Integer charge transfer states in organic light-emitting diodes: Optical detection of hole carriers at the anode∣organic interface

Hole carriers that reside at the anode∣hole-transport-layer (HTL) interface, creating a dipole layer, have been observed in organic light-emitting diode (OLED) and model devices using charge modulation spectroscopic techniques. These interfacial carriers have been observed with various HTL materials and several anode compositions at voltages such that the HTL is reverse-biased. The cation density at an ITO∣CFx∣NPB interface is estimated to be approximately 1×1013/cm2, or approximately 10% of a molecular layer. [NPB is 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl. CFx is a fluorocarbon polymer.] The cations represent the integer charge-transfer states whose presence has been inferred from ultraviolet photoelectron spectroscopy studies of various conductor∣organic interfaces. Anions of an organic hole-injecting material, 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) were also observed. Conditions near the injecting contacts often determine the electric field profile and drive voltage of an OLED. Elucidation of these conditions is important to a complete understanding and detailed modeling of OLED operation.