Emissive Polymer Layer Research Articles

Photophysical dynamics of excitons in polyaniline nanotubes with enhanced resistance against exciton quenching for applications in flexible high-efficiency PLEDs

The development of a high-efficiency polymer light-emitting diode is strongly hindered by injected charge carriers in the emissive polymer layer. But real-time identification of the interactions that quench the photoluminescence is extremely difficult owing to the complex photophysics of polymers involved in it. Here, we have synthesized the polyaniline nanotubes to study interactions at the nanoscale within the emissive layer. Acid doping of PAni helped us to quantify the processes that suppress the emission, e.g., Förster resonance energy transfer (FRET) and charge transfer (CT). We have developed a material design perspective by demonstrating how an effective increase in conjugation length of PAni nanochain through 1D nanostructure growth can avoid the undesired emission quenching due to exciton quenching by the polarons. We have demonstrated how this approach helped us to preserve up to 95 % of the emission, which would otherwise be lost by exciton-polaron quenching interaction. Exploration of this new strategy could be a way forward for both fundamental and application aspects to enhance the emission efficiency of the conducting conjugate polymers and emissive layers in flexible polymer light-emitting diodes (PLEDs).

Fully Vacuum-Free Fabrication of Bi-Directional Polymer Light-Emitting Diodes Based on a Hybrid Lamination Top Electrode

This study investigates a hybrid lamination electrode based on a silver nanowire and a conductive polymer for the top electrode of bi-directional polymer light-emitting diodes (PLEDs). Through a vacuum-free hot-press lamination step, the proposed hybrid electrode employed a complete and concrete electrical contact with the underlying emissive polymer layer. In addition, by inserting a hole injecting poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) layer to increase the level of work function, the bi-directional PLED device with a laminated top electrode showed a more competitive device performance such as an operating voltage, a current efficiency, and a total external quantum efficiency compared to the counterpart device with a multilayered electrode of MoO3/Ag/MoO3 by the vacuum thermal evaporation method. An analysis carried out on the angular characteristics of bi-directional PLEDs including the variation of color coordinates and change in luminance values according to emission angles confirmed that the hybrid lamination electrode provided a more uniform angular distribution regardless of the direction of emission without any optical micro-cavity effect. Detailed optical and electrical analyses were also performed to evaluate the compatibility of the hybrid lamination electrode for the low-cost fabrication of efficient PLEDs.

Organometallic compound derivates as a novel efficient electron injection layer for hybrid light-emitting diodes

In this study we have presented the use of a organometallic compounds derivates (Keggin type polyoxometalates) for the production of powerful electron injection layer between the emissive polymer layer (PFO) and cathode layer (Al) . Electron transport studies in organometallic compounds layers showed efficient electron transport in very thin structures. High efficiency solution-processed organic light emitting diodes (OLEDs) are achieved by organometallic compounds such as Keggin type polyoxometalates used as electron injection or transport layers with superb electron mobilities and hole blocking capacities. These results indicate the potential for organometallic compounds derivates as a variable, emerging new class of efficient electron injection/transport molecular materials for high performance air-stable HyLEDs. The Keggin type polyoxometalates show unique features such as superior ionization energy and low electron affinity which conversion them as efficient electron injection/hole blocking layers and, immensely high electron mobility.

Investigation on Photoluminescence Behaviour of 2, 3-Diphenylquinoxalin-6-Vinyl Benzaldehyde

Poly (p-phenylene vinylene) is one of the most important classes of conjugated polymers, with a wide range of applications, such as lightemitting diodes, optoelectronic devices etc. Electroluminescence from OLEDs arises from the radioactive decay of excitation generated by the recombination of electrons and holes injection from two opposite electrodes into the emissive polymer layer. Quinoxaline is a useful n-type building block with high electron affinity and good thermal stability. It has been successfully incorporating small molecules to find utility as electron transport materials in multilayer OLEDs based on PPV. Hence, vinyl benzaldehyde containing quinoxaline derivatives was synthesized using 6-methyl-2, 3-diphenylquinoxaline with terephthaldicarboxaldehyde via Wittig reaction. The structures of synthesised compounds were confirmed by FT-IR, 1 H, 13C, 31P-NMR, and MASS spectral data. The result of photoluminescence studies showed that vinyl benzaldehyde capped quinoxaline exhibited green emission maxima at shorter wavelengths of 454nm.

