Ralph Pätzold

Polymer light-emitting devices for large-area lighting

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.

Trapping and charge transport in organic LEDs

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.