Hybrid organic–inorganic light emitting diodes

Specific features of plane-geometry nanocrystals (CdSe nanoplatelets) functioning in an organic–inorganic light emitting diode are analyzed.     

Two-layer light emitting diodes prepared by the sol–gel route

We have elaborated organic–inorganic hybrid light-emitting diodes (HLED). These devices emitting in the green are formed of two hybrid thin layers, exhibiting different functionalities, which are sandwiched between indium–tin oxide (ITO) and metallic electrodes. These layers have been prepared from silane precursors modified with hole transporting units and light-emitting naphthalimide moieties by the sol–gel technique. The hole transporting sol–gel layers exhibit about the same charge mobility as organic polymers having equivalent active units. The maximum external quantum efficiency of the best diode using LiF/Al cathode is about 1% and the luminance reaches 4000 cd · m ⁻² .     

Er–Pr doped tellurite glass nanocomposites for white light emitting diodes

In this paper, optical glass nanocomposites (nanoparticles sizes up to 100 nm) with composition TeO₂–WO₃–PbO–xEr₂O₃–yPr₆O₁₁ (x = 0.30 mol%, y = 0.70 mol%) embedded into polymer matrices was reported. The two types of polymers chosen for present study were: photopolymer oligoetheracryalte (OEA) and polymethylmethacrylate (PMMA), respectively. The incorporation of the titled nanoparticles into the polymer matrices is analyzed optically. The fluorescence spectra of the nanocomposites were compared with the fluorescence spectra of bulk glasses. Based on the comparison of Er^3 + and Pr^3 + ions' energy level schemes, possible energy transfer processes were identified. The prepared glasses are promising candidates for the white light emitting diodes applications.     

Alternating current organic light emitting diodes based on polymer heterojunction

Most alternating current (ac) polymer EL (electroluminescent) devices to date are based on symmetrical structure. Here novel alternating current EL devices with asymmetric structure are successfully fabricated by using a hole type polymer PDDOPV [poly (2,5-bis (dodecyloxy)-phenylenevinylene)] and an electron type polymer PPQ [poly (phenyl quinoxaline)]. We report that performance of polymer devices with heterojunction in ac operation is not so sensitive to thickness of the two polymer layers as in direct current (dc) operation. This new advantage of ac operation mode over dc means easy production and cheap facilities in large-scale production in the near future. Different emission spectra are obtained when our ac devices operate in ac mode, forward and reverse bias. Emission spectrum at reverse bias includes two parts: one is from PDDOPV, the other is from PPQ.     

Synthesis and photoluminescence properties of?novel red-emitting phosphors for light emitting diodes

The novel red-emitting phosphors K₂Ba_1−x (MoO₄)₂: xEu³⁺(0.02≤x≤0.15) phosphors were prepared by solid-state reaction and their crystal structures, photo luminescent characteristics were investigated. The results show that all samples can be efficiently excited by UV (396 nm) and blue (466 nm) light, which are coupled well with the characteristic emission from UVLED and blue LED, respectively. Their emission spectra show intense red emission at 616 nm with line spectra due to the ⁵D₀–⁷F₂ transition of Eu³⁺. The XRD and photoluminescence experimental results indicate that the K₂Ba(MoO₄)₂: Eu³⁺ phosphor crystallization optimum annealing temperature occurs at about 800°C. The optimum doping concentration of Eu³⁺ is 0.10 mol, and the critical transfer distance (Rc) among Eu³⁺ ions is calculated to be about 11.126 ?. The approach to charge compensation was used: Ba²⁺→Eu³⁺+X⁻ (X=F, Cl, Br), and the charge compensation influence on the luminescent intensity of phosphors is investigated.     

Internal quantum efficiency drop induced by the heat generation inside of light emitting diodes （LEDs）

The reasons for low output power of AlGaInP Light Emitting Diodes (LEDs) have been analysed. LEDs with AlGaInP material have high internal but low external quantum efficiency and much heat generated inside especially at a large injected current which would reduce both the internal and external quantum efficiencies. Two kinds of LEDs with the same active region but different window layers have been fabricated. The new window layer composed of textured 0.5 μm GaP and thin Indium-Tin-Oxide film has shown that low external quantum efficiency (EQE) has serious impaction on the internal quantum efficiency (IQE), because the carrier distribution will change with the body temperature increasing due to the heat inside, and the test results have shown the evidence of LEDs with lower output power and bigger wavelength red shift.     

