The cost of organic light-emitting diodes (OLEDs) is commensurate with the complexity of their architecture, which traditionally has relied on the use of many materials within an increasing number of layers. This complexity is driven by the optimization of the device performance in terms of efficacy and lifetime, and generally requires the co-deposition of multiple materials within the OLED’s light-emitting layer, to balance carrier transport and reduce light quenching.

With the help of DOE funding, researchers at the Georgia Institute of Technology (Georgia Tech) have developed a new generation of organic emitters that challenge conventional wisdom by yielding high performance when used as neat single-component layers in OLED devices with simplified geometry. The team's approach is based on the search for new molecules within the known class of materials that exhibit thermally activated delayed fluorescence (TADF), which provides an alternative to phosphorescent emission.

In contrast to conventional phosphorescent emitters, TADF molecules do not require the inclusion of heavy elements to harvest energy from both singlet- and triplet-excited states for electroluminescence. Instead, the molecular design utilizes the common elements used in carbon-based materials (carbon, hydrogen, nitrogen, and oxygen) and is based on controlling the subtle electronic coupling of electron-donor and electron-acceptor groups.

In a first set of experiments, the Georgia Tech team fabricated blue-emitting OLEDs with TADF molecules having a photoluminescence quantum yield of 82% in neat films, yielding devices with an external quantum efficiency of 13.7% at a luminance of 1,000 cd/m². More recently, the team achieved an external quantum efficiency of 18% at 1,000 cd/m² with a neat film of yellow-green-emitting TADF molecules – considerably higher than the maximum efficiency of OLED devices based on fluorescent emitters.

In view of these promising results, one could envision OLEDs with 100% internal efficiency being fabricated from single-component layers instead of blends of different molecules, significantly simplifying their manufacture and decreasing their cost, and also potentially increasing their lifetime. (September 2019)