Organic light emitting devices (OLEDs) have been intensively studied for more than 20 years, for they offer a broad range of emitting colors, high efficiency, wide viewing angle, and ultra-thin thickness that enable new-generation devices such as transparent and rollable displays and lighting panels. Indeed, efficient electrophosphorescent white OLED (WOLED) has led to commercial flat panel displays whose internal quantum efficiency (IQE) is approaching to 100%, while exhibits superior color balance along with desirable features such as continuous brightness control, high contrast, and no turn-on delay. In the past decade, with the steadily improved external efficiency of WOLED, the application of WOLED to reduce the energy consumption of interior lighting, which now costs more than $230 billion annually, has attracted considerable interest as well, since WOLED has surpassed the efficiency of commonplace fluorescent lamps.
This Thesis mainly focuses on the improvement of one of the most important characteristics of WOLED, external quantum efficiency (EQE), which is a product of IQE and the light outcoupling efficiency (ηout).
Issues such as proper combination of host and dopant materials, balanced charge recombination in the emission region, efficient utilization of excitons, etc., can lead to improved IQE of WOLED. These are achievable through well designed device architectures. In this Thesis, methods such as utilization of fluorescence filtered phosphorescence, a stepped progression of energy barriers to charges to form multiple exciton generation regions, and vertically stacked iv WOLED connected by non-metal charge generation layers, are presented.
On the other hand, ηout for OLEDs fabricated on conventional flat glass substrates is only ∼20%, mainly due to the optical characteristics of the active layers and the substrate of OLED. This leaves considerable opportunity for improvement of EQE. This Thesis presents potentially low-cost methods to outcouple the light originally confined in the substrate, as well as the light originally in the waveguided modes in the organic and anode layers, without changing the emitting spectrum. These methods result in an enhancement factor of (2.3±0.2) in the external quantum and power efficiencies.
Through these approaches, a better understanding of WOLED operation, and several record efficiencies of WOLEDs have been demonstrated.