20 Sep 2007
Byung Doo Chin of the Korea Institute of Science and Technology has developed a novel and simplified preparation method for fabricating top-emitting organic light emitting devices (OLEDs).
The devices were made by adding spin-coated polymeric hole injection/transport layers of more than 100 nm thickness to top-emitting OLEDs. The bilayers effectively smooth the surface of the substrate, and hence reduce the probability of electrical shorts and current leakage in the operating devices (J. Phys. D: Appl. Phys. 40 5541).
The fabrication process uses a simple color patterning method known as laser thermal patterning to simultaneously transfer hole transport/light emitting bilayers onto the receptor substrates. This allows the fabrication of RGB top-emitting OLEDs with hole transport layers of various thicknesses, which enables the cavity effect to be optimized to significantly enhance the color purity.
Using optical simulations and experiments, Chin found that the optimal thicknesses of the hole transport layers were 140 nm for blue, 160 nm for green and and 230 nm for red-emitting devices.
The resulting top-emitting devices exhibit satisfactory color purity and efficiency, but require relatively high operating voltages. Future advances in the intrinsic mobility of the transport materials will allow the fabrication of more efficient top-emitting OLEDs requiring lower driving voltages based on thicker transport layers.