling-induced dark counts, a suitable design change to the detector structure is proposed.

Increasing Power and Energy in Amonix CPV Solar Power Plants

Large-scale concentration photovoltaic (CPV) power plants deliver the high energy production and low electricity cost that will allow photovoltaics to become a substantial portion of the electrical grid. High concentration minimizes the semiconductor material costs, while tracking delivers higher capacity factors and provides a better match to demand. In order to prove the net cost benefits, however, annual deployments must exceed the tens of megawatt range. CPV companies, including Amonix, are reaching this level in 2011. Ongoing performance improvements combined with aggressive cost reductions are the means for Amonix to win the projects that are needed to gain more economies of scale. Energy modeling has been used to increase both the rated power and energy yield. Solar power generators deployed in 2011 exceed previous performance by more than 10%. Rated output now exceeds 60 kW AC-PTC, and ac system efficiency exceeds 27%. A similar increase in performance is expected in 2012.

Geiger mode operation of an In/sub 0.53/Ga/sub 0.47/As-In/sub 0.52/Al/sub 0.48/As avalanche photodiode

Low-temperature photon counting with gated mode quenching is demonstrated with separate absorption, charge, and multiplication avalanche photodiodes that have an In/sub 0.52/Al/sub 0.48/As multiplication layer. A minimum of ten dark counts per second and single-photon detection efficiency of 16% were achieved at 130 K.