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Sunny skies and spectroscopy

Tom Eddershaw looks at the spectroscopy solutions used in the development of solar cells

The global green energy sector is growing as countries aim to meet clean energy targets. This is increasing the focus on technologies such as solar power, which in turn is providing a healthy vein of revenue for spectrometer producers for quality assurance tasks and for R&D.

However, subsidies offered by the Chinese market for the development and production of solar cells are making it hard for European companies to compete on price. Government legislation has been implemented to attempt to slow the flow of cheaper cells into Europe and to assist the local producers, but the obstacles to Asian producers can be avoided by distribution through other nations, such as India.

On top of this, European governments are divided on where they are investing money: into research or implementation. Ger Loop, Avantes product manager, said: ‘Each country has its own green target and it depends on the government as to how they push the green energy, if they put money into wind energy, solar energy, or water energy. Some countries are subsidising projects on solar cells to get this green amount higher, but other governments are spending less and lagging behind on it.

‘It’s quite difficult to generalise this, but I believe Germany is doing quite well at implementing solar cells to generate electricity. [The solar panels] are mostly coming from China. As I understand it, the European companies producing solar panels are having a difficult time because of the competition.’

Loop added: ‘Overall, we will see a growing market, but as to where the centre point is in the coming years, it will be a constant battle. We at Avantes are trying to be a global player.’

So, it would seem that European producers of spectroscopy equipment used in the production of solar cells must be able to operate on an intercontinental level in order to make the most of the green market. Once involved however, the solar energy market is a good one for companies to be in.

‘The solar market has provided good opportunities for the photonics industry for quite some time now,’ said Rob Morris, marketing operations manager at Ocean Optics, ‘but the interest has certainly been gradually ramping up, both in the research and development side of things and in the production side as well.’

He explained that there are many types of spectrometers used for characterising photovoltaic cells. Essentially, the techniques are similar to other types of applications – absorbance, transmission and reflectance of, for example, cell materials.

The primary goal of photovoltaics research and panel testing is to produce solar cells that collect and convert solar radiation to electricity as efficiently and cost effectively as possible. Morris continued: ‘Here’s an example: One of our customers, a manufacturer of thin film photovoltaic panels, requested NIR reflectivity analysis of several coated glass samples. Measurements were conducted from 1,200nm to 2,100nm under ambient lab lighting conditions.

‘Because the absorbance of photovoltaic panels is so critical, determining the reflectivity at panel edges and elsewhere is a good indicator of the light loss at those areas. The use of anti-reflective coatings and glass dopants are among the approaches manufacturers may evaluate to improve panel efficiency. Other folks are monitoring the deposition layers of substrates that comprise the panel, using familiar thin film measurement techniques typical of the semiconductor industry.’

Avantes also provides spectrometers for thin film coating control. Loop explained that the spectrometer, in combination with a light source and a probe, can measure the thickness of the coating when applied to the solar cell. This is done by recording the reflected light from the top and bottom of the coating and measuring the interference pattern produced.

Light reflected from the top of the surface will have travelled a slightly shorter distance than the light reflected by the bottom. The different light paths will then cause an interference pattern that can be measured and, when used in combination with the refractive index of the coating material, will accurately determine the thickness of the coating.

This is where Avantes sees most of its spectrometers used regarding photovoltaic cells, Loop said: ‘Each production facility can use a spectrometer to monitor the fabrication of the thickness – volume-wise, this one of the most important [PV markets].’

Avantes also offers spectrometers that are used to periodically characterise solar simulator lamps, which are required to calculate the efficiency of a photovoltaic cell. This process is more sophisticated than checking thin films and occurs in smaller volumes – Loop said there are normally only one or two set-ups per factory.

Loop said: ‘You have to monitor it over time to see how much energy is coming from the lamp because the lamp degrades over time.’ These light sources are used to check the solar cells after production – by comparing the power of the lamp to the power produced, the efficiency of the cell can be calculated.

In order to know what the input power is, spectrometers are used to measure light emitted by the lamp. ‘Our spectrometers are used as an irradiance measurement system,’ Loop continued. ‘It consists of multiple spectrometers in order to cover a broad wavelength range, 200nm up to 2,000nm. So, there are multiple spectrometers, a probe with a measurement head, and the total system has to be irradiance calibrated.’

