Naphthalimide polymers for organic light-emitting diodes

We used a side-chain polymer based on a high-electron affinity (EA) naphthalimide moiety (PNI), to fabricate single and double-layer light-emitting diodes (LEDs) with improved efficiency in the green spectral region. The chromophore is attached to a polymethacrylate backbone through a spacer and is characterised by a 30% photoluminescence quantum yield. In single-layer light-emitting diodes we find that the electroluminescence efficiency is not limited by A1 cathodes as for poly(p-phenylene vinylene), PPV, as expected from consideration of the EA. We also report maximum internal quantum efficiencies of about 1.7% for Ca and 0.9% for Al in double-layer devices where PPV serves as both hole-injector and emitter. Tuning of emission in the red is possible by dye-doping (at high concentration) the PNI and causing the emission to happen in this layer. Unexpectedly, not only does the dye-doping of PNI red-shifts the emission spectrum, but also affects significantly the charge transport properties and in particular reduces the driving field necessary for electroluminescence in both single- and double-layer LEDs and we propose this effect as one of the factors for the lifetime increase upon doping recently reported in the literature.     

A new precursor to electroconducting conjugated polymers: synthesis and opto-electrical properties of luminescent devices based on these PPV derivatives

A variety of novel conjugated PPV-related polymers have been synthesised using a synthetic approach that allows the tailoring of the chemical structure of the polymer backbone. In this way we have been able to prepare not only poly(p-phenylene vinylene) (PPV) (1), but also some electron-rich PPV derivatives such as poly(2,5-dimethyl-1,4-phenylene vinylene) (PDMetPV) (2) and poly(2,5-dimethoxy-1,4-phenylene vinylene) (PDMeoPV) (3) and PPV derivatives with an enlarged aromatic system, poly(4,4′-bisphenylene vinylene) (PBPV) (4) and poly(2,6-naphtaln vinylene) (PNV) (5). These materials were characterised in terms of molecular weight (GPC), elimination temperature (TGA, FT-IR), optical properties (UV-VIS), photoluminescence (PL) and device characteristics of the LEDs.     

Characterization of 6PV phenylene vinylene oligomer thin films

Investigations of phenylene vinylene oligomer thin films with 7 phenyl, 6 vinyl units (6PV) have been performed by optical and electrical measurements. The optical characterizations including infrared, Raman and UV-VIS spectroscopies were discussed by comparing the results with those obtained in poly(p-phenylene vinylene) (PPV). The electrical properties of oligomer based diodes were studied by measuring the current-voltage-temperature characteristics and the thermally stimulated currents (TSC). At high fields, tunneling of carriers through the interfacial barrier would occur while at lower fields, two distinct temperature ranges were observed. In the high temperature range (> 250 K), a field assisted mechanism occurs involving localized states in the oligomer bulk whereas in the low temperature range, hopping conduction is probable. The TSC spectrum shows two relaxation peaks at 219 and 261 K which correspond to trap depths of 0.4 and 0.55 eV. The deep traps appeared as a characteristic of phenylenevinylene oligomers.     

Use of ultra-thin aluminum oxide layer to reduce photoluminescence decay in poly(p-phenylene vinylene) films

The degradation of conjugated polymer poly(p-phenylene vinylene) (PPV) in the atmosphere upon exposure to ultraviolet light limits its applications as the emitting layer in organic light emitting diodes. In this paper we show that a thin layer of aluminum oxide around 10 nm in thickness prevents photoluminescence degradation of PPV during exposure to blue light in the atmosphere but not at lower excitation wavelengths. This oxide film is free of cracks and pinholes from transmission electron microscopy analysis.     

Tin oxide as a transparent electrode material for light-emitting diodes fabricated with poly (p-phenylene vinylene)

We have fabricated light emitting diodes (LED) with poly (p-phenylene vinylene) (PPV) as the emissive layer and tin oxide (TO) as the transparent conducting positive electrode and aluminium as the negative electrode. The fabrication conditions are optimized for visible light emission in these TO/PPV/Al LEDs. The threshold voltage for substantial charge injection for visible light emission in these LEDs lies below 10V. The device fabrication and electrical characterization of TO/PPV/Al LEDs are discussed in this communication.     

