Sky-blue Emission Research Articles

Achieving ultra-high efficiency by tuning hole transport and carrier balance in fluorescent blue organic light-emitting diode with extremely simple structure

Ultra-high efficiency in fluorescent blue organic light-emitting diode (OLED) with extremely simple structure is demonstrated by tuning hole transport and carrier balance. Using 2-methyl-9,10-bis(naphthalen-2-yl)anthracene doped with 3 wt% 1-4-di-[4-(N,N-diphenyl)amino]styryl-benzene ([MADN: 3 wt%DSA-Ph]) as hole transport and emissive layer, the OLED gives sky-blue emission with 1931 Commission Internationale d'Eclairage color coordinates of (0.159, 0.281), luminous efficiency of 14.7 cd/A, power efficiency of 13.0 lm/W and external quantum efficiency of 10.0%. The efficiencies have been enhanced by 54.7% (11.4%), 49.4% (11.1%) and 38.9% (5.3%), respectively, in comparison with the counterpart having hole transport layer of NPB (MADN). Current versus voltage characteristics and impedance spectroscopy analysis of hole only cells elucidate that [MADN: 3 wt%DSA-Ph] shows reduced hole mobility as a result of trapping or scattering effect of DSA-Ph. Efficient hole injection and appropriate hole transport favorably tune holes entering into the emissive layer and improve carrier balance. Extra-fluorescent emission generated by triplet-triplet annihilation and broad emission zone by integrating hole transport and emission as a single layer are also responsible for ultra-high device efficiency.

Excited state engineering for efficient reverse intersystem crossing.

Reverse intersystem crossing (RISC) from the triplet to singlet excited state is an attractive route to harvesting electrically generated triplet excitons as light, leading to highly efficient organic light-emitting diodes (OLEDs). An ideal electroluminescence efficiency of 100% can be achieved using RISC, but device lifetime and suppression of efficiency roll-off still need further improvement. We establish molecular design rules to enhance not only the RISC rate constant but also operational stability under electrical excitation. We show that the introduction of a second type of electron-donating unit in an initially donor-acceptor system induces effective mixing between charge transfer and locally excited triplet states, resulting in acceleration of the RISC rate while maintaining high photoluminescence quantum yield. OLEDs using our designed sky-blue emitter achieved a nearly 100% exciton production efficiency and exhibited not only low efficiency roll-off but also a marked improvement in operational stability.

The investigation of sky-blue emitting anthracene-carbazole derivatives: Synthesis, photophysics and OLED applications

Abstract New well-defined structurally different 9,10-bis(9-hexyl-6-((E)-styryl)-9H-carbazol-3-yl)anthracene derivatives, shortly named “6a, 6b and 6c”, with side groups having electron donating and withdrawing were systematically synthesized using Suzuki cross-coupling and Vilsmeier-Haack reactions. The impact of electron donating and withdrawing groups and their influence on the molecules photophysical properties has been investigated. The materials showed sky-blue emissions with high internal quantum efficiencies. Based on photophysical investigations the most promising molecule (6a) has been selected and high efficiency OLEDs with external quantum efficiency at very low current efficiency (∼1 mA/cm2) reaching 5% (doped) were obtained.

Multifunctional blue AIE emitters for efficient organic light emitting devices with low efficiency roll-off

Blue organic light-emitting diodes (OLEDs) have gained tremendous attraction owing to their promising potential for flat-panel display technologies and lighting application. To make the OLEDs more useful, efficient materials are highly desired. Herein, two novel multifunctional sky-blue emitters, TPE-m-PCzI and TPE-m-TPAI, have been developed with the integration of imidazole, carbazole, triphenyl amine and tetraphenylethene groups. These materials show high photoluminescence quantum yields. Aggregation-induced emission (AIE) behavior, thermal stability and glass transition temperatures of these materials are also examined. Non-doped OLEDs fabricated employing these materials as emitters exhibit sky-blue electroluminescence and demonstrate high efficiencies of 2.22% (4.56 cd/A, 3.15 lm/W) and 3.1% (5.74 cd/A, 3.31 lm/W) for TPE-m-PCzI and TPE-m-TPAI. High performance of TPE-m-TPAI based device shows a bright prospect of imidazole and AIEgen integration to prepare efficient fluorescent materials for OLED devices.

