Polymer-based Organic Light-emitting Diodes Research Articles

Effect of Drying Condition of Emitting Layer Formed by Ink-Jet Coating on Optical Property and Film Morphology of Polymer-Based Organic Light-Emitting Diodes

The effect of a controlled drying process (CDP) of a polyfluorene [poly(9,9-dioctylfluorenyl)-2,7-diyl, PFO] based emitting layer formed by ink-jet coating on the optical properties and film morphologies of organic light-emitting diodes (OLEDs) was studied. The dominant wavelengths (DWLs) obtained from chromaticity shifted stepwise from 469 to 488 nm along neighboring cells of the emitting layer in a coating batch without the controlled drying process (without CDP). In contrast, the DWLs with the controlled drying process (with CDP), in which solvent mist is blown over the surface of the cell during ink-jet coating, did not show any chromaticity shift. The PFO film thicknesses measured using an optical interferometer also varied stepwise from 0.18 to 0.21 μm in neighboring cells in the coating batch without CDP, while those with CDP did not vary. Moreover, the surface profiles of each cell with CDP were flat and uniform. Therefore, we found that applying a CDP was effective in forming uniform PFO films with homogeneous optical properties.

Kinetics of thermally activated triplet fusion as a function of polymer chain packing in boosting the efficiency of organic light emitting diodes

Understanding the photophysical process governing the operation of the organic light emitting diodes (OLEDs) and how they are affected by film morphology is crucial to the efficient design of future OLEDs. In particular, delayed fluorescence (DF), is known to contribute a significant fraction of the light emission from polymer-based OLEDs, but its mechanism remains unclear. Here, we investigate the origin of DF in the state of the art OLED polymer Poly (9, 9-dioctylfluorene-alt-benzothiadiazole) (F8BT), under both optical and electrical excitation using time-resolved emission spectroscopy (TRES) as a function of film thickness, excitation fluence, magnetic-field, and temperature. The temperature dependence of the DF for various film thicknesses suggests that thermally activated triplet migration is the dominant process controlling DF at room temperature. We found that thermal activation energy (Eeff) of triplet migration decreases from 179 ± 31 meV to 86 ± 11 meV as film thickness varied from ~110 nm to ~560 nm, respectively. The Eeff of triplet migration is found to be a function of the molecular packing of polymer chains as determined from synchrotron grazing incidence wide angle x-ray scattering (GIWAXS) studies and steady-state photoluminescence studies. Quantum chemical calculations of reorganization energy and singlet–triplet exchange energy gap in F8BT molecule as a function of the dihedral angle between donor & acceptor moiety also confirm the experimental results. Our results show that DF in polymer OLEDs is significantly affected by parameters such as the film thickness and disorder, allowing for a high degree of control over the underlying photophysics to be achieved.

Current density of anodes, recombination rate and luminance in MEH-PPV, MDMO-PPV, and P3HT polymers in polymer light-emitting diodes

The present study examined polymer-based organic light-emitting diodes (PLEDs) with the different active layers using Silvaco Atlas software. The electrical and optical responses of the devices were evaluated through the characteristics of current-voltage, luminance, and recombination rate. The performance of the device strongly depends on the physical properties of the active layer. A better understanding of the effect of varying the parameters of the active layer, such as mobility, lifetime, and band gap, on the functioning of the device is necessary to achieve further improvement in these applications. This work presents a theoretical study of PLEDs having different active layers. Poly[2-methoxy-5-(3’,7’-dimethyloctyloxy)-1,4-phenylenevinylene], poly(3-hexylthiophene), and poly[2-methoxy-5-(2’-ethylhexyloxy)-p-phenylene vinylene] organic layers were deposited on an ITO substrate and the structure of the device was completed by a calcium cathode with a work function of 2.9 eV. The Langevin and Poole-Frenkel models were used to analyze the devices. The Langevin model was used to analyze the recombination rate and investigate the relationship between luminance, recombination rate, and the parameters of the active layer that have been less studied in previous works. Analysis of anode current density, luminance, and recombination rate indicate that poly(3-hexylthiophene) PLED shows better performance than others.