Multifunctional silicon-based light emitting device in standard complementary metal–oxide–semiconductor technology

A three-terminal silicon-based light emitting device is proposed and fabricated in standard 0.35 μ m complementary metal--oxide--semiconductor technology. This device is capable of versatile working modes: it can emit visible to near infra-red (NIR) light (the spectrum ranges from 500 nm to 1000 nm) in reverse bias avalanche breakdown mode with working voltage between 8.35 V--12 V and emit NIR light (the spectrum ranges from 900 nm to 1300 nm) in the forward injection mode with working voltage below 2 V. An apparent modulation effect on the light intensity from the polysilicon gate is observed in the forward injection mode. Furthermore, when the gate oxide is broken down, NIR light is emitted from the polysilicon/oxide/silicon structure. Optoelectronic characteristics of the device working in different modes are measured and compared. The mechanisms behind these different emissions are explored.     

Room-temperature 2 μm luminescence from Tm doped silicon light emitting diodes and SOI substrates

Room temperature electroluminescence in the eye safe region of the spectrum over the range 1.7–2.1 μm is demonstrated from a thulium doped silicon diode. The same room temperature photoluminescence can be attained on a silicon-on-insulator substrate. The emission lines are from the first excited to ground state of the Tm³⁺ ion, ³F₄→³H₆. A detailed study has been made to establish the optimum implant and processing conditions for efficient room temperature luminescence. The importance of the correct placement of the thulium ions with respect to the depletion region edge and dislocation loops formed upon boron implantation has been established. Tm³⁺ has been demonstrated to lase in other systems and is the basis of widely applied, commercial, optically pumped 2 μm lasers. The demonstration of electroluminescence in silicon and luminescence on an SOI platform are necessary prerequisites for the potential development of Tm injection lasers and optical amplifiers.     

Optical properties of Fabry–Perot microcavity with organic light emitting materials

We investigated the optical properties of the tris(8-hydroxyquinoline)aluminum (Alq₃) organic film with Fabry–Perot microcavity by measuring photoluminescence (PL) and transmittance. We have simulated the phase change on reflection as a function of wavelength. The Fabry–Perot microcavity structures were designed according to the simulation results and the resonant wavelength corresponding to the maximum of PL spectrum of a bare Alq₃ film. These structures were fabricated in three types of microcavities, such as type A [air|metal|Alq₃|metal|glass], type B [air|dielectric|Alq₃|dielectric|glass], and type C [air|metal|Alq₃|dielectric|glass]. A bare Alq₃ layer on glass, [air|Alq₃|glass], showed a PL peak around 514 nm and its full width at half maximum (FWHM) was about 80 nm. The broad FWHM of the bare Alq₃ film was reduced to 15–27.5, 7–10.5 and 16–16.6 nm for three types by cavity effects. Also, the control of the resonant wavelength can be achieved by the spacer length as well as the phase change on reflection on mirror.     

Optical properties of bulk gallium nitride single crystals grown by chloride–hydride vapor-phase epitaxy

A gallium nitride crystal 5 mm in thickness was grown by chloride–hydride vapor-phase epitaxy on a sapphire substrate, from which the crystal separated during cooling. At an early stage, a three-dimensional growth mode was implemented, followed by a switch to a two-dimensional mode. Spectra of exciton reflection, exciton luminescence, and Raman scattering are studied in several regions characteristic of the sample. Analysis of these spectra and comparison with previously obtained data for thin epitaxial GaN layers with a wide range of silicon doping enabled conclusions about the quality of the crystal lattice in these characteristic regions.     

