In eight minutes, light (in the form of emitted sunlight) crosses one hundred and fifty million kilometers and reaches the earth, bringing enough energy to power our energy-intensive lifestyles many times over. Expressed numerically, this eight-minute journey results in our earth absorbing 3,850,000 exajoules (EJ; 1EJ= 1E18 Joule) per year to power our societies and of this amount, only a fraction (500 EJ) is required to sustain the world’s energy consumption.
In more recent times, this free energy resource has been utilized to generate electricity, traditionally by using photovoltaic (PV) cells to capture the incoming sunlight. However, and rather unfortunately, solar cell technology has been afflicted with either cost issues (commonly found in the manufacture of first-generation monocrystalline silicon solar cells) or efficiency rate issues (commonly found in the majority of second-generation copper indium gallium selenide (CIGS) or cadmium telluride (CdTe) thin films). This has held back the widespread deployment of solar energy technology for the generation of electricity. This situation has improved over the last few years and this is partly due to technologies that use different ways to concentrate the power of sunlight.
From the basic approach of deploying a silicon solar cell, technology (through research) has advanced to include forms of PV technology that concentrate the power of sunlight. Such methods are referred to as concentrated solar technologies, with concentrated solar power (CSP) and concentrated photovoltaic (CPV) making up the two technologies in this category.
Concentrating the power of sunlight
The theory underpinning concentrated solar technologies is that sunlight is directed via mirrors to a small area where heat is produced that drives a steam turbine engine (CSP) or directed via optics to a highly-efficient solar cell array (CPV). The belief is that concentrating light onto an array or to power a steam engine will reduce the need for large solar projects that use expensive silicon and take up a sizable amount of land, thus reducing the costs associated with installing such a system. While CSP is indeed a fascinating technology and will certainly play a part in the global PV market, advancements in the area of CPV, such as the integration of quantum wells, quantum dots, and luminescence CPV, will be the main thrust of this article.
CPV Systems Multiplying the ‘Sun’
CPV is a relatively new technology and after more than thirty years in the basic research stage, this technology has finally moved to the commercial stage. Using a set of optics, and mounted on a tracking device (applicable to only certain situations), CPV systems are able to ‘follow’ the light from the sun and direct sunlight to a small and very efficient solar cell array. The ‘concentrated’ energy is then measured and expressed as a multiplication of the power of the sun and referred to as ‘suns’. One ‘sun’ can be expressed as being equal to the power of solar incidence at noon on a clear summer day.
CPV systems are classified into three types: low, medium, and high. Low concentration can reach a magnification ratio of less than ten suns. The magnification ratio of medium concentration photovoltaic systems is between 10 and 100 suns. High concentration systems have magnification ratios of between 100 and 1000 suns.
Overcoming the limitation of CPV Systems
Magnifying sunlight to equal the power of multiple suns is not without an inherent set of problems; chief among them is the issue of heat as producing so much heat may actually impede the CPV system. For low concentrator systems, this heat issue is not a significant one, but for medium and high CPV systems (and especially high CPV systems), the issue is considerable and requires a heat sink to cool the system down otherwise the heat loss would be unmanageable and thermal destruction would take place.
To counter this issue in high CPV systems, researchers at IBM leveraged their decades of expertise in chip cooling systems and nanotechnology and produced a liquid metal interface that was able to reduce the temperature of the solar cell from 1600 degrees Celsius to 85 degrees Celsius. This was achieved by producing a thermal interface layer (a thin liquid metal amalgam of gallium and indium) for use in the laboratory prototype, and placing this thermal interface layer between the cooling block component (a copper plate) of the mock-CPV system and the laboratory-scale solar chip.
By concentrating the equivalent of 2000 suns onto the cell, IBM researchers directed a temperature high enough to melt the solar chip. However, with the addition of the thermal interface layer, researchers at IBM managed to efficiently reduce the temperature from 1600 degrees Celsius to 85 degrees Celsius. This groundbreaking advancement, originally developed for the betterment of computer chips, turned out to benefit the growing solar energy industry. In doing so, this advancement has shed light on the crossover potential(s) as both technologies have the same genesis with silicon as the base material.
