Improving light efficiency of white polymer light emitting diodes by introducing the TPBi exciton protection layer

We fabricated and evaluated the efficient white polymer light emitting diode (WPLED) by introducing TPBi exciton protection layer with ITO/PEDOT:PSS/PFO:MEH-PPV/TPBi/LiF/Al structure. PFO and MEH-PPV were prepared by the spin coating as the light emitting host and guest materials. TPBi was used as exciton protection material. The dependences of the MEH-PPV concentrations into the PFO (PFO:MEH-PPV) on the optical and electrical properties of the WPLEDs were investigated. The effect of the introduction of TPBi layer was studied by means of the property comparison between the samples with and without TPBi layer. The maximum luminance with 1480 cd/m² was obtained at the MEH-PPV concentration of 1.0 wt.% for the ITO/PEDOT:PSS/PFO:MEH-PPV/LiF/Al structure. In addition, the maximum luminance and current efficiency of the WPLED with TPBi layer were 7560 cd/m² at 12 V and 7.8 cd/A at 10 cd/m², respectively. The CIE color coordinates for WPLED with 1.0 wt.% MEH-PPV concentration was found to be (x, y) = (0.36, 0.33), showing pure white color.     

Synthesis of a red electrophosphorescent heteroleptic iridium complex and its application in efficient polymer light-emitting diodes

The preparation and characterization of a heteroleptic iridium complex [2-(benzo[b]thiophen-2-yl)pyridine]Ir(III)[2-(4H-1,2,4-triazol-3-yl)pyridine] [(Btp)₂Ir(PZ)] were reported (2-(benzo[b]thiophen-2-yl)pyridine = Btp; 2-(4H-1,2,4-triazol-3-yl)pyridine = PZ). Electrophosphorescence was investigated in the device structure [indium-tin-oxide (ITO)/poly(ethlenedioxythiophene) (PEDOT)/poly(vinylcarbazole)(PVK)/Poly(9,9-dioctylfluorenyl-2,7-diyl) end capped with dimethylphenyl (PFO): (Btp)₂Ir(PZ)/Ba/Al] by using this iridium complex as guest and PFO as host. The red electrophosphorescent devices showed a peak emission at approximately 604 nm and shoulder at 654 nm with the Commission International de'Eclairage (CIE) coordinates of (0.64, 0.35) and external quantum efficiency of 7.7% at a doping concentration of 8 wt.% without an electron-transporting material in the emitting layer.     

Modified 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetra-methyljulolidyl-9-enyl)-4H-pyran-containing red fluorescent emitters for efficient organic light-emitting diodes

Efficient red fluorescent compounds Red 1 and Red 2 based on bulky bicyclo[2,2,2]octane groups in the pyran moiety and tert-butyl or isopropyl group in the julolidine moiety of the 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetra-methyljulolidyl-9-enyl)-4H-pyran (DCJTB) skeleton were synthesized and characterized. As red-emitting dopants in an Alq₃ single-host emitting system, Red 1 and Red 2 exhibited improved color purity and enhanced luminous efficiency compared to DCJTB. Moreover, a device using Red 1 as a dopant in a rubrene-Alq₃ co-host emitting system exhibited improved electroluminescence performance with a luminous efficiency and power efficiency of 6.89 cd/A and 3.09 lm/W at 20 mA/cm², respectively, and CIE x,y coordinates of (x = 0.64,y = 0.36) at 7.0 V, approaching saturated red emission.     

A novel heteroleptic iridium complex with multifunctional ligands used for polymeric light-emitting diodes

A novel iridium complex using 3-(5-(4-(pyridin-2-yl)phenyl)-1,3,4-oxadiazol-2-yl)-9-hexyl-9H-carbazole containing both hole-transporting group and electron-transporting group as main ligand and 2-(5-p-tolyl-2H-1,2,4-triazol-3-yl)pyridine as ancillary ligand was synthesized and investigated in this paper. The iridium complex possesses high solubility, high thermal stability with 5% weight-reduction temperature (ΔT_5%) of 390 °C and glass-transition temperature (T_g) of 218 °C. Polymeric light-emitting diodes (PLEDs) based on structure of ITO/PEDOT: PSS/PVK: PBD: iridium complex/TPBI/CsF/Al fabricated by solution-processed technology were measured. The PLED using the iridium complex as phosphorescent dopant at 4 wt.% doping concentration is optimal which shows the maximum luminance of 7746 cd·cm⁻², maximum current efficiency of 14.0 cd A⁻¹ and maximum external quantum efficiency of 5.8% with CIE coordinates of (0.48, 0.50). The EL emission originates from both monomers and exciplexes formed from the iridium complex and PVK.     

