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The Role of Transition Metal Oxides in Charge‐Generation Layers for Stacked Organic Light‐Emitting Diodes
Author(s) -
Hamwi Sami,
Meyer Jens,
Kröger Michael,
Winkler Thomas,
Witte Marco,
Riedl Thomas,
Kahn Antoine,
Kowalsky Wolfgang
Publication year - 2010
Publication title -
advanced functional materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.069
H-Index - 322
eISSN - 1616-3028
pISSN - 1616-301X
DOI - 10.1002/adfm.201000301
Subject(s) - oled , materials science , optoelectronics , doping , diode , x ray photoelectron spectroscopy , layer (electronics) , electron , ultraviolet photoelectron spectroscopy , nanotechnology , physics , nuclear magnetic resonance , quantum mechanics
Abstract 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 Cs 2 CO 3 ‐doped 4,7‐diphenyl‐1,10‐phenanthroline layer and a WO 3 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 WO 3 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.