Counting The Watts

Light Source And Its Life Span, Bifacial Measurement And Measuring Cells Strips Are Key Topics For Cell Testers

Counting The Watts

In our first PERC Cell Production Equipment market survey we have also looked at cell testers - shown is a device from market leader halm from Germany (photo source: halm)

  • In our PERC Cell Production Equipment 2022 Market Survey we also cover cell testers
  • While xenon lamps were the state of the art in the past, LED based illumination has been evolving as a reliable alternative with a long list of advantages
  • Testing cell strips without loosing throughput is the new line of development in cell testers
  • TaiyangNews cell production equipment survey features 6 IV cell testers from 4 companies, of which 2 have a collective market share of close to 80%

In our PERC Cell Production Equipment 2022 we have also looked at cell testers. These devices are sitting at the end of cell production lines – and are similar to teller machines which count the number of watts a cell can produce, and in turn gives an estimate of the revenue it can generate. After all, PV products are sold on a per-watt basis. The main purpose of the cell testers is to simulate sunlight indoors and conduct IV testing at standard test conditions (STC).

A solar cell testing setup mainly consists of a light source, an electronic load, a testing chuck and a contacting probe. The test setup must meet several requirements. Maintaining the cell’s temperature at 25 °C during the testing procedure is very important. Then the tester quality is ranked A, B, or C based on 3 criteria – spectrum, uniformity and stability – following IEC guidelines. That means, when a tester scores an A on all these metrics, the product is labeled as a AAA class tester. However, there are a few products available that provide precision levels way beyond what is prescribed by the IEC. It is an industry-wide practice to label such products with a “+”; for example, A+AA.

halm, Wavelabs, Sinton Instruments and ASYS are the 4 companies that are part of this survey. The first two command more than 80% share of the market between themselves. It is quite surprising that these 2 German companies have been able to maintain their leadership during a time when Chinese tool suppliers dominate nearly every segment of production equipment supply. According to halm’s technical sales director Felix Lange, one reason his company has been able to keep its leadership is because it mostly executes sales and service directly with the customer and stay closer to them. But an even more compelling reason is it constantly upgrades the technology at a pace that its Asian counterparts can hardly catch up. In a way, the industry has also played its part, with constant demands of technically advanced characterization solutions. The PV industry is quite young and there have been major developments in a short time – for example, bifacial technology a couple of years ago and the larger wafer quite recently – which need such solutions. The devices in use even 3 to 4 years back appear completely outdated compared to today’s tools. On the other hand, Lange points out, “Our modular platform can be upgraded to latest requirements even when there is a significant upgrade in technology, and it is what keeps us ahead. By adding a few components, our systems become up to date.”

Coming to the products, the survey features 6 IV testers from the above 4 companies. A major technological difference between the cell testers featured here, which also reflects the market in general, is the light source. While xenon lamps were the state of the art in the past, LED based illumination has been evolving as a reliable alternative. Wavelabs has been a strong advocate of LED based illumination. Below is the long list of advantages of a LED based light source compared to xenon according to Wavelabs:

  • Longer lifespan
  • Steady state operating ability
  • High temperature stability
  • No short- or long-term drift of the spectrum due to aging
  • The LED spectra does not contain emission spikes, thus does not require special filters
  • Avoids unnecessary heating of the cell due to the absorption beyond the bandgap
  • IV characterization can be combined with spectral resolved analysis

An undeniable benefit of the LED light source indeed is the longer lifespan. The Wavelabs cell tester, for example, comes with a rated lifetime of 20,000 operational hours. While it cannot be directly compared to the flashers, there is a 1 or 2 order of magnitude difference. On the other hand, the ‘old’ xenon technology is also being improved to compensate to a certain extent. For example, the flash tube integrated into halm’s tester comes with a rated guaranteed lifetime of 1 million flashes, while Lange emphasizes that the flash tube can actually sustain up to 3 million flashes easily in real life production. In addition, the company now has developed a new flash tube that has a life span that is 10 times higher, meaning the stipulated warranty is as high as 10 million flashes, and Lange estimates that the non-guaranteed performance attribute would not change. While this does narrow the gap, LED still seems to have some advantages. These testers have the possibility of evaluating the blue and red response of the solar cells, which enables early detection of process related issues. For example, the blue response is governed by the quality of the emitter and front surface passivation, while the red light helps in evaluating rear-surface engineering.

In terms of recent developments, all cell testers are now able to handle the larger wafer sizes. Lange says that replacement of single system components allow to upgrade halm’s IV testers to handle larger wafers, achieve higher throughputs or to add unique measurement features for future technologies.

