Solution processed deep‐blue organic light emitting diodes (OLEDs) with high external quantum efficiency (EQE) and a long operational lifetime are still constrained. In this context, two thermally activated delayed fluorescence (TADF) emitters are synthesized utilizing a new design strategy of twisted interlocked acceptor core integrated with carbazole (KCCz) and tert‐butylcarbazole (KCTBC) as donors, respectively, for solution processed deep‐blue TADF OLEDs. Twisting of the acceptor core by two methyl groups results in complete separation of highest occupied molecular orbital and lowest unoccupied molecular orbital, along with cyanide group facilitating the generation of low‐lying triplet excited states as suggested by theoretical simulation. The combined effect of both results in tuning of emission in ultradeep blue region through the efficient population of triplet excitons and concurrently reverse intersystem crossing to produce highly efficient devices. A doped device based on KCTBC shows EQEmax of 9.0% along with low efficiency roll‐off with long operational device half lifetime of 72 min at initial brightness of 1000 cd m−2, and Commission Internationale de L'Eclairage (CIE) coordinates of (0.17, 0.13). In addition, with 12.5 wt% of 4CzFCN as assistant dopant/cohost the performance of the KCTBC‐based device is enhanced to an EQEmax of 13.9% and CIE coordinates of (0.18, 0.13). Further, a high‐efficiency warm white OLED adopting the TADF hybrid approach is realized with EQEmax of 9.0%.
Phenanthroimidazole (PI) has great proficiency to design
efficient
near UV deep blue emitting materials due to its bipolarity behavior.
The tuning of N1 and C1 centers of the PI further reveals the improvement
of the optical and electronical properties. Thorough investigation
is also executed to explore the influence of the alkyl chain on the
optical and electroluminescence (EL) properties of these emissive
materials. The incorporation of the alkyl chain in the materials can
tune the optical properties, leading to better solid state emission
and enhancement of the photoluminescence quantum yield (PLQY) of thin
film (∼74%) as compared to solution (∼65%). The EL spectra
were also exhibited between 395 and 420 nm (in the near-UV spectral
region). Furthermore, for boosting of the device efficiency and color
purity, the OLED device is fabricated by doping CBP as host matrix.
Selectively, CBP is used as a host because of the similar energy level
(HOMO and LUMO) of the synthesized emitters which assist efficient
charge trapping. Predominantly, the emitters in OLED device blended
with 0.5 wt % concentration achieved brilliant device efficiency and
luminance properties. Among all the synthesized fluorophores, PIPP
(0.5 wt %) doped OLED device showed 4.4% maximum external quantum
efficiency (EQEmax), 1202 cd/m2 highest luminance,
2.2 lm/W power efficiency (PE), 2.7 cd/A current efficiency (CE) with
Commission International de L’Eclairage (CIE) coordinates x = 0.17, y = 0.10 (nearer the NTSC standard
value).
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