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12 Dec 2006
A US consortium is aiming to smash the solar-cell efficiency record with a radical design that uses a lateral architecture and a dispersive concentrator. If they are successful, soldiers will be freed from the burden of carrying a 20 lb load of spare batteries. Richard Stevenson reports.
The US Defense Advanced Research Projects Agency (DARPA) believes that solar cells can be used to recharge the batteries that power a soldier's radio, night-vision goggles, GPS navigation system and other electronic gadgets, and it is supporting a programme that will receive up to $53 m (€28 m) to develop photovoltaics with the required efficiency.
The main disadvantage of batteries is their weight. Despite advances in technology, spare batteries still account for one-fifth of a soldier's 100 lb battlefield load.
Batteries also have to be replaced regularly because a 20 lb stock will only last for 3–7 days. Although their weight and lifetime are major drawbacks, solar-cell technologies are not efficient enough to offer viable alternatives. The space available on the soldier's gadgets for a solar cell varies. It is too risky to fix cells on larger pieces of equipment and use them to power other gadgets because damage to an individual device can have knock-on effects. Instead, each gadget must have its own cell, which places an upper limit on the size of the solar panel of 10 cm2.
Silicon falls well short of this 10 cm2 criterion. If a silicon solar cell were used to recharge tactical battlefield flashlights over a period of eight hours (and other devices used by soldiers that have similar requirements), it would need an active area of 33 cm2 and an efficiency of 15%. Today's best triple-junction cells can get closer to the target with cell efficiencies approaching 40% – although they require 500 × concentrations that are not suitable for portable applications – but a hike in efficiency to 50% is needed to meet the 10 cm2 target.
To reach this goal, DARPA launched its Very High Efficiency Solar Cell (VHESC) programme in November 2005. Led by Allen Barnett from the University of Delaware, US, this project is using multiple-junction solar cells and involves contributions from 21 additional institutions including Emcore, BP Solar, Corning, the National Renewable Energy Laboratory and various high-profile US universities.
The team aims to deliver 1000 prototype 10 cm2 modules with a minimum efficiency of 50% and an output power of 0.5 W by 2009. The weight of these cells is not an issue: "Even if [each module] was made out of solid metal it would weigh less than one set of spare batteries," said DARPA programme manager Douglas Kirkpatrick. Cost is not a big issue either because even if the price for solar material was $10,000 per square metre, a 10 cm2 cell would cost $10, which is a fraction of the total cost of a soldier's equipment.
Barnett believes that the 50% target cannot be met by just making refinements to the most efficient photovoltaics ever built. A radically different approach is needed that combines a lateral architecture. This was first proposed many years ago, with non-imaging concentrators that disperse and focus various portions of the incident spectrum onto different cells (see figure 1).
The concentrator's design uses dichroic coatings to manipulate and direct the incident radiation. For the concentrator to be effective, 90% of the photons have to land on the correct solar cell. "We have exceeded 90%," enthused Barnett, "and there's a chance we could go over 95%."
The concentrator is fixed and a wide-acceptance-angle optical element captures a high proportion of diffused light, which typically makes up one-tenth of the incident power in the solar spectrum. Tracking concentrator designs were also considered, but DARPA thinks that they are too risky to use in the desert, where many of the US Army's soldiers operate.
The concentrator used by Barnett and his team delivers a lot of freedom in terms of the number of cells that can be used in the design. However, theoretical calculations have determined a constraint that at least six junctions are required to make a practical device operating at over 50% efficiency. These six junctions could be fabricated as six separate cells. However, this is not the most efficient way to do it because a higher proportion of the light is steered onto the wrong cell as the number of spectral bins increases. So the team is using a design that divides the radiation into just three bins – a high-, mid- and low-energy bin – and focuses them onto different solar cells that have up to three junctions (figures 1 and 2). These three solar cells can be independently contacted, which avoids the current-matching issues that complicate conventional multijunction cells. Lattice-matching constraints are also relaxed, which allows more freedom in the choice of bandgap for the device. The architecture is also very flexible because improvements made to one type of cell can be implemented without detriment to the others.
Working with the new design rules
Part of the project involves establishing the best material combination for each of the three cells. Nitrides are being investigated for the high-energy cell and a GaInP/GaAs tandem design for the mid-energy cell. Silicon is being used as part of the low-energy triple-junction cell, alongside materials with bandgaps of 0.95 and 0.7 eV that could be made from either III-Vs or germanium-based compounds. The specific bandgaps for these cells, their thermodynamic efficiencies and their practical efficiencies at 20× and 100× concentrations are listed in figure 3. "We've built our whole approach on a silicon platform because it is well known, well established and you're standing on the shoulders of something that's known to be good," explained Barnett.
Research has already been carried out for each type of cell. GaInP/GaAs tandem cells for the mid-energy bin have been built with efficiencies of 28.4 and 29% at solar-cell concentrations of 20 and 40 suns, respectively. These results are encouraging, says Barnett, because the theoretical maximum for a two-junction cell is only 41.6%. If all three bins had devices offering these performance levels, then the DARPA's target of 50% would be comprehensively broken.
GaInAsP/GaInAs tandem cells are being investigated for the low-energy bin. These MOCVD-grown devices feature 0.95 and 0.74 eV subcells on an infrared-transparent iron-doped InP substrate interconnected with an essentially transparent tunnel junction. The back contact is attached to the device mesa and the front contact is made through a highly conductive window layer on the front side of the device. This solar cell produced an efficiency of 3.5% at a concentration of 5.7 suns and improvements are expected when a two-layer antireflection coating is added and changes are made to allow higher concentration ratios.
Efforts are also being directed at developing InGaN-based cells for the high-energy bin that draw upon advances made in GaN-LED and photodetector technologies. PIN structures have been grown by MOCVD on 2 inch sapphire substrates that feature a low-temperature GaN nucleation layer, a 2 μm-thick GaN template and InGaN layers with an indium concentration ranging from 0 to 40%, which correspond to bandgaps of 3.4–2.4 eV. Devices ranging in size from 1 × 1 to 5 × 5 mm were fabricated from these wafers. Internal quantum efficiency measurements on these cells, which have an In0.05Ga0.95N absorption layer, show quantum efficiencies of up to 60% at 3.26 eV. Improvements to the cell efficiency can be made by decreasing the light absorption in the current-spreading metal layer and reducing recombination at heterojunction interfaces.
The consortium is also investigating alternative lower-cost growth techniques, such as bio-fabrication processes that can make electronically active material at room temperature and pressure. The processes are capable of yields greater than 50% of the starting feedstock, which is at least five times more than typical MOCVD processes. At the University of California, Santa Barbara, US, Daniel Morse, Birgit Schwenzer and co-workers have used this approach to form GaN and other optoelectronic materials at very low temperatures using ammonolysis – a biochemically inspired process route.
The progress made on low-temperature processing techniques, devices for each of the three energy bins, and the dispersive concentrator have been described by Kirkpatrick as phenomenal. "I think we will turn the solar world on its ear over the next 18 months. This architecture gives everybody the freedom to succeed, and I think that's going to make a big difference."
•This article originally appeared in the October issue of Compound Semiconductor.
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