Combinatorial methods for screening and optimization of materials and device parameters in organic light-emitting diodes

An experimental set-up was used to optimize the layer thicknesses of hole transport materials and electron transport/emitter material in multi-layer light emitting diodes by combinatorial methods. The method is based on a movable mask/shutter technique and simultaneous evaporation of organic molecules resulting in linear gradients of layer thickness. This allows the preparation of different devices in one single experiment under identical conditions. In the first experiment, we studied the influence of the Alq<SUB>3</SUB> layer thickness on photometric and power efficiency in two layer devices using various TPD derivatives as hole transport material at a constant thickness. Some new low molecular weight TPDs and a polymeric TPD were utilized. Both photometric and power efficiencies depend considerably on the thickness of the Alq<SUB>3</SUB> layer. In a second experiment, the efficiency dependence on both the TPD and Alq<SUB>3</SUB> layer thickness was investigated simultaneously by preparing a landscape library with two orthogonal linear gradients of TPD and Alq<SUB>3</SUB>. It was observed that the device efficiencies depend on both TPD and Alq<SUB>3</SUB> layer thicknesses and additionally on the total thickness of the organic layer.