Ethoxylated polyethylenimine as an efficient electron injection layer for conventional and inverted polymer light emitting diodes

We report inverted light emitting devices using ethoxylated polyethylenimine (PEIE) as a single electron injection layer for indium tin oxide cathode, which possess comparable efficiency to those using ZnO/PEIE double electron injection layers. Implementation of a PEIE layer between light emitting polymer layer and aluminum has been shown to significantly enhance device efficiency as well. Improvement of device efficiency can be attributed to increased electron injection due to the reduced work function of PEIE modified cathode as well as the hole blocking effect of PEIE layer. Furthermore, PEIE serves as an efficient electron injector for a range of light emitting polymers with wide distribution of energy levels.

Fully Solution-Based Fabrication of Flexible Light-Emitting Device at Ambient Conditions

Organic light-emitting devices (OLEDs) have long been perceived as a low-temperature, low-cost technology that can be fabricated through solution-based processes. However, how to achieve all-solution-processed OLEDs has been a challenge. Here, we report the fabrication of fully solution processed polymer light-emitting electrochemical cells (PLECs) as an alternative emissive device by spin-coating, rod-coating, and/or blade-coating at ambient conditions. The all-solution-based device, which can be fabricated under ambient air and in large-size, exhibits good flexibility and uniform light emission. The entire fabrication process of the PLEC, include the formation of electrodes, emissive polymer layer, and substrate, are carried out by solution processing under ambient conditions. Moreover, it is notable that employing a solution-processed and transparent SWCNT/AgNW bilayer composite electrode as anode, this fully solution processed PLEC shows even higher device performance than the conventional control dev...

Polymer light-emitting electrochemical cells: Recent developments to stabilize the p-i-n junction and explore novel device applications

Polymer light-emitting electrochemical cells (PLECs) employ a thin layer of a luminescent conjugated polymer admixed with an ionic source and an ionic conductor for the in-situ formation of p-i-n junction and subsequent efficient injections of both electrons and holes. The junction formation enables the use of air-stable conductors as the cathode and a relatively thick emissive polymer layer that is more compatible with low-cost solution-based processes. This paper overviews the operation mechanism of the PLECs, the properties and drawbacks of the devices. The employment of crosslinkable ionic conductors to stabilize the p-i-n junction is reviewed. The resulting static junction electroluminesces light at high brightness, high efficiency, and prolonged lifetime. Silver paste and carbon nanotubes can be used as the cathode, thus, PLECs were fabricated by lamination. Using single wall carbon nanotubes coated elastic substrate as both anode and cathode, the PLECs can be made highly stretchable.

Disubstituted Liquid Crystalline Polyacetylene Derivatives That Exhibit Linearly Polarized Blue and Green Emissions