Multifunctional Dithiadiazolyl Radicals: Fluorescence, Electroluminescence, and Photoconducting Behavior in Pyren-1′-yl-dithiadiazolyl

The pyren-1'-yl-functionalized dithiadiazolyl (DTDA) radical, C16H9CNSSN (1), is monomeric in solution and exhibits fluorescence in the deep-blue region of the visible spectrum (440 nm) upon excitation at 241 nm. The salt [1][GaCl4] exhibits similar emission, reflecting the largely spectator nature of the radical in the fluorescence process, although the presence of the radical leads to a modest quenching of emission (ΦF = 98% for 1+ and 50% for 1) through enhancement of non-radiative decay processes. Time-dependent density functional theory studies on 1 coupled with the similar emission profiles of both 1+ and 1 are consistent with the initial excitation being of predominantly pyrene π-π* character. Spectroscopic studies indicate stabilization of the excited state in polar media, with the fluorescence lifetime for 1 (τ = 5 ns) indicative of a short-lived excited state. Comparative studies between the energies of the frontier orbitals of pyren-1'-yl nitronyl nitroxide (2, which is not fluorescent) and 1 reveal that the energy mismatch and poor spatial overlap between the DTDA radical SOMO and the pyrene π manifold in 1 efficiently inhibit the non-radiative electron-electron exchange relaxation pathway previously described for 2. Solid-state films of both 1 and [1][GaCl4] exhibit broad emission bands at 509 and 545 nm, respectively. Incorporation of 1 within a host matrix for OLED fabrication revealed electroluminescence, with CIE coordinates of (0.205, 0.280) corresponding to a sky-blue emission. The brightness of the device reached 1934 cd/m2 at an applied voltage of 16 V. The crystal structure of 1 reveals a distorted π-stacked motif with almost regular distances between the pyrene rings but alternating long-short contacts between DTDA radicals. Solid state measurements on a thin film of 1 reveal emission occurs at shorter wavelengths (375 nm) whereas conductivity measurements on a single crystal of 1 show a photoconducting response at longer wavelength excitation (455 nm).

Reaction between alkyl isocyanides and 3-formylchromones in the presence of monocyclic unsaturated acyl compounds: Competition between Friedel-Crafts acylation and Diels-Alder cycloaddition

The highly reactive alkylaminofurochromenone intermediate, produced from the reaction between 3-formylchromones and alkyl isocyanides, was trapped by monocyclic unsaturated acyl compounds such as maleic anhydride or maleimide derivatives to yield new types of organic fluorophores 4-(alkylamino)-1H-furo[3,4-b]xanthene-1,3,10-triones or 4-(alkylamino)chromeno[2,3-f]isoindole-1,3,10(2H)-triones, which exhibited strong sky blue emission in solution. The reaction is very simple from an experimental point-of-view and allows the creation of a fused benzene ring with concomitant formation of three new C–C bonds in a single operation.

Platinum-based metallomesogens bearing a Pt(4,6-dfppy)(acac) skeleton: synthesis, photophysical properties and polarised phosphorescence application.

Polarised phosphorescence has a bright future in backlighting for conventional liquid crystal displays due to its theoretical 100% internal quantum efficiency and low cost. However, there are scarce reports on polarised phosphorescence from metallomesogens. In this contribution, a platinum-based metallomesogen containing a mesogenic biphenyl (Pt1) was prepared and characterised. To further explore the effect of the substituent on mesophase and emission properties, a related complex Pt2 containing a tetraphenylethene (TPE) moiety was also synthesised. Both complexes melt at elevated temperatures but thereafter do not appear to crystallise on cooling. Complex Pt1 shows an enantiotropic nematic phase from which a broad emission can be seen when spread as a film; in solution, an intense, sky-blue emission is observed. For Pt2, which shows a monotropic SmA phase, the emission in the condensed phase is suppressed and there is only weak emission in solution. Polarisation-dependent photoluminescence with a polarised ratio of 5.4 was obtained for the aligned film of a Pt1:polyimide mixture. Using Pt1 as an emissive layer, non-doped, polarised organic light-emitting diodes presented a broad emission spectrum in the range of 450-900 nm with a polarised ratio of 1.33 and the highest external quantum efficiency of 1.1%. This research has an important significance for achieving broad-based polarised phosphorescence from platinum complex-based metallomesogens.

