Xu-Lin Chen

Highly efficient cuprous complexes with thermally activated delayed fluorescence and simplified solution process OLEDs using the ligand as host

Two strongly luminescent complexes, [Cu(czpzpy)(PPh3)]BF4 (1) and [Cu(czpzpy)(POP)]BF4 (2), bearing a new carbazole-based diimine ligand (czpzpy = 2-(9H-carbazolyl)-6-(1H-pyrazolyl)pyridine, PPh3 = triphenylphosphine, and POP = bis[2-(diphenylphosphine)phenyl]ether) were synthesized and characterized. Complex 2 emits intense thermally activated delayed fluorescence (TADF) with high photoluminescence quantum yields (PLQYs) of up to 0.98 at room temperature in the solid state. A highly efficient solution-processed OLED, with an emissive layer was spin-coated using a mixture of the diimine ligand czpzpy and the starting material, [Cu(CH3CN)2)(POP)]BF4, was fabricated with peak efficiencies of 6.36% (external quantum efficiency, EQE) and 17.53 cd A−1 (current efficiency, CE) and a maximum brightness of 3251 cd m−2. These performances are nearly identical to those of the device whose emissive layer was comprised of complex 2 and czpzpy, indicating that this simplified process, which omits the preparation and purification of the emissive complex, is practicable in preparing efficient solution-processed OLEDs. In this approach, czpzpy, which bears a carbazole unit, was used both as a ligand to form the emissive cuprous complex and as a host matrix for the formed emitter.

Highly Efficient Cuprous Complexes with Thermally Activated Delayed Fluorescence for Solution-Processed Organic Light-Emitting Devices

Two mononuclear cuprous complexes [Cu(PNNA)(POP)]BF4 (1) and [Cu(PNNA)(Xantphos)]BF4 (2) (PNNA = 9,9-dimethyl-10-(6-(3-phenyl-1H-pyrazol-1-yl)pyridin-3-yl)-9,10-dihydroacridine, POP = bis[2-(dipenylphosphino)phenyl]ether, Xantphos =4,5-bis(diphenylphosphino)-9,9-dimethylxanthene), with intense bluish-green luminescence based on a new diimine ligand were designed and synthesized. Their structural, electrochemical, and photophysical properties were characterized by single-crystal X-ray analysis, cyclic voltammetry, temperature dependence of spectroscopy, time-dependent emission spectroscopy, etc. The complexes exhibit high photoluminescence quantum yields in doped films (up to 74.6%) at room temperature. Thermally activated delayed fluorescence based on intraligand charge transfer was observed by grafting a strong electron-donor moiety, 9,9-dimethylacridan, on the diimine ligand, which is supported by the density functional theory calculations on two complexes. Highly efficient solution-processed OLEDs based on these two complexes were fabricated, among which the electroluminescent device using 2 as dopant shows a peak external quantum efficiency of 7.42%, a peak current efficiency of 20.24 cd/A, and a maximum brightness of 5579 cd/m(2).

Syntheses, Photoluminescence, and Electroluminescence of a Series of Sublimable Bipolar Cationic Cuprous Complexes with Thermally Activated Delayed Fluorescence

Three thermally activated delayed fluorescence cationic cuprous complexes [Cu(POP) (ECAF)]PF6 (1, POP = bis(2-diphenylphosphinophenyl)ether, ECAF = 9,9-bis(9-ethylcarbazol-3-yl)-4,5-diazafluorene), [Cu(POP) (EHCAF)]PF6 (2, EHCAF = 9,9-bis(9-ethylhexylcarbazol-3-yl)-4,5-diazafluorene), and [Cu(POP) (PCAF)]PF6 (3, PCAF = 9,9-bis(9-phenylcarbazaol-3-yl)-4,5-diazafluorene) with bipolar 4,5-diazafluorene ligand substituted by bis-carbazole have been successfully prepared, and their UV absorption, photoluminescent properties, and electrochemical behaviors were investigated. At room temperature, complexes 1, 2, and 3 exhibit efficient yellowish-green emission with peak maxima of 550, 549, and 556 nm, respectively, and lifetimes of 5.7 μs. In powder states, the quantum yields (ϕPL) of 22.4% for 1, 18.5% for 2, and 20.0% for 3, respectively, are found. These metal phosphors can be vacuum-evaporated and applied in the organic light-emitting diodes (OLEDs) of indium tin oxide/poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (40 nm)/4,4,4″-tri(9-carbazoyl)triphenylamine (15 nm)/cuprous complexes (10 wt %): 1,3-bis(9-carbazolyl)benzene (30 nm)/1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (50 nm)/LiF (0.5 nm)/Al (100 nm). Complex 1-based device D1 achieved a maximum luminance of 11u202f010 cd m-2, a current efficiency of 47.03 cd A-1, and an external quantum efficiency of 14.81%. The high electroluminescence efficiencies of these complexes are assumed to be due to their good thermal stabilities and capture of both singlet and triplet excitons. The research presented here provides a powerful tool toward highly efficient and cheap OLED devices.

