The Role of Transition Metal Oxides in Charge-Generation 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 Cs2CO3‐doped 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. Charge generation in transition metal oxide (TMO)‐based interconnecting units of stacked organic light‐emitting diodes occurs between the WO3 layer and its neighboring hole‐transport material TCTA. The role of the adjacent n‐type doped electron‐transport layer BPhen:Cs2CO3 is only to facilitate electron injection from the TMO into the adjacent organic light‐emitting unit.