The role of transition metal oxides in chargegeneration layers for stacked organic light-emitting diodes

The mechanism of charge generation in transition metal oxide (TMO)-based charge-generation layers (CGL) used in stacked organic light-emitting diodes (OLEDs) is reported upon. An interconnecting unit between two vertically stacked OLEDs, consisting of an abrupt heterointerface between a Cs₂CO ₃doped 4,7-diphenyl-1,10-phenanthroline layer and a WO₃ film is investigated. Minimum thicknesses are determined for these layers to allow for simultaneous operation of both sub-OLEDs in the stacked device. Luminance - current density - voltage measurements, angular dependent spectral emission characteristics, and optical device simulations lead to minimum thicknesses of the n-type doped layer and the TMO layer of 5 and 2.5 nm, respectively. Using data on interface energetic determined by ultraviolet photoelectron and inverse photoemission spectroscopy, it is shown that the actual charge generation occurs between the WO₃ layer and its neighboring hole-transport material, 4, 4', 4'-tris(N-carbazolyl)-triphenyl amine. The role of the adjacent n-type doped electron transport layer is only to facilitate electron injection from the TMO into the adjacent sub-OLED.