Performance Of Polymer Light-emitting Diodes Research Articles

Effect of adjusting charge transport on optoelectronic performances of polymer light-emitting diodes based on SY-PPV

Effect of adjusting charge transport on optoelectronic performances of polymer light-emitting diodes based on SY-PPV

Improved performance of polymer light-emitting diodes using graphene oxide doped with silver nanoparticles as the hole-transport layer

This study successfully demonstrates the fabrication of a thin film of graphene oxide doped with silver nanoparticles (GO:AgNPs) using electrochemical exfoliation and photochemical synthesis. The GO:AgNPs thin film exhibits a higher work function and conductivity compared to poly[3,4-ethylenedioxythiophene] polystyrene sulfonate (PEDOT:PSS), making it a promising candidate for the hole-transport layer in polymer light-emitting diodes (PLEDs). Among various configurations, a PLED incorporating GO:AgNPs as the interlayer, which is prepared under blue-light illumination, exhibits optimal luminescent performance. This configuration achieves the highest luminance of 6341 cd/m2 and the highest average current efficiency of 1.87 cd/A. Notably, this novel PLED demonstrates a remarkable 168 % enhancement in average current efficiency and an impressive 637 % increase in electroluminescence intensity compared to a PLED employing a PEDOT:PSS interlayer.

Tuning photophysical properties of PFO conjugated polymer for enhancing polymer light emitting diode performance via addition of SiO2/TiO2 nanocomposite

Tuning photophysical properties of poly(9,9′-di-n-octylfluorenyl-2.7-diyl) (PFO) by addition SiO2/TiO2 nanocomposites (STNCs) with different weight ratios is demonstrated. The PFO/STNCs thin films were prepared successfully by solution blending method followed by spin coating technique. The structure properties were investigated via X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM). The optical properties were determined by using UV–Vis and photoluminescence spectrophotometer, while a Keithley 238 measurement system and HR2000 Ocean Optic Spectrometer were employed to investigate the optoelectronic properties of the PFO/STNCs based polymer light emitting diodes (PLED). The dominant phase in the PFO/STNCs thin films was for the broad hump of PFO and no variation on structure of PFO was observed upon increment the STNCs content. The aggregation of the thin films with higher STNCs content was evidenced theoretically by S-factor calculations and experimentally by FE-SEM images. The extension in the PFO conjugation length upon addition the STNCs, which plays unique role for improvement the performance of PLED devices as proved here by current density and luminance measurements, was evidenced by the systematic red shift of absorption peak of the PFO and vibronic band transitions and extended absorption tail as well.

Controllable β-phase formation in poly(9,9-dioctylfluorene) by dip-coating for blue polymer light-emitting diodes

The performance of polymer light-emitting diodes based on poly(9,9-dioctylfluorene) (PFO) is strongly affected by the presence of β-phase in the polymer films. In this work, β-phase PFO films were developed by a facile method dip-coating in one step. Absorption spectra with an additional peak at 435 nm and red-shifted emission spectra demonstrate the formation of β-phase in the dip-coated films. Notably, β-phase content in the PFO films can be tailored by controlling the withdrawal speed. The experimental results show that the content of β-phase in PFO films increases from 1.4 % to 23.2 % with the decreasing of the withdrawal speed from 60 mm/min to 5 mm/min, and the device efficiency and color purity for the blue emission get improved due to the formation of β-phase. When the withdrawal speed is 5 mm/min, the devices possess the optimal performance with a current efficiency of 2.47 cd/A, which is 3.6 times as high as that of the device with the spin-coated PFO film as emitting layer. These results demonstrate that as a simple methodology, the dip-coating technology can effectively induce the molecular chain orientation in the film to regulate the optoelectronic properties for applications in efficient optoelectronic devices.

Highly efficient polymer light-emitting devices based on sodium compounds electron injection layer

Enhancing the injection of electron is an effective strategy to improve the performance of polymer light-emitting diodes (PLEDs). In this work, electron injection was improved by inserting sodium compounds NaCl, NaBr, NaI and Na2CO3, respectively, between emissive layer (EML) and electron injection layer (EIL). A maximum 49.6% increase in current efficiency (CE) of PLEDs from 10.85 to 16.24 cd/A was achieved based on NaCl. The Vbi values of PLEDs were investigated to analyze the performance enhancement of PLEDs, which can be ascribed to the migration of Na+. The effect of NaCl, NaBr, NaI and Na2CO3 on the performance of PLEDs were compared to investigate the influence of the anion of sodium compounds. Our work provides an effective and simple method to improve the performance of PLEDs, which is also compatible with the roll-to-roll technology.

