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 Alq<SUB>3</SUB> layer thickness for two-layer devices ITO/TPD/Alq<SUB>3</SUB>/Al is confirmed and such an optimized two-layer structure could not be improved by adding an additional hole blocking layer to the optimum Alq<SUB>3</SUB> layer. But an improvement of photometric efficiency can be obtained by replacing the optimum Alq<SUB>3</SUB> layer thickness by certain combinations of Alq<SUB>3</SUB>/spiro-Quinoxaline layers.