We synthesized disubstituted liquid crystalline polyacetylene (di-LCPA) derivatives by polymerizing acetylene derivatives consisting of LC moieties either directly or indirectly attached to the main chain through flexible alkyl spacers. The di-LCPA derivatives show either enantiotropically thermotropic LC or lyotropic LC behavior. The origin of emission of substituted PAs, with respect to their substituents and structure, was elucidated. Depending on the substituents of the side chains, the polymers exhibit blue (470–485 nm) and green (500–540 nm) photoluminescence (PL) in chloroform and in cast film. The di-LCPA derivatives were macroscopically aligned using rubbing technique, and the aligned structures of the polymers are characterized in terms of main chain and side chain type alignments through XRD measurements. The emission color and alignment direction toward an external force in the di-LCPA derivatives are crucially determined by both the linkage forms (direct or indirect attachment) between the main chain and side chains and the molecular moieties (alkyl or aromatic moiety) of the side chains. The macroscopically aligned films of the polymers exhibit linearly polarized photoluminescence (LPL) with notable dichroic ratios. We fabricate multilayer electroluminescence (EL) devices using the polymers as the emissive polymer layer that emit 480 nm light with promising EL properties. We emphasize that although substituted PA derivatives are usually nonluminescent, the di-LCPA derivatives emit intense fluorescence with notable linear dichroism, and they could be promising for optically anisotropic luminescent materials.

Ion beam assisted deposition of an organic light emitting diode electrode

Abstract This work presents the electro-optical characterization of metal-organic interfaces prepared by the Ion Beam Assisted Deposition (IBAD) method. IBAD applied in this work combines simultaneously metallic film deposition and bombardment with an independently controlled ion beam, allowing different penetration of the ions and the evaporated metallic elements into the polymer. The result is a hybrid, non-abrupt interface, where polymer, metal and ion coexists. We used an organic light emitting diode, which has a typical vertical-architecture, for the interface characterization: Glass/ Indium Tin Oxide (ITO)/Poly[ethylene-dioxythiophene/poly{styrene sulfonic acid}]) (PEDOT:PSS) /Emitting Polymer/Metal. The emitting polymer layer comprised of the Poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-{2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene}] (PFO) and the metal layer of aluminum prepared with different Ar + ion energies varying in the range from 0 to 1000 eV. Photoluminescence, Current–Voltage and Electroluminescence measurements were used to study the emission and electron injection properties. Changes of these properties were related with the damage caused by the energetic ions and the metal penetration into the polymer. Computer simulations of hybrid interface damage and metal penetration were confronted with experimental data.

Post Doping by Wet Deposition Process in Polymer Light-Emitting Diode Fabrication for Color Tuning and Performance Improving

We present a wet deposition process for doping of fluorescent dye and then metal salt by diffusion into a single emitting polymer layer of triazole-end-capped polyfluorene (TazPFO), which can provide color tuning and performance improvement for polymer light-emitting diode fabrication. The original blue emission from TazPFO can be tuned into sky-blue, green, and orange-yellow after its thin film is dipped in the solutions of the fluorescent dyes 2,5,8,11-tetra-tert-butylperylene (TBPe), 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]-pyrano[6,7,8-ij]quinoli-zin-11-one (C545T), and 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), respectively. Further dipping the dye-doped polymer films into a solution of cesium carbonate to deposit cesium carbonate in the polymer surface region results in an enhancement of maximum current efficiency (and brightness) by a factor of 5.9 (13.5), 4.0 (17), and 1.4 (3.6) for TBPe-, C545T-, and DCJTB-dop...

69.2: High Efficiency Red‐Emission Phosphorescent Polymer Lighting Emitting Diodes Based on Ir(piq)3 Derivatives

AbstractIn this paper, several kinds Ir(piq)3 derivatives are used as dopants in polyfluorene to fabricate high performance pure red emission phosphorescent polymer light‐emitting‐diodes. Electron injection/hole blocking layer Cs2CO 3 is inserted between light emitting polymer layer and Al cathode. By modulating the electron injection ability from cathode using Cs2CO3 nano‐thick layer, electron current can be controlled precisely to balance hole current. Peak efficiencies of 16.9 cd/A and 17.6 lm/W at 220cd/m2 is achieved.