Polymorphism of derivatives of tert-butyl substituted acridan and perfluorobiphenyl as sky-blue OLED emitters exhibiting aggregation induced thermally activated delayed fluorescence

Blue multi-functional emitters with a perfluorobiphenyl moiety exhibiting polymorphism and changing emission colour.

Dual-emissive 2-(2'-hydroxyphenyl)oxazoles for high performance organic electroluminescent devices: discovery of a new equilibrium of excited state intramolecular proton transfer with a reverse intersystem crossing process.

The photoluminescence (PL) and electroluminescence (EL) properties of two highly efficient excited state intramolecular proton transfer (ESIPT) molecules, 2-(2'-hydroxyphenyl)oxazoles containing one triphenylamine (TPA) (1) and two TPAs (2) respectively, are studied systematically. The enol-forms of both 1 and 2 possess highly hybridized local and charge transfer (HLCT) excited state character, while their excited-state keto-forms are not of obvious HLCT character. A 1-based device exhibits green-white electroluminescence with Commission Internationale d'Eclairage (CIE) coordinates of (0.25, 0.41) and a high external quantum efficiency (EQE) up to 5.3%, which is the highest EQE value recorded for single molecular white light-emitting materials. A 2-based device shows sky-blue emission with CIE coordinates of (0.18, 0.16) and an EQE of 8.0%, which is the highest EQE in the reported HLCT materials. The fluorescence intensities of the enol-forms of 1 and 2 in EL spectra are increased remarkably relative to their PL spectra. Experimental data and theoretical calculations reveal a new ESIPT equilibrium with a reverse intersystem crossing (RISC) process arising from the HLCT character. In EL, the RISC of the enol-form excitons from the triplet state to the singlet state triggers an increase in the number of enol-form singlet excitons, which further leads to a shift of the ESIPT equilibrium towards an enhanced enol-form emission. Thus, the difference between the ESIPT equilibria in PL and EL may be ascribed to the HLCT character of the enol-form excited state.

High-efficiency blue OLEDs based on dendritic dinuclear iridium (III) complexes grafted with fluorene core and blue fluorescence chromospheres

Two novel dendrimer-like blue-emitting dinuclear cyclometalated iridium (III) complexes, namely (DNaTPA)2DBF(FIrpic)2 and (DPyTPA)2DBF(FIrpic)2, have been successfully synthesized and characterized. In which FIrpic is an iridium (III) bis[(4,6-difluorophenyl)pyridinato-N,C2′]picolate blue-emitting phosphorescent chromophore core, DBF is a 2,7-diphenyl-9H-fluorene bridging core, DNaTPA and DPyTPA are deep blue-emitting fluorescent chromophores composed by rigid high-triplet-energy dendrons of triphenylamine-functionalized naphthalene or pyrene units, and the peripheral dendrons are connected with the ancillary ligand of the emitting core through nonconjugated ether linkage. Their photophysical, thermal, electrochemical, as well as electrophosphorescent properties were primarily studied. Both iridium (III) complexes exhibit high efficient blue emission in solution (38.5% and 19.2%) and a typical FIrpic emission in 1,3-bis(N-carbzolyl)benzene (mCP) matrix (27.0% and 24.1%). Simple bilayer phosphorescent organic light-emitting diodes (PHOLEDs) with a configuration of ITO/PEDOT:PSS/mCP:dopants/TmPyPB/Liq/Al achieved high efficiencies of 12.96 cd/A for current efficiency (CE), 6162 cd/m2 for brightness, 6.22% for external quantum efficiency (EQE), and 3.13 lm/W for power efficiency (PE) with Commission International de L'Eclairage (CIE) coordinates of (0.19 ± 0.01, 0.35 ± 0.02) at only 2 wt% blend of (DNaTPA)2DBF(FIrpic)2. (DPyTPA)2DBF(FIrpic)2-doped devices also reach efficiencies of (9.14 cd/A, 7167 cd/m2, 4.41%, 2.61 lm/W) at the same doping concentration. The results demonstrate that the introduction of dendritic blue-emitting fluorescent chromophore grafted into the blue phosphorescent chromosphere core through nonconjugated linkage is an efficient way to achieve high-efficiency sky-blue emission.