Such a reduction makes CPV technology very attractive, as this would then reduce the cost of either installing or adding extra components to handle the very high temperatures. In addition to the lower manufacturing costs, the second benefit to industry would be higher efficiencies. By lowering the temperature and providing a workable environment for the solar cell to operate, the efficiency can be maintained: as the solar cell is usually a highly-efficient multijunction solar cell, efficiencies of around 40% can be reached.
Quantum wells to improve multijunction solar cells
A p-n junction is the interface between a P-type and N-type semiconductor, and this junction is the reason electronic transfer can take place in devices such as solar cells. In a traditional silicon solar cell, with one p-n junction, the efficiency rate is around 22% (with very pure monocrystalline silicon solar cells reaching 40%). With the recent advances made to multijunction solar cells, the efficiency rate has been recorded at between 39% and 41%.
In tandem stack or multi-layer stacks, the material is chosen to fully utilize the electromagnetic (EM) spectrum. For example, in a conventional triple-stack p-n junction, the three layers are normally a combination of indium, gallium, germanium, and arsenide. While the resultant triple-junction solar cell is extremely efficient, the combination is not ideal. By employing quantum wells, the bandgaps of the semiconducting material used in the triple-junction can be tuned to absorb more of the EM spectrum.
Under ideal conditions and concentrated sunlight, a multijunction stack could have a theoretical efficiency rate of almost 90%. Compared to an efficiency of just over 20% with traditional silicon solar cells, it is quite clear why multijunction PV cells are of interest. By reducing heat loss and increasing efficiency in concentrated sunlight, the multijunction PV cell is an ideal candidate for use in CPV systems.
Practical applications of quantum well-multijunction solar cells
Visible light is found between 380 nanometers (nm) and 780 nm, yet the light from the sun constitutes the entire EM spectrum. As current solar technology focuses on absorbing visible light, this implies a lot of energy from other parts of the spectrum is wasted. By being able to tune the solar cell, and more specifically the triple-junction, the wider bandgap could then absorb other parts of the EM spectrum such as the infrared (between 700 nm and 1E6 nm).
Research by Professor Barnham of Imperial College (UK) demonstrated the increased efficiency of triple-junction solar cells using quantum wells. By balancing the stress in the three layers, Professor Barnham was able to introduce 65 quantum wells with no negative effect, widening the absorption band edge and thus increase the range at which EM spectral absorption took place. In its basic form, the quantum wells stress-balanced the device and had the effect of re-absorbing the photon passing through the device, boosting the efficiency to 30% (for single junction solar cells) and higher for multijunction solar cells.
From basic research to commercialization
QuantaSol, a company based on the commercialization of Professor Barnham’s research into quantum dots and quantum wells, now offers highly efficient solar cells with a particular focus on CPV systems, which will help mainstream CPV technology.
In addition to QuantaSol, a company based in San Jose (California, USA) has also adopted the use of stress-balancing to increase the efficiency of the solar cell; Solar Junction produced a triple-junction solar cell using their A-SLAM™ (Adjustable Spectrum Latticed Matched) architecture, which had a recorded and verified efficiency rating of 43.5%. The patented architecture allowed for the solar device to be tuned between 0.8 electronvolts (eV) to 1.42 eV, which facilitates for absorption in the (near) infrared region of the EM spectrum.
An Alternative CPV System
A promising development in CPV technology is the use of luminescent plates (containing either luminescent dyes or luminescent thin films) to generate electricity at a concentration ratio of 3 suns. The technology, still at the research stage, does look encouraging and offers an interesting route in capturing the energy from the sun especially as this technology is able to absorb not only direct sunlight but also diffuse light (which allows for operation even on days that are cloudy).
