Lasers—indispensable for organic electronics

The ultra-thin solar films weigh only 500 g/m².

Displays based on organic light emitting diodes (OLEDs) have reached the mass market. Organic photovoltaics (OPV) will soon join them. In both cases, lasers are paving the way.

This smartphone is worth a second look: when you open it, you have an AM (active matrix) OLED display in tablet format. Samsung claims that its Infinity Flex Display can be opened and closed hundreds of thousands of times without a problem. Mass production will begin in a few months, reported the manufacturer at the world premiere in San Francisco at the start of November. The market-ready foldable display is a milestone in the more than 30-year development history of organic displays.

It’s not just by chance that there is also an increasing number of success stories in the field of organic photovoltaics (OPV). Both technologies are closely connected and profit from the advances being made in ultrashort pulse laser technology. The challenges of producing extremely thin, flexible OLED displays and flexible OPV films are similar. In both cases the aim is to cut and structure heat-sensitive material sandwiches without causing any adverse effects with regard to the functions. The demand for very high production rates and the highest possible precision don’t make the task any easier.

The laser process guarantees durability

In the case of the OPV films, precision is closely connected to durability. OPV manufacturer Heliatek attaches its cells to flexible PET film. A very thin barrier layer is applied directly to the film as protection against the effects of weather. Even very small amounts of water or oxygen degrade the organic materials and reduce the power of the solar cells. In a roll-to-roll process, Heliatek first applies the conductive, transparent front electrode, followed by the organic active material and then the back electrode. The process is complicated because the electrode and active layers require several structuring and cutting processes in which the barrier layer below must absolutely not be damaged. This structuring should be carried out parallel to the coating process at five meters per second with a foil width of 1.2 meters.

This can be done only with laser technology. In the EU-funded project ALABO, a consortium involving 3D-Micromac AG and the Fraunhofer Institute for Material and Beam Technology, IWS, has accepted the task. Center stage are pico and femtosecond (ps/fs) lasers for gentle ablation of the electrodes and the wafer-thin organic active layer. The laser removes the mate-rial in pulses by breaking open its atomic and molecular bonds. The thermal influence on the surrounding material is minimal. The sensitive barrier layer remains intact. Parallel to this, the ALABO project has developed a LIBS (laser induced breakdown spectroscopy) process monitoring system. In the future, lasers will also increase the surface area of the cells. For this purpose, IWS scientists are introducing nanometer-fine groove patterns using a “light plow.” Offset laser beams create an interference pattern. Material is removed only where the laser beams superimpose. The structuring should increase energy yield by more than a third.

Productive OLED production with ultrashort pulse lasers

Many OPV manufacturers use lasers. The same applies to display manufacturers who are increasingly opting for OLED technology. This is possible because lasers dramatically increase the productivity of OLED production for manufacturers such as Coherent and Trumpf. Initially, flexible displays are also applied to glass. Lasers then separate them with the lift-off process—once again, the focus is on protecting the sensitive organic semiconductor layers. This applies especially in the following cutting process. To prevent the damaging spread of heat, all layers of the multi-layer sandwich should be able to absorb the laser light. This is the case with UV wavelengths (355 nm). In several test series, Coherent has determined that the highest processing efficiency can be achieved with a 30-watt laser and pulse frequencies up to 1600 kilo-hertz. So that the cut edges remain smooth despite the very high process speeds even with changes of direction, a symmetrical laser beam focused to 10 to 20 micrometers is also needed. If the beam were to be distorted, the precise atom-by-atom material ablation would no longer be guaranteed. But for optimum curve maneuvers, Coherent relies not only on beam shaping. The current HyperRapid NX series also has a control system that closely monitors the pulse energy and can vary the repeat rates on-the-fly. This ensures that manufacturers can use the laser for many different material sandwiches. After all, a lot is happening with regard to OLED technology—Samsung recently demonstrated this with its foldable display.

More information on the current state of research and on application areas of printed electronics is available at LOPEC, international trade fair and conference. LOPEC takes place from March 19 to 21, 2019 at the ICM, Internationales Congress Center München.