Influence of the polymer matrix on the efficiency of hybrid solar cells based on silicon nanowires

Poly (N-vinylcarbazole) (PVK):SiNWs and poly (2-methoxy, 5-(2-ethyl-hexyloxy)-p-phenyl vinylene) (MEH-PPV):SiNWs bulk-heterojunctions (BHJ) have been elaborated from blends of SiNWs and the polymer in solution from a common solvent. Optical properties of these nanocomposites have been investigated by UV-vis absorption and photoluminescence (PL) spectral measurements. We have studied the charge transfer between SiNWs and the two polymers using the photoluminescence quenching of PVK and MEH-PPV which is a convenient signature of the reduced radiative recombination of the generated charge pairs upon exciton dissociation. We found that PVK and SiNWs constitutes the better donor-acceptor system. In order to understand the difference between PVK:SiNWs or MEH-PPV:SiNWs behaviours, photoluminescence responses were correlated with the topography (SEM) of the thin films. The photovoltaic effect of ITO/PEDOT:PSS/SiNWs:PVK/Al and ITO/PEDOT:PSS/SiNWs:MEH-PPV/Al structures was studied by current-voltage (I-V) measurements in dark and under illumination and interpreted on the basis of the charge transfer differences resulting from the morphologies.     

Synthesis and characterization of a new monophosphate (5-Cl-2,4-(OCH3)2C6H2NH3)H2PO4

Chemical preparation, crystal structure and NMR spectroscopy of a new organic cation 5-chloro(2,4-dimethoxy)anilinium monophosphate H₂PO₄ are given. This new compound crystallizes in the monoclinic system, with the space group P2₁/c and the following parameters: a = 5.524(2) Å, b = 9.303(2) Å, c = 23.388(2) Å, β = 90.66(4), V = 1201.8(2) Å³, Z = 4 and D_x = 1.573 g cm⁻³. Crystal structure has been determined and refined to R = 0.031 and R_w = 0.080 using 1702 independent reflections. Structure can be described as an infinite (H₂PO₄)_nⁿ⁻ corrugated chains in the a-direction. The organic groups (5-Cl-2,4-(OCH₃)₂C₆H₂NH₃)⁺ are anchored between adjacent polyanions through multiple hydrogen bonds. This compound is also investigated by IR, thermal, and solid-state, ¹³C, ³¹P MAS NMR spectroscopies.     

Preparation and characterization of poly(3,4-ethylenedioxythiophene)–poly(styrene sulfonate) composite thin films highly loaded with platinum nanoparticles

In this work, we propose a simple and efficient, low-temperature (∼120 °C) process to prepare transparent thin films of poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS) loaded with high concentration (up to 22.5 wt%) of platinum (Pt) nanoparticles. Firstly, an improved polyol method was modified to synthesize nano-sized (∼5 nm) and mono-dispersed Pt particles. These nanoparticles were incorporated into the matrix of PEDOT:PSS thin films via a spin coating/drying procedure. The electrochemical activities of the PEDOT:PSS thin film modified electrodes with respect to the I⁻/I₃⁻ redox reactions were investigated. It was found that the modified electrode of PEDOT:PSS thin film containing 22.5 wt% Pt exhibited the electrochemical activity comparable to the conventional Pt thin film electrode, suggesting that this electrode has good potential to serve as a counter electrode in dye-sensitized solar cells.     

Effect of cadmium sulphide quantum dot processing and post thermal annealing on P3HT/PCBM photovoltaic device

The present study demonstrates the effect on photovoltaic performance of poly(3-hexylthiophene) (P3HT) on doping of cadmium sulphide (CdS) quantum dots (QDs). The P3HT/CdS nanocomposite shows a 10 nm blue shift in the UV-vis absorption relative to the pristine P3HT. The blue shift in the absorption of the P3HT/CdS nanocomposite can be assigned to the quantum confinement effect from the CdS nanoparticles. Significant PL quenching was observed for the nanocomposite films, attributed to additional decaying paths of the excited electrons through the CdS. Solar cell performance of pure P3HT and dispersed with CdS QDs have been studied in the device configuration viz indium tin oxide (ITO)/poly(3,4-ethylendioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)/P3HT:PCBM/Al and ITO/PEDOT:PSS/ P3HT:CdS:PCBM/Al, respectively. Incorporation of CdS QDs in the P3HT matrix results in the enhancement in the device efficiency (?) of the solar cell from 0.45 to 0.87%. Postproduction thermal annealing at 150 °C for 30 min improves device performance due to enhancement in the device parameters like FF, V_OC and improvement in contact between active layer and Al.     

