Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet electrofluorescence,blue and green phosphorescent OLEDs

A high efficient UV–violet emission type material bis[4-(9,9′-spirobifluorene-2-yl)phenyl] sulfone (SF-DPSO) has been synthesized by incorporating electron deficient sulfone and morphologically stable spirobifluorene into one molecule. The steric and bulky compound SF-DPSO exhibits an excellent solid state photoluminescence quantum yield (Φ_PL = 92%), high glass transition temperature (T_g = 211 °C) and high triplet energy (E_T = 2.85 eV). In addition, the uniform amorphous thin film could be formed by spin-coating from its solution. These promising physical properties of the material made it suitable for using as UV–violet emitter in non-doped device and appropriate host in phosphorescent OLEDs. With SF-DPSO as an emitter, the non-doped solution processed device achieved an efficient UV–violet emission with the EL peak around 400 nm. By using SF-DPSO as a host, solution processed blue and green phosphorescent organic light emitting diodes showed a high luminous efficiency of 13.7 and 30.2 cd A⁻¹, respectively.     

Deep blue/ultraviolet microcavity OLEDs based on solution-processed PVK:CBP blends

There is an increasing need to develop stable, high-intensity, efficient OLEDs in the deep blue and UV. Applications include blue pixels for displays and tunable narrow solid-state UV sources for sensing, diagnostics, and development of a wide band spectrometer-on-a-chip. With the aim of developing such OLEDs we demonstrate an array of deep blue to near UV tunable microcavity (μc) OLEDs (λ ∼373–469 nm) using, in a unique approach, a mixed emitting layer (EML) of poly(N-vinyl carbazole) (PVK) and 4,4′-bis(9-carbazolyl)-biphenyl (CBP), whose ITO-based devices show a broad electroluminescence (EL) in the wavelength range of interest. This 373–469 nm band expands the 493–640 nm range previously attained with μcOLEDs into the desired deep blue-to-near UV range. Moreover, the current work highlights interesting characteristics of the complexity of mixed EML emission in combinatorial 2-d μcOLED arrays of the structure 40 nm Ag/x  nm MoO_x/∼30 nm PVK:CBP (3:1 weight ratio)/y  nm 4,7-diphenyl-1,10-phenanthroline (BPhen)/1 nm LiF/100 nm Al, where x = 5, 10, 15, and 20 nm and y = 10, 15, 20, and 30 nm. In the short wavelength μc devices, only CBP emission was observed, while in the long wavelength μc devices the emission from both PVK and CBP was evident. To understand this behavior simulations based on the scattering matrix method, were performed. The source profile of the EML was extracted from the measured EL of ITO-based devices. The calculated μc spectra indeed indicated that in the thinner, short wavelength devices the emission is primarily from CBP; in the thicker devices both CBP and PVK contribute to the EL. This situation is due to the effect of the optical cavity length on the relative contributions of PVK and CBP EL through a change in the wavelength-dependent emission rate, which was not suggested previously. Structural analysis of the EML and the preceding MoO_x layer complemented the data analysis.     

Fluorene-based Tröger’s base analogues: Potential electroluminescent materials

Two new Λ-shaped fluorene-based Tröger’s base (TB) analogues with aryl substitutions are successfully synthesized and their photophysical and electroluminescent properties are examined in detail. Both compounds exhibit strong fluorescence emission in dilute solutions and aggregated states. Some abnormal photophysical behaviors have been observed; that is, the amorphous films of the two TB analogues show multiple blue–green emissions similar to the emissions of some polyfluorenes and oligofluorenes, while both the dilute solutions and the polycrystalline powders of two compounds show single blue–violet emission. Furthermore, the emissions of the amorphous film are obviously red-shifted in comparison with the polycrystalline powders. Organic light emitting diodes (OLEDs) using the two compounds as non-doped emitters with device structure of ITO/NPB (30 nm)/TBFB-BP or TBFB-FB (40 nm)/TPBI (40 nm)/LiF (1 nm)/Al (80 nm) were fabricated and high brightness (22047 cd/m² for TBFB-BP and 13434 cd/m² for TBFB-FB), high efficiency (2.78 cd/A, 1.82 lm/W for TBFB-BP and 2.76 cd/A, 1.93 lm/W for TBFB-FB) and low turn-on voltage (4.6 V for TBFB-BP and 4.5 V for TBFB-FB) were obtained. Our studies suggest that TB analogues could be excellent light emitting materials for OLED applications.     