A theoretical and experimental evaluation of Ⅲ–nitride solar-blind UV photocathode

We have developed a superior solar-blind ultraviolet(UV) photocathode with an Al_xGa_(1-x)N photocathode(x ～ 0.45)in semi-transparent mode, and assessed spectra radiant sensitivity related to practical use. Before being grown over a basal plane sapphire substrate by low-pressure metal organic chemical vapor deposition(MOCVD), a reasonable design was made to the photocathode epitaxy structure, focusing on the Al_xGa_(1-x)N: Mg active layer, then followed by a comprehensive analysis of the structural and optical characterization. The spectra radiant sensitivity is peaked of 41.395 mA/W at wavelength 257 nm and then decreases by about 3 to 4 decades at 400 nm demonstrating the ability of this photocathode for solar-blind application prospects.     

Reactivation of damaged rare earth luminescence centers in ion-implanted metal–oxide–silicon light emitting devices

Charge trapping and quenching of electroluminescence (EL) in SiO₂ layers implanted by Ge and rare earth (RE) ions during hot electron injection were investigated. In case of the SiO₂:Ge layer the EL quenching is caused by the transformation of the luminescent defects (Ge–Si or Ge–Ge) to optically inactive centers during hot electron excitation, whereas the EL from rare earth centers is quenched due to the electron trapping by RE-centers or their surroundings, but not due to their optical deactivation. Therefore, the flash lamp post-injection annealing releasing trapped electrons reactivates RE centers and increases the operating time of metal–oxide–silicon light emitting devices (MOSLEDs). PACS 72.20.Jv; 73.40.Qv; 73.50.Gr     

UV–blue photoluminescence from ZrO2 nanopowders prepared via glycine nitrate process

This work is motivated by heavy-ion irradiation of materials exposed to high hydrostatic pressures in a diamond anvil cell (DAC). Those studies require the complete passage of ions through one of the two anvils consisting of natural diamond. Due to typical anvil lengths of about 2–3 mm, ion energies of a few hundred MeV/u, as provided by the SIS heavy-ion synchrotron of GSI, are needed. Data of energy loss and range of ions in this energy regime being scarce for diamond, a set of experiments was devoted to range measurements by irradiating a single DAC diamond and an immediately following stack of 100 polycarbonate foils (each 30-m thick) with ²³⁸U ions of 50.3 GeV (211.5 MeV/u). Ion range and range straggling in the stack were determined by etching the tracks and counting the pores. The experimentally determined ion range in the polymer foil stack is consistent with energy-loss values in diamond and polymer obtained with the SRIM and ATIMA computer codes and also confirms the reliability of these codes for diamond. Therefore, SRIM and ATIMA will be used in further experiments aimed at heavy-ion irradiation of materials pressurized in DACs. PACS 07.35.+k; 61.80.Jh; 61.85.+p; 81.05.Uw     

Ultra-high-speed single red–green–blue light-emitting diode-based visible light communication system utilizing advanced modulation formats

We report a 3.75-Gb/s visible light communication(VLC) system thatusessingle-carrier frequency-domain equalization(SC-FDE) based on a single red–green–blue(RGB) light-emitting diode(LED),with the measured bit error rates(BERs) under a pre-forward-error-correction threshold of 3.8×10-3.The fundamental characteristics of an RGB-LED-based VLC system are measured.We also compare SC-FDE with OFDM in terms of peak-to-average power ratio and BER performance,which shows that SC-FDE outperforms the OFDM modulation scheme.     

Effect of sintering temperature on luminescence properties of borosilicate matrix blue–green emitting color conversion glass ceramics

The color conversion glass ceramics which were made of borosilicate matrix co-doped(SrBaSm)Si_2O_2N_2:(Eu~(3+)Ce~(3+)) blue–green phosphors were prepared by two-step method in co-sintering. The change in luminescence properties and the drift of chromaticity coordinates(CIE) of the(SrBaSm)Si_2O_2N_2:(Eu~(3+)Ce~(3+)) blue–green phosphors and the color conversion glass ceramics were studied in the sintering temperature range from 600℃ to 800℃. The luminous intensity and internal quantum yield(QY) of the blue–green phosphors and glass ceramics decreased with the sintering temperature increasing. When the sintering temperature increased beyond 750℃, the phosphors and the color conversion glass ceramics almost had no peak in photoluminescence(PL) and excitation(PLE) spectra. The results showed that the blue–green phosphors had poor thermal stability at higher temperature. The lattice structure of the phosphors was destroyed by the glass matrix and the Ce~(3+) in the phosphors was oxidized to Ce~(4+), which further caused a decrease in luminescent properties of the color conversion glass ceramics.     

