Despite more than 20 years of research, the root cause of the impractically short lifetimes of blue phosphorescent organic light-emitting diodes (PHOLEDs) has remained unclear. To overcome this, the authors investigate how the electrical properties of the emission layer (EML) of blue PHOLEDs affect degradation of the devices. It is found that a large density of dopant carriers is the dominant factor triggering triplet-polaron annihilation (TPA), which is a major defect-generation and hence lifetime-reduction mechanism. In order to reduce the generation of the TPA-induced defects to ensure long device lifetimes, the dopant carrier density should be minimized by suppressing the spontaneous charge transfer from the host to the dopant initially and by supplying sufficient charges with opposite polarity into the EML. However, there exists another critical factor that offsets the low overall density of defects against device lifetimes-that is, the non-uniform distribution of defects leading to intense exciton quenching. These two degradation factors are predetermined, and hence can be controlled, by the charge mobilities of the PHOLED EML. Given these considerations, it is demonstrated that the long-lifetime blue PHOLEDs can be realized.

To utilize thermally activated delayed fluorescence (TADF) technology for future displays, it is necessary to develop host materials which harness the full potential of blue TADF emitters. However, no publication has reported such hosts yet. Although the most popular host for blue TADF, bis[2‐(diphenylphosphino)phenyl]ether oxide (DPEPO) guarantees high‐maximum external quantum efficiency (EQEmax) TADF devices, they exhibit very short operational lifetimes. In contrast, long‐lifespan blue TADF devices employing stable hosts such as 3′,5‐di(9H‐carbazol‐9‐yl)‐[1,1′‐biphenyl]‐3‐carbonitrile (mCBP‐CN) exhibit much lower EQEmax than the DPEPO‐employed devices. Here, an elaborative approach for designing host molecules is suggested to achieve simultaneously stable and efficient blue TADF devices. The approach is based on engineering the molecular geometry, ground‐ and excited‐state dipole moments of host molecules. The engineered hosts significantly enhance delayed fluorescence quantum yields of TADF emitters, as stabilizing the charge‐transfer excited states of the TADF emitters and suppressing exciton quenching, and improve the charge balance. Moreover, they exhibit both photochemical and electrochemical stabilities. The best device employing one of the engineered hosts exhibits 79% increase in EQEmax compared to the mCBP‐CN‐employed device, together with 140% and 92‐fold increases in operational lifetime compared to the respective mCBP‐CN‐ and the DPEPO‐based devices.

A molecular design platform of blue thermally activated delayed fluorescence (TADF) emitters to boost the external quantum efficiency and efficiency roll-off of blue TADF organic light-emitting diodes (OLEDs) was developed.

We present a high-performance blue phosphorescent organic light-emitting diode exhibiting a low operating voltage (4.1 V), high external quantum efficiency (23.4%, at 500 cd m−2) with a low efficiency roll-off (4.7%), and a long operation lifetime (time at which the luminance reaches 95% of its initial value, LT95 = 232 h).

Intramolecular charge transfer (ICT) plays a critical role in determining the photophysical properties of organic molecules, including their luminescence efficiencies. Twisted intramolecular charge transfer (TICT) is a process in which...