Effect of the Hole Injection Layer Conductivity on the Performance of Polymer Light-Emitting Diodes

Thus, our results show that the performance of PLEDs can be significantly enhanced by simple modification of the conductivity of polymer HILs using low-cost and solution-processed organic dopant molecules.

Synthesis, characterization, and optoelectronic properties of phenothiazine-based organic co-poly-ynes

We present the synthesis of seven new organic co-poly-ynes P1–P7 incorporating phenothiazine (PTZ) motif and evaluate their optoelectronic properties and performance in polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs).

Mechanism study on highly efficient polymer light-emitting diodes utilizing double-layered alkali halide electron injection layer

Enhancing the injection of electron is an effective strategy to improve the performance of polymer light-emitting diodes (PLEDs). In this work, we reported a 286% improvement in current efficiency (CE) of PLEDs by using double-layered alkali halide electron injection layer (EIL) NaCl/LiF instead of LiF. A significant enhancement of electron injection was observed after inserting the NaCl layer. To understand the mechanism of such improvement, the devices with KBr/LiF and CsF/LiF as EILs were also investigated. Experimental results show that metal cation migrated under the effect of built-in electric field (Vbi), which plays the main role on the improvement of electron injection in PLEDs.

Efficacy improvement in polymer LEDs via silver-nanoparticle doping in the emissive layer.

The coupling of surface plasmons and excitons in the emissive layer (EML) can improve the performance of polymer light-emitting diodes (PLEDs). Silver nanoparticles (Ag-NPs) with a decahedron structure are prepared by the chemical reduction and photochemical methods and doped directly into the EML after the phase-transfer process. The surface plasmon resonance effect of Ag-NPs, which makes full use of quenched excitons and increases the efficiency of excitons in the EML in a PLED, enhances the current efficacy by a factor of 75 relative to that of the undoped reference device (from 0.22 to 16.64cd/A). These results demonstrate that Ag-NPs can assist in simple and low-cost fabrication of high-performance polymer optoelectronic devices.

Highly Promoting the Performances of Polymer Light-Emitting Diodes via Control of the Residue of a Polar Solvent on an Emissive Layer.

The polar solvent dimethylformamide (DMF) was used to treat the emissive layer (EML) of polymer light-emitting diodes (PLEDs). The formation of a dipole layer at the EML/cathode interface after DMF treatment was proven, which led to a reduction of the electron-injection barrier. The dipole layer was formed mainly because of the intrinsic polarity of DMF. By control of the residue of DMF on the EML, a maximum enhancement of the peak luminous efficiency from 5.33 ± 0.57 to 12.05 ± 1.2 cd/A was achieved. This study suggests that solvent treatment is a simple and efficient approach to realizing highly efficient PLEDs with a high-work-function metal cathode.

Polymer Light-Emitting Diode Prepared by Floating-Off Film-Transfer Technique

Floating-off film-transfer technique was used for the formation of semiconducting polymer multi-layers and the effect on the performance of polymer light-emitting diode (PLED) was studied. This method made it possible to avoid the solvent compatibility problem that was typically encountered in successive coating of polymeric multilayer by solution processing. F8BT and MEH-PPV were used for electron transporting layer (ETL) and for emissive layer, respectively. Current-voltage-luminance characteristics and luminescence efficiency results showed that the insertion of ETL by floating-off film-transfer technique followed by proper heat treatment resulted in a significant improvement in PLED operation due to its electron-transporting and hole-blocking abilities.

Methanol treatment on low-conductive PEDOT:PSS to enhance the PLED's performance

In order to achieve high device efficiency in blue-emitting polymer light-emitting diodes (PLED) and white-emitting PLED, low-conductive PEDOT:PSS Clevios™ P CH 8000 (8000) has been used to replace widely adopted high-conductive PEDOT:PSS Clevios™ P Al 4083 (4083). In a blue PLED with poly (dibenzothiophene-S,S-dioxide-co-9,9-dioctyl-2,7-fluorene) (PF-FSO) as the emission layer, though the 8000 device has a higher efficiency and a lower turn-on voltage than the 4083 device, the 8000 device exhibits low peak luminance, sharp efficiency roll-off, and permanent degradation of the emission material. By analyzing the hole-only device and the X-ray photoelectron spectroscopy spectra, it's revealed that the insulating PSS layer on 8000 surface is responsible for the inefficient hole injection. A simple methanol treatment on the 8000 surface effectively removes the redundant PSS. Without the insulating PSS layer, the hole injection becomes efficient which extends the recombination zone. As the result, the methanol-treated 8000 device not only retains low turn-on voltage and the high device efficiency of the untreated 8000 device, but also achieves the peak luminance, mild efficiency roll-off, and device stability of the 4083 device.

