Pure Green Emission Research Articles

Green upconverted emission enhancement of ZrO2 : Yb3+–Ho3+ nanocrystals

ZrO2 : Yb3+–Ho3+ nanocrystals with different concentrations of codopants were synthesized with a 0.0082 molar ratio of surfactant Pluronic F-127 using a sol–gel micelle method, and annealed at 1000 °C for 5 h. The dominant crystallite phase was tetragonal, with an average crystallite size of 20 nm. The upconverted signal produced by the codoped nanocrystals is an almost pure green emission centred at 540 nm caused by the radiation emitted by the 5F4 + 5S2 → 5I8 relaxation of Ho3+. The highest efficiency for the Ho3+ green emission, with a decay constant (τeff) of 360 µs, was obtained with a 2/0.001 mol% Yb/Ho composition. This yields a colour coordinate of (0.289, 0.699) with a maximum luminance of 8.7 × 10−3 lm after excitation with a 970 nm continuous wave laser diode. These results can be explained in terms of the energy transfer (ET) from the donor (Yb3+) to the acceptor (Ho3+). The ET efficiency (η) for this sample was only 5%, although an increase to 42% is possible for larger concentrations of Ho3+. Highly concentrated samples have shortened decay times, which cause highly saturated samples to exhibit larger ET efficiency values.

Green polymer-light-emitting-diodes based on polyfluorenes containing N-aryl-1,8-naphthalimide and 1,8-naphthoilene-arylimidazole derivatives as color tuner

A novel series of green light emitting single polymers were prepared by end-capping of N-aryl-1,8-naphthalimide and 1,8-naphthoilenearylimidazole derivatives into polyfluorene. The electroluminescence (EL) spectra of polymers (P1∼P5) exhibit greenish-blue, bluish-green, pure green, and yellowish-green emission (λmax=465nm, 490nm, 500nm, and 545nm, respectively) from compounds (M1∼M5). It was found that by the introduction of a small amount of compounds (M1∼M5) (5mol-%) into polyfluorene, the emission color can be tuned from the blue to green region. The color tuning was found to have gone through charge trapping and Förster energy transfer. The device of P4 emits pure green light with Commission Internationale de l'Eclairage (CIE) coordinates of (0.20, 0.41), and exhibits a maximum brightness of 11500cd/m2 at 12V with a structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) [PEDOT:PSS]/PVK/emission layer/Ca/Ag. The device of P5 emits yellowish green light with Commission Internationale de l'Eclairage (CIE) coordinates of (0.36, 0.56), and exhibits a maximum brightness of 6534cd/m2 at 17V.

Tetraalkoxylated-PPV derivative: An efficient and pure green electroluminescent polymer with good solution processability

A new solution-processable tetraalkoxy-substituted poly(1,4-phenylenevinylene) derivative, poly{[2-(3′,7′-dimethyloctyloxy)-3,5,6-trimethoxy]-1,4-phenylenevinylene} (TALK-PPV), was synthesized through a dehydrohalogenation polymerization route, and its light-emitting properties were investigated. The TALK-PPV showed highly blue-shifted UV–visible absorption and PL emission spectra compared to the dialkoxy-substituted PPV derivatives. This is because of the disturbance to the π-conjugation caused by a steric hindered structure. The TALK-PPV thin film exhibited an absorption peak at 446nm, with an onset at 515nm. Its PL emission maximum was at 554nm. Cyclic voltammetric analysis showed the HOMO and LUMO energy levels of the TALK-PPV to be 5.77 and 3.36eV, respectively. Light-emitting devices were fabricated with an ITO (indium-tin oxide)/PEDOT/polymer/Ca/Al configuration. The TALK-PPV component leads to pure green light emission with a CIE 1931 chromaticity of (0.20, 0.74) at 100cd/m2 brightness, which is very close to the standard green (0.21, 0.71) demanded by the NTSC (National Television System Committee). The maximum brightness of this device was 24,900cd/m2 with an efficiency of 1.45cd/A.

Highly efficient green organic light-emitting diodes from single exciplex emission

Spectral single and stable green exciplex emission was demonstrated from organic light-emitting diodes (OLEDs) with 4,4′,4″-tris[3-methylphenyl(phenyl)amino] triphenylamine and 4,7-diphenyl-1,10-phenanthroline that function as electron donor (D) and acceptor (A), respectively. As 8-hydroxyquinoline aluminum (Alq3) was attached to the acceptor layer, electroluminescent (EL) properties of the two exciplex-type OLEDs with D/A-bilayer and D:A mixture layer configurations were markedly improved, i.e., a peak current efficiency of 7.6cd∕A at 2.38mA∕cm2 in three-layer device and a maximum luminance of 6620cd∕m2 at 8.7V in blend layer device were obtained, respectively, without changing the peak position (535nm) and the shape of EL spectrum. Discussion is given on the harvest of the pure green exciplex emission and enhancement of luminance which is obtained by inserting Alq3 layer.

