An efficient solution-processable hybridized local and charge-transfer (HLCT)-based deep-red fluorescent emitter for simple structured non-doped OLED

Benzothiadiazole‐Based Green Hybridized Local and Charge‐Transfer (HLCT) Fluorophores as Solution‐Processed Non‐Doped Emitters for Simple Structured Electroluminescent Devices

“…Both molecules were explicitly characterized by standard spectroscopic methods ( 1 H‐NMR, 13 C‐NMR, and high‐resolution MS), and the results were consistent well with their chemical structures. They exhibited good solubility in most organic solvents owing to the presence of the 9,9‐dioctylfluorene as a solubilizing moiety, [18b,22] which allows the fabrication of good quality thin film could be done using simple solution‐processed casting techniques.…”

“…Hence, in this work, we report the design, synthesis, and properties of high solid‐state fluorescence compounds, namely CBzF and CBzFC (Scheme 1), having a collective aspect of HLCT and solution processability as an effective non‐doped emitter for a simple solution‐processed OLED. In this design, one side of the electron accepting benzothiadiazole (Bz) is substituted by an electron donating carbazol‐9‐yl)phenyl unit to formulate a donor‐acceptor (D‐A) HLCT fragment, while another side of the Bz ring is directly linked with either 9,9‐dioctylfluorene and (7‐(3,6‐di‐ tert ‐butylcarbazol‐9‐yl)‐9,9‐dioctylfluorene moieties as an aromatic substituent holding combined features of good organic solvent solubility and hole‐transporting capability [18b,21] . Indeed, both CBzF and CBzFC emit an intense green color with a HLCT property and high solid state fluorescence efficiency, and so, their simple solution‐processed non‐doped OLEDs achieve admirable EL performances (EQE max =5.29–5.59 %; CE max =11.24–12.24 cd A −1 ) and high exciton utilization efficiencies of 31–33 %.…”

Dual Hybridized Local and Charge Transfer Characteristic Fluorophore as an Efficient Nondoped Emitter for Electroluminescent Device

“…[ 34 ] A superior EL performance of the solution‐processed BCBF emitter could be associated with a combination of its dual HLCT characteristic and better hole mobility. Extensively, this BCBF ‐based bilayer device performance represents one of the best results of solution‐processed HLCT‐based OLEDs reported up to now [ 4,12,11a ] and is superior to the EL performance of the thermally evaporated multilayered OLEDs of its related HLCT molecule 4‐(4‐(carbazol‐9‐yl)phenyl)‐7‐phenylbenzothiadiazole (CzP‐BZP) (EQE max = 6.95%) [10f] . To further analyze the utilization of excitons in the EL process, the exciton utilization efficiency (EUE) was calculated using EUE = EQE/( η rec × η out × Φ PL ), [ 35 ] where η out is the light outcoupling efficiency which is roughly 20% for glass substrates, Φ PL is the absolute PL quantum yield of the EML in a thin film (Table 1), and η rec is the fraction of exciton formation of the injected charge carriers, which is 100% for ideal charge recombination.…”

“…Among them, the HLCT mechanism that involves conversions between much short exciton lifetime high‐lying triplet states and singlet states ( T n → S m ) along the “hot exciton” channel presents several advantages: [ 4 ] 1) the lowest excited state ( S 1 ) possessing the combined and compatible transition features of the local excitation (LE) and charge transfer (CT) characters gives rise to simultaneously high luminous efficiency and high ratio of radiative singlet excitons; [ 5 ] 2) HLCT OLEDs usually have good device stability since RISC occurs on a nanosecond timescale rate, thus enabling to reduce the generation and accumulation of T1 state excitons, which can efficiently avoid annihilation because of the long‐lived T 1 excitons; [ 6 ] 3) ability to realize pure‐blue and deep‐blue fluorophores since HLCT molecules could be constructed from a moderate donor and acceptor pairs; [ 7 ] and 4) most HLCT OLEDs are generally nondoped devices allowing a simple fabrication process. [ 8 ] So far, many highly efficient HLCT emitters have been developed including phenanthroimidazole derivatives as blue to deep‐blue HLCT fluorophores, [2a,7a,9] triarylamine‐benzothiadiazoles as green to red‐orange HLCT fluorophores, [5a,b,10] and triarylamine‐naphthothiadiazoles as red to near‐infrared (NIR) HLCT fluorophores [8a,11] . However, all of them were fabricated by thermal vacuum deposition, involving heating a solid material inside a high‐vacuum chamber at high temperatures and in a small area which is not applicable for fabricating flexible and large area‐sized devices and scaling up for commercial applications.…”

“…Thus, the implementation of solution‐processable fabrication methods can alleviate this issue by offering a low‐cost route for the fabrication of flexible and large area‐size devices with a much greater utilization rate of materials. To this point, there are only a few examples of solution‐processed HLCT‐based OLEDs, [ 4,11a,12 ] and their EL performances are still inferior compared with the thermally evaporated ones. [ 4,13 ] Therefore, the development of more efficient solution‐processable HLCT emitters is undeniably vital and a massive challenge.…”