Synthesis and electroluminescence properties of benzothiazole derivatives

Benzothiazole-based blue fluorescent materials N-(4-(benzo[d]thiazol-2-yl)phenyl)-N-phenylbenzenamine (BPPA) and N-(4-(benzo[d]thiazol-2-yl)phenyl)-N-phenylnaphthalen-1-amine (BPNA) were synthesized for use in organic light-emitting diodes (OLEDs). Electroluminescent device with a configuration of ITO/NPB/BPPA/BCP/Alq₃/LiF/Al showed a maximum brightness of 3760 cd/m² at 14.4 V with the CIE coordinates of (0.16, 0.16). A current efficiency of 3.01 cd/A and an external quantum efficiency of 2.37% at 20 mA/cm² were obtained from this device. Molecules derived from BPPA and BPNA with incorporated dicyanomethylidene, which is a functional group for most red fluorescent molecules, were designed, synthesized and characterized to study the red fluorescence properties of the benzothiazole derivatives.     

Electroluminescent properties of dimeric bis(2-(2′-hydroxyl phenyl)benzthiazolate)zinc (II) complex

A blue shifted photoluminescent emission in bis(2-(2′-hydroxyl phenyl)benzthiazolate)zinc (II) complex, ZBZT, arises out of the dimeric structure, typical of the localized electron density around the non-bridged ligand in the excited state of the complex. An average decay lifetime of 4.8 and 3.0 ns for the ligand and the complex, respectively indicates an energy transfer from the ligand to the metal. A PL quantum efficiency of about ?_ZBZT=0.45 in DMF solution is observed, in comparison to the Alq₃, complex, ?_Alq3=0.116. Semi empirical ZINDO/S-SCF-CI calculations support the dominance of non-bridged ligand moiety in controlling the photoluminescent properties. An unusually broad white light (FWHM ∼220 nm) electroluminescent emission in the two layer device structure brings out the features of an exciplex formation between the active layer ZBZT/TPD interface, which is studied at different current densities. Such a broadened emission is verified for different thicknesses of the active layer substantiating the role of exciplex formation.     

Blue and green-blue-variable light-emitting diodes based on the blend of polymer and organic molecules

Organic light-emitting diodes based on the blend of poly (p-phenylene vinylene) (PPV) derivative and naphthyl-imine–gallium complex have been fabricated by spin-coating method. Blue emission and blue-green variation depending on the ratio of the PPV derivative to the complex and the applied voltage have been observed. The investigation on PL (photoluminescence) and EL (electroluminescence) properties demonstrates that the improvement of the luminescent efficiency is related to the injection balance between holes and electrons, and the color variation is attributed to the variation of the recombination zone. Received: 7 July 1999 / Accepted: 11 October 1999 / Published online: 8 March 2000     

Near-Infrared Emission from Organic Light-Emitting Diodes Based on Copper Phthalocyanine with a Periodically Arranged Alq3：CuPc/DCM Multilayer Structure

We demonstrate near-infrared organic light-emitting devices with a periodically arranged tris（8-quinolinolato）aluminum （Alq3）：copper phthalocyanine （CuPc）/4-（dicyanomethylene）-2-methyl-6-（4-dimethylaminost-yry）-4H-pyran （DCM） multilayer structure. DCM and Alq3 doped with CuPc were periodically deposited. Room-temperature electrophosphorescence was observed at about 1.1 μm due to transitions from the first excited triplet state to the singlet ground state （T1 - S0） of CuPc. In this device, we utilize the overlap between the Q band πr - π^＊ at about 625nm of the absorption spectra of CuPc and the PL spectra of the DCM. The near-infrared emission intensity of the CuPc-doped Alq3 device with DCM increases about 2.5 times larger than that of the device without DCM. We attribute the efficiency enhancement to the better overlap between the PL spectra of DCM and the absorption spectra of CuPc.     

Electroluminescence from a ZnO homojunction device grown by pulsed laser deposition

A ZnO homojunction light emitting device was grown on n+ GaAs substrate by pulsed laser deposition. As-doped ZnO film by diffusion of As from the substrate was used for the p-type side and Al-doped ZnO film for the n-type side of the device. A distinct electroluminescence emission consisting of a dominant emission peak at ∼2.5 eV and a weak shoulder centered at ∼3.0 eV was observed at room temperature. The I-V characteristic of the ZnO homojunction showed a good rectifying behavior with a turn-on voltage of ∼4.5 V and a reverse breakdown voltage of ∼9 V.     