Improvement of device performances by thin interlayer in conjugated polymers/methanofullerene plastic solar cell

Recent studies have reported that a thin interlayer between poly(3, 4-ethylene dioxythiophene)-poly(styrene sulfonic acid) (PEDOT: PSS) and an emissive polymer layer leads to a large increase in the performances of polymer light-emitting diodes (PLEDs) by preventing significant quenching of the radiative excitons at the PEDOT: PSS interface; therefore, acting as an efficient exciton-blocking layer. Using the similar idea, a thin interlayer was fabricated between PEDOT: PSS and the active layer of conjugated polymers/methanofullerene composites in a plastic solar cell. The interlayer consisted of a poly(fluorene)-based hole transporter spin-coated directly on top of the PEDOT: PSS layer. The devices with the interlayer exhibited a higher efficiency than in those without the interlayer.

Improvement of Device Efficiency by Cross-Linkable Interlayer in Blue Polymer Light-Emitting Diodes

We report that high efficiency blue polymer light-emitting diodes obtained by adding a thin interlayer between a poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonic acid) (PEDT:PSS) hole transporter and emissive polymer. The one of OLED hole transporters, N,N′-bis(4-methylphenyl)-N,N′-diphenyl-1,4′-phenylenediamine (PDA), was modified with cross-linkable trichlorosilyl groups and used for the interlayer material. The devices, with configuration of indium tin oxide (ITO)/PEDT:PSS (65 nm)/interlayer (10–20 nm)/emitting polymer layer (70 nm)/BaF2 (2 nm)/Ca (50 nm)/Al (300 nm), were fabricated by spin coating and thermal evaporation. In this device structure, the cross-linkable interlayer is more adherent and mechanically robust as well as impervious to spin coating of next emitting polymer layer and shows a better stability. In addition, the devices with the interlayer exhibit a higher luminescence and current efficiency than those in devices without the interlayer.

The use of cross-linkable interlayers to improve device performances in blue polymer light-emitting diodes

High efficiency blue PLEDs were fabricated by adding a thin interlayer between PEDT:PSS and emitting polymer layers. Two different cross-linkable alkoxysilane-based interlayer materials, X-NPB and X-PDA, were synthesized based on N, N′-bis(4-methylphenyl)- N, N′-diphenyl-1,4-phenylenediamine (PDA) and N, N′-diphenyl- N, N′-bis(1-naphthyl)-1,1-biphenyl-4,4-diamine (NPB) which are well-known OLED HTLs. The devices, with configuration of indium tin oxide (ITO)/PEDT:PSS (65 nm)/interlayer (10–20 nm)/emitting polymer layer (70 nm)/BaF 2 (2 nm)/Ca (50 nm)/Al (300 nm), were fabricated by spin coating and thermal evaporation. In this device structure, the cross-linked X-NPB or X-PDA interlayers are more adherent and mechanically robust as well as impervious to spin coating of next emitting polymer layer. In addition, the devices with these interlayers exhibit a higher luminescence and current efficiency than those without interlayers because interlayers have two functions which are blocking electrons and preventing from severe quenching by PEDT:PSS.

Synthesis and characterizations of a polyimide containing a triphenylamine derivative as an interlayer in polymer light-emitting diode

Polyimide containing triphenylamine derivative (TPD-PI) was synthesized to prepare a polymer interlayer having insolubility in common nonpolar solvent for light-emitting polymers. N,N′-diphenyl- N,N′-bis(4-aminophenyl)-1,1-biphenyl-4,4′-diamine, as a new triphenylamine-containing diamine monomer, was synthesized by the palladium-catalyzed amination reaction between 4-nitrodiphenylamine and 4,4′-dibromobiphenyl and subsequent reduction of the nitro-intermediate. The TPD-PI was prepared from the synthesized diamine monomer and 4,4′-(hexafluoropropylidene)-diphthalic anhydride by the standard two-step polymerization method, which involved ring-opening polymerization and subsequent cyclodehydration. The structures and properties of the monomer and the resulting polyimide were characterized with 1H and 13C NMR, elemental analysis, gel permeation chromatography, UV–visible spectroscopy, etc. The TPD-PI is readily soluble in aprotic polar solvents such as N-methyl-2-pyrrolidinone and N,N-dimethylformamide and insoluble in nonpolar solvents such as toluene and xylene. The highest occupied molecular orbital (HOMO) level of the TPD-PI was measured to be 5.5 eV by a photoelectron spectrometer in air, and the band gap was calculated as 3.1 eV from the onset of UV–vis spectrum. The polymer light-emitting diode with the thin TPD-PI layer between a hole injection layer and an emitting polymer layer was fabricated to examine the performance of the polyimide as an polymer interlayer. Although the luminous efficiency of the device is lowered by the introduction of the TPD-PI interlayer, the lifetime of the device is improved.

