Recent Advances in White Organic Light‐Emitting Materials and Devices (WOLEDs)

WOLEDs offer new design opportunities in practical solid‐state lighting and could play a significant role in reducing global energy consumption. Obtaining white light from organic LEDs is a considerable challenge. Alongside the development of new materials with improved color stability and balanced charge transport properties, major issues involve the fabrication of large‐area devices and the development of low‐cost manufacturing technology. This Review will describe the types of materials (small molecules and polymers) that have been used to fabricate WOLEDs. A range of device architectures are presented and appraised.     

Small Molecule Host Materials for Solution Processed Phosphorescent Organic Light‐Emitting Diodes

Solution processed phosphorescent organic light‐emitting diodes (OLEDs) have been actively developed due to merits of high quantum efficiency of phosphorescent materials and simple fabrication processes of solution processed OLEDs. The device performances of the solution processed phosphorescent OLEDs have been greatly improved in the last 10 years and the progress of the device performances was made by the development of small molecule host materials for solution processes. A hybrid host of polymer and small molecules, a single small molecule host and a mixed host of small molecule hosts have effectively enhanced the quantum efficiency of the solution processed phosphorescent OLEDs. Therefore, this paper reviews recent developments in small molecule host materials for solution processed phosphorescent OLEDs and provides future directions for the development of the small molecule host materials.     

OLEDs: Degradation Mechanisms in Small‐Molecule and Polymer Organic Light‐Emitting Diodes (Adv. Mater. 34/2010)

Degradation in organic light‐emitting diodes (OLEDs) is a complex problem. Depending upon the materials and the device architectures used, the degradation mechanism can be very different. In this Progress Report, using examples in both small molecule and polymer OLEDs, the different degradation mechanisms in two types of devices are examined. Some of the extrinsic and intrinsic degradation mechanisms in OLEDs are reviewed, and recent work on degradation studies of both small‐molecule and polymer OLEDs is presented. For small‐molecule OLEDs, the operational degradation of exemplary fluorescent devices is dominated by chemical transformations in the vicinity of the recombination zone. The accumulation of degradation products results in coupled phenomena of luminance‐efficiency loss and operating‐voltage rise. For polymer OLEDs, it is shown how the charge‐transport and injection properties affect the device lifetime. Further, it is shown how the charge balance is controlled by interlayers at the anode contact, and their effects on the device lifetime are discussed.     

Recent Advances in Alternating Current‐Driven Organic Light‐Emitting Devices

Organic light‐emitting devices (OLEDs), typically operated with constant‐voltage or direct‐current (DC) power sources, are candidates for next‐generation solid‐state lighting and displays, as they are light, thin, inexpensive, and flexible. However, researchers have focused mainly on the device itself (e.g., development of novel materials, design of the device structure, and optical outcoupling engineering), and little attention has been paid to the driving mode. Recently, an alternative concept to DC‐driven OLEDs by directly driving devices using time‐dependent voltages or alternating current (AC) has been explored. Here, the effects of different device structures of AC‐driven OLEDs, for example, double‐insulation, single‐insulation, double‐injection, and tandem structure, on the device performance are systematically investigated. The formation of excitons and the dielectric layer, which are important to achieve high‐performance AC‐driven OLEDs, are carefully considered. The importance of gaining further understanding of the fundamental properties of AC‐driven OLEDs is then discussed, especially as they relate to device physics.