Preparation, photo- and electro-luminescent properties of a novel complex of Tb (III) with a tripod ligand

A novel Tb(III) complex TbL (L = tris[2-(2-carboxyphenoxy)ethyl]amine, H 3L) was synthesized and characterized by means of elemental analyses, IR spectra, thermal analyses, and molar conductivity measurement. The photoluminescent properties of the complex were investigated. In addition, PVK doping Tb(III) complex was fabricated as the emissive layer by spin-coating and its electroluminescent properties were studied, in which the structure of the device was ITO (indium tin oxide)/PVK (polyvinylcarbazole)/PVK: TbL/PBD (2-(4-biphenylyl)-5-(4- tert-butylphenyl)-1,3,4-oxadiazole)/LiF/Al. It was indicated that pure green and narrow bandwidth emission at 545 nm from photoluminescence of TbL complex film and the organic electroluminescent device is the characteristic emission of Tb(III) ion, and the electroluminescence spectrum of the device was very similar to that of the photoluminescence of TbL complex film. The lowest triplet level of the ligand was calculated from the phosphorescence spectrum of GdL in N, N-dimethyl formamide (DMF) dilute solution determined at 77 K, and the energy transfer mechanisms in TbL complex were discussed.

Electroluminescent devices based on rare-earth complex TbY(p-MBA) 6(phen) 2

A new rare-earth complex TbY(p-MBA) 6(phen) 2 was synthesized and studied. Pure green emission was observed from the devices that were fabricated with the structure as follows: Device 1: ITO/PVK: TbY(p-MBA) 6(phen) 2/LiF/Al; Device 2: ITO/PVK: TbY(p-MBA) 6(phen) 2/AlQ/LiF/Al. PVK was used to improve the film-forming and hole-transporting properties of the TbY(p-MBA) 6(phen) 2. The characteristics of these devices were investigated and compared with Tb(p-MBA) 3phen, which had been reported. The results show that TbY(p-MBA) 6(phen) 2 has much better performance than Tb(p-MBA) 3phen. This improvement is attributed to the introduction of yttrium. The highest EL brightness of Device 1 is 37.5 cd/cm 2 at a fixed bias of 19 V, and the highest EL brightness of Device 2 reaches 333.7 cd/cm 2 at 23 V.

Conjugated random and alternating 2,3,4,5-tetraphenylsilole-containing polyfluorenes: Synthesis, characterization, strong solution photoluminescence, and light-emitting diodes

A novel series of soluble conjugated random and alternating copolymers derived from 9,9-dioctylfluorene (FO) and 1,1-dimethyl-2,3,4,5-tetraphenylsilole (PSP) were synthesized by Suzuki coupling reactions. The feed ratios of FO to PSP were 95:5, 90:10, 85:15, 70:30, and 50:50. Chemical structures and optoelectronic properties of the copolymers were characterized by elemental analysis, NMR, UV absorption, cyclic voltammetry, photoluminescence (PL), and electroluminescence (EL). The elemental analyses of the copolymers indicated that FO and PSP contents in the copolymers were very close to that of the feed compositions. Unlike the weak PL emission of PSP small molecules in a solution, PFO-PSP solutions could emit strong lights with PL quantum yields between 13 and 30%, indicating that the incorporation of the PSP into the conjugated rigid main chain could greatly restrict the rotations of the phenyl groups of the PSP units even in a solution. Compared with the solution PL, complete PL excitation energy transfer from the PFO segments to the PSP units could be achieved by film PL at lower PSP content. The films of the copolymers exhibited high absolute PL quantum yields between 55 and 84%. EL devices with a configuration of ITO/PEDOT/PFO-PSP/Ba/Al demonstrated that the PSP units could serve as powerful exciton traps, giving exclusively pure green EL emissions. A maximum external quantum efficiency of 1.51% was achieved using the PFO-PSP15 as the emissive layer.

Enhanced Blue and Green Upconversion in Hydrothermally Synthesized Hexagonal NaY1‐xYbxF4:Ln3+ (Ln3+: Er3+ or Tm3+).

AbstractFor Abstract see ChemInform Abstract in Full Text.