Interference phenomena in polymer light-emitting diodes: photoluminescence and modelling

We report an experimental and theoretical study of the effects of interference in polymeric light-emitting diodes (LEDs). These effects are due to the complex optical structures of the devices, which include many layers of materials with different refractive indices, and are of considerable importance since they affect spectral distribution and intensity of the absorption and emission in a significant way. By way of comparison, they can also provide a flexible, noninvasive optical probe of the electroluminescent processes, such as, for example, the spatial distribution of the recombination inside the LED. In this paper we analyse single-layer diodes with indium-tin oxide (ITO) and Al electrodes, where poly(p-phenylene vinylene) (PPV) is the luminescent polymer. We find that photo-induced excitation of the radiative species produce different spectral shapes depending on the excitation energy (and hence on the profile of excited chromophores) which we can describe in terms of interference phenomena. The theoretical analysis is conducted by means of multilayer stack theory and transfer matrix calculations, and takes into account additional quenching effects due to In contaminations from the ITO electrode. The theoretical results are in good agreement with the experiment.     

The effects of phenyl-di-substitution of PPV on its photophysical and photostability properties

We present measurements of photoluminescence (PL) and optical absorption of the phenylated PPV (poly(p-phenylene vinylene)) derivative poly(1;3-phenylene diphenyl-vinylene) (m-PPV-DP) and the model compound tetraphenylethylene (TPE). We also assess the effect of the di-phenylsubstitution in both m-PPV-DP and PDPV (poly-(4,4′-diphenylene diphenylvinylene)) on their photostability. Our work confirms that the meta-link in m-PPV-DP breaks the conjugation and leaves the polymer with a longest fully conjugated unit identical to the molecule TPE. This allows to transfer our quantum chemical modelling results for TPE to m-PPV-DP and to show that the photophysical properties of the polymer are controlled by ring-torsional relaxation of the excited states rather than by effects of chain-extended conjugation. The UV-photodegradation is shown to be slowed down significantly by the phenylation in comparison to PPV as is evident from the measurements of the PL decay. We show by means of FTIR studies that the degradation is an oxidation process.     

Blue emitting diodes based on self-assembled multilayers of cationic oligo(p-phenylene vinylene) and sulfonated polyaniline

Boladication form oligo(p-phenylene vinylene) (OPV) was synthesized and mixed boladication–polyanion multilayer thin films have been self-assembled by a process based on electrostatic interaction. Electroluminescent spectral fine structures of these self-assembled thin films showed differences in vibronic coupling intensity and space. The poly(aniline-co-N-propane-sulfonic acid aniline) (PAPSAH) provided better charge transport ability than poly(sulfonated styrene) (PSS) and the turn-on voltage of the device based on boladication OPV/PAPSAH multilayer thin films is lower than that of the PSS counterpart.     

Density-functional-based tight-binding approach to polarons in conjugated polymers

Whether or not charge injection leads to polaron formation in conjugated polymers remains a matter of controversy with profound qualitative discrepancies between different methods. In this paper we show that the Density-functional-based tight-binding (DFTB) approach of Porezag et al. predicts delocalization of an excess charge and thus no polaron formation in infinite conjugated polymer chains. Since real polymers have finite conjugation lengths, we also present the first comparative study of electron and hole polaron binding energies in oligomers of the conjugated polymers trans-polyacetylene (tPA), poly(para-phenylene) (PPP) and poly(para-phenylene vinylene) (PPV).     

Polymer/gold nanoparticles light emitting diodes utilizing high work function metal cathodes

This investigation reports the luminescent and electric properties of phenyl-substituted poly(para-phenylene vinylene) copolymer (HY-PPV) in which Au nanoparticles were incorporated. Incorporating a few amounts of Au nanoparticles into HY-PPV film enhances the electroluminescence (EL) efficiency of HY-PPV-based polymer light emitting diodes (PLEDs) using Al and Cu as cathodes. Blue-shifts in the emission spectra and a decrease in the capacitance of the emitting layer were observed. These unique behaviors are due to the increase in the density of the effective hole trap centers and affect the electronic configuration of HY-PPV, induced by the Au nanoparticles, which increase the probability of hole–electron recombination and shift the emission spectra toward higher photo energies.     

