The device exhibits an EL peak maximum at 617 nm with x/y CIE chromaticity coordinates of 0.64/ 0.36 and a FWHM of 44 nm. In addition, the device shows a longer device lifetime of 954 h at 3000 cd m -2 . Wang group reported a red HF-OLED using TADF emitter T15 and conventional red fluorescence emitter F6 (Table ). The HF-OLED fabricated using this combination resulted in a maximum EQE of 16.4% and x/y CIE chromaticity coordinates of 0.65/0.35. Recently, HF-OLEDs with MR-TADF materials as terminal emitters have been developed rapidly owing to their strong and narrow emission. MR-TADF materials (Fig. ) are fused polycyclic aromatic compounds with suitably placed electron-deficient and electron-rich centers, enabling their HOMO and LUMO separation on different atoms due to the opposite resonance/mesomeric effects. This reduces ΔEST and triggers TADF emission. The rigid frameworks and short-range charge transfer features of MR-TADF materials make them ideal candidates for OLEDs realizing narrow emission with high color purity and high efficiency. The following are some representative examples of this class of HF -OLEDs. Adachi group used TADF emitter T16 to sensitize the narrow emission of MR-TADF emitter MR-T1 (Figs. and). The presence of a bulky m-terphenyl (instead of carbazole) unit at the para-position of T16 triggers a greenish-blue emission at ~ 475 nm in solution, which is close to the absorption maximum of MRT1, enabling efficient energy transfer between them. In thin film with 0.5 wt.% of MR-T1 and 20 wt.% of T16 the emission maximum observed at 485 nm with a PL of 86%. High kRISC i.e., 9.2×10 5 s -1 in thin-film, attested an excellent TADF behavior. Moreover, a high FRET efficiency of about 64% from T16 to MR-T1, indicates an efficient excited energy transfer from T16 to MRT1. This combination resulted in an efficient narrow-emitting (FWHM = 19nm) HF-OLED with a maximum EQE of 41% and good stability (Table ). Zhang group reported a blue HF-OLED using the T17: MR-T1 couple. The donor-void-acceptor structural feature of T17 justifies its high PL of 92% and displays an emission maximum at 440 nm in toluene.