Influence of doped Alq3 layer on performance of MEH-PPV, MDMO-PPV, and P3HT polymer light-emitting diodes

The present study examined the influence of a doped Tris(8-hydroxyquinolinato) aluminium (Alq3). Layer on the performance of polymer-based organic light-emitting diodes (PLEDs) using Silvaco Atlas software. The luminance and recombination rate of a device depends on the physical properties of the active layer and of the effective layers on the active layer. Insertion of a doped Alq3 layer between the active layer and cathode increased the recombination rate of the middle layer that had been limited by the anode and cathode of the device. Better understanding the effect of the doped Alq3 layer on the parameters of mobility, lifetime, and band gap of the active layer and the performance of the device is necessary to achieve further improvement in these applications. This work presents a theoretical study of the influence of the Alq3 layer on PLEDs having different active layers. Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), poly(3-hexylthiophene) (P3HT), and poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) organic layers were deposited onto an ITO substrate that was completed using calcium as a cathode with a work function of 2.9 eV. The Langiven model was used to analyze the recombination rate and luminance and indicated that the doped Alq3 layer between the active layer and cathode increased performance of the middle layer and improved the P3HT, MEH-PPV, and MDMO-PPV PLEDs. The MEH-PPV device showed the most improvement in comparison with the MDMO-PPV and P3HT devices. This originated from appropriate overlapping of the doped Alq3 layer and the MEH-PPV active layer.

Cost, energy and emissions assessment of organic polymer light-emitting device architectures

Proponents for sustainable alternative lighting and display options advocate for organic light-emitting diodes (OLEDs), particularly polymer-based organic light-emitting diodes (P-OLEDs), because of their potential for low-cost fabrication, more versatile device formats and lower power consumption compared to traditional options. Here, an economic, energy and CO2 emissions assessment is carried out for four different laboratory-scale, blue-emitting P-OLED device architectures: bottom-emitting conventional; bottom-emitting inverted; top-emitting conventional; and top-emitting inverted. Additionally, comparisons with a standard, commercial-scale, blue inorganic light-emitting diode (LED) device architecture are made. The various P-OLED device architectures are investigated due to their potential to increase operational lifetime (inverted) and light out-coupling efficiency (top-emitting). The following metrics are used in this assessment: device cost per area; yearly operating cost; optical power cost; CO2 emissions from device production; and yearly operating CO2 emissions. We show that the top-emitting inverted device architecture significantly reduces the device cost per area, yearly operating cost, optical power cost and CO2 emissions for the P-OLED devices, due to elimination of indium tin oxide and its comparatively high luminous efficacy and longer lifetime. In addition, the top-emitting inverted P-OLED device architecture performs competitively at the laboratory scale with commercial-scale inorganic LEDs for all metrics. However, if top-emitting P-OLEDs are to be manufactured on a large scale, the luminous efficacy assumed for laboratory-scale devices needs to remain constant throughout development to remain competitive.

Recent progress of high performance polymer OLED and OPV materials for organic printed electronics

The development of organic printed electronics has been expanding to a variety of applications and is expected to bring innovations to our future life. Along with this trend, high performance organic materials with cost-efficient fabrication processes and specific features such as thin, light weight, bendable, and low power consumption are required. A variety of organic materials have been investigated in the development of this field. The basic guidelines for material design and the recent progress of polymer-based organic light-emitting diodes (OLEDs) and organic photovoltaic cells (OPVs) are reported.

Comparison of light out-coupling enhancements in single-layer blue-phosphorescent organic light emitting diodes using small-molecule or polymer hosts

Single-layer blue phosphorescence organic light emitting diodes (OLEDs) with either small-molecule or polymer hosts are fabricated using solution process and the performances of devices with different hosts are investigated. The small-molecule device exhibits luminous efficiency of 14.7 cd/A and maximum power efficiency of 8.39 lm/W, which is the highest among blue phosphorescence OLEDs with single-layer solution process and small molecular hosts. Using the same solution process for all devices, comparison of light out-coupling enhancement, with brightness enhancement film (BEF), between small-molecule and polymer based OLEDs is realized. Due to different dipole orientation and anisotropic refractive index, polymer-based OLEDs would trap less light than small molecule-based OLEDs internally, about 37% better based simulation results. In spite of better electrical and spectroscopic characteristics, including ambipolar characteristics, higher carrier mobility, higher photoluminescence quantum yield, and larger triplet state energy, the overall light out-coupling efficiency of small molecule-based devices is worse than that of polymer-based devices without BEF. However, with BEF for light out-coupling enhancement, the improved ratio in luminous flux and luminous efficiency for small molecule based device is 1.64 and 1.57, respectively, which are significantly better than those of PVK (poly-9-vinylcarbazole) devices. In addition to the theoretical optical simulation, the experimental data also confirm the origins of differential light-outcoupling enhancement. The maximum luminous efficiency and power efficiency are enhanced from 14.7 cd/A and 8.39 lm/W to 23 cd/A and 13.2 lm/W, respectively, with laminated BEF, which are both the highest so far for single-layer solution-process blue phosphorescence OLEDs with small molecule hosts.

