Durable blue emitters boost OLED lifetime
White OLEDs are driving a new generation of vivid, thin, and efficient panels, emitting light that is brighter, more uniform and more energy efficient compared to fluorescent lights. In general, white OLEDs are produced by combining three colours of phosphorescent materials (blue, green and red). However, creating a stable and high-efficiency blue light-emitting material has been difficult to realise until now. To make efficient white OLEDs that are free of rare earth metals, researchers working on the EU-funded project PHEBE focused on a process known as thermally activated delayed fluorescence (TADF). Exploring ground-breaking research in the field, they unveiled innovative blue emitters that are cheaper and better for the environment. Addressing the lifetime challenge squarely Currently, the short lifespan of blue emitters is a major issue that prevents phosphorescent OLED emitters from being used in commercial lighting applications. Phosphorescent OLEDs are made of a host material (typically a polymer) upon which an organometallic complex based on a rare earth metal such as iridium is added as a dopant. “Until now, the lifetime of OLEDs was thought to be independent of the host material. Identifying and designing appropriate combinations of a light-emitting material and a host material has been found the key to extending the lifetime of OLED,” notes project coordinator Giles Brandon. Careful selection of the organic material used for the emitter is another significant factor that should enable OLEDs to gain a major fraction of the lighting markets. “Our research showed that it is extremely important to use ultra-pure organic material as high as 99.9 % for the TADF emitters to optimise their lifetime. This requires careful consideration of the synthesis path for producing the organic material,” adds Brandon. Uncovering the underlying photophysics of TADF In general, what significantly reduces OLED quantum efficiency is that that the radiative decay from the metastable triplet state to the ground singlet state is forbidden. Involving fluorescent instead of phosphorescent materials, TADF moves away from this problem and allows creation of a high-efficiency blue emitter. Molecules exhibiting this mechanism are designed so that the energy difference between the excited singlet state and metastable triplet state is much smaller than in typical organic molecules. This small energy gap enables reverse intersystem crossing (RISC) to occur. PHEBE researchers deemed it critical to identify the factors that influence the RISC rate in TADF emitters to improve the efficiency of OLEDs. The produced results fundamentally enhance understanding of the photophysics behind TADF by focusing on a three-state instead of a two-state model for RISC. The new model shows that spin-orbit coupling between the lowest singlet and triplet states is mediated by a third triplet state. This spin-vibronic coupling mechanism significantly enhances the rate of RISC. In particular, their research on intramolecular charge transfer systems and intermolecular exciplex charge transfer systems that enable TADF has demonstrated promising improvements in energy efficacy. “Our highly efficient blue OLED TADF emitter achieves around 18 % external quantum efficiency – similar to the best blue phosphorescent emitters,” notes project coordinator Giles Brandon. However, the 50 % lifetime for these blue emitters was very short – just two hours. The consortium is still quite a way off from having a new material that can be used commercially for OLED lighting. However, the future looks bright.
Keywords
PHEBE, organic light-emitting diode (OLED), thermally activated delayed fluorescence (TADF), lifetime, blue emitters, reverse intersystem crossing (RISC), quantum efficiency, OLED lighting