Molecular orientation of a new anthracene derivative for highly-efficient blue fluorescence OLEDs

A new anthracene derivative of 9,10-bis(2,5-dimethyl-4-(naphthalen-2-yl)phenyl)-2,3-diphenylanthracene (BDNPA) was designed and synthesized as a non-doped blue emitter in organic light emitting diodes (OLEDs). BDNPA has highly rigid structure and thermal stability with decomposition temperature (corresponding to 5% weight loss) of 490 °C, because p-naphthyl xylene groups in 9,10-positions were highly twisted to anthracene core due to steric hindrance of xylene groups, and because the 2,3-diphenyl groups were also twisted about 45–49° to anthracene. OLEDs with BDNPA non-doped emitter showed high efficiency of 5.21 cd/A due to the carrier mobility with well-aligned π-stacking structure toward out-of-plane by the face-on orientation by grazing incidence X-ray diffraction.     

Efficient non-doped pure blue organic light-emitting diodes based on an asymmetric phenylenevinylene with cis double bond

We reported an asymmetric phenylenevinylene with a cis double bond 2-(4-(p-tolyl)styryl)-1,4-dip-tolylbenzene (cis-TSDTB) and its use as efficient deep-blue emitter for organic light-emitting diodes (OLEDs) applications. The crystal structure of cis-TSDTB showed torsion configuration and asymmetric geometry, which make it packing in a reduced intermolecular interaction arrangement. And its single crystals showed excellent fluorescence owing to this unique molecular configuration. Typical OLEDs using cis-TSDTB as non-doped emitters exhibited saturated blue light with the CIE 1931 coordinates of (0.15, 0.10), which is quite close to the National Television Standards Committee (NTSC) blue standard. High luminescence efficiency (3.4 cd A⁻¹) and high brightness (9855 cd m⁻²) have been realized in the device. All of these outstanding results indicated that cis-phenylenevinylene will be a promising candidate as blue light-emitting materials.     

Molecular modification on bisphenanthroimidazole derivative for deep-blue organic electroluminescent material with ambipolar property and high performance

Efficient deep-blue fluorescent emitters are of particular significance in organic light-emitting devices (OLEDs). An ambipolar deep-blue emitter, 4,4′-bis(4-(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-1,1′-binaphthalene (2NBTPI), was designed, synthesized and applied in a high-efficiency deep-blue emitting OLED. By modifying with binaphthyl, 2NBTPI exhibits a high thermal stability, deep blue emission as well as spatially separated HOMO and LUMO orbits. Comparing with its mononaphthyl counterpart 1,4-bis(4-(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)naphthalene (NBTPI), 2NBTPI shows more balanced charge transport properties, better color purity (color index: (0.15, 0.09) versus (0.15, 0.11)), higher external quantum efficiency (EQE) (5.95% versus 5.73%) and slower efficiency roll-off (EQE roll-off at 100 mA cm⁻²: 13.1% versus 27.6%). To the best of our knowledge, OLED performances of 2NBTPI are comparable to the best reported non-doped deep-blue emitters.     

Efficient non-doped monochrome and white phosphorescent organic light-emitting diodes based on ultrathin emissive layers

Efficient red, orange, green and blue monochrome phosphorescent organic light-emitting diodes (OLEDs) with simplified structure were fabricated based on ultrathin emissive layers. The maximum efficiencies of red, orange, green and blue OLEDs are 19.3 cd/A (17.3 lm/W), 45.7 cd/A (43.2 lm/W), 46.3 cd/A (41.6 lm/W) and 11.9 cd/A (9.2 lm/W). Moreover, efficient and color stable white OLEDs based on two complementary colors of orange/blue, three colors of red/orange/blue, and four colors of red/orange/green/blue were demonstrated. The two colors, three colors and four colors white OLEDs have maximum efficiencies of 30.9 cd/A (27.7 lm/W), 30.3 cd/A (27.2 lm/W) and 28.9 cd/A (26.0 lm/W), respectively. And we also discussed the emission mechanism of the designed monochrome and white devices.     

Solution-processed cathode-interlayer-free deep blue organic light-emitting diodes

Efficient non-doped deep blue organic light-emitting diodes (OLEDs) were fabricated by solution-processing method by using a series of small molecules consisting of various contents of triphenylamine and phosphonate-featured fluorene units as the emitting layer. Without any electron-injection layer, one of the optimal devices with a simple double-layer device configuration exhibits a maximal current efficiency of 2.59 cd A⁻¹ at 6.8 V (1.72 mA cm⁻²) with a CIE coordinates of (0.163, 0.097). These double-layer devices are demonstrated with excellent color-stability under a wide range of operating current density. The current work indicates that electron-rich triphenylamine moiety incorporated with phosphonate-featured fluorene units could be utilized as building blocks to construct a multi-functional platform combining good electron-injection property, carrier-transport property, and efficient electroluminescence. It also provides an approach to achieve a structure-simplified color-stable efficient blue OLED.     