Análisis de la característica corriente-voltaje de diodos orgánicos emisores de luz (OLEDs) basados en polímeros depositados por spin coating.

<p>Polymer-based organic light-emitting diodes (OLEDs) with the structure ITO / PEDOT:PSS / MDMO-PPV / Metal were prepared by spin coating. It is known that electroluminescence of these devices is strongly dependent on the material used as cathode and on the deposition parameters of the polymer electroluminescent layer MDMO-PPV. <strong>Objective.</strong> In this work the effect of i) the frequency of the spin coater (1000-8000 rpm), ii) the concentration of the MDMO-PPV: Toluene solution, and iii) the material used as cathode (Aluminium or Silver) on the electrical response of the devices, was evaluated through current-voltage (I-V) measurements. <strong>Materials and methods</strong>. PEDOT:PPS and MDMO-PPV organic layers were deposited by spin coating on ITO substrates, and the OLED structure was completed with cathodes of aluminium and silver. The electric response of the devices was evaluated based on the I-V characteristics. <strong>Results.</strong> Diodes prepared with thinner organic films allow higher currents at lower voltages; this can be achieved either by increasing the frequency of the spin coater or by using concentrations of MDMO-PPV: Toluene lower than 2% weight. A fit of the experimental data showed that the diodes have two contributions to the current. The first one is attributed to parasitic currents between anode and cathode, and the other one is a parallel current through the organic layer, in which the carrier injection mechanism is mediated by thermionic emission. <strong>Conclusions.</strong> The results fitting and the energy level alignment through the whole structure show that PPV-based OLEDs are unipolar devices, with current mainly attributed to hole transport.</p> <p><strong>Key words:</strong> organic semiconductors, OLEDs, electroluminescent polymers, MDMO-PPV, PEDOT:PSS, Spin coating, HOMO, LUMO, carrier injection, thermionic emission.</p><br />

Enhanced luminance of organic light-emitting diodes with metal nanoparticle electron injection layer

Improvement of the performance of organic light-emitting diodes (OLEDs) was achieved by implementing Magnesium-Nickel nanoparticles at the cathode–organic interface using pulsed laser deposition technique. The small geometry of Mg-Ni nanoparticles acts to enhance the localized electric field around them, thus increasing electron injection through tunneling, from the cathode to the organic layer. Improved current and luminance characteristics were demonstrated for both small molecule and polymer-based OLEDs when the nanoparticle layer was incorporated.

Highly efficient solution-processed phosphorescent multilayer organic light-emitting diodes based on small-molecule hosts

The authors report on highly efficient phosphorescent organic light-emitting diodes (OLEDs) based on a low-molecular weight electron-conducting, bis-spirobifluorene host doped with a soluble derivative of the green emitter fac-tris(2-phenylpyridine) iridium (III) [Ir(ppy)3]. All organic layers were spin coated and a strong improvement of performance was achieved by introduction of a hole-transporting double layer based on cross-linkable low-molecular weight molecules. The devices combine the easy fabrication procedure known from polymer-based OLEDs with the higher efficiency of small molecules. Maximum luminous and power efficiencies of 59cd∕A and 58lm∕W, respectively, are obtained, combined with a low driving voltage and high efficiencies even at high brightnesses. At 1000cd∕m2 the efficiencies are as high as 55cd∕A and 49lm∕W.

Polymer Light-Emitting Diode Using a New Electrophosphorescent Cyclometalated Iridium Complex

ABSTRACT A new iridium-based cyclometalated complex, namely bis (2-phenylpyridine–C2, N′) iridium (III) picolinate [(ppy)2Ir(pic)] is synthesized and investigated as electrophosphorescent dopant in polymer-based organic light-emitting diodes. The photoluminescence emission from this complex is observed at 501 nm, with a shoulder at 526 nm. Poly(N-vinylcarbazole) (PVK) is used as host polymer and single-layer devices of PVK doped with (ppy)2Ir(pic) complex are fabricated. Photoluminescence (PL) spectra of PVK films doped with different concentration of iridium complex were measured to study the possible energy transfer occurring between PVK to iridium complex. A predominant blue to a completely green shifted emission is observed when the doping concentration was increased from 0% to 5 wt%. Optical and atomic force microscopic images of the doped films indicate a smooth film formation when spin coated with chlorobenzene as a solvent as compared to chloroform. Electroluminescence spectra of ITO/PEDOT:PSS/PVK:(ppy)2Ir(pic)/Al device resembles that of the PL spectra with emission at 507 nm and Commission Internationale de l'Eclairage (CIE) coordinates (0.29, 0.57).