High external quantum efficiency in deep blue phosphorescent organic light emitting diodes using a simple device structure

Simple high efficiency deep blue phosphorescent organic light-emitting diodes were developed using a mixed host of high triplet energy host materials. A hole transport type host was used both as the hole transport layer and host in the mixed host emitting layer and an electron transport type host was mixed with the hole transport type host in the emitting layer. A three organic layer device structure of the hole transport layer/emitting layer/electron transport layer gave high external quantum efficiency of 26.4% with a color coordinate of (0.14, 0.19).     

烷氧基取代聚对苯乙炔单层电致发光器件

以对甲氧基苯酚为起始原料,用脱氯化氢法得到可溶性的聚（２－甲氧基－５－壬氧基）对苯乙炔（ＰＭＯＮＯＰＶ）,以ＰＭＯＮＯＰＶ为发光层装配了相对稳定的单层电致发光器件,其起亮电压为５伏。研究了ＰＭＯＮＯＰＶ的电致发光（ＥＬ）和光致发光（ＰＬ）性质。     

Stacking layered structure of polymer light emitting diodes prepared by evaporative spray deposition using ultradilute solution for improving carrier balance

Polymer light-emitting diodes (PLEDs) with staking layered structures are prepared by the evaporative spray deposition using ultradilute solution (ESDUS) method, which has enabled forming a polymer layer onto another polymer layer even if both polymers are soluble in a solvent used for the preparation. By this method, polymers having various HOMO and LUMO levels can be stacked as a hole transport layer, an emitting layer and an electron transport layer as commonly employed in small molecule-based organic light emitting diodes. Here we demonstrated that a PLED having a tri-layer structure using three kinds of polymers showed significant improvement in quantum efficiency compared with those having a single or bi-layer structure of corresponding polymers.     

Electroluminescent characteristics of spin-assembled multilayer films with confined layer structure

Multilayer films composed of poly(p-phenylene vinylene) (PPV) as the semiconducting polymer and poly(methacrylic acid) (PMAA) as the insulating polymer were fabricated by spin-assembly method. These films, comprising a confined layer structure, showed that the water contact angles are periodically and distinctly oscillated when the top surface layer is alternated between PPV precursor and PMAA. The turn-on voltage of the multilayer electroluminescent (EL) devices increased from 2.6 V to 9.8 V as the thickness of the PMAA layer inserted between neighboring PPV layers was increased from 0 (i.e., PPV single layer film) to 2.0 nm. Furthermore, the emission peaks in the photoluminescent and EL spectra of these devices were strongly blue-shifted due to excitons formed at the confined PPV layers. Particularly when inserting about 1.0 nm thick PMAA layers, which possibly induced a tunneling effect on the charge carriers (i.e., holes and electrons), these multilayer films decreased the mobility of the hole carriers in the PPV layers with strong hole transporting characteristics, and therefore increased the recombination probability in the emitting layer with confined geometry. As a result, the device efficiency was significantly improved in comparison with that of a PPV single layer device without PMAA layer and with that of devices with relatively thick PMAA layers of 1.4 or 2.0 nm.     

Pyridine substituted spirofluorene derivative as an electron transport material for high efficiency in blue organic light-emitting diodes

The quantum efficiency of blue fluorescent organic light-emitting diodes was enhanced by 20% using a pyridine substituted spirofluorene-benzofluorene derivative as an electron transport material. 2′,7′-Di(pyridin-3-yl)spiro[benzofluorene-7,9′-fluorene] (SPBP) was synthesized and it was used as the electron transport material to block the hole leakage from the emitting layer. The improvement of the quantum efficiency and power efficiency of the blue fluorescent organic light-emitting diodes using the SPBP was investigated.     

