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Solar & Alternative Energy

Pushing concentrator photovoltaic efficiency to the limit

Adding quantum wells to triple-junction solar cells could help deliver renewable power at costs on a par with conventionally generated electricity.
25 April 2011, SPIE Newsroom. DOI: 10.1117/2.1201104.003532

Photovoltaic technology has become commoditized, and the market for crystalline silicon photovoltaic panels is growing while prices have fallen dramatically. The efficiencies of single-junction silicon solar cells, however, are currently close to their limit, as shown by the very slow upward trend of the record value for monocrystalline silicon cells over the last decade. Physics ultimately limits the efficiency of single-junction silicon cells to no higher than 30–33% when irradiated by unconcentrated sunlight, and manufacturing a solar cell that achieves 75% of this in practice is challenging. Multicrystalline solar cells are in a similar position, and their record efficiency of 20.4% has remained unchanged for seven years. Once cells are incorporated into utility-scale systems, their output is reduced further through electrical, optical, and thermal losses.

Concentrator photovoltaics (CPVs) have the potential to challenge the market dominance of single-junction silicon and thin-film cells by developing modules and systems that significantly enhance the delivered efficiency of electricity generation. It has long been accepted that very high efficiency multiple-junction solar cells can be transferred to terrestrial use if the cost can be reduced. A multiple junction solar cell splits the spectrum up into separate bands, absorbing each band in a semiconductor with an appropriate energy gap, or bandgap. Each junction operates at a voltage proportional to the bandgap energy. Adding the output of the multiple bandgaps together, the conversion efficiency of the device rises as each spectral band is converted to electricity at a higher efficiency because overall losses in the semiconductor are reduced. A convenient way to reduce cost is to employ concentrating optics to focus the sun's rays, typically by a factor of at least 500, and direct them onto a smaller cell, in order to replace the semiconductor solar cell area with relatively cheap and robust optical components such as plastic lenses and mirrors.

The concept of CPV became attractive as the oil crisis of the mid-1970s gripped the world and the cost of energy spiraled. Early experimentation with concentration systems was limited to silicon solar cells, which were then still extremely expensive in real terms. But CPV has continued to be researched and championed, notably at the National Renewable Energy Laboratory in the US and at the Fraunhofer Institute for Solar Energy systems in Germany. Modern CPV system integrators, some with many years of in-field experience, boast CPV module arrays delivering electricity to the grid at efficiencies approaching 30%, which compared to that delivered by silicon panel systems represents an increase of 40%. In areas of high irradiance, CPV modules can deliver renewable power at costs on a par with conventionally generated electricity.1, 2

CPVs are widely used in space (almost all satellites are powered by them), and terrestrially, the market is growing. Indeed, the speed of development of cells for terrestrial concentrator use has been impressive. In 2000, the record efficiency for such cells was ∼33%. By 2010, this had risen to 41.6% with a device produced by SpectroLab Inc. Today these cells offer average efficiencies between 37 and 39% at a solar concentration of 500×. When combined with current optical concentration systems and the two-axis trackers required to keep the sun's rays incident on the optics, these systems deliver between 25 and 30% efficiency.

This level of efficiency from commercial CPV systems is a good achievement, but the downward pressure on cost, in particular the levelized cost of energy, is driving the technology to aim for 35% or beyond. Advances in the optical and electrical efficiency of the systems will be made to achieve such performance, but the item targeted for the most significant improvement is the CPV cell itself.

Figure 1 shows the fundamental efficiency limits of a triple-junction, or 3J, cell at 500× illumination. The conventional current incarnation uses germanium (Ge), indium gallium arsenide (InGaAs), and indium gallium phosphide as the three semiconductor junctions. This material combination has a fundamental efficiency limit of 47.3%, which when subject to the guideline that around three-quarters of this is realistically achievable equates to a practical limit of 36%.

Figure 1. Limit efficiencies of multiple-junction solar cells as a function of their subcell bandgaps. 3J: Triple junction. Ge: Germanium. InGaP: Indium gallium phosphide. GaAs: Gallium arsenide.

The goal of any work aiming to produce a 3J solar cell based on Ge with the maximum possible theoretical efficiency is to provide good-quality semiconducting material with a bandgap that can be chosen to suit the application. If the materials used could be chosen without real-world constraints, the theoretical maximum efficiency of a 3J Ge-based device would rise to 60.1%, opening up the path to realistic production efficiencies of 45%. Another approach is to replace the Ge junction with a different material of higher bandgap. In this instance the limiting efficiency is ∼58% (see Figure 1, third column).

