OLEDs gain more and more interest in the field of lighting applications. The OLED technology provides striking
advantages and covers completely new application fields offering a new freedom in design for next generation lighting.
Large area OLEDs might act as a 2-dimensional light source which is thin, flat and lightweight generating diffuse, nonglaring illumination.
In the first part of our report we investigate small scale inhomogeneities of polymer based OLEDs. Devices were
monitored during operation by taking pictures of the active area at constant periods of time. These pictures were
analyzed by a software tool with respect to the occurrence and evolution of defects. Initially induced inhomogeneities are
growing and dominate the performance with increasing operation time. Within the error margin of the setup no
additional spots are generated during operation.
The voltage drop inside the ITO anode due to a high resistivity plays an important role for the brightness homogeneity of
large area devices. The voltage drop causes a brightness fall-off towards the center of the device. It is maintaining with
increasing average current density and luminance, respectively. At a brightness of 1000cd/m2 the deviation at the center
exceeds 30%. The homogeneity of luminance is improved by incorporation of additional metal lines on the anode layer.
The best results were achieved with 200nm thick aluminum structures with a pitch of 1mm and a width of 60μm of each
line. At an average current density of 45mA/cm2 the decay towards the center of the device is only half of the decrease
without any additional metallization.
The transport of charge carriers in polymer-based Organic Light-Emitting Diodes (OLEDs) as determined by the hopping mobility is an important factor influencing both lifetime and performance of OLED devices. It is strongly dependent on the density and energetic distribution of trap states in the polymer material. Especially in multi-component copolymers single functional groups can act as hole or electron traps determining the optical and electrical characteristics of the device. Transient measurements of the charge carrier mobility together with steady-state current-voltage characteristics are used to investigate the behavior of three blue polyspiro-based light-emitting polymers (LEP) with varying compositions. The first material is a simple homopolymer, the second adds a hole transporting component which is copolymerized into the backbone and the third, most complex, additionally includes a blue chromophore. With some of the added components acting as charge carrier traps the electrical behaviour of the diodes changes significantly.
OLEDs for lighting applications are gaining increasing attention due to the possibility to produce large area, 2-dimensional light sources. In contrast to the existing technology e.g. based on white inorganic LEDs this offers a completely new freedom in design for applications of next generation lighting. Today, different approaches to achieve white broadband emission for organic lighting solutions are investigated ranging from devices with blue emission in combination with conversion layers to RGB-color by lateral patterning with the support of active color tunability. Within this contribution we present results of broadband emitting copolymers to achieve white emission. New requirements arising from the shift of OLEDs in a display configuration to those for lighting applications are discussed with focus on the electro-optical behavior. Furthermore, we describe challenges that result from using large active areas and investigate ways to improve large area lighting tiles.
An attractive approach to full color OLED displays is based on white emitting copolymers and color filters. In this paper the special impact of broadband emitting copolymers based on polyspirobifluorene structures is discussed. The EL spectra of broadband emitters in PLEDs are strongly influenced by interference effects as well as by the driving conditions. Experimental results could be confirmed by modelling. Adjustment of emission spectra to the color filter characteristics lead to improved efficiency.