Loss of performance due to Ultraviolet (UV) Light has become a concern for new PV cells, although previous laboratory tests may have overestimated the problem. Paul Bhardt, photovoltaic module Reliability scientist at Fraunhofer Institute for Solar Energy Systems (ISE), outlines a mechanism that may have distorted earlier results.
Can you provide some background information about recent concerns about the industry around Ultraviolet-induced demolition in PV modules?
Since about two and a half years we have been involved in various projects where larger module buyers different modulette types against each other benchmarking in terms of reliability. In this context we realized that there was a strong UV breakdown in laboratory tests for some modules. This came in addition to a few other problems, such as mechanical loads or moist heatgradation, but the UV breakdown was the most surprising for us and for others in the industry, including some module manufacturers.
Simply put, the applied UV dose in the test usually corresponds to about a year of exposure in Europe. If this laboratory degradation appeared in the field, this would mean that the economic viability of many large PV projects could be considerably affected, only due to the amount of UV-related performance loss.
And have you seen these levels of UV breakdown appear in modules that are already in the field?
This is where another recent finding comes in. We soon realized that these modules did not show any stable behavior after UV tests in the lab. Depending on the storage and lighting conditions, we sometimes saw a very strong breakdown, but also something like recovery. What the community realized is that there are two different mechanisms at work here.
We have UV-induced demolition, which, as far as we know, causes irreversible damage to the passenger layer of the cell. Then there is an extra process that happens after the UV test. The module loses power to the left in dark storage. It can lead to relegation of several percent within one or two days. But it is also reversible due to a few minutes of light.
This can be evaluated as an artifact that we only see in the lab. We need good stabilization to get rid of this artifact and evaluate the relevant part of the UV breakdown, that is actually the main report.
Is this a problem with the way in which the laboratory test is accelerated – that it might be too strong to give an accurate representation of what is happening in the field?
Not entirely, although that is a subject that we are still investigating. There are no good studies that compared indoor and outside breakdown, so for now we must assume that the gear, the higher light intensity, will cause the same demolition as for a longer period of a lower intensity.
On the one hand we have seen cases of UV -induced demolition in the field. On the other hand, looking at the number of modulette types that show this UV breakdown in the laboratory, we would expect more cases outdoors. There is uncertainty when it comes to comparing interior and outdoor results. But independently of this, we know that there are interior results by this extra dark storage effect.
Is this a need for an update of the standard test procedures?
There is a clear need to add light weeks or another stabilization procedure after the UV test. It will be discussed if we can use exactly the same procedure for the soils defined in the standard, or if we have to define a special light that is applied after UV. Changes to the UV test itself may also be necessary. For example, the working point, if modules are in short -circuit or open circuit, or in maximum Power Point tracking, in the UV room. Data is still being collected about these problems.
Is this relegation problem limited to tunneloxide -passivated contact (topcon) modules, or have you seen it with other technologies?
In principle, Passivated Emitter Achter Cell (PERC) modules can also be influenced, although it is difficult to say because there are currently not so many new commercial perc models on the market. We also see reports of heterojunction modules with at least a similar effect. When it comes to back contact, there is not much data. However, if you look at the cell architecture, there are strong similarities on the front, and because the UV light in the field usually relieves the front, we expect similar behavior.
Manufacturers suggest different solutions to arrange UV breakdown – usually with careful process control or additives for the incoming. Can you comment on its effectiveness?
We have seen cases in which module manufacturers, after contacting them, claim to have changed something. We do not know the details, but the modules showed much less UV -induced breakdown when these changes were made.
We therefore believe that this problem can be solved at cell level by, for example, changing the properties of the anti -reflecting layer, the hydrogen concentration in some layers or another factor.
The other strategy used in commercial heterojunction -modules is to add a UV blocker or down to the Encapsulant. However, this can cause additional problems in the long term if these polymer additives cancel over time.
Do you think the industry still needs to worry about UV breakdown, in Topcon or other cell technology?
We should not worry about Topcon or any technology as a whole. But thorough UV tests must be part of the normal quality control for manufacturers and buyers. The problem may not be as bad as we thought or as some previously seemed to show. But it’s still there.
As long as we have uncertainties with regard to UV breakdown and whether we can really quantify in the lab what will happen in the field in the long term, it is possible to reduce the risk by performing UV tests and buying those modulette types that do not show relegation in the laboratory. There are certainly modules where this is the case.
Interview by Mark Hutchins
This content is protected by copyright and may not be reused. If you want to work with us and reuse part of our content, please contact: editors@pv-magazine.com.