It is possible to alter the composition of many III-V compound semiconductors to change the electronic bandgap of the material. Figure 2 shows the range of materials available. The problem that arises for solar cell production comes from the lattice mismatch between materials. Solar cells rely on very high quality crystallinity to give the materials both the absorptivity and the electrical properties necessary for efficient operation, and even a very small mismatch of crystal lattice sizes (the average spacing between atoms, or the x-axis in Figure 2) can significantly disturb the crystalline structure and sharply reduce the cell's efficiency.

Figure 2. Bandgap plotted against lattice constant for a variety of compound semiconductors. The multicolored bands correspond to the visible wavelengths in the spectrum. AlAs: Aluminum arsenide. AlP: Aluminum phosphide. AlSb: Aluminum antimonide. BAs: Boron arsenide. BP: Boron phosphide. CdS: Cadmium sulfide. CdSe: Cadmium selenide. CdTe: Cadmium telluride. GaN: Gallium nitride. GaP: Gallium phosphide. GaSb: Gallium antimonide. InAs: Indium arsenide. InN: Indium nitride. InP: Indium phosphide. InSb: Indium antimonide. MgSe: Magnesium selenide. Si: Silicon. ZnS: Zinc sulfide. ZnSe: Zinc selenide. ZnTe: Zinc telluride. (Taken from J. F. Geisz and D. J. Friedman.3)

A patented approach being pioneered by QuantaSol uses extremely thin layers of lattice-mismatched material, not more than a few tens of nanometers thick, which can be grown without degrading the crystalline quality because each layer is so thin. These layers form quantum wells between barrier layers designed to relieve built-up strain in the crystal. Quantum effects increase the absorption of the quantum wells, making it possible to tune the bandgap of the host solar cell according to the design of the quantum wells. If done correctly, the crystallinity is not affected at all, and the potential efficiency of the devices is very high.4 These devices are fabricated using standard III-V semiconductor production equipment, namely, metal-organic vapor phase epitaxy. QuantaSol has developed the method by which the quantum wells are designed and fabricated on this equipment, which involves the deposition of crystal directly onto a substrate material (typically crystalline Ge) in very thin layers, carefully controlling the composition of these ternary alloys, such as InGaAs.

QuantaSol is developing structures that modify the middle-cell bandgap in a traditional 3J device design, along with approaches using quantum well structures in all the junctions in a 3J stack. In this way significant efficiency improvements can be made. In the future, QuantaSol will extend its quantum-well technology to modify both the bandgap of the top cell (indium gallium phosphide) to tune the absorption in the top cell, and also develop new material systems to allow the fabrication of a III-V solar cell with a bandgap of around 1eV. This would remove the need for the Ge substrate and open up new avenues for higher efficiency combined with lower cost of production.5,6

Tom N. D. Tibbits
QuantaSol Ltd.
Kingston-upon-Thames, UK

As director of product marketing, Tom Tibbits is the architect of QuantaSol's vision to maximize energy harvesting from multi-junction solar cells by tuning material bandgaps with proprietary quantum well structures.

1. http://www.solfocus.com/en/news-events/press-releases/2010-03-18.php Victor Valley College goes solar with SolFocus power plant. Accessed 1 April 2011.
2. http://www.vvc.edu/measurejj/091216_Program_Status_Report_and_Budget
 Describes approval for funding of a solar-generating facility on the Victor Valley Community College campus. Accessed 1 April 2011.
3. J. F. Geisz, D. J. Friedman, III-N-V semiconductors for solar photovoltaic applications, Semicond. Sci. Technol. 17, pp. 769-777, 2002.
4. M. Lumb, A. Dobbin, D. Bushnell, K.-H. Lee, T. N. D. Tibbits, Energy yield calculations of bulk and MQW (III-V) multi-junction cells under different spectral conditions, Proc. 5th World Conf. Photovolt. Energy Conversion, 2010.
5. A. Dobbin, M. Lumb, T. N. D. Tibbits, Modeling of location specific solar spectra for use in the tuning of multi-junction solar cells and energy harvest predictions, Proc. 5th World Conf. Photovolt. Energy Conversion, 2010.
6. A. Dobbin, M. P. Lumb, T. N. D. Tibbits, Increasing the energy yield of III-V multi-junction solar cells using multiple quantum wells and location specific spectral tuning, Solar Energy, forthcoming.