The luminescent plates capture sunlight and fluoresce. This fluorescence, which is then directed to the edges of the plate, strikes the embedded solar cells and generates electricity. As fluorescent-doped objects absorb a greater portion of the EM spectrum, internally reflecting this fluorescence in a device will concentrate the energy so that when the fluorescence strikes the solar cell, an efficiency rating of 20% should be achievable. Unfortunately, efficiencies have been recorded at 7% and this is due to a variety of reasons. For example, incorrect reflective angles within the device led to efficiency losses, and self-absorption by the different dyes when overlapping also caused a drop in efficiency.
Quantum dots to improve luminescent CPV systems
Quantum dots, with their unique properties, offer an alternative to the use of organic dyes found in traditional luminescent CPV systems. Quantum dots are tunable and this offers the ability to alter the absorption size.
Theoretical research carried out by Dr A Chatten from Imperial College London (UK) highlighted the use of quantum dots in luminescent solar concentrators, with positive results including a higher degree of stability over organic dyes (i.e. less degradation) and high luminescence quantum yields (in a cadmium-selenide based heterostructure, the yield was over 80%). By selecting the size of the quantum dot, the device was able to minimize the overlap between luminescence and absorption, increasing the efficiency of the device.
Professor Barnham (Imperial College, UK) focused on the use of quantum wells and quantum dots in solar concentrators. His research on the use of quantum dots also supported the results obtained by Dr A Chatten, in that quantum dots do not degrade in sunlight as easily as organic dyes and that the tunability of quantum dots allow for an expansion in the absorbing range of the device, with a predictable increase in efficiency.
The addition of these specific quantum dots into a luminescent concentrator device offers for higher efficiencies and greater stability, factors that will result in lower manufacturing and maintenance costs and be very interesting to industry.
Integrating Quantum Dotsinto Thin FilmLuminescent Solar Cells
In 2010, Dr W van Sark of Utrecht University in The Netherlands published research on integrating quantum dots with thin film solar cells to produce luminescent solar concentrators. Using select nanocrystals and tuning their absorption properties, and by tuning the bandgap of the selected thin film, Dr van Sark was able to produce a device with an efficiency of 5.9%. While nowhere near as high as conventional solar cells, this is comparable to the average efficiency rate of 7% found in luminescent solar concentrators. Further research will increase this efficiency rate even higher, indicating luminescent solar concentrators have an important role to play in the future of solar energy technology.
The Commercial Perspectives of CPV systems
There are many attractive features of installing and using CPV systems. Due to the feature of being able to concentrate sunlight, and to possessing higher efficiencies, CPV systems will be beneficial in countries that receive a great deal of insolation. However, there are other factors that make this technology an attractive one: low-capital manufacturing costs and low water consumption rates. As one of the drivers in the PV industry is to reduce manufacturing costs, developing and introducing a technology that is not capital intensive will be a cost-effective way to make the transition to alternative technologies.
By introducing the research highlights mentioned in this article, such as the thermal interface layer, the integration of quantum wells and the incorporation of quantum dots, next generation concentrated photovoltaic systems will be manufactured that will be ingrained with higher efficiencies (due to progress in the already-high efficiencies) and possibly even lower manufacturing costs.
Current Situation of CPV Projectsaround the World
For worldwide CPV installation projects, the advancements listed in this article will be of particular interest. In the Asia-Pacific region, projects such as Guascor Fotón’s pre-operational 59 megawatt (MW) project in Taiwan, Delta Electronics operational 0.5 MW project in Taiwan, and Silex System’s 154 MW project in Australia might consider implementing quantum dot advances or quantum well structures to increase efficiency rates.
Projects located in Europe include the operational Amonix/Guascor Fotón 7.8 MW CPV system, and two good examples of CPV projects in America is the 1 MW Solfocus project in California and the 2 MW Greenvolt CPV project in California; as the state of California receives a great deal of insolation, utilizing CPV systems is a sensible option.
The Outlookfor CPV Systems
By utilizing concentrating technology, such as CPV, costs can be reduced by using fewer solar cells and by extension, less of the expensive raw material, and by utilizing less land for installing projects. Efficiency rates are already high and are predicted to go even higher, resulting in a technology that promises to generate more electricity for less. This will undoubtedly benefit both industry and consumers, and ultimately, our environment.
