An Alternative Host Material for Long‐Lifespan Blue Organic Light‐Emitting Diodes Using Thermally Activated Delayed Fluorescence

It has been challenging to find stable blue organic light emitting diodes (OLEDs) that rely on thermally activated delayed fluorescence (TADF). Lack of stable host materials well‐fitted to the TADF emitters is one of the critical reasons. The most popular host for blue TADF, bis[2‐(diphenylphosphino)phenyl] ether oxide (DPEPO), leads to unrealistically high maximum external quantum efficiency. DPEPO is however an unstable material and has a poor charge transporting ability, which in turn induces an intrinsic short OLED operating lifespan. Here, an alternative host material is introduced which educes the potential efficiency and device lifespan of given TADF emitters with the appropriateness of replacing the most popular host material, DPEPO, in developing blue TADF emitters. It simultaneously provides much longer device lifespan and higher external quantum efficiency at a practical brightness due to its high material stability and electron‐transport‐type character well‐fitted for hole‐transport‐type TADF emitters.

After stirring for 1 h, 3-bromo-5-fluorobenzonitrile (6.55 g, 32.89 mmol) was dissolved in anhydrous DMF (150 mL) and it was added to the stirred reaction mixture under a nitrogen atmosphere. The solution was allowed to be stirred at 120 o C for 12 h. The reaction mixture was extracted with dichloromethane and distilled water, and then the separated dichloromethane layer was dried using anhydrous magnesium sulfate and concentrated in vacuo. The reaction product was purified by silica gel chromatography using a mixture of chloroform and n-hexane (1/2) as an eluent. A white product was obtained in 8.8 g (85%) yield.

Synthesis of mCPyPO
Scheme 4. Synthetic route of mCPyPO

Photoluminescence of BDPyInCz
Figure S13. Absorption and photoluminescence spectra of BDPyInCz.

Photoluminescence of DPyInCz
Figure S14. Absorption and photoluminescence spectra of DPyInCz. Figure S15. Absorption and photoluminescence spectra of mCBP-CN

Device fabrication and characterization
The organic layers were consequently deposited on pre-cleaned ITO glass substrates using a thermal evaporation system with a vacuum pressure of < 1.0  10 -6 torr. A 1-nm-thick Liq and 100-nm-thick Al (or Ag) were deposited as a cathode by thermal evaporation. The deposition rates of the organic and metal layers were about 0.1 nm s -1 and 0.5 nm s -1 , respectively. That of the Liq layer was about 0.01 nm s -2 . The active device area of 4 mm 2 was defined by the overlapped area of the ITO and Al electrodes. The HOD structure is ITO/HAT-CN(10 nm)/NPB(50 nm)/test material (30 nm Figure S21. Change of the driving voltages of a) EODs and b) HODs of mCBP-CN, mCBP-2CN, mCBP-3CN and DPEPO keeping the driving current constant. The EOD of the three new hosts are much more stable than that of DPEPO but no difference is observed between the three. The HOD of mCBP-CN among the three new hosts is however most stable but the difference is not so much remarkable though.