The 2015 United Nations Conference on Climate Change (COP21) held in Paris last December initiated noteworthy steps toward a clean energy future. Among these was the launch of the Global Solar Council. The principal members of the organisation, whose aim is to unify the solar power sector at an international level, are solar associations from established and emerging markets. As noted by SolarCity founder Elon Musk at the 2015 American Geophysical Union meeting, ‘you could take basically a corner of Utah and Nevada and power the whole United States with solar power’.
But while the international community acknowledges the value of solar power and the sun offers sufficient energy, the photovoltaics industry has a way to go before it can sustain our society’s demand for energy. Part of the challenge lies in storage technology; but equally important is the efficiency of solar cells themselves.
Silicon dominates the solar market, but modern panels can only harness a single wavelength of light. This means that only a fraction of the sun’s available energy is actually captured. Developing materials that can not only harness multiple wavelengths but also do so efficiently and affordably is critical for the photovoltaics industry. Along with specialised materials comes the need for specialised devices to characterise their properties and bring the industry to the same level as other key energy sources.
Silicon records
There are two ways of structuring silicon solar cells. Both-sides-contacted devices have metal current-carrying contacts on front and back, while back-contacted devices have contacts only on the rear. The record efficiency for back-contacted cells is 25.6 per cent, and in principle can be higher than both-sides-contacted cells. However, back-contacted devices require extensive structuring processes, making production costly.
‘The market share of both-sides-contacted cells is over 99 per cent and there are a lot of improvements in... technologies that help to boost [their] efficiency,’ explained Martin Hermle, head of the high efficiency solar cells department at Germany’s Fraunhofer Institute for Solar Energy Systems (ISE).
Last autumn, Fraunhofer ISE set a new efficiency record of 25.1 per cent for both-sides-contacted silicon solar cells. Hermle led the development of the record-setting Tunnel Oxide Passivated Contact ‘TOPCon’ technology, which features a novel full-area passivated back contact. The selective passivated contact enables electrons to tunnel through, while preventing recombination. A silicon coating completes the combination of layers that allows electrical current to flow from the cell with nearly zero loss.
Most solar cells have an aluminium-alloy contact covering the entire back surface. But the contact’s high recombination and low internal reflection limits efficiency. Therefore, the industry retrofits production to incorporate Passivated Emitter Rear Cell (PERC) technology. PERC increases efficiency by adding a dielectric passivation layer in small areas, to reduce surface recombination. However, it requires additional patterning steps and leads to longer current conduction paths in the silicon wafer. TOPCon reduces these losses, is easier to manufacture, and closes the gap on the efficiency record for back-contacted solar cells.
A solar cell built from two
‘The last few years have seen a number of research groups break the long-standing silicon solar cell efficiency record of more than 25 per cent. One of these groups is near production at that efficiency level, which is a big breakthrough,’ said David Young, senior staff scientist at the US Department of Energy’s National Renewable Energy Laboratory (NREL) in the High Efficiency Crystalline PV group.
One way to boost silicon’s practical efficiency is by stacking individual films, each of which is optimised to collect a specific band of wavelengths. ‘Growing a material directly on top of silicon is one option, but efficiencies are not very high because the lattice constants need to be just right. The easier way is to grow the top cell separately, lift it off its parent wafer, and glue it onto the silicon wafer,’ explained Young.
Young’s team recently set a record for mechanically stacked cells, achieving 29.8 per cent efficiency for a dual-junction device. The top cell, made of gallium indium phosphide (GaInP), was developed by NREL. The bottom cell was developed by the Swiss Centre for Electronics and Microtechnology using silicon heterojunction technology.
Although multi-junction cells made of semiconductor materials similar to GaInP can have efficiencies of up to 40 per cent, they are very expensive and are used only in satellites or high concentration applications. ‘Our 29.8 per cent efficiency got attention because it’s a new way to boost silicon efficiency. If you could...grow the top cell cheaply, you could slap that onto a silicon cell and increase efficiency relatively cheaply,’ added Young. When they recognised the value of their GaInP cell, it was a natural choice to connect it to silicon.
