41‐1: Invited Paper: Highly Efficient and Stable Deep‐blue Organic Light‐emitting Diode Using Phosphor‐sensitized Thermally Activated Delayed Fluorescence

Phosphorescent and thermally activated delayed fluorescent (TADF) blue organic light‐emitting diodes (OLEDs) have been developed to overcome the relatively low efficiency of fluorescent OLEDs. However, device instability originating from triplet excitons and polarons limits the application of such blue OLEDs. Herein, we propose host and dopant molecules contributing to a longer device lifetime and higher color purity. Also, we discuss the devices with phosphorescence emitter (PhOLED) and phosphor‐sensitized TADF emitter (PST). Our PhOLED shows CIE‐y 0.197 and a long device lifetime of 150hrs at T95 of initial luminance of 1,000 nits. The PST shows CIE‐y 0.165 and a lifetime of 72.9hrs at the bottom‐emitting device. We will also discuss the device performance of top‐emitting structure with strong microcavity. These devices are achieved by suppressing triplet‐exciton‐ and polaron‐initiated degradation, while maximizing exciton utilization. The triplet exciton density of the TADF emitter was controlled by facilitating reverse intersystem crossing and Förster resonant energy transfer from a Pt phosphor working as a singlet exciton sensitizer. The polaron density was controlled by aligning the highest occupied molecular orbital level of the emitter and phosphor sensitizer.