Dual luminescent organic LEDs with tunable emission characteristics

One of the greatest challenges of developing a multi-colored organic light-emitting diode is matching the electronic properties of individual molecules to preserve their distinctive emission behavior. Hammer et al. accomplished this by using only one molecular species as the organic component, zinc-phthalocyanine (ZnPc).

Hammer et al. demonstrated an organic thin film device with a stable dual luminescent emission behavior that can be spectrally tuned. They chose to use ZnPc, because it is easy to process via evaporation, non-toxic, highly stable and can be chemically tuned for a specific purpose.

“ZnPc constitutes a model compound to explore the possibility and limitations in influencing the emission properties by the underlying, thermally adjustable morphology,” said author Sebastian Hammer. “We believe this is an alternative approach to using combinations of different molecules with complementary spectral emission features.”

The authors tuned the spectral emission of ZnPc thin films by a thermally induced structural transformation between two structural phases (termed α- and β-phase) of the material. Because the emitting states have high quantum efficiency, only a small number of β-phase crystallites is needed to balance emission, which minimizes the thermal stress on samples.

By using a multi luminescent thin film, the plasmon coupling could be used to enhance the emission of a specific wavelength along a certain spatial direction, resulting in an orientation dependent multicolor emission.

“Thinking further ahead, we would like to transfer the presented concept of structural induced spectral tuning to other organic semiconductors and their integration in more complex photonic circuitries,” said Hammer.

The next steps in implementation of this dual luminescent device are to improve its overall quantum efficiency by fine tuning the electrical characteristics of the utilized substrate and layers.

Source: “Phase transition induced spectral tuning of dual luminescent crystalline zinc-phthalocyanine thin films and OLEDs,” by S. Hammer, T. Ferschke, G. v. Eyb, and J. Pflaum, Applied Physics Letters (2019). The article can be accessed at https://doi.org/10.1063/1.5132698 .