Thermally activated delayed fluorescence (TADF) materials can achieve 100% internal quantum efficiency (IQE) by harvesting both singlet and triplet excitons. In 2012, Adachi and co-workers first reported organic light-emitting diodes (OLEDs) that showed IQEs of 100% based on organic donor-acceptor (D-A) type TADF emitters. Successively, a wide variety of D-A type TADF emitters have been developed, resulting in devices showing high external quantum efficiencies (EQEs) of over 20% for blue, green, and red emission colors. Nonetheless, D-A type TADF emitters usually possess broad emission bands that result in devices with poor color purity pixels. To achieve BT.2020 standard for high-definition displays, narrowband electroluminescence in devices that show high EQE is highly desired.