While not new, bifacial characterization is one notable technology milestone in the characterization section of cell manufacturing. The IEC standard legitimately allows characterizing a bifacial device from single side measurement and calculating the rear side power from the present values (for background, check our TaiyangNews Bifacial Solar Technology Report 2021). halm has been a strong advocate of measuring the bifacial cell from both sides using two light sources — now part of the company’s standard configuration. “About 80% our testers we are shipping are bifacial compatible,” said Lange. Wavelabs supports the philosophy of measuring the bifacial from both sides with 2 illumination sources as well.

When it comes to the characterization sequence, Lange said that his company’s systems support 3 different kinds of measurements – only front side measurement at STC, only rear side illumination at STC, and simultaneous exposure of front and rear to light. But in production, the majority of cell makers are just using STC measurement on the front and rear separately to collect all required data for front and rear side process and quality control, according to Lange. The reason could be that while bifaciality attracts a slight premium, the gain itself does not qualify as a sales metric, at least not yet.

The most important development in the cell tester segment, according to halm, is testing the cell strips (half cell, 1/3 cell) individually without affecting the throughput. The company has already developed a measurement solution which allows to measure multiple cell stripes (e.g. half cell, 1/3 cell) within a single flash. Major advantage of this measurement approach is that all cell stripes are measured in parallel and without increasing the pulse duration. This allows to measure two half-cut or three 1/3 cells at a time with the same throughputs as it would be for the measurement of full cells. Every single cell strip is characterized individually, and the data is then fed to the sorter and the module assembler to configure the module appropriately. This measurement feature is as well combinable with all further measurement and analysis features (e.g. bifacial measurement, EL, IR) that halm offers.

As to the listed specifications, halm’s data is the most elaborate. It is listing only one model called cetisPV-IUCT-3600-BF. The product features 2 light sources based on a xenon lamp and with an illumination intensity range of 20 to 110 w/m2 and a A+A+A+ class light source. The tool is integrated with the so-called advanced hysteresis method, allowing reliable measurement of any known silicon-based technology, including TOPCon and HJT. In addition, the tool can also perform SunsVoc, 2-diode analysis, infrared imaging, 2-wavelength spectral response measurement, grid resistance measurement, EL imaging with automated image evaluation. It has a rated throughput of 4,000 cells per hour with standard system configuration, while higher throughputs are possible as well with additional flash power booster.

ASYS has provided data for one tool, called AIRON CT, the light engine of which has been developed by Botest, a subsidiary of ASYS. The tool is a AAA-class LED flasher with 3 quadrant electronic load that comes with 10 to 120 mW/cm2 illumination intensity. The IV tester features a patented contacting system that can be adapted to different cell layouts, for example bifacial and busbar less measurement, according to ASYS solar sales director Matthies Drews. The tester can perform hysteresis, dark measurement as part of the advanced characterization package, while EL, IR and grid resistance are among the additional characterization methods the tool can take up. For bifacial, while ASYS is providing single side measurement as standard, it is also open to integrate a second light source on special requests.

We received data for 2 tester modules from Wavelabs, and both are based on an LED light engine. While the SINUS-70 is a lab-scale system with 51 x 51 mm illumination area, the SINUS-300 is a commercial tool designed for production. It has 2 light sources to support the bifacial measurement. This light generator can work in both steady-state as well as flasher modes and qualifies for an A++AA++ rating.. The tester is designed to collect 4,000 data points to generate an IV curve in 20 to 205 ms. It has a rated average throughput of 4,000 cells per hour.

Sinton Instruments sent data for 2 models, FCT- 750 and FCT-650. Both come with very similar specifications: they are based on a xenon flasher with a lifespan of 10,000,000 flashes and a class A+A+A+ light generator. The devices both support pulsed or quasi steady-state illumination methods. The testers feature patented Voltage Modulation to neutralize the capacitive effects in IV measurements. In addition to IV testing, these 2 products can also undertake EL, PL and hotspot tests, while at the same time accomplish advanced characterization, including substrate doping, power loss analysis, SunsVoc, and minority carrier lifetime curves. Both the tools have the same rated average throughput of 3,600 wafers per hour.

The Text is an excerpt from TaiyangNews recent report on PERC Cell Production Equipment 2022, which can be downloaded for free hereIf you are at REI India this week, you can also pick up your printed copy, which is being distributed during the show.

About The Author

Shravan Chunduri

Shravan Chunduri is Head of Technology at TaiyangNews. Shravan caught the solar bug vey early in this career, starting 20 years ago in research, followed by solar manufacturing, then writing and consulting. His responsibility spans from writing technology articles and reports.

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