The EU PVSEC exhibition, which this year takes place in Amsterdam, is far from beeing as big as it once was. However, equipment suppliers in some segments are still very much present. These include manufacturers of flashers, also because particularly in the cell flasher market, European suppliers are still strong. The same is true for the Research at European institutes. The conference program is packed with presentations on high efficiency technologies, silicon feedstock and other topics and the discussion with the scientific community is one reason why these machine suppliers still come to the event.
Concerning flasher-development, the discussion is still about the adjustments that become necessary in the course of recent technological innovations. On the cell side, PERC and heterojunction require special measurement methods, possibly also bifacial cells and modules. On the device level, the question arises as to how flashers can be compared to each other, especially if they use different light sources, as LED illumination is developing as a competitor to the standard xenon lamps.
Bifacial modules make use of the reflected light which hits the backside. A group of experts argues that these cells and modules in production don’t have to be measured by both sides, but it is sufficient to flash from one side with higher power. If one assumes ten percent surplus yield and a light sensitivity of the back side which is 70 percent of the front side, this would correspond to 1070 watts per square meter illumination per square meter, instead of 1000 watts as is specified in IEC standards.
Detlev Berger, CEO of Berger Lichttechnik, contradicts and says that the higher power is not imperative. The dependencies of the cell output power from the irradiation power and other parameters are well known, and three measurement points at different illuminations would theoretically be enough to calculate the power output at 1070 W illumination. In a white paper, he underlines this with the results of the measurement on two modules. Another difficulty of the measurement at higher illumination is that clients should not take this value for granted. It depends on the environment of a module, how much light is reflected on the back side. It can also be more than ten per cent.
New cell flasher from Berger Lichttechnik
Berger Lichttechnik has now upgraded its cell flasher. At the PVSEC, Detlev Berger showed that they had implemented everything in the PSS Gen 3-C “that others reproached us as missing”. The A+A+A+ class cell tester is fast and can measure up to 6000 cells per hour. It contains, according to the company, the “latest high efficiency measurement technology” for PERC, HJT and other cell types, is adaptable for half cut cells, and allows the integration of EL and infrared measurement.
In order to quickly measure high-efficiency cells, Berger developed the so-called over- and undershooting concept years ago. During flashing the measuring points on the I-V curve are approached by quickly switching a resistance, which acts as load, to different values. If the curve is measured in the direction of increasing load, the resistance is decreased step by step. Undershooting means that, before each measuring point, the resistance value is briefly decreased further to accelerate the transient oscillation. Also the number of measurement points was significantly increased in the new device compared to its predecessors.
Halm considers bilateral measurement of bifacial cells to be useful
Halm, on the other hand, measures in its cell testers with a variable voltage applied to the cells. This voltage only compensates the losses in the cables and contacts, says Klaus Ramspeck, Director Development at Halm, at EU PVSEC. This allows longer leads. The company provides also EL and IR measurements integrated in the cell tester and has also developed algorithms for analysis. According Ramspeck, it is market leader in this segment and has already sold a three-digit number of these cell testers.
For the measurement of bifacial cells, Ramspeck considers it useful to measure from both sides. The variation of the sensitivity between front and back side in production can be even so large that the error exceeds the range of a cell class into which the cells are sorted, he sais.
Halm can implement mirrors in its cell testers that the cells are illuminated also from the back side. Photo: Halm
The variation of cell or module properties in production is also one of the fundamental difficulties in assessing the quality of the light spectrum of a flasher. This is a big topic at the EU PVSEC, as a new standard is currently being discussed. One point is that a so-called coverage factor could be introduced. Up to now, the standard compares only how the flasher illuminates in six defined areas of the light spectrum. This makes it possible that in principle with six LEDs emitting exactly six spectral lines, each in one of the six areas, it is possible to reach the same pro forma quality as devices which make use of more LEDs or xenon lamps, which emit a more continuous spectrum. The coverage factor would indicate how good coverage is overall, so it would better differentiate between different light sources. It is also under discussion to introduce a factor for spectral mismatch. This describes how a cell reacts to deviation of the light spectrum of the flasher from the defined sunlight spectrum.
None of these approaches can solve the difficulty that it depends on the application and way, a flasher is operated, how accurate the measurement results are at the end in production. If the variation of the spectral mismatch in production is low enough and the flasher is calibrated correctly, in principle it can be possible to measure accurately enough even with a lower coverage, some experts say.