Pure white phosphorescent organic light-emitting diodes using a phosphine oxide derivative as a high triplet energy host material

Pure white phosphorescent organic light-emitting diodes were developed using a high triplet energy (3.02 eV) host material (PPO1) with a carbazole and a phosphine oxide unit. Deep blue emitting tris((3,5-difluoro-4-cyanophenyl)pyridine) iridium was doped into the wide triplet bandgap PPO1 host material and the blue emitting layer was combined with a red/green phosphorescent emitting layer. The doping concentration, thickness of the emitting layer and the electron transport material were managed to optimize the device performance of the white device. A pure white color with a color coordinate of (0.29,0.33) and a high current efficiency of 27.2 cd/A was achieved from the white organic light-emitting diodes with the PPO1 host after optimization.     

Enhancement of the luminance efficiency for top-emitting DB-PPV polymer light-emitting diodes with Ca/Ag cathode

The enhancement of the luminance efficiency of the top-emitting 2,3-dibutoxy-1,4-polyphenylene vinylene (DB-PPV) polymer light-emitting diodes (PLEDs) with Ca/Ag cathode has been investigated in this work. The luminance efficiency of 2.9 cd A⁻¹ (at 10 mA) for the thickness of Ca/Ag cathode of 10/15 nm is achieved by our devices. In this paper, partially oxidized CaO_x film is proposed to have the capability of hole-blocking as well as the buffer layer on condition of the suitable Ag thickness of Ca/Ag cathode. The tuning ability of colors by simply adjusting the thickness of the DB-PPV emitting layer is available for the devices.     

Investigation of defects in polyhedral oligomeric silsesquioxanes based organic light emitting diodes

We have investigated novel poly(p-phenylene vinylene) (PPV) derivative based organic light emitting diodes (OLEDs). We have used poly(2,3-diphenyl-1,4-phenylenevinylene) (DP-PPV) as an emitter in which an inorganic core of polyhedral oligomeric silsesquioxanes (POSS) have been incorporated. The hybrid structure obtained shows an improvement on the stability and an enhancement of electroluminescence properties. Charge-based deep level transient spectroscopy (Q-DLTS) has been used to study the defect states in indium-tin-oxide (ITO)/polyethylene dioxythiophene:polystyrene sulfonate (PEDOT:PSS)/POSS-DP-PPV/CaAl light emitting devices. Analysis of the Q-DLTS spectra obtained in devices, reveal at least six trap levels. The mean activation energies of traps are distributed in the range 0.3–0.5 eV within the band gap of the hybrid polymer and capture cross sections are of the order of 10¹⁶–10²⁰ cm². The trap densities are in the range of 10¹⁶–10¹⁷ cm^?3. Although the origin of these traps remains not clearly established, we suggest that the trap states with a large capture cross-section would likely to be originated from the inorganic part of hybrid material while those with lower capture cross-section would be related to its organic part.     

聚对苯乙炔-b-聚苯乙烯刚柔嵌段共聚物的合成与自组装

在分子设计的基础上,合成了分别以CHO、OH和Br为端基的PPV低聚体,并以后者作为大分子引发剂引发苯乙烯的原子转移自由基聚合,成功合成了一种新的结构明确的PPV-b-PS二嵌段共聚物。采用IR、UV-VIS、1H-NMR和GPC等手段表征了其结构,并研究了其在CS2溶液中的自组装行为。SEM观察证明,在温度、氩气流速和湿度分别为19.2℃,0.8 m3/h和67.8%RH的条件下,PPV-b-PS在质量分数0.2%的CS2溶液中能够自组装成高度有序的六角蜂窝状结构薄膜,其空穴的平均直径为1.15μm。     

High efficiency phosphorescent white organic light-emitting diodes using a spirofluorene based phosphine oxide host material

High efficiency phosphorescent white organic light-emitting diodes (PHWOLEDs) were developed by using a spirofluorene based phosphine oxide (SPPO1) as a host material in blue emitting layer. A stack structure of red:green/blue with an interlayer was used and the device performances of PHWOLEDs were investigated according to host composition in red:green emitting layer and the interlayer thickness. The use of SPPO1 as a host in the blue emitting layer resulted in a high quantum efficiency of 13.5% and a current efficiency of 27.6 cd/A with a color coordinate of (0.37, 0.43).