Optimized Performances of Thick Film Organic Lighting-Emitting Diodes

The performance of organic light-emitting diodes (OLEDs) with thick film is optimized. The alternative vanadium oxide (V₂O₅) and N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) layers are used to enhance holes in the emissive region, and 4,7-dipheny-1,10-phenanthroline (Bphen) doped 8-tris-hydroxyquinoline aluminium (Alq₃) is used to enhance electrons in the emissive region, thus ITO/V₂O₅ (8nm)/NPB (52nm)/V₂O₅ (8nm)/NPB (52nm)/Alq₃ (30 and 45nm)/Alq₃:Bphen (30wt%, 30 and 45nm)/LiF (1nm)/Al (120nm) devices are fabricated. The thick-film devices show the turn-on voltage of about 3V and the maximal power efficiency of 4.5lm/W, which is 1.46 times higher than the conventional thin-film OLEDs.     

Studies of luminescence properties of ZnO and ZnO:Zn nanorods prepared by solution growth technique

Pyramidal ZnO nanorods with hexagonal structure having c-axis preferred orientation are grown over large area silica substrates by a simple aqueous solution growth technique. The as-grown nanorods were studied using XRD, SEM and UV-vis photoluminescence (PL) spectroscopy for their structural, morphological and optical properties, respectively. Further, the samples have also been annealed under different atmospheric conditions (air, O₂, N₂ and Zn) to study the defect formation in nanorods. The PL spectra of the as-grown nanorods show narrow-band excitonic emission at 3.03 eV and a broad-band deep-level emission (DLE) related to the defect centers at 2.24 eV. After some mild air annealing at 200 °C, fine structures with peaks having energy separation of ∼100 meV were observed in the DLE band and the same have been attributed to the longitudinal optical (LO) phonon-assisted transitions. However, the annealing of the samples under mild reducing atmospheres of N₂ or zinc at 550 °C resulted in significant modifications in the DLE band wherein high intensity green emission with two closely spaced peaks with maxima at 2.5 and 2.7 eV were observed which have been attributed to the V_O and Zn_i defect centers, respectively. The V-I characteristic of the ZnO:Zn nanorods shows enhancement in n-type conductivity compared to other samples. The studies thus suggest that the green emitting ZnO:Zn nanorods can be used as low voltage field emission display (FED) phosphors with nanometer scale resolution.     

Enhanced electroluminescence of ZnO nanocrystalline annealing from mesoporous precursors

An improved sol-gel method was used to prepare ZnO nanoparticles. EL results showed that slowing the addition of LiOH solution and heating in vacuum to obtain gel precipitation made the final ZnO samples’ emission peak blue shift to 520 nm. Simultaneously, the peak value of the sample processed with no templates was enhanced 4.68 times and that of the sample processed with ODA was enhanced 0.71 times. Two copolymers Pluronic P123 (P123) and Pluronic F-127 (F-127) were adopted respectively as template reagents. The obtained mesoporous ZnO precursors exhibited a surface area of 69.21 m²/g and 103.57 m²/g and an average pore size of 6.61 nm and 5.70 nm, respectively. After calcining in a muffle furnace in air, the obtained ZnO nanocrystalline samples from these precursors revealed stronger green emission than the samples dealt with ODA. Compared to the magnification multiple of 0.89 times of the sample processed with ODA, the peak intensity of the sample processed with P123 was 2.03 times higher than that of the sample processed with no template reagents, and the intensity of the sample processed with F-127 was 3.3 times higher. This may be due to the larger surface area of samples from the longer molecule chains of the two template reagents.     

Efficient polymer white-light-emitting diodes with a single-emission layer of fluorescent polymer blend

Efficient polymer white-light-emitting diodes (WPLEDs) have been fabricated with a single layer of fluorescent polymer blend. The device structure consists of ITO/PEDOT/PVK/emissive layer/Ba/Al. The emissive layer is a blend of poly(9,9-dioctylfluorene) (PFO), phenyl-substituted PPV derivative (P-PPV) and a copolymer of 9,9-dioctylfluorene and 4,7-di(4-hexylthien-2-yl)-2,1,3-benzothiadiazole (PFO-DHTBT), which, respectively, emits blue, green and red light. The emission of pure and efficient white light was implemented by tuning the blend weight ratio of PFO: P-PPV: PFO-DHTBT to 96:4:0.4. The maximum current efficiency and luminance are, respectively, 7.6 cd/A at 6.7 V and 11930 cd/m² at 11.2 V. The CIE coordinates of white-light emission were stable with the drive voltages.     