High efficiency polymeric light-emitting diodes with a blocking layer

In this paper, we describe a simple method for fabrication of high efficiency multilayered polymeric light-emitting diodes (PLEDs). In the device configuration of ITO/HIL/BL/EML/BaF 2:Ca:Al (ITO: indium tin oxide, BL: blocking layer, HIL: hole injection layer as PEDOT:PSS, EML: emitting layer as poly(alkylfluorene) derivative), poiy(9-vinyl carbazole) (PVK) was selected as an BL because it has a hole transporting property and a relatively higher band gap, especially a lower LUMO level than the emitting polymer and higher HOMO level than PEDOT:PSS. As a result, the PVK layer could effectively block the electrons from metal cathode and confine them in the emitting polymer layer. In addition, it could properly control the amount of holes from ITO anode by partial blocking. The charge balance was achieved and the device with a PVK. layer exhibits a higher brightness and luminance efficiency than those in devices without PVK. Also, the thickness of PVK was easily controlled by solvents for emitting polymer and the device with a thicker PVK layer shows better performance.

Synthesis and Luminescent Properties of Fluorene Copolymers Bearing DCM Pendants

A series of copolymers, poly[(9,9-dioctylfluorene-2,7-diyl)-co-(4-dicyanomethylene-2-methyl-6-[4-(diphenylamino)styryl]-4H-pyran-4‘,4‘ ‘-diyl)], were synthesized by polymerizing 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene with mixtures of 2,7-dibromo-9,9-dioctylfluorene and 4-dicyanomethylene-2-methyl-6-[bis(4‘-bromophenyl)amino]styryl]-4H-pyran (a DCM derivative) by the palladium-catalyzed Suzuki coupling reaction. The copolymers were characterized by molecular weight determination, elemental analysis, 1H NMR, FT-IR spectroscopy, DSC, TGA, UV−vis spectroscopy, and photoluminescence (PL) and electroluminescence (EL) spectroscopy. The copolymers showed two absorption peaks at 380 and 485 nm, and the long-wavelength absorption increased with increasing the fraction of the DCM comonomer. The PL spectra of copolymers in chloroform solution displayed emission from both the main chain (420 nm) and DCM units (620 nm). In the solid state, however, PL spectra of copolymers showed only the long wavelength red emission at 620 nm with no trace of emission from the main chain, which implies a facile exciton migration or energy transfer to the lower energy sites from the fluorene part to the DCM part. This results in emission of only the red light originating from the latter segments. A study on time-resolved PL rise and decay of the polymers clearly supports the energy transfer mechanism. Light-emitting diode (LED) devices were fabricated to have the configuration of ITO (indium−tin oxide)/PEDOT/polymer/Li:Al alloy. EL spectra of the devices showed only red emissions as observed in the PL spectra of the polymers' thin films. EL efficiency decreased with increasing DCM contents. When a tris(8-hydroxyquinolinato)aluminum (Alq3) layer was inserted between the emitting polymer layer and the cathode to make the ITO/PEDOT/polymer/Alq3/Li:Al alloy configuration, the device efficiencies became much higher (∼10-2%) than those (5 × 10-5−5 × 10-3%) of single-layer devices.

