Typically high efficient OLED device structures are based on a multitude of stacked thin organic layers prepared by
thermal evaporation. For lighting applications these efficient device stacks have to be up-scaled to large areas which is
clearly challenging in terms of high through-put processing at low-cost. One promising approach to meet cost-efficiency,
high through-put and high light output is the combination of solution and evaporation processing. Moreover, the
objective is to substitute as many thermally evaporated layers as possible by solution processing without sacrificing the
device performance. Hence, starting from the anode side, evaporated layers of an efficient white light emitting OLED
stack are stepwise replaced by solution processable polymer and small molecule layers. In doing so different solutionprocessable
hole injection layers (= polymer HILs) are integrated into small molecule devices and evaluated with regard
to their electro-optical performance as well as to their planarizing properties, meaning the ability to cover ITO spikes,
defects and dust particles. Thereby two approaches are followed whereas in case of the "single HIL" approach only one
polymer HIL is coated and in case of the "combined HIL" concept the coated polymer HIL is combined with a thin
evaporated HIL. These HIL architectures are studied in unipolar as well as bipolar devices. As a result the combined HIL
approach facilitates a better control over the hole current, an improved device stability as well as an improved current
and power efficiency compared to a single HIL as well as pure small molecule based OLED stacks. Furthermore,
emitting layers based on guest/host small molecules are fabricated from solution and integrated into a white hybrid stack
(WHS). Up to three evaporated layers were successfully replaced by solution-processing showing comparable white
light emission spectra like an evaporated small molecule reference stack and lifetime values of several 100 h.
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.
This study investigates magnetic layer structures suitable for devices measuring mechanical responses such as stress, strain and pressure. The sensors are based either on giant magnetoresistiance (GMR) structures or on magnetic tunneling junctions (MTJ's) both intentionally prepared with highly magnetostrictive free layer materials. Results for magnetostrictive Fe50Co50 materials or amorphous Co- or Fe-based alloys serving as sensing (or “free”) layers are discussed in view of possible applications. In general, gauge factors in the range of 300-600 have been obtained for strain sensors based on MTJ's, whereas gauge factors of 2-4 are typical for metal based thin film, and 40-180 for piezoresistive strain gauges.