‘If you look at the solar spectrum and work out the two best bandgaps, it’s 1.1 eV for the bottom and 1.7 eV for the top cell,’ said Young. With a ~1.8 eV bandgap, GaInP achieves 20.8 per cent efficiency by itself. ‘Gluing’ it on top of a 1.1 eV silicon cell optimises the tandem for the solar spectrum. Engineering the optical coupling of the two cells, such that none of the bottom cell was in the dark and most of the unused light in the top cell passed through to the bottom cell, optimised the power from the tandem.
Work continues to raise the individual efficiencies of high-bandgap, single-junction cells, from tried-and-true III-V materials to inexpensive but unstable perovskites. Finding a high-bandgap material that could easily attach to silicon offers a valuable way forward.
Simulating the sun
Techniques of metrology play an important role in advancing solar cell efficiency, by providing a detailed understanding of cell and module performance.
Solar simulators allow developers to test a cell’s response to light in a controlled environment. Xenon lamp-based simulators have been around for 40 years and have found their way into the commercial and industrial sectors for solar cell production; now, LEDs also allow simulation of response to selected wavelengths.
Excelitas Technologies manufactures a broad array of lighting components and subassemblies for companies that design finished solar simulators. It’s Cermax product line includes components that deliver power, mainly from xenon lamps, as part of a fully integrated system. ‘We are not constrained to design solely around xenon. We have the capability to deliver a 100 per cent xenon-based cell, but we also use argon and krypton to shift the spectrum,’ noted Jim Clemens, product manager for Cermax.
Often working with distribution partner Atlas Specialty Lighting in Florida, Excelitas created a unique product that incorporates a reflector, lamp, and power supply in a single package. ‘The product is very transportable, and people tend to like the results. We see the continued use of xenon for small systems that could go into a lab, or for a large platform that tests hundreds of solar cells in an array,’ added Atlas general manager Ralph Felton.
Because of the maturity of the xenon-based line of simulators, lamps can be manufactured within a broad range of output levels and specifications. ‘Imagine xenon as the brute force method. It’s going to produce a spectrum of many wavelengths, and you get what you get. But with LEDs, you can dial in [specific wavelengths] and be more precise,’ said Felton.
Moving away from xenon-based light sources to LEDs will be a natural progression, thanks to the many advantages this technology brings. ‘You can simulate situations by varying 13 to 18 different wavelengths, which offers advantages for testing and analysis. But what you really want is to look at specific wavelengths that let you improve efficiency grading of the solar panel itself,’ explained Mark Gaston, Excelitas product manager for solid state lighting.
While LED solar simulators exist today, integrated systems are challenging to develop. ‘Since the LED side is new and complex, people need an integrated solution. We offer an approach where we select LEDs, design optics and the appropriate heat sink, add them to a circuit board and build the final solar product,’ said Gaston.
WaveLabs Solar Metrology Systems, based in Germany, grew out of its founders’ desire to revolutionise solar metrology with LED technology. According to WaveLabs sales engineer Jason Nutter, ‘LEDs offer options that are simply not available with other light sources. With the flexibility of LEDs, debugging can happen live on the production floor.’
Their Sinus-220 solar simulator offers the combined benefits of high throughput and a tunable, stable, and flexible spectrum. ‘Because we are able to flash only specific bandwidths of light, we are...equipped to understand why our cells and modules react to light the way they do,’ added Nutter. The inline solar simulator has been used by institutes including Fraunhofer ISE and Seris. Studies have used the tunable spectrum to model light that has gone through module encapsulation, and rapid testing of external quantum efficiency, using individually adjustable bias light.
With LED technology, calibration and replacement of ageing parts can be a thing of the past. A large draw of the SINUSinus system is its consistency in achieving the required end spectrum, allowing the user to focus on production and optimisation.
