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

Abstract

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.