Highly Efficient Organic Light-Emitting Diode Based on a Hidden Thermally Activated Delayed Fluorescence Channel in a Heptazine Derivative

Considerable progress in organic light-emitting diodes (OLEDs) has triggered intensive effort to develop efficient solid-state electroluminescent (EL) materials over the past two decades. Among the many classes of materials being investigated, transition metal complexes are highly attractive because phosphorescent OLEDs containing Ir (III), Pt (II) and Os (II) complexes exhibit very high external quantum efficiencies (ηext). This is because such complexes effectively harvest triplet excitons, so their efficiencies are four times higher than that of conventional fluorescent OLEDs. However, phosphorescent OLEDs containing transition metal-based compounds are rather expensive and unsustainable because they contain rare metals. While OLEDs containing Cu (I) complexes that exhibit high ηext comparable to those with transition metal complexes have been examined as an alternative, the relatively low reliability and high driving voltage of such OLED are fundamental problems. Therefore, a novel way to realize high EL efficiency is required. Although fluorescent OLEDs have been assumed to show limited efficiency because of the branching ratio of singlet and triplet excitons of 1:3, the most recent fluorescence-based OLEDs have overcome this limitation using triplet-triplet annihilation and thermally activated delayed fluorescence (TADF) [1-2]. In particular, we have developed promising blue and green TADF materials [3-4]. However, the design of efficient orange or red emitters is inherently difficult because the photoluminescence (PL) quantum efficiency tends to decrease as the emission wavelength increases according to the energy gap law.