Infrared electroluminescence from erbium-doped spark-processed silicon

The infrared (IR) electroluminescence (EL) of erbium-doped spark-processed silicon (sp-Si) was investigated. For this, a device was constructed which consisted of a silicon wafer on which an erbium layer was vapor deposited, followed by spark-processing and rapid thermal annealing for 15 min at 900 °C in air. The metallization consisted of a 200 nm Ag layer (above the spark-processed area) and a 50 nm thick Al film (on the “back side”), containing a window through which the light could escape. Maximal light emission occurred near 1.55 μm, that is, at a wavelength where commonly used fiber optical materials have their minimum in energy loss. The processing parameters for most efficient light emission were an Er thickness of 200-300 nm, a spark-processing time of about 30 s, an n-type Si wafer having a low (3-5 Ω cm) resistivity, an operating temperature near room temperature, and an operating voltage between 25 and 40 V under reverse bias. The results are interpreted by postulating an energy transfer from sp-Si to the Er³⁺ ions involving the first excited state ⁴I_13/2 to ground state ⁴I_15/2. Further, impact excitation and hot electrons that are accelerated into the erbium doped sp-Si by the applied field (100 kV/cm) are considered.     

Crystallographic and luminescent characterizations of blue-emitting BaAl2S4 : Eu electroluminescent thin films

We characterized the crystallization and luminescence of blue-emitting BaAl₂S₄ : Eu electroluminescent thin films, prepared using switching electron-beam evaporation with two targets. From the photoluminescence intensity and decay profile of the activated Eu²⁺ ions in the BaAl₂S₄ : Eu, we found that the optimum annealing conditions for preparing highly luminescent thin films are a temperature of around 900°C and an annealing time of 2 min. We analyzed the crystalline properties using cross-sectional transmission electron microscope images. Evaluation of the cathodoluminescence spectra in the cross-sections showed that the BaAl₂S₄ : Eu emitting layer was luminously inhomogeneous on the depth of the layer and that the main luminescent area was near the surface of the emitting layer. We discuss here the relationship between the crystalline and luminescent properties.     

Room temperature electroluminescence from ZnO/Si heterojunction devices grown by metal-organic chemical vapor deposition

Heterojunction light-emitting diodes with ZnO/Si structure were fabricated on both high-resistivity (p) and low-resistivity (p⁺) Si substrates by metal-organic chemical vapor deposition technology. Fairly good rectifications were observed from the current-voltage curves of both heterojunctions. Ultraviolet (UV) and blue-white electroluminescence (EL) from ZnO layer were observed only from ZnO/p⁺-Si heterojunction under forward bias at room temperature (RT), while strong infrared (IR) EL emissions from Si substrates were detected from both ZnO/p-Si and ZnO/p⁺-Si heterojunctions. The UV and IR EL mechanisms have been explained by energy band structures. The realization of RT EL in UV-visible and IR region on Si substrate has great applicable potential for Si-based optoelectronic integrated circuits.     

An Electroluminescence Delay Time Model of Bilayer Organic Light-Emitting Diodes

Based on the mechanism of injection, transport and recombination of the charge carriers, we develop a model to calculate the delay time of electroluminescence （EL） from bilayer organic light emitting diodes. The effect of injection, transport and recombination processes on the EL delay time is discussed, and the relationship between the internal interface barrier and the recombination time is revealed. ＂]~he results show that the EL delay time is dominated by the recombination process at lower applied voltage and by the transport process at higher applied voltage. When the internal interface barrier varies from 0.15 eV to 0.3 eV, the recombination delay time increases rapidly~ while the internal interface barrier exceeds about 0.3eV~ the dependence of the recombination delay time on applied voltage is almost undiversified, which may serve as a guideline for designing of a high-speed EL response device.     

Application of indium nanowires to donor-acceptor pair luminescence

A novel, self-supported nanostructured powder electroluminescence (EL) device, consisting of a dilute layer of conductive nanowires, which directly contacts a ZnS phosphor layer and functions as rear electrodes, is reported in this paper. Indium nanowires fabricated by using a porous alumina template and hydraulic pressure injection technique induce a localized field in the phosphor that is much higher than the average applied field and hence excite the phosphor to luminesce. The mechanism for light generation from the nanostructured contact EL device can be understood from the bipolar field-emission model.     