Phase transitions, structure and ion conductivity of zirconium hydrogen phosphates, H1±XZr2−XMX(PO4)3·H2O (M = Nb, Y)

Structure transformations and proton conductivity of hydrogen zirconium phosphates with the NASICON structure, H_1±XZr_2−XM_X(PO₄)₃·H₂O (X = 0, 0.02 and 0.1, M = Nb, Y), were studied by X-ray powder diffraction, calorimetry, IR- and impedance spectroscopy. Substitution of zirconium by niobium leads to decrease of the lattice parameters, while yttrium doping leads to their increase. H_0.9Zr_1.9Nb_0.1(PO₄)₃ structure was determined at 493 and 733 K. This phase crystallizes in rhombohedral space group with lattice parameters a = 8.8564(5) Å, c = 22.700(1) Å at 493 K and a = 8.8470(2) Å, c = 22.7141(9) Å at 733 K. The a parameter and lattice volume were found to decrease with temperature increasing. Structure transformations upon heating are caused mainly by the decrease of the M1 site and C cavities. Ion conductivity of obtained materials was found to increase in humid atmosphere. Activation energies of conductivity were calculated. Rhombohedral-triclinic phase transition found by X-ray powder diffraction was proved by calorimetry data. According to XRD and IR spectroscopy data hydrogen bond in HZr₂(PO₄)₃ was found to be weaker than in hydrated material.     

Blue-violet phosphate phosphor thin films for EL

Photoluminescence (PL) and electroluminescence (EL) in blue-violet emission were observed in newly developed phosphate phosphor thin films such as Ba₃(PO₄)₂:Eu and Ba₃(PO₄)₂:Ti. These phosphate phosphor thin films were first deposited on thick BaTiO₃ ceramic sheets by r.f. magnetron sputtering using powder targets and then post-annealed in various atmospheres. Blue-violet PL and EL emissions were obtained in Ba₃(PO₄)₂:Eu and Ba₃(PO₄)₂:Ti phosphor thin films that were deposited in an Ar + H₂ (10%) gas atmosphere and then post-annealed above about 900 °C in an Ar + H₂ (10%) gas atmosphere. In particular, the EL observed in Ba₃(PO₄)₂:Eu thin films exhibited two peaks, a red emission peaking at about 615 nm and a blue-violet emission peaking at about 420 nm. A luminance of 2.0 cd/m² in blue-violet emission was obtained in a thin-film EL device using a two step post-annealed Ba₃(PO₄)₂:Eu thin-film emitting layer: step 1, post-annealing at 1000 °C in air for 1 h, and step 2, post-annealing at 1000 °C in an Ar + H₂ atmosphere.     

Synthesis and characterization of a new 2,4,6-triaminopyridinium dihydrogendiphosphate dihydrate (C4H8N5)2H2P2O7·2H2O

Structural properties of the 2,4,6-triaminopyridinium dihydrogendiphosphate dihydrate are discussed on the basis of an X-ray structure investigation. (C₄H₈N₅)₂H₂P₂O₇·2H₂O is monoclinic, C2/c, with a = 10.414(1) Å, b = 13.365(1) Å, c = 13.736(2) Å, β = 98.39(4)°, and Z = 4. The structure has been solved by a direct method and refined to a reliability R factor of 0.0375 (R_w = 0.0961) using 2751 independent reflections. The structural arrangement can be described as inorganic infinite ribbons, , spreading along the c direction; the organic groups, [C₄H₈N₅]⁺, link the precedent ribbons, via their hydrogen bonds, to form a three-dimensional network. The present work, deals with crystal structure, thermal behavior and IR analysis of this new compound.     

Properties of PEDOT:PEG/ZnO/p-Si heterojunction diode

The zinc oxide (ZnO) and poly(3,4-ethylenedioxythiophene) bis-poly(ethyleneglycol) (PEDOT:PEG) films were deposited on p-Si substrate by sputter and spin coating methods, respectively. An organic/inorganic heterojunction diode having PEDOT:PEG/ZnO on p-Si substrate was fabricated. The barrier height (BH) and the ideality factor values for the device were found to be 0.82 ± 0.01 eV and 1.9 ± 0.01, respectively. It has been seen that the value of BH is significantly larger than those of conventional Au/p-Si metal–semiconductor contacts. The PEDOT:PEG/ZnO/p-Si heterostructure exhibits a non-ideal I–V behavior with the ideality factor greater than unity that could be ascribed to the interfacial layer, interface states and series resistance. The modified Norde's function combined with conventional forward I–V method was used to extract the parameters including the barrier height and series resistance. At the same time, the physical properties of ZnO and PEDOT:PEG films deposited by sputter and spin coating technique, respectively, were investigated at room temperature. The obtained results indicate that the electrical parameters of the diode are affected by structural properties of ZnO film and PEDOT:PEG organic film.     

