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Forschungsarbeit

Light extraction in organic light-emitting diodes

By Jörg Frischeisen (24.04.2012)

Organic light-emitting diodes (OLEDs) are flat large-area light sources having a pleasant diffuse light emission. A typical OLED structure has a total thickness of only a few hundred nanometers and consists of several organic layers sandwiched between two electrodes (Fig. 1).

Figure 1: Simplified illustration of a two-layer OLED stack (left). Four different OLEDs demonstrating the facile manufacturing of arbitrary shapes and colors (right).[Bildunterschrift / Subline]: Figure 1: Simplified illustration of a two-layer OLED stack (left). Four different OLEDs demonstrating the facile manufacturing of arbitrary shapes and colors (right).

Besides the implementation of OLEDs in displays, these light sources have great potential for applications in general lighting. OLEDs are already far superior to incandescent lamps regarding their lifetime and efficacy. However, OLEDs still suffer from a poor light extraction, i.e. only a small fraction of light can finally leave the device. In a typical OLED with optimized cavity structure, only about 20% of the light is directly emitted into air, and roughly the same amount is trapped inside the glass substrate owing to total internal reflection at the interface between glass and air. The remaining power is lost to waveguide modes, which propagate inside the organic layers, or lost to surface plasmons, i.e. guided electromagnetic surface waves traveling along the interface between the organic material and the metallic cathode. This PhD thesis summarizes existing methods and presents novel ways of enhancing OLED efficiency by either recovering some of the optical losses or by reducing coupling to unfavorable optical channels.

One of the three major approaches for enhanced light outcoupling studied in this work is the implementation of grating structures which allows for outcoupling of bound modes by Bragg scattering (Fig. 2). This effect is investigated by using periodic one-dimensional grating structures fabricated by nanoimprint lithography. In order to demonstrate that grating coupling can also be cost-efficient, the extraction of surface plasmons by means of a common DVD structure is presented.

Figure 2: AFM image of a structure obtained by imprint of a line grating with 555nm period (left) and of a sample after imprinting a DVD stamp into a PMMA layer (right). The DVD track runs diagonally from the top right corner to the bottom left corner.[Bildunterschrift / Subline]: Figure 2: AFM image of a structure obtained by imprint of a line grating with 555nm period (left) and of a sample after imprinting a DVD stamp into a PMMA layer (right). The DVD track runs diagonally from the top right corner to the bottom left corner.

Another interesting method is based on a reversed Kretschmann configuration using a thin metallic cathode and a layer with a high refractive index in order to extract waveguide modes and surface plasmons (Fig. 3).

Figure 3: Top view photograph of a sample comprising a thin silver film adjacent to a 20 nm thick organic layer. The organic emitter is excited by a UV laser and the emission into a high-index prism is measured.[Bildunterschrift / Subline]: Figure 3: Top view photograph of a sample comprising a thin silver film adjacent to a 20 nm thick organic layer. The organic emitter is excited by a UV laser and the emission into a high-index prism is measured.

Finally, a completely different approach is to reduce the initial coupling of excited molecules to unfavorable optical channels. A very efficient method is based on the orientation of the transition dipole moment of the molecule (Fig. 4). A novel straightforward measurement technique is presented which allows for a determination of orientation even for doped films, which are usually used in modern small molecule OLEDs. To demonstrate the efficiency boost of horizontally oriented emitters in OLEDs, two devices are analyzed having a similar layout but different degrees of emitter orientation.

Figure 4: Comparison of the simulated power dissipation in an OLED structure assuming a completely isotropic emitter orientation (left) and a perfectly horizontal orientation (right).[Bildunterschrift / Subline]: Figure 4: Comparison of the simulated power dissipation in an OLED structure assuming a completely isotropic emitter orientation (left) and a perfectly horizontal orientation (right). The emission to air can be enhanced by around 60% by using horizontally oriented molecules.

By using one of the presented approaches or by combining several of them it should be possible to achieve very high efficiencies, which is a basic requirement for a widespread application of OLEDs in general lighting.


Stations
  • 2006 - 2011
  • PhD Student in the IDK "Materials Science of Complex Interfaces“, University of Augsburg
  • 2008 and 2009
  • Visiting researcher at the Kyushu University, Fukuoka, Japan
  • 2000 - 2006
  • Physics at the University of Regensburg
  • 2003 - 2004
  • Physics at University of Colorado, Boulder, USA

Scientific Prizes and Awards
  • 2003 - 2004
  • Jahresstipendium des DAAD
  • 2010
  • “Photonic Devices + Applications Best Student Paper Runner-Up Award” (SPIE Optics + Photonics 2010 Conference)
  • 2011
  • Kulturpreis Bayern der E.ON Bayern AG und
  • 2012
  • 1. Preis des Forschungsclusters Green Photonics der Fraunhofer-Gesellschaft für die Doktorarbeit „Light extraction in organic light-emitting diodes“

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