Enhanced blue and green upconversion in hydrothermally synthesized hexagonal NaY 1− xYb xF 4:Ln 3+ (Ln 3+ = Er 3+ or Tm 3+)

Hexagonal NaY 1− x Yb x F 4:Ln 3+ ( x=0.05–1.00, Ln 3+=Er 3+ or Tm 3+) have been prepared through hydrothermal synthesis. Powder X-ray diffraction (XRD) patterns have been presented to characterize the synthesized samples. The concentration of doped rare earth and pumping power on the upconversion emissions have been extensively investigated under 980 nm excitation. NaY 1− x Yb x F 4:Er 3+ emit green ( 2 H 11/2 , 4 S 3/2→ 4 I 15/2 ), red ( 4 F 9/2→ 4 I 15/2 ), weak violet ( 2 H 9/2→ 4 I 15/2 ) and weak UV ( 4 G 11/2→ 4 I 15/2 ) upconversion emission. NaY 1− x Yb x F 4:Tm 3+ exhibit intense blue ( 1 G 4→ 3 H 6 , 1 D 2→ 3 F 4 ), weak red ( 1 G 4→ 3 F 4 ) and UV ( 1 D 2→ 3 H 6 ) upconversion emission. A two-photon process account for the green and red emission of an Er 3+ ion with the energy transfer from Yb 3+ ion. One blue emission ( 1 G 4→ 3 H 6 ) of Tm 3+ results from a three-photon excitation, and the other ( 1 D 2→ 3 F 4 ) from a four-photon process. The measured data about emission intensity suggest that hexagonal NaYF 4:Yb 3+/Er 3+ is a more efficient upconverting phosphor than cubic one. Comparison of the measured properties in our work with values from the reported literature suggests that hydrothermally synthesized NaYF 4:Yb 3+/Er 3+ (or Tm 3+) samples emit more pure green or blue emission than solid-state synthesized samples with certain concentration of doping rare earth ions.

Synthesis and Characterization of a New Light-Emitting Fluorene−Thieno[3,2-b]thiophene-Based Conjugated Copolymer

A new alternating polyfluorene copolymer, poly(9,9‘-dioctylfluorene-alt-thieno[3,2-b]thiophene) (PFTT), containing a thiophene-condensed thieno[3,2-b]thiophene moiety has been synthesized via a palladium-catalyzed Suzuki coupling reaction. The synthesized polymer was successfully characterized by 1H NMR, 13C NMR, and elemental analysis. It shows good thermal stability and displays unique phase transition behavior between the crystalline and liquid-crystalline states. The ionization potential and electron affinity of PFTT are −5.38 eV and −2.40 eV, respectively, as determined by cyclic voltammetry. Thus, PFTT has an electrochemical band gap of approximately 2.98 eV, which is smaller than that of common polyfluorene (PF) homopolymers. As a film, PFTT exhibits UV−vis and photoluminescence maxima at 471 and 511 nm, respectively. A light-emitting diode device fabricated with an ITO/PEDOT/PFTT/LiF/Al configuration exhibits pure green light emission with the full width at half-maximum (fwhm) of only 57 nm and a low turn-on voltage of 3.3 V. Especially, this emission has the CIE coordinates (0.29, 0.63), which are very close to the standard for green used by the National Television System Committee. In addition, PFTT exhibits better electroluminescence performance than other similar PF homopolymers and than fluorene- and thiophene-based copolymers.

Dependence of structural and luminescent characteristics of Y2O3:Er thin film phosphors on substrate

The dependence of the structural and cathodoluminescent (CL) characteristics of green-emitting Y2O3:Er thin films phosphors deposited by electron beam evaporation on substrate used for the Y2O3:Er thin film deposition has been investigated. Quartz glass, ITO glass and c-axis-oriented ZnO thin film substrates were used as the substrate. Best crystallinity and CL luminance were obtained from the Y2O3:Er thin film deposited on the ZnO thin film substrate. One of the reason for this fact might be due to high conductivity the substrate. The other might be due to better crystallinity of Y2O3:Er thin film deposited on the ZnO thin film substrate than that on the quartz and ITO substrates, because of the best crystallinity of the ZnO thin film substrate among three substrates. The Y2O3:Er thin film deposited at 500°C on the ZnO thin film and annealed at 800°C for 1h in O2 flow showed CL luminance of 70cd/m2 with pure green emission under excitation by electron beam with 3kV, 60μA/cm2.

Phosphorescent top-emitting organic light-emitting devices with improved light outcoupling

A dielectric capping layer has been used to increase the light output and to tune the spectral characteristics of top-emitting, phosphorescent organic light-emitting devices (OLEDs). By controlling the thickness of the dielectric layer deposited on top of a thin metal cathode, the transmittance of the top electrode can be adjusted. Maximum light output is not achieved at highest cathode transmittance, indicating that the interplay between different interference effects can be controlled by means of the capping-layer thickness. Furthermore, we demonstrate that the electrical device characteristic is not influenced by the capping layer. The strength of our concept in particular lies in the fact that the optical and the electrical device performance can be optimized separately. Using the capping-layer concept, we have achieved an OLED efficiency of 64 cd/A with pure green emission.