Systematically tuning of optoelectronic properties from electron donating to accepting substituents on bicarbazole/cyanobenzene hybrids: Host to dopant materials for phosphorescent and delayed fluorescence OLEDs

Abstract Six bicarbazole/cyanobenzene hybrid compounds are designed and synthesized by changing the substituents from electron-donating phenylcarbazole (PCzCNBCz, 1), phenoxy (OPCNBCz, 2) and methyl (MCNBCz, 3), neutral hydrogen (HCNBCz, 4) to electron-accepting trifluomethyl (CF3CNBCz, 5) and cyano (DCNBCz, 6) moieties, respectively at the same meta-position of cyanobenzene (refer to carbazole). The substituted units are judiciously designed with both conjugated (phenylcarbazole and cyano) and non-conjugated (phenoxy, methyl and trifluomethyl) units. Their HOMO and LUMO energy levels are rationally adjusted from 5.17 to 5.46 eV and 2.13–2.50 eV, respectively. Compared to bare hydrogen-attached 4 with triplet energy (T1) of 2.7 eV, the conjugated moiety based 1 and 6 lowered T1 to ≤2.50 eV, while other compounds bearing either non-conjugated donating or accepting units unexpectedly maintain T1 as high as ∼2.70 eV. Moreover, the singlet-triplet bandgaps (ΔEST) can also be tailored from 0.01 to 0.32 eV, therefore, delayed fluorescence characteristics are observed in the electron-accepting trifluomethyl and cyano substituted 5 and 6. The high triplet 2, 3 and 4 are served as host materials for blue phosphorescent OLEDs, with maximum external quantum efficiency (EQE) up to 20.5%, while 5 and 6 are used as sky-blue and greenish-blue emitters in delayed fluorescence OLEDs, with maximum EQE of 4.9 and 10.0%, respectively.

Non-Doped Sky-Blue OLEDs Based on Simple Structured AIE Emitters with High Efficiencies at Low Driven Voltages.

Blue organic light-emitting diodes (OLEDs) are necessary for flat-panel display technologies and lighting applications. To make more energy-saving, low-cost and long-lasting OLEDs, efficient materials as well as simple structured devices are in high demand. However, a very limited number of blue OLEDs achieving high stability and color purity have been reported. Herein, three new sky-blue emitters, 1,4,5-triphenyl-2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole (TPEI), 1-(4-methoxyphenyl)-4,5-diphenyl-2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole (TPEMeOPhI) and 1-phenyl-2,4,5-tris(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole (3TPEI), with a combination of imidazole and tetraphenylethene groups, have been developed. High photoluminescence quantum yields are obtained for these materials. All derivatives have demonstrated aggregation-induced emission (AIE) behavior, excellent thermal stability with high decomposition and glass transition temperatures. Non-doped sky-blue OLEDs with simple structure have been fabricated employing these materials as emitters and realized high efficiencies of 2.41 % (4.92 cd A-1 , 2.70 lm W-1 ), 2.16 (4.33 cd A-1 , 2.59 lm W-1 ) and 3.13 % (6.97 cd A-1 , 4.74 lm W-1 ) for TPEI, TPEMeOPhI and 3TPEI, with small efficiency roll-off. These are among excellent results for molecules constructed from the combination of imidazole and TPE reported so far. The high performance of a 3TPEI-based device shows the promising potential of the combination of imidazole and AIEgen for synthesizing efficient electroluminescent materials for OLED devices.

