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Phosphorescent and thermally activated delayed fluorescence (TADF) blue organic light-emitting diodes (OLEDs) have been developed to overcome the low efficiency of fluorescent OLEDs. However, device instability, originating from triplet excitons and polarons, limits blue OLED applications. Here, we develop a phosphor-sensitized TADF emission system with TADF emitters to achieve high efficiency and long operational lifetime. Peripheral carbazole moieties are introduced in conventional multi-resonance–type emitters containing one boron atom. The triplet exciton density of the TADF emitter is reduced by facilitating reverse intersystem crossing, and the Förster resonant energy transfer rate from phosphor sensitizer is enhanced by high absorption coefficient of the emitters. The emitter exhibited an operational lifetime of 72.9 hours with Commission Internationale de L’Eclairage chromaticity coordinate y = 0.165, which was 6.6 times longer than those of devices using conventional TADF emitters.

A novel bipolar host architecture was investigated to improve the external quantum efficiency (EQE) of green phosphorescent organic light-emitting diodes (PhOLEDs). The host was developed by incorporating carbazole as a hole-transport unit and fused rigid benzo[4′,5′]imidazo[2′,1′:2,3]imidazo[4, 5, 1-jk]carbazole (BzICz) as a new electron transport unit. The primary goal of the BzICz-based host design was to achieve a high triplet energy and bipolar charge transport characteristics. The green PhOLEDs fabricated using the new BzICz and carbazole-based host demonstrated a high EQE of 26.6% due to their high triplet energy and good bipolar charge transporting characteristics.

The origin of spectrum broadening in blue phosphorescent organic light-emitting diodes (OLEDs) was investigated in order to improve color purity of the OLED device aiming to realize efficient blue phosphorescent OLED with deep blue emission. Assuming the formation of exciplex between host and guest (H-G) material from intermolecular charge transfer (CT) state of the two, the experiment was performed controlling the distance between H-G for the verification. The exciplex formation was observed dependent on the distance of H-G, indicating that the spectrum broadening occurred due to H-G CT state. The critical distance to avoid H-G CT state was found which can be further utilized in designing host and dopant materials for blue phosphorescent OLEDs. The EL efficiency and the stability of the blue phosphorescent device were enhanced by 1.2 and 5 times, respectively, by minimizing H-G CT state.

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