Jörg Frischeisen
Light extraction in organic light-emitting diodes
Supervisor: Prof. Dr. Wolfgang Brütting [Experimental physics IV]
Date of oral examination: 07/26/2011
291 pages, english
Organic light-emitting diodes (OLEDs) are flat large-area light sources with a 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. Besides the implementation of OLEDs in displays, these light sources have a great potential for applications in general lighting. However, the overall efficiency of OLEDs is still far below the theoretical maximum. This is mainly due to a rather low light extraction with losses resulting from total internal reflection in the OLED multilayer stack, i.e. the excitation of substrate and waveguide modes. In addition, a large fraction of the power is dissipated to surface plasmons (SPs), i.e. guided electromagnetic surface waves traveling along the interface between the organic material and the metallic cathode. This work 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. All methods are studied both from an experimental point of view as well as by optical simulations. 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. 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 SPs by means of a common DVD structure is presented. Another interesting method is the high-index coupling, which uses a layer with a high refractive index in order to extract waveguide modes and SPs. It is demonstrated that not only substrate modes, but in particular waveguide modes and even SPs can be partially recovered from a state-of-the-art OLED by high-index coupling. Finally, a completely different approach is to reduce the initial coupling of the excited molecules to unfavorable optical channels. A very efficient method is based on the orientation of the transition dipole moment of the molecule. A novel straightforward method 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. Moreover, it is shown that even phosphorescent emitters which usually have a rather bulky structure can show a considerable non-isotropic orientation.