Mukundan Thelakkat

Efficient screening of electron transport material in multilayer organic light-emitting diodes by combinatorial methods

A combinatorial approach combining vapor deposition of organic molecules and a mask technique was used to prepare on one substrate a matrix of 49 organic light emitting diodes (OLEDs) with different configuration and layer thickness. A landscape library with two orthogonal, linear gradients of an emitter and a hole blocking electron transport material on top of a hole transport layer of constant thickness was prepared. The aim of this experiment was to investigate the influence of an additional electron transport material on the efficiency. Using a semi-automated measurement set-up, the device parameters for each of the 49 OLEDs were evaluated. The existence of an optimum Alq3 layer thickness for two-layer devices ITO/TPD/Alq3/Al is confirmed and such an optimized two-layer structure could not be improved by adding an additional hole blocking layer to the optimum Alq3 layer. But an improvement of photometric efficiency can be obtained by replacing the optimum Alq3 layer thickness by certain combinations of Alq3/spiro-Quinoxaline layers.

Influence of the sensitizer on the dynamic behavior of an organic photorefractive material

Predictions made by two models describing dynamic processes in organic photorefractive materials are verified for a photorefractive polymer, consisting of a Poly-TPD, a dicyano dye and the sensitizer C60. One of the models can describe our measurements under the assumption of optically active traps as dominant trap species. This is confirmed by altering the sensitizer concentration, which has an influence on the charge transport properties of the material. Also, increasing the sensitizer concentration yielded a significant increase of the grating erasure rate in our material.

Variation of the glass transition temperature in organic photorefractive materials: plasticizer versus novel synthetic approaches

Most photorefractive (PR) materials require plasticizers in order to decrease the glass transition temperature, allowing for orientational enhancement by the chromophores. Introduction of the plasticizer, however, alters not only the viscosity but also the photoconductive properties of the material. This can be shown by comparing two different plasticizers which were introduced into a bifunctional low-molecular-weight PR glass and into a polyfluorene guest-host polymer. The latter reaches response times down to 600 microsecond(s) at a writing intensity of 1 W/cm2. We have recently improved the concept of low-molecular-weight PR glasses. A suitable, photoconducting unit allows the synthesis of a bifunctional system with a glass transition of 22.6 degree(s)C. Therefore, no plasticizer is needed. The material is based on a triphenyldiamine (TPD) moiety to which a nonlinear-optical chromophore is directly attached. The system is the first representative of a whole class of TPD molecules and polymers for photorefractive applications.