Photoluminescence properties of Ce3＋ and Mn2＋-activated Ba9Sc2Si6O24 phosphor for white light emitting diodes

A single-phased silicate compound (Ba1-xCex)9(Sc1-yMny)2Si6O24 was prepared by solid-state reaction at high temperature. From powder X-ray diffraction (XRD) analysis, the formation of Ba9Sc2Si6O24 with an R3 space group was confirmed. In the photoluminescence spectra under ultraviolet (UV) ray excitation, the Ba9Sc2Si6O24:Ce3+,Mn2+ phosphor emits two distinctive color light bands: a blue one originating from Ce3+and a red one caused by Mn2+. The energy transfer process from Ce3+ to Mn2+ was confirmed, the critical radius as well as the transfer efficiency was calculated, and the energy transfer mechanism was discussed. In addition, the decay-time testing indicates that the energy transfer efficiencies from Ce(1) to Mn2+ and Ce(2) to Mn2+ are different. The emission chromaticity of Ba9Sc2Si6O24:Ce3+,Mn2+ phosphor could be tuned from blue to red by altering the Ce3+/Mn2+ concentration ratio.     

Room temperature hydrogen sensing of multiple networked ZnO/WO3 core–shell nanowire sensors under UV illumination

ZnO/WO₃ core–shell nanowires were synthesized by thermal evaporation of a mixture of ZnO and graphite powders (ZnO:C = 1:1) followed by sputter-deposition of WO₃. The sensing properties of multiple networked ZnO-core/WO₃-shell nanorod sensors toward H₂ gas was examined. The responses of pristine ZnO and ZnO-core/WO₃-shell nanorods to 1000 ppm H₂ at room temperature under UV illumination were ∼236% and ∼645%, respectively. The responses of the core–shell nanowires increased from ∼118 to ∼645% with increasing the UV illumination intensity from 0 mW/cm² to 1.2 mW/cm². The enhanced sensing performance of the ZnO-core/WO₃-shell nanowires induced by encapsulation with WO₃ was explained based on a combination of surface depletion and potential barrier-controlled carrier transport models. The origin of the enhanced sensing properties of ZnO-core/WO₃-shell nanorods toward H₂ under UV illumination was also discussed.     

Enhanced red fluorescence in Sr2Si1−xGexO4:Eu3+ phosphors by the substitution of Si by Ge for white light emitting diodes

Eu³⁺-doped Sr₂Si_1?xGe_xO₄ (x=0–1) phosphors have been prepared by the high temperature solid-state reaction method. The luminescent properties of these phosphors were investigated. Red fluorescence of Eu³⁺ is enhanced gradually in the samples with increasing substitution of Si by Ge upon the excitation of 393 nm light. The intensity is increased by 50% with full substitution of Si by Ge. These results are originated from the structural changes and the phonon energy reduction in the samples due to the substitution of Si by Ge. The CIE chromaticity coordinates of the phosphors vary slightly around (0.62, 0.37) and all are in the red color region. The results indicate that these phosphors could be promising red phosphors for white light emitting diodes.     

Evaluating the coupling strength of electron–hole pairs and phonons in a 0.9 μm-wavelength silicon light emitting diode using dressed-photon–phonons

We investigated the coupling strength between electron–hole pairs and phonons in a silicon light emitting diode (Si-LED) fabricated by dressed-photon-assisted annealing. This Si-LED emitted light in the 1.4 eV photon energy (0.9 μm wavelength) band, and phonon sidebands were observed in the emission spectrum. From a comparison with simulation results, these sidebands were found to be due to coupling of electron–hole pairs with LO-mode and TO-mode coherent phonons via dressed-photon–phonons. The value of the Huang–Rhys factor, $S$ , representing the coupling strength between the electron–hole pairs and the phonons was estimated to be $4.08 \pm 0.02$ .