Study on ZnGa2O4:Cr a.c. powder electroluminescent device

An a.c. powder electroluminescent (EL) device using ZnGa₂O₄:Cr³⁺ phosphor was fabricated by the screen printing method. Optical and electrical properties of the device were investigated. The fabricated device shows a red emission at 695 nm driven by the a.c. voltage. The emission is attributed to the energy transfer from hot electrons to Cr³⁺ centers via self-activated Ga-O groups. Luminance (L) versus voltage (V) matches the well-known equation of L = L₀exp(− bV ^− 1 / 2) and luminance increases proportionally with frequency due to the increase of excitation probability of host lattice or Cr³⁺ centers. The diagram of the charge density (Q) versus applied voltage (V) is based on a conventional Sawyer-Tower circuit. At 280 V and 1000 Hz, the luminance and the luminous efficiency of the fabricated powder EL device are about 1.0 cd/m² and 13 lm/W, respectively. And under the high field, the device fabricated with the oxide-based phosphor of ZnGa₂O₄:Cr³⁺ shows excellent stability in comparison with the conventional sulfide powder EL device.     

Properties of one-step synthesized Pt nanoparticle-doped poly(3,4-ethylenedioxy thiophen):poly(styrenesulfonate) hybrid films

Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) thin films incorporating Pt nanoparticles were synthesized by a simple method using H₂PtCl₆? xH₂O. The surface morphology, chain structure, bonding states, and electronic structure of the films were investigated. Unusual formation of a PEDOT-rich surface was accompanied with interactions between PEDOT:PSS and Pt nanoparticles due to agglomeration of PEDOT:PSS around Pt nanoparticles and destabilization of chain conformation. High doping of the PEDOT moiety occurred during the reduction of H₂PtCl₆·xH₂O so that an increased work function and a decreased energy gap between the edge of the highest occupied molecular orbital and the Fermi level were observed. Intermediate products including chlorine and [PtCl₆]⁻, and the ionic nature of the Pt nanoparticles were responsible for the resultant properties of Pt-PEDOT:PSS. Products generated during the formation of Pt nanoparticles served as a third anionic dopant like PSS and as an inhibitor of segregation.     

Hydrothermal synthesis and photoluminescence properties of Gd2Zr2O7:Tb phosphors

This paper presents hydrothermal synthesis, characterization, and photoluminescence (PL) properties of novel green-emitting phosphors, Gd₂Zr₂O₇:Tb³⁺. Their crystal structure, morphology and photoluminescence properties were investigated by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM) and fluorescence spectrophotometer. The results revealed that one-dimensional Gd₂Zr₂O₇:Tb³⁺ nanorods with diameter of about 30 nm and length of 150-300 nm were formed, and the products exhibited a fluorite-type structure. PL study revealed that Gd₂Zr₂O₇:Tb³⁺ phosphors presented dominant green emission luminescence, which was attributed to the transitions from ⁵D₄ excited states to ⁷F_J (J = 3-6) ground states of Tb³⁺. The luminescence intensity of Gd₂Zr₂O₇:Tb³⁺ with different Tb³⁺ concentration was also investigated and reported, and an obvious concentration quenching was observed when Tb³⁺ ion concentration was 5 at.%.     

Synthesis and photoluminescence properties of Cl-doped ZnS nanoparticles prepared by a solid-state reaction

ZnS:Cl⁻ nanoparticles with strong blue emission have been synthesized successfully by a solid-state reaction at low temperature. The dependence of photoluminescence (PL) properties of ZnS:Cl⁻ nanoparticles on the Cl⁻ contents was researched, and the influences of the annealing ambience and using polyvinyl alcohol (PVA) during the synthesis on the PL of ZnS:Cl⁻ (Cl/Zn = 0.35) nanoparticles were discussed. X-ray power diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and ultraviolet-visible spectroscopy were used to characterize their structure, chemical state, diameter, surface states, and PL properties. The results showed that ZnS:Cl⁻ nanoparticles had a cubic blende crystal structure and an average crystallite size of 17.40–19.16 nm. The most intensity blue emission peaking at about 425 nm was obtained when Cl/Zn = 0.35 under 330 nm excitation at room temperature. The emission intensity of ZnS:Cl⁻ (Cl/Zn = 0.35) was increased 3-fold than that of ZnS. The results showed that the PL of ZnS:Cl⁻ (Cl/Zn = 0.35) nanoparticles was enhanced after annealing or using PVA during the synthesis, and annealing in vacuum had a stronger effect in improving the luminescence properties of ZnS nanoparticles than in air. This work suggests that it is an effective method to improve the PL intensity of ZnS nanocrystals by doping with Cl⁻ in ZnS.     