Efficient blue emitting organic light emitting diodes based on fluorescent solution processable cyclic phosphazenes

Solution processable blue fluorescent dendrimers based on cyclic phosphazene (CP) cores incorporating amino-pyrene moieties have been prepared and used as emissive layers in organic light emitting diodes (OLEDs). These dendrimers have high glass transition temperatures, are monodisperse, have high purity via common chromatographic techniques, and form defect-free amorphous films via spin/dip coating. The solution processable blue light emitting OLEDs reach current efficiencies of 3.9 cd/A at brightness levels near 1000 cd/m². Depending on the molecular bridge used to attach the fluorescent dendron to the inorganic core, the emission wavelength changes from 470 to 545 nm, corresponding to blue and green light respectively. Via dilution experiments we show that this shift in emission wavelength is likely associated with molecular stacking of the amino-pyrene units.     

Uniform Aerosol Jet printed polymer lines with 30 μm width for 140 ppi resolution RGB organic light emitting diodes

We demonstrate the patterning of top emitting organic light emitting diodes (OLEDs) by direct-write Aerosol Jet printing in air from non-halogenated solvents. Indane was determined to be a suitable single solvent to dissolve the archetypal host poly(N-vinylcarbazole) and guest emitting dopants complexes for red, green, and blue OLEDs, and to print on both PEDOT:PSS and MoO₃ transport layers. The Aerosol Jet deposition parameters were studied, including: atomization flow rate, exhaust flow rate, focusing sheath flow rate, nozzle-to-substrate distance, nozzle speed, and substrate temperature. A line width of 30 μm and thickness of 30 nm was achieved, resulting in RGB OLEDs fabricated side-by-side with a pixel density of 140 ppi.     

Green and blue–green light-emitting electrochemical cells based on cationic iridium complexes with 2-(4-ethyl-2-pyridyl)-1H-imidazole ancillary ligand

The ionic iridium complexes, [Ir(ppy)₂(EP-Imid)]PF₆ (Complex 1) and [Ir(dfppy)₂(EP-Imid)]PF₆ (Complex 2) are used as the light-emitting material for the fabrication of light-emitting electrochemical cells (LECs). These complexes have been synthesized, employing 2-(4-ethyl-2-pyridyl)-1H-imidazole (EP-Imid) as the ancillary ligand, 2-phenylpyridine (ppy) and 2-(2,4-difluorophenyl)pyridine (dfppy) as the cyclometalated ligands, which were characterized by various spectroscopic, photophysical and electrochemical methods. The photoluminescence (PL) emission spectra in acetonitrile solution show blue–green and blue light emission for Complexes 1 and 2 respectively. However, LECs incorporating these complexes resulted in green (522 nm) light emission for Complex 1 with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.33, 0.56) and blue–green (500 nm) light emission for Complex 2 with the CIE coordinates of (0.24, 0.44). Using Complex 1, a maximum luminance of 1191 cd m⁻² and current efficiency of 1.0 cd A⁻¹ are obtained while that of Complex 2 are 741 cd m⁻² and 0.88 cd A⁻¹ respectively.     

Color stable white organic light-emitting diode based on a novel triazine derivative

A novel red–orange emitting material with a branched molecular structure, 2,4,6-tris[2-(N-ethyl-3-carbazole)carboxethenyl]-1,3,5-s-triazine (TC3), has been synthesized and characterized using UV–visible, photoluminescence (PL) and electroluminescence (EL) spectroscopy. White EL devices were fabricated using TC3 as a red–orange emitter and 8-hydroxyquinolinolato lithium (Liq) as a blue–green emitter. N,N-bis(3-methylphenyl)-N,N-diphenylbenzidine (TPD) as the adjustor for charge carrier mobility was introduced between the two emitting layers to improve the stability of the white emission color on bias voltage. The EL devices of ITO/poly(N-vinylcarbazole) (PVK):TC3 (56 nm)/TPD (5 nm)/Liq (30 nm)/Mg:Ag exhibited good quality white emission. The Commission Internationale De L’Eclairage chromaticity coordinates are (0.34, 0.39) and are stable on the bias voltage.     

