Fraunhofer’s Contactless Method To Gauge Solar Cell Efficiency

Fraunhofer Institute Research scientists for Solar Energy Systems ISE havedeveloped a measurement method to determine the performance of back-contacted solar cells without contact in the production line. Measuring the current-voltage characteristic curve is the most important test in solar cell quality control. Eliminating physical contact with the solar cell saves time and thus allows significantly higher throughput rates in production. It also eliminates mechanical stress on the increasingly thinner solar cells during measurement and reduces the maintenance costs of the measuring system. The researchers tested the method for the first time in 2022 and have now successfully transferred it to IBC (interdigitated back contact) cell architectures in a production-related environment. IBC, along with Perovskite Tandem cells is widely seen as the next generation of solar technology that is already finding its feet in China.  

Contactless Power Measurement

The new measurement method is based on photoluminescence and contactless electroluminescence images under different excitation conditions, as well as spectral reflection measurements. In combination with a model, the complete current-voltage (IV) characteristic curve and thus the performance parameters of the solar cell, such as open-circuit voltage, short-circuit current density, fill factor, and efficiency can be derived.

In addition, contactless measurement enables higher production rates, which reduces investment costs when setting up new production lines.

“The results of conventional contacted measurement and the new contactless measurement method show very good agreement in our tests,” says Johannes Greulich, group manager for inline solar cell analysis and simulation at Fraunhofer ISE. “We intend to develop the adjustments required for use in industrial mass production and accelerate the measurement of the reverse behavior in a future research project with partners.”

New Solar Cell Efficiency Records

New efficiency records for silicon solar cells have been achieved in recent years with IBC cell architectures, and the current record of 27.8 percent efficiency is also held by a back-contact solar cell. Since both metal contacts are located on the rear side of this cell architecture, the cells must be contacted on one side from the rear for quality control, which requires special contacting devices and severely limits throughput due to the complexity of the contracting process.

“Our contactless method is therefore particularly suitable for measuring these single-sided metallized solar cells as well as the bottom cells of perovskite-silicon tandem solar cells, which have no contacts at all,” adds Johannes Greulich.

“The inline-capable, contactless solar cell power measurement methodology opens the door to productivity increases of more than 10,000 wafers per hour and to the production of even thinner solar cells. With this innovation, we are enabling more cost-efficient large-scale production of next-generation solar cells,” adds Director Ralf Preu.

The organization said, “As part of its PV-TEC research line, Fraunhofer ISE operates an automated solar cell tester that enables, among other things, both contacted IV measurements and contactless measurements on solar cells up to G12 format (210 mm x 210 mm) as well as on shingle solar cells in high quantities. In its latest study, the research team measured 150 IBC solar cells with different efficiencies using both the contactless approach and the classic contact method with a flasher from halm elektronik GmbH and compared the results.”

It added, “The development of the measurement approach was funded by the German Federal Ministry for Economic Affairs and Climate Action (BMWK) as part of the “NextTec” research project and by the German National Academic Foundation (Studienstiftung des deutschen Volkes e.V.) as part of a doctoral studies program.”