Long-term accelerated current operation of white light-emitting diodes

Long-term degradation tests regarding white light-emitting diodes based on InGaN were performed under accelerated current conditions, and the half-life of the light's output was estimated. An estimated mean half-life of 1.5×10⁴ h was obtained under the recommended 20-mA operating condition. The change in the emission spectrum was found to be slight, and the color quality was considered generally satisfactory over the long term.     

Thermal quenching of electroluminescence assisted by an electric field in thin films of tris-(8-hydroxyquinolinato) aluminum (III)

Temperature dependence of the electroluminescence (EL)-current efficiency of tris-(8-hydroxyquinolinato) aluminum (III) (Alq₃)-based organic light-emitting diodes (OLEDs) operated at a constant current density was investigated. The effects of temperature and electric field on photoluminescence (PL) efficiency of Alq₃ thin layers were also investigated. On the basis of these results, it was found that the EL efficiency decreases more markedly with increasing temperature than does PL efficiency. The temperature dependence of the EL efficiency can be interpreted in terms of the thermal dissociation of excitons that is assisted by the electric field.     

Isotope effect in the linewidth distribution of single-molecule spectra in doped toluene at 2 K

The low-temperature dynamics of two different varieties of a low-molecular glass—protonated toluene (C₆H₅CH₃) and perdeuterated toluene (C₆D₅CD₃)—weakly doped with tetra-tert-butylterrylene (TBT) molecules was investigated with single-molecule spectroscopy. In both glassy matrices the distributions of the spectral widths were measured for a large number of single TBT molecules at T=2 K. The marked shift of the distribution upon deuteration of matrix was observed and attributed to an isotope effect which is qualitatively analyzed. It was found that at this temperature the tunneling dynamics of the matrix dominates the broadening behavior and hydrogen/deuterium atoms are involved in the tunneling motions.     

Two modes of electroluminescence from single‐walled carbon nanotubes

The electroluminescence from single‐walled carbon nanotube field effect transistors is spectrally resolved, and shows two distinct modes of light emission. The vast majority of nanotubes have spectrally broad emission consistent with the spectrum of blackbody radiation. Much more rarely, superposed on the broad emission is a single narrow (<50 meV) peak which is consistent with expectation for electron–hole recombination. The narrow emission is strong even at lower biases and in general has greater peak intensity than the broadband emission. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Efficient White Light Emission Using a Single Copolymer with Red and Green Chromophores on a Conjugated Polyfluorene Backbone Hybridized with InGaN-Based Light-Emitting Diodes

We report an efficient white-light emission based on a single copolymer/InGaN hybrid light-emitting diode. The single copolymer consists of a conjugated polyfluorene backbone by incorporating 2,1,3-benzothiadiazole （BT） and 4,7-bis（2-thienyl）-2,1,3-benzothiadiazole （DBT） as green and red light-emitting units, respectively. For the single copolymer/InGaN hybrid device, the Commission Internationale de 1＇Eclairage （CIE） coordinates, color temperature Tc and color rendering index Ra at 20mA are （0.323,0.329）, 5960K and 86, respectively. In comparison with the performance of red eopolymer PFO-DBT15 （DOF：DBT=85：15 with DOF being 9＇9- dioctylfluorene） and green copolymer PFO-BT35 （DOF：BT=-65：35） blend/InGaN hybrid white devices, it is concluded that the chemically doped copolymer hybridized device shows a higher emission intensity and spectral stability at a high driving current than the polymer blend.     

Transient behavior of the strong violet electroluminescence of Ge-implanted SiO2 layers

Si-based light emitters will be a key element of future optoelectronics. One of the most promising approaches is Ge implantation into thin SiO₂ films on crystalline Si. This system exhibits a strong violet electroluminescence with a power efficiency up to 0.5% [18], but the mechanism of electrical excitation is not yet fully understood. In this paper the electrical excitation of the luminescence centers is investigated by means of electrical and electroluminescence transient measurements. It is found that the most probable way to excite luminescence centers is the impact excitation by hot electrons. Whereas the injection is explained by trap-assisted tunneling of electrons from the substrate into the oxide, the electrons will be transported via traps or in the SiO₂ conduction band. Furthermore, the electroluminescence rise and decay time is estimated to be of the order of 100 μs. Received: 26 September 2001 / Published online: 29 November 2001