Blue light-emitting OLED using new spiro[fluorene-7,9′-benzofluorene] host and dopant materials

A new spiro-type compound, 2-(10-biphenylanthracene)-spiro[fluorene-7,9′-benzofluorene] (BH-3B) containing anthracene moiety was prepared for the blue host material. Also new dopant materials, 2-[4′-(phenyl-4-vinylbenzeneamine)phenyl-spiro[fluorene-7,9′-benzofluorene] (BH-3BD) and 4-[2-naphthyl-4′(phenyl-4-vinylbenzeneamine)]phenyl (BD-1N) were successfully synthesized and a blue OLEDs were made from them. The structure of the device was as follows; ITO/DNTPD/α-NPD/Host:5% dopant/Alq₃/Al-LiF. Among all of the devices, the device obtained from BH-3B host doped with 5% BH-3BD showed the best electroluminescence characteristics. The emission peak of EL is at 456 nm and the CIE value is (0.15, 0.14). The brightness of the device is up to 5407 cd/m² at 10 V with the maximum EL efficiency of 3.4 cd/A.     

Extremely low driving voltage electrophosphorescent green organic light-emitting diodes based on a host material with small singlet–triplet exchange energy without p- or n-doping layer

In order to achieve low driving voltage, electrophosphorescent green organic light-emitting diodes (OLEDs) based on a host material with small energy gap between the lowest excited singlet state and the lowest excited triplet state (ΔE_ST) have been fabricated. 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a] carbazole-11-yl)- 1,3,5-triazine (PIC–TRZ) with ΔE_ST of only 0.11 eV has been found to be bipolar and used as the host for green OLEDs based on tris(2-phenylpyridinato) iridium(III) (Ir(ppy)₃). A very low onset voltage of 2.19 V is achieved in devices without p- or n-doping. Maximum current and power efficiencies are 68 cd/A and 60 lm/W, respectively, and no significant roll-off of current efficiency (58 cd/A at 1000 cd/m² and 62 cd/A at 10,000 cd/m²) have been observed. The small roll-off is due to the improved charge balance and the wide charge recombination zone in the emissive layer.     

Surfactant free hydrothermally derived ZnO nanowires,nanorods, microrods and their characterization

ZnO nanowires, nanorods and microrods have been prepared by an organic-free hydrothermal process using ZnSO₄ and NaOH/NH₄OH solutions. The powder X-ray diffraction (PXRD) patterns reveal that the ZnO nano/microrods are of hexagonal wurtzite structure. The Fourier transform infrared (FT-IR) spectrum of ZnO powder shows only one significant spectroscopic band at around 417 cm^?1 associated with the characteristic vibrational mode of Zn–O bonding. The thickness 75–300 nm for ZnO nanorods and 0.2–1.8 μm for microrods are identified from SEM/TEM images. UV–visible absorption spectra of ZnO nano/microrods show the blue shift. The UV band and green emission observed in photoluminescence (PL) spectra are due to free exciton emission and singly ionized oxygen vacancy in ZnO. Finally, the mechanism for organic-free hydrothermal synthesis of the ZnO nano/microrods is discussed.     

Color stable multilayer all-phosphor white organic light-emitting diodes with excellent color quality

The color stability of all-phosphor white organic light-emitting diodes (WOLEDs) is crucial and remains a challenge that must be overcome before the wide application of phosphor WOLEDs technology. Besides, color stable all-phosphor WOLEDs should also offer high color rendering index (CRI) and ideal correlated color temperature (CCT) simultaneously to make the technology competitive against other alternative technologies such as inorganic LEDs. In this work, we demonstrate a series of color stable all-phosphor WOLEDs with two emitters (blue and yellow), three emitters (blue, green/red, and yellow) and four emitters (blue, green, yellow and red) by introducing tris (phenylpyrazole) Iridium [Ir(ppz)₃] as interlayer. The results show that appropriate thickness of Ir(ppz)₃ interlayer not only can control exciton distribution in the emission zone, but also can improve the spectra stability. In particular, one efficient four-color device with double-interlayer yields fairly high CRI of 92 and 90, ideal CCT of 3703 K and 3962 K at illumination-relevant luminance of 100 cd m^–2 and 1000 cd m^–2, respectively, which is very appropriate to indoor lighting application. By further employing appropriate hosts to regulate the carrier injection, ultrahigh stable four-color devices with applicable CRI are finally achieved.     