Highly twisted organic molecules with ortho linkage as the efficient bipolar hosts for sky-blue thermally activated delayed fluorescence emitter in OLEDs

Abstract Two new host materials, CzDPPy and tCzDPPy, were designed and synthesized through the Ullmann-coupling reaction between carbazole/3,6-di- tert -butyl-carbazole and 2,6-bis(2-bromophenyl)pyridine. Their single-crystal structure, thermal, electrochemical, opt-electronic and bipolar carrier-transporting properties were fully investigated. Due to the steric hindrance of carbazole at the ortho positions of diphenylpyridine, both CzDPPy and tCzDPPy adapted highly twisted molecular conformation, which could effectively minimize their π conjugation and endow them the high triplet energies of 2.67 and 2.64 eV, respectively. Organic light-emitting devices (OLEDs) were fabricated by using CzDPPy and tCzDPPy as the host materials and 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene (2CzPN) as the sky-blue TADF emitter. The peak current efficiency of 34.8 cd A −1 , power efficiency of 33.1 lm W −1 and external quantum efficiency of 16.0% were realized for the CzDPPy-based TADF OLED, along with the satisfactory CIE coordinate of (0.18, 0.34) at 100 cd m −2 .

Strongly Luminescent Cyclometalated Gold(III) Complexes Supported by Bidentate Ligands Displaying Intermolecular Interactions and Tunable Emission Energy.

A series of charge-neutral AuIII complexes, which comprise a dicarbanionic C-deprotonated biphenyl ligand and bidentate ancillary ligands ([Au(C^C)(L^X)]; L^X=β-diketonate and relatives (O^O), quinolinolate and relatives (N^O), and diphosphino (P^P) ligands), were prepared. All the complexes are emissive in degassed CH2 Cl2 solutions and in thin-film samples with Φem up to 18 and 35 %, respectively, except for 5 and 6, which bear (N^O)-type ancillary ligands. Variation of the electronic characteristics of the β-diketonate ancillary ligand was demonstrated to be a viable route for tuning the emission color from blue-green (peak λem at ca. 466 nm for 1 and 2; 501 nm for 4 a and 4 b) to orange (peak λem at 585 nm for 3), in contrast to the common observations that the ancillary ligand has a negligible effect on the excited-state energy of the AuIII complexes reported in the literature. DFT/time-dependent (TD) DFT calculations revealed that the energies of the 3 ππ*(C^C) and the 3 ILCT(O^O) excited states (ILCT=intraligand charge transfer) switch in order on going from O^O=acetylacetonate (acac) to aryl-substituted β-diketonate ligands. Solution-processed and vacuum-deposited organic light-emitting diode (OLED) devices of selected complexes were prepared. The vacuum-deposited OLED fabricated with 2 displays a sky-blue emission with a maximum external quantum efficiency (EQE) of 6.71 % and CIE coordinates of (0.22, 0.40). The crystal structures of 7 and 9 reveal short intermolecular AuIII ⋅⋅⋅AuIII contacts, with intermetal distances of 3.408 and 3.453 Å, respectively. DFT/TDDFT calculations were performed on 7 and 9 to account for the noncovalent interactions. Solid samples of 1, 3, and 9 exhibit excimeric emission at room temperature, which is rarely reported in AuIII complexes.

