Lanthanide imidodiphosphinate complexes: Synthesis,structure and new aspects of electroluminescent properties

The lanthanide imidodiphosphinate complexes Ln(pip)₃ (Ln = Ce, Nd, Tb, Ho) were prepared from the corresponding Ln[N(SiMe₃)₂]₃ with [Ph₂P(O)]₂NH in a quantitative yield. X-ray single structure analysis of Ce(pip)₃ has revealed that the lanthanide ion in these compounds coordinated by three bidentate [Ph₂P(O)NP(O)Ph₂]⁻ ligands and THF molecule. With respect to the organic light emitting diodes (OLED) application, the simple electroluminescent (EL) devices with a multilayer configuration ITO/TPD/Ln(pip)₃/Yb, ITO/CBP/Ln(pip)₃/Yb, ITO/TPD/Ln(pip)₃/Alq₃/Yb, ITO/TPD/Alq₃/Ln(pip)₃/Yb were fabricated. It was found that the lanthanide imidodiphosphinate complexes possess the hole-blocking and electron-transporting ability and it was also observed the exciplex emission between these complexes and TPD.     

Optimizing efficiency of polycrystalline p-Si anode organic light-emitting diode

Optimizing efficiencies of organic light-emitting diodes (OLEDs) with a structure of Al/glass/nanometer-thick polycrystalline p-Si (NPPS) anode/SiO₂/N′-bis-(1-naphthl)-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/tris (8-hydroxyquinoline) aluminum (Alq₃)/4,7-diphenyl-1,10-phenanthroline (BPhen):Cs₂CO₃/Sm/Au were studied. The NPPS anodes were fabricated by magnetron sputtering (MS) Si and Ni layers followed by Ni-induced crystallization of the amorphous Si layers. By adjusting the resistivity of the p-Si target adopted in MS, the electroluminescent efficiency of the OLED was optimized. When the resistivity of the p-Si target is 0.01 Ω·cm, the current and power efficiencies of the NPPS anode OLED reach maximum values of 6.7 cd·A^?1 and 4.64 lm·W^?1, respectively, which are 2.7 and 3.1 times those of the resistivity-optimized bulk p-Si anode counterpart and 2.9 and 3.7 times those of the indium tin oxide (ITO) anode counterpart, and then, the physical reasons were discussed.     

A flexible textile structure based on polymeric photovoltaics using transparent cathode

This paper presents results for photovoltaic performance obtained from the application of different bulk heterojunction blends onto flexible polypropylene (PP) substrates for textile applications. Organic photovoltaic devices were fabricated onto non-transparent PP tapes and ITO coated glasses. The layer of poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) and PEDOT:PSS with 100 nm of silver (Ag) metal layer constituted anode structure and substituted indium tin oxide (ITO) layer in this study. The blends of poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl-1-phenyl(6,6)C₆₁ (P3HT:PCBM) or poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)-1-4-phenylene vinylene] and 1-(3-methoxycarbonyl)-propyl-1-phenyl(6,6)C₆₁ (MDMO-PPV:PCBM) were utilized as the absorbing materials. The thin metal layers of lithium fluoride (LiF) and aluminum (Al) were deposited on top of the devices by evaporation. All photovoltaic devices were characterized by measuring current–voltage (I–V) characteristics under simulated AM 1.5 conditions. The morphology of these devices using MDMO-PPV:PCBM and P3HT:PCBM blends was also investigated using atomic force microscopy (AFM).     

Pure exciplex electroluminescence in blended film of small organic molecules

3,6-diarylcarbazole derivatives, 3,6-di-(p-cyanophenyl)-N-hexylcarbazole (CNHC) and 3,6-di-(p-acetylphenyl)-N-hexylcarbazole (ANHC) were synthesized, characterized and energy levels determined. Steady state fluorescence of these molecules showed blue emission in thin films. Blends of these molecules with N,N′-diphenyl-N,N′-(bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) showed exciplex emission as the dominant one. Single layer organic light emitting devices (OLEDs) of CNHC, ANHC and their blends with TPD were fabricated using ITO as anode and Ca as cathode. The blend film devices showed pure exciplex electroluminescence.     

Self-assembled monolayer effect on the characteristics of organic diodes

In this paper, we study the effect of self-assembled monolayers (SAMs) on the electric behavior of organic diodes based on sexithiophene (6T) sandwiched between indium tin oxide (ITO) and aluminum. We have used molecules of SAMs based on a thiol with functional groups of oligothiophene (3T(CH₂)₆SH). Wettability measurements have been performed to characterize ITO surface energy and its modification upon deposition of SAMs. The results of contact angle measurements and surface energies demonstrate the homogeneity and rigidity of grafting surface. The current vs. applied voltage characteristics of devices show that conduction in weak biasing follow Richardson–Schottky behavior. Beyond 1.5 V, J–V characteristics can be successfully modeled by space-charge limited current (SCLC) theory followed by a trap charge limited current (TCLC). The electrical as well as optical characteristics of 6T layer are clearly affected by the presence of the SAM. The differences between ITO/SAM and bare ITO samples are interpreted in terms of structural effect induced by the self-assembled monolayer of 3T(CH₂)₆SH.     