‘As with all transformative technologies, time is needed for markets to fully comprehend [their] advantages. Current solar cell efficiency already allows for photovoltaics to occupy a much larger portion of the current energy mix, and we believe it is just a matter of time before it is widely utilised,’ concluded Nutter.
Solar spectroscopy
Not only does the solar industry need to test products using simulated sunlight, but it also needs to conduct spectroscopy measurements during development and production. Spectrometer manufacturer Avantes, based in the Netherlands, tailors products to meet quality assurance needs.
Evaluating the exact spectral output of solar simulators can be challenging, because each manufacturer offers different features. ‘Characterisation [of a simulator] tells you something about how well it emulates the outside solar light, so you need a spectrometer which is calibrated for intensity,’ explained Ger Loop, Avantes product manager.
To meet this need, Avantes developed a calibrated spectroradiometer that can measure pulsed and continuous wave solar simulators. This requires special hard- and software to measure the timing and triggering of light flashes in pulsed simulators, and to detect false readings given by stray light reflected from optical benches. Similar instruments are useful for measuring environmental solar radiation and positioning solar panels to use. ‘Spectrometers for outdoor use give both diffuse and direct sunlight measurements. You can use this to identify the optimum positioning for solar cells outdoors,’ added Loop.
Spectroscopy provides quality assurance at all stages of the design and production process. Monitoring solar panel manufacturing requires high speed, constant spectral data acquisition over long production runs. Avantes’ multichannel spectrometers achieve this, along with inspection of thin film thickness. When subjected to light, beams are reflected from the coating and from the substrate, creating an interference pattern from which the thickness of the material can be derived.
‘Our devices enable you to post accurate data sheets for your own product,’ said Loop. ‘Ultimately, you want to know how well your solar panel translates light energy into electricity, and you have to define that very accurately.’ By doing so, a developer can show the unique features that distinguish themselves from competitors.
Increasing electricity, increasing markets
Another means to improve solar efficiencies dramatically is by a quantum phenomenon called singlet fission. In conventional semiconductors like silicon, the absorption of a photon leads to the formation of one free electron that can be harvested as electrical current. Some of the photon’s energy is lost as heat. But in singlet fission, which occurs in some organic materials, the absorption of a photon leads to the formation of two energetically excited particles. If singlet fission could be incorporated into solar cell materials, the available electrical current could be doubled.
The most recognised means to incorporate singlet fission into a solar cell material is to add molecules to dye-sensitised cells. Combining suitable dyes and singlet fission molecules, the theoretical energy conversion efficiency has been shown to increase by up to 44 per cent. Alternative designs have recently been proposed, exploiting observations of exciton transfer from singlet fission materials such as quantum dots. According to Alex Chin of the University of Cambridge’s Cavendish Laboratory, ‘the basic idea is to avoid energy losses by producing more charges per photon (the ideal quantum efficiency is 200 per cent) and these designs differ in the means of harvesting the fission products.’
Now, a team led by Chin have made the first real-time observations of one of the primary mechanisms behind singlet fission. Shining ultrafast laser pulses on a sample of organic material pentacene, they showed an elusive intermediate state where the two excited particles are entangled, or share a single quantum state. When vibrated by the laser pulses, the pentacene molecules changed shape and briefly absorbed light, allowing them to be observed.
‘While we already have a large number of effective singlet fission materials, many are not optimised for solar energy conversion. We hope that understanding how to “control” fission could allow us to do both,’ added Chin. As scientists’ understanding of quantum dynamics of real materials grows, so does the potential for optimising electronic properties for clean energy markets.
In 1931, Thomas Edison said: ‘I’d put my money on the sun and solar energy – what a source of power. I hope we don’t have to wait until oil and coal run out, before we tackle that.’
With newfound support resulting from the COP21 agreement, developments in solar cell efficiency and manufacturing are poised to expand markets and bring a reliable source of energy to the world’s growing population.
