Samsung Advanced Institute of Technology

Simultaneously achieving both a high efficiency and long lifetime in deep-blue organic light-emitting diodes (OLEDs) is challenging. Here we report thermally activated delayed fluorescence (TADF) OLEDs that aim to meet this goal by combining a new design of blue TADF materials with a triplet-exciton-recycling protocol. Two TADF materials, one distributing and one emitting, were doped into a host to form triplet-exciton-distributed (TED) TADF devices. The singlet excitons were transferred from the host to the emitter via the distributing TADF material by cascade energy transfer, whereas the triplet excitons were transferred to the emitter as singlet excitons by a triplet exciton recycling process between the low triplet energy host and the distributing TADF material. The resulting TED-TADF devices achieved a high external quantum efficiency (EQE) of 33.5±0.1, a colour coordinate corrected current efficiency over 400 cd/A, a lifetime of >5,000 h, and a y colour coordinate < 0.10.