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New Methods for the Investigation of Organic Thin-Film Devices
(2004)
- We have developed new techniques for the investigation of organic thin-film devices and have focussed on properties on the molecular scale as well as on macroscopic properties of organic devices. Scanning probe techniques were used to obtain spatially resolved information on morphology and electro-optical properties. Structural changes in composite-based devices were found to have an important influence on device performance. Furthermore, two modes of electroluminescence detection have been developed. Local luminescence detection in the optical near-field by a scanning near-field optical microscope allowed us to monitor the light emission around a dark spot with a resolution better than 134 nm and to observe the electrode ablation. Finally, we have established a new scanning probe technique, named SELM, "Scanning Electroluminescence Microscopy". The simultaneous detection of a PtIr-tip-induced electroluminescence and shear force allows us to distinguish between topography and conductivity. This technique has revealed a strong spatial variation in the electro-optical properties of Alq3 films on ITO substrates. The existing combinatorial preparation method has been supplemented by a variable testing setup that permits the simultaneous investigation of 64 devices under nearly identical conditions. Both OLEDs and photovoltaic cells have successfully been tested over more than 300 hours of continuous operation so that it was possible to study the influence of material combinations and layer thicknesses on the performance and on the degradation of the devices. Variable-angle spectroscopic ellipsometry has been used for the optical characterisation of materials and an automation has been provided for the analysis of combinatorially prepared device arrays. Furthermore, a Mathematica program has been developed for the theoretical description of the short-circuit current in photovoltaic cells. By this means it was possible to explain in detail the observed performance enhancement in heterojunction solar cells, induced by an additional TiO2 layer. The optical and electronic contribution could only be identified by the variation of both layer thickness and device type. The strength of the setup presented is its ability to produce and to test devices under nearly identical conditions and to yield reliable data, which in turn can be used to test physical models. Finally, we have addressed the degradation process of OLEDs. The experiments have shown that inert gas plays an essential role in protecting against degradation, not only by the exclusion of reactive species but also by its heat-transport capabilities. These investigations are only just beginning and further combinatorial studies paired with AFM measurements are currently being developed.
