FEATURE SOLAR
➤ 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
Developing materials that can not only harness multiple wavelengths but also do so efficiently and affordably is critical for the photovoltaics industry
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.
Cermax xenon lamp as part of an integrated solution 18 ELECTRO OPTICS l MARCH 2016
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
TEM-image showing TOPCon structure developed at Fraunhofer ISE for silicon solar cells
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. l
@electrooptics |
www.electrooptics.com
Excelitas
Fraunhofer ISE
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