Electroluminescent behavior of polydiacetylene films

We observed the electroluminescent behavior of films of the polydiacetylene poly[1,6-bis(3,6-dipalmitoyl-N-carbazolyl)-2,4-hexadiyne] (polyDPCHD) by preparing organic light-emitting diodes (OLED) based on a single emitting polymer layer. We measured their optical and electrical characteristics and obtained a typical power efficiency of 10−6. The observed rapid quenching of the electroluminescence was supposed to be related to the formation of charged traps enhanced both by local disorder of polymer chains and by current flow.

Color tuning and efficiency enhancement through interfacial energy transfer and thickness control in light-emitting diodes

We report a possible interfacial energy transfer phenomenon in double-layer light-emitting diodes (LEDs) consisting of a poly(p-phenylene vinylene) (PPV) hole-transport layer and an emitting polymer layer containing either of two blue emitting polymers (I and II). The polymers in film form have almost the same photoluminescence (PL) spectra though they have different band gaps as determined from their absorption and PL spectra. This is due to intermolecular∕intramolecular energy transfer in polymer II films since the latter structure contains two differing oligomeric chromophores regularly interspersed by long nonconjugated blocks. In double-layer LEDs, PPV functions not only as a hole-transport layer which improves hole injection but also as a chromophore participating in the tuning of the LED emission color through interfacial energy transfer. The significant spectral overlap between the absorption spectrum of PPV and the emission spectra of I or II supports this energy transfer. The presence of PPV can modify the LED color from blue to blue-green or green and also contributes to a high external quantum efficiency. Poly(3,4-ethylenedioxythiophene)∕poly(styrene sulfonate) was also used as a hole-transport layer and its LEDs were compared to PPV-containing devices. Thicker films of the conjugated polymer chromophores produced red shifts in both PL and electroluminescence (EL) spectra which was attributed to molecular packing and self-absorption effects. At higher applied voltage, these LEDs showed blue shifts, and the emission corresponding to a higher energy transition was enhanced at the expense of emissions from the lower energy transitions. Chromophore heating is also held responsible for some of the blueshifts in EL. The thicker LEDs showed lower efficiency because of self-absorption.

Charge transport and trapping in Cs-doped poly(dialkoxy-p-phenylene vinylene) light-emitting diodes

Al/Cs/MDMO-PPV/ITO (where MDMO-PPV stands for poly[2-methoxy-5-${(3}^{\ensuremath{'}}{\ensuremath{-}7}^{\ensuremath{'}}$-dimethyloctyloxy)-1,4phenylene vinylene] and ITO is indium tin oxide) light-emitting diode (LED) structures, made by physical vapor deposition of Cs on the emissive polymer layer, have been characterized by electroluminescence, current-voltage, and admittance spectroscopy. Deposition of Cs is found to improve the balance between electron and hole currents, enhancing the external electroluminescence efficiency from 0.01 cd ${\mathrm{A}}^{\mathrm{\ensuremath{-}}1}$ for the bare Al cathode to a maximum of 1.3 cd ${\mathrm{A}}^{\mathrm{\ensuremath{-}}1}$ for a Cs coverage of only $1.5\ifmmode\times\else\texttimes\fi{}{10}^{14}{\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{m}}^{2}.$ By combining $I\ensuremath{-}V$ and admittance spectra with model calculations, in which Cs diffusion profiles are explicitly taken into account, this effect could be attributed to a potential drop at the cathode interface due to a Cs-induced electron donor level 0.61 eV below the lowest unoccupied molecular orbital. In addition, the admittance spectra in the hole-dominated regime are shown to result from space-charge-limited conduction combined with charge relaxation in trap levels. This description allows us to directly determine the carrier mobility, even in the presence of traps. In contrast to recent literature, we demonstrate that there is no need to include dispersive transport in the description of the carrier mobility to explain the excess capacitance that is typically observed in admittance spectra of \ensuremath{\pi}-conjugated materials.