Enhancement of device efficiency in CuPc/C60 based organic photovoltaic cells by inserting an InCl3 layer

An InCl₃ dipole layer is inserted into a copper phthalocyanine (CuPc) and fullerene (C₆₀) based organic photovoltaic cell (OPV) to modify the indium-tin-oxide (ITO) anode surface. The work function of the ITO is improved from 4.63 eV to 5.47 eV. In addition, a 30% enhancement in absorption coefficient is achieved due to the strong interaction between CuPc and InCl₃ molecules, which induces a configuration change of the CuPc stacks from perpendicular to parallel along the ITO substrate. Therefore, the power conversion efficiency of the OPV devices has a 30% improvement because of the improved work function of the ITO anode and the enhanced absorption coefficient of the devices.     

Organic light emitting devices performance improvement by inserting thin parylene layer

An organic light emitting device (OLED) structure with a thin parylene layer deposited by low-temperature chemical vapour deposition (CVD) at the anode–organic interface was fabricated. Such a structure gives off higher efficiency, a smaller number and smaller size dark non-emissive areas, slower growth rate of the dark areas and a longer device lifetime compared to one without the parylene layer. The parylene modified indium tin oxide (ITO) surface shows an increased work-function and a reduced surface roughness compared to that of the bare ITO surface. The interface optimisation contributes to the device performance improvement.     

Effect of UV–ozone treatment on ITO and post-annealing on the performance of organic solar cells

In this paper, the influence of the ultraviolet (UV)–ozone treatment of indium tin oxide (ITO) surface and the active layer post-annealing treatment on the performance of organic solar cells were investigated. Bulk heterojunction organic solar cells based on the blend of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) were fabricated. It is found that the devices with the UV–ozone treatment for 5 min on ITO substrates show the better performance, compared with the devices without this treatment. The results demonstrate that the short-circuit current density (J_sc) and fill factor (FF) could be improved by the post-annealing treatment. The devices with both treatments together show the best performance, with the increase of J_sc from 2.68 mA/cm² to 4.13 mA/cm² and the enhancement of FF from 32.2% to 38.8%. Therefore, the power conversion efficiency is improved from 0.62% to 1.08%. The morphology of the active layers with and without the post-annealing treatment was characterized by atomic force microscopy.     

Fabrication and characterization of electro-phosphorescent organic light-emitting devices with a ferromagnetic cathode for observation of spin injection effect

Organic light-emitting devices (OLEDs) with a phosphorescent molecule Ir(ppy)₃ as the emitter and with an Fe cathode as the spin injector were fabricated for the observation of the large degree of circular polarization. The OLED structure was a glass-substrate/ITO/α-NPD/CBP doped with Ir(ppy)₃/BCP/Al-oxide/Fe/Al. The mixing ratio of CBP and Ir(ppy)₃ in the emissive layer was optimized for high luminescence efficiency. The OLEDs showed circular polarization, and the maximum degree of circular polarization of the OLEDs was 0.4% at the applied magnetic field of 1.6 T at room temperature. On the other hand, no circular polarization was observed from the OLEDs with an Al cathode.     

Top emission organic light emitting diode with a Cr/Al/Cr anode

A top emission organic light emitting diode (TEOLED) comprised of a Cr anode on PES film/NPB/Alq₃/cathodes has been fabricated. The triple layer structure of Cr/Al/Cr allowed for fabrication of a crack-free anode and provided better thermal stability and higher work function than a conventional ITO anode. For the Cr/Al/Cr anodes, a series of Cr layers with various surface morphology has been tested. A Cr layer with a smooth surface morphology was found to be optimal. The TEOLED fabricated on PES film having good anode surface morphology showed similar device characteristics to that on a Si wafer. TEOLEDs on PES film and Si wafer exhibited a maximum luminous efficiency of 2.87 and 3.0 cd/A, respectively, at 1000 cd/m² with a NPB/Alq₃/LiF/Al/Ag structure on a Cr/Al/Cr anode.     

