Interface conditioning to improve efficiency and lifetime of organic electroluminescence devices

Abstract

An organic light emitting diode (“OLED”) display or device is typically comprised of: a transparent anode on a substrate; a hole injection/transporting layer; a light emitting layer (“emissive layer”); and a cathode, where one or more of these layers are organic in nature. When a forward bias is applied, holes are injected from the anode into the hole injection/transporting, and the electrons are injected from the cathode into the emissive layer. Both carriers are then transported towards the opposite electrode and allowed to recombine with each other. The location of this recombination is called the recombination zone and due to the recombination, the emissive layer produces visible light.

There is some suggestion in a published patent application that incorporation of metal nano-particles within a polymer-based light emitting layer suppresses photo-oxidation and enhances luminous stability [Publication number US 2004/0217696 A1]. Yet another patent application suggests that acceleration of the radiative processes is achieved by incorporation of metal nano-particles within the hole transporting layer or within the light emitting layer of phosphorescence based OLEDs [Publication number US 2005/0035346]. The acceleration of the radiative processes is achieved by the interaction of the light emitting species with surface plasmon resonances in the vicinity of metal nano-particles. Non-radiative Förster-type processes are efficiently suppressed by encasing each nano-particle in organic capping molecules. In all of the above approaches, metal nano-particles were blended in one or more layers of the OLEDs.

However direct incorporation of metal nano-particles into the active region or other layers within the OLED can cause additional negative effects. For example, it has been demonstrated that incorporation of a gold nano-particle even at low volume fraction of 3×10−5 within a light emitting polymer layer introduces strong hole blocking effects and a large increase in operating voltage [Publication number US 2004/0217696 A1 and Jong Hyeok Park et al., Chem. Mater. 2004, 16, 688]. Furthermore, incorporation of metal nano-particles in both fluorescence-based and phosphorescence-based OLEDs will likely quench emission and strongly deteriorate device performance. Encasing nano-particles in organic capping molecules is suggested to achieve an optimum balance between quenching and acceleration of the radiative processes of the triplets states [Publication number US 2005/0035346]. However achieving such a condition is not straightforward and capping of the metal nano-particles is not a well known procedure.