Solar power research

Along with quality control, there is also a lot of research into photovoltaics being conducted in Europe, which provides opportunities for spectrometer manufacturers. WITec supplies Raman spectrometers to research institutes and universities that are trying to understand why malfunctions occur. Dr Thomas Dieing, director of customer support at the company, commented: ‘Our systems are capable of investigating photovoltaic cells in great detail with a variety of measurement techniques, so if something went wrong in the manufacturing process, they’ll see it. That being said, they weren’t designed for the sort of in-line, on-the-factory-floor quality control that some customers are looking for. Rather than a yes or no verdict, they provide in-depth characterisation of materials.’

The WITec systems can answer questions like whether the material was heated up too much. Dieing noted: ‘You can look much closer and take the diagnostics much deeper and do more of the detective work: where and why was there a fault.’

Considering spectroscopy is a light-based technology, monitoring a material that is designed to absorb as much light as possible and convert it into energy brings in issues of its own. Dieing said: ‘You want to make sure that as many photons as possible reach the detector. For the Raman effect, only about one in a million photons are frequency shifted; but then you still only have one [photon] and you need thousands to build a spectrum. You need to be as effective as possible within the collection range.’

In other applications, in order to increase the signal it is common to increase the power of the laser. Dieing explained that this isn’t an option here as putting more laser power in may introduce changes to the material that are non-reversible. This could create potentially misleading results when looking at the causes of failure in a material. ‘In most cases, you start with a very low [laser] power and a long integration time to see the spectra; then you raise the power. Through this, you can evaluate when you see spectral changes and then you know what power to stay below.’

WITec specialises in high-resolution, high-throughput Raman systems allowing the user to collect tens of thousands of spectra in minutes and generate an image that illustrates, for example, the stress distribution within a sample.

Dieing noted that Germany is putting a lot of investment into research into photovoltaics, adding that other countries like France are increasing their focus on solar cell research.

Again, Dieing doesn’t expect a massive increase in the photovoltaics market at the moment, but he puts this down to other aspects of the technology causing progress and uptake to slow. He said: ‘I expect battery technology to be the bigger field in terms of renewable energy. From my point of view, this is the bottleneck at the moment [in photovoltaics] and this is where most of the research will go into to open up this bottle neck.’

But this isn’t bad news for the spectrometer market. He said: ‘Here you can use Raman to look at the different cathode and anode materials, how they change during charging and discharging; there are various institutes looking at this area of research.’

He explained: ‘For batteries, there is an anode and a cathode. When you go through the charging and discharging cycle, then it’s about ions attaching to these anodes or cathodes. People are looking at the speed at which you can charge [the battery], how many charging and discharging cycles you can have, the capacity of the battery itself, and of course the efficiency. What good is it if you increase your photovoltaic cells by half a per cent but you throw away 50 per cent when you charge your battery? You want to have your battery as effective and low-loss as possible.’

The big advantage with Raman spectroscopy is that it is a non-destructive testing method. Raman looks at the vibrations of molecules, which change as ions dock to the material.

Dieing, who is based in Germany, continued: ‘I have solar cells on my roof; I would love to store all of the electricity from that, it would be plenty for my house. When I have a lot, I don’t need it – when I need it in the evening or at night, I don’t have a lot. There is at the moment no effective way to generate these sort of island solutions. The alternative to this is integration into the grid; this so far has happened in Germany. The excess is sold back to the grid but they need to buffer that. On a day where there is lots of wind and it’s very sunny, you get a lot of energy. On other days, it’s overcast and still and you don’t get any energy.’

He concluded: ‘It’s a real challenge for the German grid to handle this. But if you had good batteries, then you could easily say, “okay, let’s make it attractive for people to put their own batteries in their basements and then buffer for these extremes.”’

Ocean Optic’s Morris thinks the future of spectroscopy in solar cell production is reliant on easily integrated instruments for process lines. ‘When referring to the tools that are used to evaluate and monitor solar cell materials, then modular spectroscopy products are already evolving to meet the changing needs of the solar industry. There likely will be more interest in handheld instruments, and in spectral sensors that are smaller and less expensive than today’s options, so that integrating them into process lines is simpler and more efficient.’

Morris believes that as developers of photovoltaic materials seek improvement in cell efficiency, the need for convenient and scalable analytical tools to evaluate glass coatings, dopants and other materials is great. Spectral sensing systems such as NIR spectrometers, thin film measurement systems and solar simulator testing units will become even more easily configured for both the research lab and process line applications.

About the author

Tom Eddershaw is a technical writer for Electro Optics, Imaging & Machine Vision Europe, and Laser Systems Europe.

You can contact him on tom.eddershaw@europascience.com or on +44 (0) 1223 275 478.

 Find us on Twitter at @ElectroOptics, @IMVEurope, @LaserSystemsMag and @ESTomEddershaw.