PURE GREEN AND NARROW BANDWIDTH ELECTROLUMINESCENCE FROM ORGANIC LIGHT-EMITTING DIODES 

Pure green and narrow bandwidth emission from organic bilayer electroluminescent devices were presented by using Tb3+∶(SA)3 as hole-transporting layer and emissive layer, and high fluorescent material Alq3 acting as electron-transporting layer. It proves that the emission comes from carriers' recombination after their tunneling through the inner interface (Tb3+∶(SA)3/Alq3) in the bilayer ITO/Tb3+∶(SA)3(40nm)/Alq3(55 nm)/Al diode. The spectrum of the bilayer devices varies with the thickness of the electron-transporting layer. Keeping the thickness of Tb3+∶(SA)3 layer constant, there is no emission from Alq3 layer when Alq3 layer reduces to 12 nm; meanwhile, pure green and narrow bandwidth emission can be observed. Thus, it provides a simple way to obtain high luminance and long-term stability by reducing electron- or hole-transporting layer, especially, by using a lanthanide complex for emissive layer.

Current status and future prospects of ZnSe-based light-emitting devices

Device lifetime has been improved to over 100 h by reducing dark-spot density (DSD) to less than 3×10 3 cm −2. The lifetime of II–VI laser diodes is no longer limited by a rapid degradation but by gradual degradation due to microscopic point defects. Lifetime has been significantly improved up to ∼500 h at 20°C under CW operation with LDs grown in Se-rich conditions for the active layer. If we know the growth-condition dependence of degradation behaviors, it may be possible to improve the reliability of ZnSe-based LDs. ZnSe-based LDs are being developed for applications such as display systems and printing systems utilizing their pure green emission.

An organic electroluminescent dot-matrix display using carbon underlayer

A carbon layer has been inserted between ITO and a hole-transport layer in an organic electroluminescent (EL) device with the aim of improving its stability and efficiency. The structure of the proposed device is ITO/carbon/hole-transporting material/Alq3 + dopant/cathode (Al-Li). The carbon layer is quite effective for improving the threshold voltage and the contrast. These advantages are explained in terms of ionization potentials, effectiveness in adhesion and its light absorption. By using the carbon layer, an EL display panel with 16 × 96 dots has been fabricated. The display exhibits excellent performance: a half decay period of 4000 h at an initial luminance of 500 cd cm −2 and strong and pure green emission. In addition, no dark spots are detectable during the operation.

High-Efficiency ZnCdSe/ZnSSe/ZnMgSSe Green Light-Emitting Diodes

Green light-emitting diodes (LEDs) were fabricated employing a ZnCdSe/ZnSSe triple quantum-well (TQW) active region surrounded by ZnMgSSe cladding layers grown on an n-type (100) GaAs substrate by molecular beam epitaxy (MBE). A 3.5 mW pure green emission was observed for the surface-emitting LED device at a peak wavelength of 513.3 nm (2.415 eV) with a spectral half-width of 11.7 nm (55 meV) under a 20 mA (4.6 V) direct current at room temperature (25°C). These correspond to an external quantum efficiency of 7.2%, a power conversion efficiency of 3.8%, a luminous current efficiency of 66 lm/A, and a luminous efficiency of 14 lm/W.

Bright Pure Green Emission from N-free GaP LED's

The pure green emission with high external efficiency of 0.12% has been obtained in nitrogen free GaP LED grown by applying the optimum phosphorus pressure at a constant temperature solution growth. The high efficiency N-free LED has a p+n- structure and the peak emission wavelength is 5560 Å at room temperature. The minority carrier lifetime is 1.4 µsec. The crystalline quality has been improved under the optimum phosphorus pressure and the concentration of deep levels in GaP decreases. Therefore, the minority carrier lifetime becomes longer, and the high efficiency pure green GaP LEDs have been produced.

Effects of Vapor Pressure on GaP LED's

The pure green emission with high external quantum efficiency of 0.03% is reported in the GaP diodes grown by the temperature difference method under controlled vapor pressure. The emission and photocapacitance spectra show remarkable variations with phosphorus pressure. The concentrations of deep levels are suppressed by an application of the optimum phosphorus pressure. Also, in nitrogen-doped diode, it has become clear that the density and energy of nitrogen levels in GaP depend largely on phosphorus pressure applied.