Synthesis and characterization of 5,7-dimethyl-8-hydroxyquinoline and 2-(2-pyridyl)benzimidazole complexes of zinc(II) for optoelectronic application

Bis(5,7-dimethyl-8-hydroxyquinolinato)zinc(II) (Me₂q)₂Zn and 5,7-dimethyl-8-hydroxyquinolinato(2-(2-pyridyl)benzimidazole) zinc(II) Me₂q(pbi)Zn have been synthesized and characterized by various techniques. These metal complexes have high thermal stability (>300 °C) and high glass transition temperatures (>150 °C). The vacuum deposited films of these materials show good film forming property and are suitable for opto-electronic applications. Multilayered organic electroluminescent (EL) devices have been fabricated having structure ITO/α-NPD/zinc complex/BCP/Alq₃/LiF/Al, which produce emission with chromaticity having Commission Internationale d’Eclairage (CIE) coordinates x = 0.506 and y = 0.484 for (Me₂q)₂Zn; x = 0.47 and y = 0.52 for (Me₂q)(pbi)Zn complex. The electroluminescence spectra show peak emission centered at 572 and 561 nm respectively for these materials.     

Electrical and gas sensing properties of self-aligned copper-doped zinc oxide nanoparticles

Electrical and gas sensing properties of nanocrystalline ZnO:Cu, having Cu X wt% (X = 0.0, 0.5, 1.0, and 1.5) in ZnO, in the form of pellet were investigated. Copper chloride and zinc acetate were used as precursors along with oxalic acid as a precipitating reagent in methanol. Material characterization was done by X-ray diffraction (XRD), scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM) and inductive coupled plasma with optical emission spectrometry (ICP-OES). FE-SEM showed the self-aligned Cu-doped ZnO nano-clusters with particles in the range of 40-45 nm. The doping of 0.5% of copper changes the electrical conductivity by an order of magnitude whereas the temperature coefficient of resistance (TCR) reduces with increase in copper wt% in ZnO. The material has shown an excellent sensitivity for the H₂, LPG and CO gases with limited temperature selectivity through the optimized operating temperature of 130, 190 and 220 °C for H₂, LPG and CO gases, respectively at 625 ppm gas concentration. The %SF was observed to be 1460 for H₂ at 1% Cu doping whereas the 0.5% Cu doping offered %SF of 950 and 520 for CO and LPG, respectively. The response and recovery time was found to be 6 to 8 s and 16 s, respectively.     

Synthesis and characterization of a new organic dihydrogen phosphate-arsenate: [H2(C4H10N2)][H2(As, P)O4]2

Chemical preparation, crystal structure, calorimetric and spectroscopic investigations (IR and RMN) are given for a new non-centrosymmetric organic-cation dihydrogen phosphate-arsenate [H₂(C₄H₁₀N₂)][H₂(As, P)O₄]₂. This compound is triclinic P1 with the following unit-cell parameters: a = 7.082(2) Å, b = 7.796(1) Å, c = 12.05(3) Å, α = 95.37(2)°, β = 98.38(3)°, γ = 62.98(1)°, Z = 2, V = 586.2(1) Å³ and D_x = 1.836 g cm⁻³. The crystal structure has been solved and refined to R = 0.03 using 2328 independent reflections. The structure can be described as infinite (H₂XO)_n chains spreading parallel to the b direction. These chains are themselves interconnected by a set of NH?O hydrogen bonds generated by the organic entities, alternating with the chains. Solid-state ¹³C, ¹⁵N and ³¹P MAS NMR spectroscopies are in agreement with the X-ray structure.     

Synthesis and crystal sructure of a new dihydrogenomonophosphate (4-C2H5C6H4NH3)H2PO4

Chemical preparation, crystal structure, calorimetric, and spectroscopic investigations are given for a new organic-cation dihydrogenomonophosphate, (4-C₂H₅C₆H₄NH₃)H₂PO₄ in the solid state. This compound crystallizes in the orthorhombic space group Pbca with the following unit cell parameters: a=8.286(3) Å, b=9.660(2) Å, c=24.876(4) Å, Z=8, V=1991.2(7) Å³, and D_X=1.442 g cm⁻³. Crystal structure was solved with a final R=0.054 for 3305 independent reflections. The atomic arrangement coaled described as H₂PO₄⁻ layers between which are located the 4-ethylanilinium cations.