Almost 100% internal quantum efficiency (IQE) is achieved with a green fluorescent organic light-emitting diode (OLED) exhibiting 30% external quantum efficiency (EQE). The OLED comprises an exciplex-forming co-host system doped with a fluorescent dye that has a strong delayed fluorescence as a result of reverse intersystem crossing (RISC); the exciplex-forming co-hosts stimulate energy transfer and charge balance in the system. The orientation of the transition dipole moment of the fluorescent dye is shown to have an influence on the EQE of the device.

For electroluminescence with delayed fluorescence, the azasiline unit has been introduced for the first time as a donor in a thermally activated delayed fluorescence (TADF) material. The TADF material (DTPDDA) shows strong intramolecular charge transfer (CT) character with large spatial separation with the acceptor of triazine leading to narrow splitting of singlet and triplet excited states for the efficient reverse intersystem crossing (RISC). A blue organic light emitting diode (OLED) based on DTPDDA not only displays deep blue in the Commission Internationale de L’Eclairage (CIE) coordinates of (0.149, 0.197) but also exhibits a high external quantum efficiency (EQE) of 22.3% which is the highest value ever reported for a blue fluorescent OLED. Theoretical prediction based on transient photoluminescence (PL) and optical simulation result agrees well with the achieved EQE indicating the successful conversion of triplet excitons to singlet in the blue fluorescent OLED by using DTPDDA.

COMMUNICATION take advantage of narrow emission spectra and high stability of fl uorescent molecules on the one hand, and effi cient RISC of the triplet exciton to the singlet exciton in the TADF molecules on the other hand, to harvest both singlet and triplet excitons by the Förster energy transfer from the TADF molecules to the fl uorescent dopant. [ 23,24 ] This concept has been quite successful to reach the EQE over 10% using TADF materials as host or sensitizers recently in fl uorescent OLEDs. Zhang et al. reported EQE of 10% using 2-biphenyl-4,6-bis(12-phenylindolo)-[2,3-a] carbazole-11-yl)-1,3,5-triazine as TADF host and 3,11-diphenylamino-7,14-diphenylacenaphtho[1,2-k]fl uoranthene as a conventional fl uorescent emitter [ 23 ] and Nakanotani et al. reported EQE of 18% using TADF materials as assistant dopant. [ 24 ] However, in order to utilize the RISC effi ciently in a fl uorescent dye doped OLEDs, it is desirable to form excitons on TADF materials rather than direct formation on the fl uorescent dopant. In this regard, exciplex forming cohost systems are good candidates as host for fl uorescent dopants because the bimolecular recombination on host molecules is dominant rather than the trap assisted recombination as reported in an exciplex host doped with a phosphorescent dye. [ 22 ] Here, we report a red fl uorescent OLED with unprecedented high EQE of 10.6% using an exciplex system as host for a conventional fl uorescent dopant not showing delayed fl uorescence. The fraction of the radiative exciton is over 35% by using the exciplex host, clearly indicating the triplet harvesting in the system. Figure 1 a shows the chemical structures of the host and the red-emitting dopant used in this study. The red fl uorescent dopant 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) was used as the emitter which showed a high photoluminescence (PL) quantum yield and negligible delayed fl uorescence with the EQEs of 3%-6% in previous reports. [25][26][27][28] 4,4′,4″-tris( N -carbazolyl)-triphenylamine (TCTA) and 4,6-bis[3,5-(dipyrid-4-yl)phenyl]-2-methylpyrimidine (B4PYMPM) [ 29 ] were used as the exciplex host.The PL spectra of TCTA, B4PYMPM and the mixed fi lms with 1:1 ratio are shown in Figure 1 b. The peak wavelengths were 393 nm for TCTA, 420 nm for B4PYMPM, and 509 nm for the mixed fi lm, respectively. The mixed fi lm showed a broad featureless emission red shifted from the consisting materials, indicating that the mixed fi lm forms exciplex upon excitation. The delayed PL spectrum of the exciplex measured after 10 µs at 30 K was almost the same as the one measured at room temperature, suggesting almost the same energy levels of the triplet and the singlet exciplexes. The PL quantum yield of TCTA:B4PYMPM exciplex measured using a 50 nm thick mixed fi lm on a quartz substrate in an intergrating sphere was

The mixed cohosts of 1,3-bis(N-carbazolyl)benzene and 2,8-bis(diphenylphosphoryl)dibenzothiophene have been developed for a highly efficient blue fluorescent oragnic light emitting diode (OLED) doped with a thermally activated delayed fluorescence (TADF) emitter [4,5-di (9H-carbazol-9-yl) phthalonitrile (2CzPN)]. We have demonstrated one of the highest external quantum efficiency of 21.8% in blue fluorescent OLEDs, which is identical to the theoretically achievable maximum electroluminescence efficiency using the emitter. Interestingly, the efficiency roll-off is large even under the excellent charge balance in the device and almost the same as the single host based devices, indicating that the efficiency roll-off in 2CzPN based TADF host is related to the material characteristics, such as low reverse intesystem crossing rate rather than charge imbalance.

Azasiline-based blue thermally activated delayed fluorescence emitters for blue fluorescent organic light emitting diodes.