A novel dimesitylboron-substituted indolo[3,2-b]carbazole derivative: Synthesis,electrochemical, photoluminescent and electroluminescent properties

A novel indolo[3,2-b]carbazole derivative containing B(Mes)₂ groups, 5,11-dibutyl-2,8-bis(dimesitylboryl) indolo[3,2-b]carbazole (DBDMBICZ), was synthesized and structurally characterized by elemental analysis, NMR, MS. The thermal, electrochemical and photophysical properties of DBDMBICZ were characterized by thermogravimetric analysis, electrochemical methods, UV–vis absorption spectroscopy and fluorescence spectroscopy. DBDMBICZ exhibited high fluorescence quantum yields (Φ_max = 0.76) in solution and excellent thermal stability (T_d = 290 °C, T_g = 170 °C) and electrochemical stability. The multi-layered OLEDs devices with the configuration of ITO/NPB/CBP/light-emitting layer/Bphen/LiF/Al are fabricated by using DBDMBICZ as light-emitting layer. The devices show the same pure blue emissions at different voltages and relative good electroluminescent performances. The results indicate that DBDMBICZ has potential applications as an excellent optoelectronic material in optical field.     

Fluorene trimers with various 9,9′-substituents: The synthesis,characteristics, condensed state structures,and electroluminescence properties

The four fluorene-based trimers with various aromatic and alkyl substituents (T1–T4) are synthesized and characterized. These oligomers show the similar electronic absorption and emission characteristics (e.g., absorption peak at 351 nm, and highly efficient deep blue emission at 394 nm in solution), indicating that the major electronic properties of the core chromophore are essentially independent of the substituents. However, the condensed state structures and thermal properties of four trimers are found to be different from each other, from crystalline (full alkyl (T1) or full aromatic (T2) substituted trimers) to amorphous (mixed aromatic and alkyl (T4) substituted trimers). The effect of different condensed state structures on electroluminescence device properties is presented: The blue light-emitting devices with accordant structure of ITO/PEDOT:PSS/TCTA (40 nm)/trimers (40 nm)/BCP (10 nm)/Alq₃ (20 nm)/LiF/Al exhibit different EL efficiency (2.9% of T2, 1.8% of T3 and 2.7% of T4). Using amorphous T4, the white light-emitting device of ITO/TCTA (40 nm)/rubrene (0.1 nm)/T4 (8 nm)/Alq₃(52 nm)/LiF/Al is fabricated with high efficiency (6.15 cd A⁻¹), high brightness (9500 cd m⁻²) and good white light CIE coordinates (0.32, 0.37).     

Blue/green luminescence based on Zn(S)Se/GaAs heterostructures

Substantial advances have been realized in the aim to achieve blue–green light emitting devices based on Zn(S)Se wide band gap II–VI semi-conductor materials. Two light emitting diodes p on n and n on p heterostructures were grown on GaAs substrate by molecular beam epitaxy. The active layer was a single ZnCdSe quantum well, with ZnSSe guiding layers and ZnSe cladding layers. p-GaInP, p-AlGaAs and p-CdZnSe buffer layers were deposited at the p-ZnSe/GaAs interface to reduce the valence band offset in the case of n on p heterostructures. Electrical and optical properties were investigated using current voltage, capacitance voltage, electroluminescence, photoluminescence and photocurrent measurements at room temperature. Blue–green luminescence centered at 516.7 nm is observed. The highest luminescence intensity is observed under 7 V forward bias. Photoluminescence spectrum shows two wide peaks at 2.2 and 1.9 eV energies. These energies are attributed to the transitions between ZnSe and GaAs conduction bands and the deep level at E_v−0.6 eV. Absorption process from ZnSe and ZnSSe conduction bands to the shallow nitrogen acceptor level (2.6 and 2.8 eV, respectively) have been observed using photocurrent measurements. From these results we present a band alignment diagram which confirms the presence of the two levels at 0.1 and 0.6 eV from the valence band of ZnSe.     

A flexible moisture barrier comprised of a SiO2-embedded organic–inorganic hybrid nanocomposite and Al2O3 for thin-film encapsulation of OLEDs

We demonstrated a high performance flexible multi-barrier containing a silica nanoparticle-embedded organic–inorganic hybrid (S–H) nanocomposite and Al₂O₃. The multi-barrier was prepared by low-temperature Al₂O₃ atomic layer deposition and with a spin-coated S–H nanocomposite. The moisture barrier properties were investigated with a water vapor transmission rate (WVTR), estimated by a Ca test at 30 °C, 90% R.H.. Moisture diffusion was effectively suppressed by the sub-700 nm thick multi-barrier incorporating well-dispersed silica nanoparticles in the organic layer. A low WVTR of 1.14 × 10^?5 g/m² day and average transmittance of 85.8% in the visible region were obtained for the multi-barrier. After bending under tensile stress mode, the moisture barrier property of the multi-barriers was retained. The multi-barrier was successfully applied to thin-film encapsulation of OLEDs. The thin-film encapsulated OLEDs showed practicable current–voltage–luminance (I–V–L) characteristics and stable real operation over 700 h under ambient conditions.