Efficient light emitting diodes with Teflon buffer layer

The polytetrafluoroethylene (Teflon) was utilized as buffer layer to improve the performance of organic light emitting diodes (OLEDs). In the ITO/Teflon/N,N′-diphenyl-N,N′-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)/tris(8-hydroxy-quinoline) aluminum (Alq₃)/Ca/Ag device, the Teflon film helped to enhance the hole tunneling injection and effectively impede indium diffusion from the ITO electrode. Compared with the devices without Teflon, the turn-on voltage was lowered by 1.5 V due to the introduction of Teflon buffer, and the optimized devices exhibited a luminous efficiency double that of the devices without Teflon layer, and the device lifetime proved to be dramatically increased.     

Transient electroluminescence in organic alloy light emitting diodes

In this paper, we report on transient electroluminescence (EL) studies in (ITO/TPD/alloy/Alq₃/Al) organic light emitting diodes. The alloy in the active layer is a co-evaporated mixture of TPD + Alq₃ in the ratio TPD:Alq₃ = 1:4. These results were compared with the transient EL response of a standard device (ITO/TPD/Alq₃/Al). The EL response of the alloy device consists of two components – a fast component (10–20 μs) and a slow component (200–300 μs). It is shown that the slow component arises due to the leakage of electrons from the Alq₃ layer into the alloy layer and subsequent exciton formation in the alloy layer. The magnitude of the fast component depends on the pulse repetition rate and temperature. This is shown to be related to the presence of deep traps in the alloy layer. The presence of deep traps is also confirmed by current transients in the alloy device.     

Influence of the counter ion on the properties of poly(o-toluidine) thin films and their Schottky diodes

The influence of various counter ions on the electrochemical, morphological and optical properties of poly(o-toluidine) (POT) thin films and on the electrical properties of POT-based diodes was investigated. The POT thin films were deposited on indium tin oxide (ITO)-coated glass substrates by electrochemical polymerization under cyclic voltammetric conditions from o-toluidine monomer in aqueous solutions of HCl, H₂SO₄ and HNO₃. The deposited films were characterized by cyclic voltammetry, scanning electron microscopy and UV–vis spectrophotometry. It was observed that the current densities and optical absorption spectra are influenced by the counter ions of the electrolyte solutions.On the other hand, ITO/POT/Al devices were fabricated by thermal evaporation of aluminum circular electrodes on the as-deposited POT films. The current–voltage (C–V) characteristics of these devices are non linear. The diode parameters were calculated from current–voltage (I–V) characteristics using the modified Shockley equation. The C–V and capacitance–frequency (C–F) characteristics were also measured. These measurements revealed that the junction electrical parameters depend strongly on the type of supporting electrolytes used in the synthesis of the polymer.     

Enzyme electrodes stabilized by monolayer-modified nanoporous Au for biofuel cells

Open-cell nanoporous Au (np-Au) electrodes with pore size of approximately 40 nm were fabricated by dealloying of Au–Ag, and surfaces of the electrodes were modified with a self-assembled monolayer (SAM) of 4-aminothiophenol to enhance the electrocatalytic activities of immobilized laccase and glucose oxidase. Enzyme-immobilized SAM-modified np-Au working electrodes exhibited additional reduction–oxidation peak pairs in cyclic voltammograms in buffer solution (pH = 5.0). Thus, the SAM on the np-Au facilitated electron transfer between the electrode and reactants. First-principles calculations of perfect and defective Au (111) surfaces indicated that the atomic defects at nanoligament surface of np-Au are critically responsible for the electron transfer enhancement. For the utilization of these results, a glucose/O₂ biofuel cell composed of these enzyme-immobilized SAM-modified np-Au electrodes was preliminarily fabricated, and it exhibited a maximum power density of 52 μW/cm² at 20°C. Further optimization of nanoporous structures and kinds of SAM will improve the performance of biofuel cells.     

Efficient multilayer organic light emitting diode

Some organic light emitting diodes with Alq₃ as emission layer, multilayer composed of m-MTDATA, NPB, TPD or SA as hole transport layer and lithium doped Alq₃ as electron injection assistant layer have been investigated. The current voltage characteristics and the electroluminescent (EL) output voltage characteristics have been investigated. It is found that the m-MTDATA is suitable as a hole injection layer close to the hole injection electrode ITO, NPB is suitable in contact with the emission layer, SA and TPD can be inserted between them to form an energy ladder structure, with which the efficiency of device has been increased. When thin lithium doped Alq₃ layer is inserted between Alq₃ and the aluminium electrode, the driving voltage of the devices has been clearly decreased while the current density and the EL output increased.     