Effect of the bipyridine ligand substituents on the emission properties of phosphorescent Ir(III) compounds

To investigate the effect of bipyridine ligand substituents on the emission properties of Ir(III) compounds, four homoleptic iridium(III) compounds, specifically, mer-Ir(Mepypy)3 (1), mer-Ir(Me2pypy)3 (2), fac-Ir(OMe2pypy)3 (3) and Ir(OMe2Bupypy)3 (4), where Mepypy = 2'-methyl-2,3'-bipyridine, Me2pypy = 2′,6'-dimethyl-2,3'-bipyridine, OMe2pypy = 2′,6'-dimethoxy-2,3'-bipyridine and OMe2Bupypy = 2′,6'-dimethoxy-4-tert-butyl-2,3'-bipyridine, were prepared via a one-pot reaction of the corresponding methyl- or methoxy-substituted ligand with Ir[(COD)]BF4 as the starting material. Under the same reaction conditions, the major isolated products differed depending on the substituent used. The methyl substituents resulted in Ir(III) compounds with a meridional geometry, while methoxy-substituted Ir(III) compounds with a facial geometry were isolated in high yields when the methoxy substituent was used. Compounds 1 and 2 emit a green phosphorescence (ΦPL = 0.3) with a λmax = 459–463 nm, while 3 and 4 show a bright, sky-blue emission (ΦPL = 0.5). The dimethoxy-substituted bipyridine ligand was advantageous in terms of yield, thermal stability and quantum efficiency, and its iridium compound is a good candidate for triplet emitters in phosphorescence organic light-emitting diodes (PHOLEDs).

Fluorine-free blue-emitting cationic iridium complexes with oxadiazole-type cyclometalating ligands and their use in light-emitting electrochemical cells

Fluorine-free, blue-emitting cationic iridium complexes, namely [Ir(Tipr-dphoxd)2(bpy)]PF6 (1) and [Ir(Tipr-dphoxd)2(pzpy)]PF6 (2), which use 2-phenyl-5-(2,4,6-triisopropylphenyl)-1,3,4-oxadiazole (Tipr-dphoxd) as the cyclometalating ligand (CΛN) and 2,2’-bipyridine (bpy) or 2-(1H-pyrazol-1-yl)pyridine (pzpy) as the ancillary ligands (NΛN), have been synthesized and fully characterized. In solution, complex 1 emits efficient blue-green light centered at 505 nm with notable charge-transfer (CT) (Ir/CΛN → NΛN) character; while in the lightly doped film, it emits efficient sky-blue light peaked at 474 and 502 nm with much CΛN-centered 3π-π∗ character. In both solution and films, complex 2 always emits sky-blue light peaked around 468 nm and 500 nm with dominant CΛN-centered 3π-π∗ character. The sky-blue light emission from complexes 1 and 2 is among the bluest reported for fluorine-free, blue-emitting cationic iridium complexes. Theoretical calculations reveal that complex 1 bears energetically close-lying CT and CΛN-centered 3π-π∗ states, and its emission can arise from either or both of them; while for complex 2, its emission always comes from the lowest-lying CΛN-centered 3π-π∗ state. Because the CT state contributes considerably to the emission of complex 1, complex 1 exhibits more rapid radiative decays, higher luminescent efficiencies and shorter excited-state lifetimes than complex 2 in both the solution and lightly doped film. Complexes 1 and 2 have been tested as emitting materials in solid-state light-emitting electrochemical cells (LECs), with red-shifted electroluminescence compared to the photoluminescence. The green-yellow LEC based on complex 1 and the blue-green LEC based on complex 2 show peak current efficiencies of 12.4 and 5.0 cd A−1, respectively.

Stable and efficient sky-blue organic light emitting diodes employing a tetradentate platinum complex

A tetradentate Pt(II) complex platinum (II) [10-(9-(4-tert-butylpyridin-2-yl-κN)-9H-carbazol-2-yl-κC1)-9,10-dihydro-9,9-dimethyl-3-(1H-pyrazol-1-yl-κN2)acridine-1-yl-κC1] (PtN′1N-tBu) incorporating pyrazolyl-acridine as the lumophore was demonstrated to act as a stable and efficient sky-blue emitter. Phosphorescent organic light-emitting diode (OLED) employing PtN′1N-tBu without the electron blocking layer (EBL) achieved a high external quantum efficiency (EQE) of 15.9% and an estimated operational lifetime LT70 of 635 h at an initial luminance of 1000 cd/cm2. The device efficiency could be further improved by adding TrisPCz as EBL, reaching EQE of 17.3% and operational lifetime up to 482 h at 1000 cd/cm2.