Four highly efficient cuprous complexes and their applications in solution-processed organic light-emitting diodes

Four mononuclear cationic Cu(I) complexes featuring functional C-linked pyrazolyl pyridine diimine ligands have been synthesized and characterized. Under UV 365 nm at room temperature, these complexes emit similar orange light in solution and green light in thin film, which could be attributed to the limited charge and steric influence of these methyl or trifluoromethyl substituted phenyl appendages at the pyrazol ring. Moreover, multilayer organic light-emitting diodes (OLEDs) based on these Cu(I) complexes had different performance efficiencies. The device with complex 1 as the light emitting material had the highest current efficiency of 17.8 cd A−1 and an external quantum efficiency (EQE) of 6.4%, while the device with complex 2 which shows the highest photoluminescence quantum yields gave the poorest device efficiency with a current efficiency of 13 cd A−1 and an EQE of 4.7%.

Combining Charge‐Transfer Pathways to Achieve Unique Thermally Activated Delayed Fluorescence Emitters for High‐Performance Solution‐Processed, Non‐doped Blue OLEDs

Two efficient blue thermally activated delayed fluorescence compounds, B-oCz and B-oTC, composed of ortho-donor (D)-acceptor (A) arrangement were designed and synthesized. The significant intramolecular D-A interactions induce a combined charge transfer pathway and thus achieve small ΔEST and high efficiencies. The concentration quenching can be effectively inhibited in films of these compounds. The blue non-doped organic light emitting diodes (OLEDs) based on B-oTC prepared from solution processes shows record-high external quantum efficiency (EQE) of 19.1u2009%.

Photo- and electro-luminescence of three TADF binuclear Cu(I) complexes with functional tetraimine ligands

Three new binuclear cuprous complexes with similar tetraimine ligands [Cu2(pytzph)(POP)2](BF4)2 (1), [Cu2(pytzphcf)(POP)2](BF4)2 (2) and [Cu2(pytzphcz)(POP)2](BF4)2 (3), (pytzph = 6,6′-(1-phenyl-1,2,4-triazole-3,5-diyl)bis(2-methylpyridine), pytzphcf = 6,6′-(1-(4-(trifluoromethyl)phenyl)-1,2,4-triazole-3,5-diyl)bis(2-methylpyridine), pytzphcz = 9-(4-(3,5-bis(6-methylpyridin-2-yl)-1,2,4-triazol-1-yl)phenyl)-carbazole and POP = bis[2-(diphenylphosphine)phenyl]ether), have been synthesized and characterized in order to compare the different effects of substituent groups on the photoluminescence (PL) and electroluminescence (EL) properties. These complexes exhibit highly efficient green thermally activated delayed fluorescence (TADF) with short decay times (5.5–16 μs) and high photoluminescence quantum yields (up to 79%) at room temperature in the solid form. These complexes have essentially identical emission energy. However, the influence of the substituents on the photoluminescence and electroluminescence efficiencies is evident. Complex 3 with the carbazole group shows the highest efficiency in terms of both PL and EL, exhibiting an EQE of 8.3%, a CE of 27.1 cd A−1 and a peak brightness of 2525 cd cm−2 in the solution-processed OLED, while complex 2 with a trifluoromethyl appendage exhibits poorer quantum efficiency than the others.

Anion-π Interaction-Induced Room-Temperature Phosphorescence of a Polyoxometalate-Based Charge-Transfer Hybrid Material

Room-temperature phosphorescence (RTP) was realized for the first time in a polyoxometalate-based charge-transfer (CT) hybrid material bearing polyoxometalates (POMs) as electron-donors (D) and rigid naphthalene diimides (NDIs) as electron-acceptors (A), meanwhile, this hybrid material displayed photochromism as well. The significant D-A anion-π interaction induced an additional through-space charge-transfer pathway. The resulting suitable D-A CT states can efficiently bridge the relatively large energy gap between the NDI-localized 1 π-π* and 3 π-π* states and thus trigger the ligand-localized phosphorescence (3 π-π*).

Doped polyaniline-hybridized tungsten oxide nanocrystals as hole injection layers for efficient organic light-emitting diodes

Solution-processed tungsten oxide nanocrystals (WO3 NCs) hybridized with a conductive polymer, polyaniline:poly(styrene sulfonate) (PANI:PSS), are introduced as a hole injection layer (HIL) in organic light-emitting diodes (OLEDs). These devices exhibited much higher current efficiency (CE) and power efficiency (PE) compared to regular devices without HILs and devices using pristine WO3 NCs as HILs. This enhanced device performance is mainly attributed to the facilitated hole injection due to the improvement in the energy level alignment at the interface between the ITO anode and the organic semiconductor layer. Besides, PANI:PSS can efficiently reduce surface defects and enhance the film-forming properties of WO3 NCs without affecting the device conductivity. These results suggest that PANI:PSS–WO3 could be a promising candidate for charge injection layers in optoelectronic devices.