Low temperature processing of high conductivity and high transparency indium–tin-oxide/Ag alloy/indium–tin-oxide multilayered thin films

Indium–tin-oxide (ITO) thin films and ITO/Ag alloy/ITO multilayered thin films were deposited on glass substrates using a vertical in-line multilayer sputtering system. Ceramic ITO targets were used for the deposition of ITO layers at low substrate temperature of 100 °C. It was observed that the sheet resistance and light transmission of ITO films were affected by the oxygen pressure significantly. The ITO/Ag alloy/ITO multilayered thin films made in the present work had a low sheet resistance (6.9 Ω/□) and a high transmission (87.1%) at 550 nm. Atomic force microscopy (AFM) investigation showed that through selecting proper processing parameters, the surface roughness could be significantly reduced. The surfaces of ITO films were found very smooth by using pulsed-direct current (pulsed-dc) sputtering with introduction of H₂O. The work functions of the ITO films and ITO/Ag alloy/ITO multilayered films were increased with oxygen plasma treatment.     

Fabrication and characterization of Schottky diodes and thin films based on poly(o-toluidine) deposited by spincoating technique

Poly(o-toluidine) (POT) thin films were fabricated by spin coating on bare glass and indium–tin–oxide (ITO)-coated glass substrates, from a solution of poly(o-toluidine) in chloroform. The optical transmittance of the as-deposited and doped films was measured in the 250–1200 nm wavelength range. These measurements showed that the optical band gaps of the undoped and doped polymer films are on the order of 3.28 and 2.7 eV, respectively, and that doping increases absorption in the near infrared region. The FT-Raman measurement on spincoated POT film is comparable to that of polyaniline. The electrochemical properties of those thin films are presented using cyclic voltammetry. ITO/POT/Al devices were fabricated by thermal evaporation of aluminum circular contacts on films deposited on ITO-coated glass. The current–voltage characteristics of the devices indicate a Schottky diode-type behavior. The current–voltage characteristics can be fitted using the modified Shockley equation. The diode parameters were calculated from I–V characteristics and discussed. On the other hand, capacitance of these structures decreased with increasing frequency.     

Structural, ferroelectric and dielectric properties of In2O3:Sn (ITO) on PbZr0.53Ti0.47O3 (PZT)/Pt and annealing effect

Ferroelectric indium tin oxide (ITO) on PbZr_0.53Ti_0.47O₃ (PZT)/Pt structure, prepared by RF sputtering onto SiO₂/Si substrates, is studied in order to investigate the effect of ITO as a top electrode in these systems. X-ray diffraction, scanning electron microscopy (SEM) and atomic force microscopy (AFM) experiments were performed to study the structure and the surface morphology of the samples. From X-ray diffraction, we observe that the ITO thin film grows with the (1 1 1) texture and the peaks attributed to PZT are all from the perovskite phase. The average roughness (RMS) of the PZT surface is found to be 1.650 nm from AFM experiment. The ferroelectric and dielectric properties were inferred from polarization hysteresis loops, capacitance and dielectric constant measurements. These properties have been compared to those of the widely studied Pt/PZT/Pt system prepared under the same conditions. The effect of ITO/PZT/Pt annealing has been studied. Annealing at 400 °C leads to 13% increase in the dielectric constant ?_r.     

Field effect transistor behavior of organic light-emitting diodes with a modified configuration of ITO anode

We have developed the organic light-emitting diodes (OLEDs) with a modified configuration of ITO anode in which a thin channel was etched to form a bottom-contact field effect transistor (FET) using ITO and MgAg as a source/drain electrode and a gate electrode, respectively. The hole injection layer in OLEDs functioned as an active layer of FET and the other organic layers as insulator-like layer. The devices were found to exhibit a behavior of FET due to horizontal charge migration between source and drain, and an electro-optical transfer characteristic due to vertical charge transport and recombination. We have investigated the dependence of drain current on the channel length from 5 to 30 μm and found that the modified channel length could change drain current directionally and quantitatively.     

The effect of film microstructures on cracking of transparent conductive oxide (TCO) coatings on polymer substrates

We investigate the effects of film microstructures such as crystallinity and surface roughness on cracking of TCO coated on polymer substrates. Indium tin oxide (ITO), which is the most extensively used TCO for electrodes in flexible electronics, is sputter-deposited on polyethylene terephthalate (PET). Varying thickness (t_f) and deposition condition, amorphous ITO (a-ITO) with different surface roughness (R_rms) and a partially crystallized ITO with similar R_rms to a-ITO of the same t_f are coated on PET. Cracking in ITO under tension is analyzed by uniaxial tension test. The mode of cracking and the fracture strength are related to the crystallinity and R_rms of ITO analyzed by high resolution transmission electron microscopy (HR TEM) and atomic force microscopy (AFM), respectively. Our experimental results reveal that the strength of ITO is significantly improved with crystallinity. The important effect of R_rms on cracking is also clearly revealed. In a-ITO, fracture strength is decreased almost linearly with R_rms that increases with t_f. In addition, cracks are always initiated and propagated along the valleys between protrusions on the surface regardless of crystallinity. Based on our experimental results, a low temperature deposition condition to obtain ITO with a high level of functional and mechanical properties is proposed.