Efficient screening of electron transport material in multi-layer 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 configurations and layer thicknesses. 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 ITO/TPD/Alq3/Al two-layer devices was confirmed and such an optimized two-layer structure could not be improved by adding an additional hole blocking layer to the optimum Alq3 layer. However, an improvement in photometric efficiency can be achieved by replacing the optimum Alq3 layer thickness by certain combinations of Alq3/spiro-quinoxaline layers.