Project develops laser scribing process for organic photovoltaic cells

AACHEN, GERMANY – Under the direction of the Fraunhofer Institute for Laser Technology (Fraunhofer ILT), five project partners from industry and research are developing technologies for the roll-to-roll production of organic photovoltaics in the EffiLayers project. This project aims to help the machine manufacturers based in the North Rhine-Westphalia region play a pioneering role in the market for flexible thin-film solar cells and printed electronics.

It is about organic photovoltaic cells (OPV cells), which are less efficient than traditional silicon-based solar cells, but are flexible and transparent. These last two advantages allow OPV cells to be functionally and decoratively integrated, for example, into building facades. By developing an efficient production process, the partners intend to boost the industrial mass production of OPV cells.

Since the individual layers of the OPV cells are only a few nanometers thick, the technology only requires small amounts of material to coat large areas of flexible substrates. This way, producers save on resources. The roll-to-roll process also makes it possible to manufacture on an industrial scale. Compared to classical silicon photovoltaic production, this process has lower manufacturing costs because energy-intensive and costly process steps are no longer necessary.

In the predecessor projects FlexLas and PhotonFlex, the partners focused on individual steps of the complex roll-to-roll production process for OPV cells (FIGURE 1). In September 2019, the EffiLayers research project was launched with the aim of implementing and integrating innovative analysis and process technologies into the production process. Individual process steps are monitored by high-resolution sensors and implemented in a process control system.

“We would like to implement the process in a manner oriented to the industry,” explains Ludwig Pongratz, a researcher at Fraunhofer ILT. “Our approach is to replace expensive, energy-intensive sputtering processes with wet-chemical coating processes”, says Pongratz.

The functional layers are applied on top of each other via wet-chemical solutions by means of heated slot-die coating. The 10- to 250-nm-thick layers are processed with various laser sources from the short-pulse and ultrashort-pulse (USP) range. In the continuous process, the laser drying and laser thin-film ablation are used to separate individual cells as well as to remove layers in the edge area. The OPV cells are then sealed by laser encapsulation with a barrier film to protect them from environmental factors.

“To produce OPV cells having an area of 10 square meters, we only need three grams of organic active material,” Pongratz explains. “Because we were able to successfully implement many innovative process steps into our plant in the first two projects, the funding of a third project was approved with funds from the European Regional Development Fund (ERDF).”

In the EffiLayers research project, a USP laser in the femtosecond regime plays an important role. It separates the individual layers so that individual cells are connected in series. “With laser scribing, we guide 11 partial beams onto the surface while the film moves,” explains Pongratz (FIGURE 2). “The laser beams selectively separate the composite layers so that, at the end, 12 serially connected subcells are produced on a single film. The challenge is to selectively ablate the individual nanometer-thick layers without damaging the underlying layers or causing short circuits [FIGURE 3].”

FIGURE 3. The ablation of nanometer-thin layers on a moving film with USP laser scribing in the femtosecond regime is a complex process.(Courtesy: Fraunhofer ILT, Aachen, Germany/Volker Lannert)

To map the entire process, Fraunhofer ILT is cooperating with the Chair of Applied Laser Technologies (LAT) at Ruhr University Bochum (Bochum, Germany). Both research institutes are working closely with the plant manufacturer Coatema Coating Machinery GmbH (Dormagen, Germany) and the communication experts of Ortmann Digitaltechnik GmbH (Attendorn, Germany) and LIMO GmbH (Dortmund, Germany), the latter of which provides the optical components for the 11 partial beams.

In the meantime, development in the field of organic materials has also reached new standards. These novel materials are incorporated into the EffiLayers project to achieve a significant increase in solar cell efficiency and to make OPV cells usable for indoor applications as well. “For us, it is important to map the process in an integrated manner and to develop the necessary process monitoring and control along with our partners,” Pongratz says.

For more information, please visit ilt.fraunhofer.de.