Phenothiazine substituted phenanthroimidazole derivatives: Synthesis, photophysical properties and efficient piezochromic luminescence

Two phenanthroimidazoles with phenothiazine substituents were synthesized. It was found that these novel molecules showed intense fluorescence in both solution and the solid state. Additionally, these phenanthroimidazoles displayed reversible piezofluorochromism. For example, a crystal of the monophenothiazinyl analogue gave an intense sky blue emission which changed to a green emission after grinding, the fluorescence emission could be recovered when the ground powders were fumed with organic vapors or heated. XRD patterns suggested that the reversible piezofluorochromism resulted from the transformation between crystalline and amorphous states. The single crystal X-ray structure of the monophenothiazinyl analogue revealed the twisted conformation and existence of weak C-H ⋯ π interactions, resulting loose packing in the crystal state, which was favorable for the piezofluorochromism. These results obtained will be of great help in designing new piezofluorochromic molecules, which might have potential applications in mechanical sensors and security printing.

High Power Efficiency Blue-to-Green Organic Light-Emitting Diodes Using Isonicotinonitrile-Based Fluorescent Emitters.

Herein, 9,10-dihydro-9,9-dimethylacridine (Ac) or phenoxazine (PXZ)-substituted isonicotinonitrile (INN) derivatives, denoted as 2AcINN, 26AcINN, and 26PXZINN, were developed as a series of thermally activated delayed fluorescence (TADF) emitters. These emitters showed reasonably high photoluminescence quantum yields of 71-79 % in the host films and high power efficiency organic light-emitting diodes (OLEDs). Sky-blue emitter 26AcINN exhibited a low turn-on voltage of 2.9 V, a high external quantum efficiency (ηext ) of 22 %, and a high power efficiency (ηp ) of 66 lm W-1 with Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of (0.22, 0.45), whereas green emitter 26PXZINN exhibited a low turn-on voltage of 2.2 V, a high ηext of 22 %, and a high ηp of 99 lm W-1 with CIE chromaticity coordinates of (0.37, 0.58). These performances are among the best for TADF OLEDs to date.

OLEDs based on the emission of interface and bulk exciplexes formed by cyano-substituted carbazole derivative

Four carbazol-9-yl and diphenylamino substituted 9-ethylcarbazole derivatives having cyano groups in the substituents were synthesized and characterized by the experimental and theoretical tools. Thermal, optical, photophysical and electrochemical properties were studied. The derivatives exhibited moderate thermal stability with 5% weight loss temperatures exceeding 300 °C. All the derivatives were found to be capable of glass formation with glass transition temperatures ranging from 77 to 111 °C. The optical band gaps of the solid samples were 2.84–3.38 eV. One of the derivatives was used for the preparation of blue non-doped emitting layer and as exciplex forming material for the fabrication of blue and yellow organic light emitting diodes with CIE color coordinates of (0.17, 0.28) and (0.40, 0.52), respectively. The electroluminescence of the yellow exciplexes based device resulted from the overlapping of sky blue bulk emission with photoluminescence quantum efficiency of 43.8% and orange interface exciplex emission with the efficiency of 3.84%. The fluorescent non-doped blue OLED exhibited maximum luminance of 2515 cd/m2 and external quantum efficiency reaching of 2%, while the yellow exciplex OLED exploiting the effect of thermally activated delayed fluorescence had maximum luminance of 6260 cd/m2 and external quantum efficiency of 5.8%. In addition, sky-blue and orange OLEDs with only one exciplex-based emitter were fabricated showing external quantum efficiencies of 4.2 and 3.2%, respectively. This work provides background for the fabrication of efficient OLEDs combining advantages of both interface and bulk exciplex emissions (maximal internal quantum efficiency of 100%, provision of exelent charge balance in emitting layer, formation of the “active” planar pn heterojunctions).