The effects of solvent vapor annealing on the performance of blue polymer light-emitting diodes

In this manuscript, we reported the effects of solvent vapor annealing on performances of blue polymer light-emitting diodes, where the blue light-emitting poly(dibenzothiophene-S,S-dioxide-co-dioctyl-2,7-fluorene) that comprising dibenzothiophene-S,S-dioxide with molar ratio of 10% was used as the emissive layer, and toluene or chloroform was use as the annealing solvents. It was noted that the performances of PLED in terms of both luminous efficiency and brightness were obviously enhanced by solvent vapor annealing treatment, which can be correlated to the improved film morphology. The formation of β-phase lead to more balanced electron and hole flux as evaluated by single carrier devices. The best device performance with luminous efficiency of 4.8 cd/A was realized, which increased about 70% relative to that of 2.77 cd/A for the pristine device. These observations demonstrated that the solvent vapor annealing approach can be an effective strategy to improve performances of polymer light-emitting diodes.

Graphene oxide hole injection layer for high-efficiency polymer light-emitting diodes by using electrophoretic deposition and electrical reduction

Graphene oxide (GO) is a fascinating nanomaterial with tremendous potential for electronic devices which show high performance when the properties of GO are properly tuned like reduced graphene oxide (RGO). Here, we report a simple, cost effective and precisely controllable methods to fabricate high-quality RGO films as a hole injection layer (HIL) for high efficiency polymer light-emitting diodes (PLEDs). First, GO is electrophoretically deposited and electrically reduced to produce RGO films. The deposition and reduction of films are electrical methods; hence the thickness and the degree of reduction for GO films can be easily manipulated by controlling electrical parameters and time. The performance of PLEDs with RGO film as HIL is measured and compared with GO and (poly(3,4-ethylenedioxythiophene)polystyrene sulfonate) (AI4083), and shows better results for luminance and current density. The optimized maximum luminance and current efficiency of the PLEDs with RGO as HIL are found to be 12830cd/m2 and 3.35cd/A, respectively.

The effects of compressive stress on the performance of organic light-emitting diodes

We investigated the effects of compressive stress on the performance of polymer light-emitting diodes (PLEDs). Vertical contraction of the PLEDs under compressive stress was induced by loading the PLEDs during operation. The current density–luminescence–voltage (J–L–V) characteristics of the PLEDs under stress were measured and compared with those of unloaded PLEDs. To explain the changes in the PLED performance due to the stress, the chemistry and electronic energy band structures of each stressed layer constituting the PLEDs were analyzed via X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS), respectively. According to the analysis, an irreversible deterioration in the fluorescent characteristics of the active layer and an increased electron energy barrier between the layers of the PLEDs were induced by the applied stress, which resulted in the significant degradation of the J–L–V characteristics.

Solution processed polymer light emitting diode with vanadium-oxide doped PEDOT:PSS

We studied the solution processed vanadium pentoxide (V2O5) doping effect in PEDOT:PSS for use in hole injection layer (HIL), and as a result, V2O5 doped PEDOT:PSS improved the performance of polymer light-emitting diode (PLED) significantly. The improvement is related to the increased conductivity of PEDOT:PSS by V2O5 doping. This improvement enhances hole injection into the emission layer, and it will improve the charge balance. The PLED with PEDOT:PSS HIL and super yellow polymer has maximum current and power efficiencies of 12.01 cd A−1 and 9.67 lm W−1, respectively. On the other hand, the device with V2O5 (10:1) doped PEDOT:PSS indicates the maximum current and power efficiencies of 15.12 cd A−1 and 12.66 lm W−1, respectively, indicating that the efficiencies increase over 20% compared to the control device. The leakage currents of the PLED decrease to ∼10% of the control PLED. Note also that V2O5 suppressed the diffusion of indium from anode to PEDOT:PSS, and also prevented the acidic damage from sulfate ions of PEDOT:PSS to IZO and active layer.

An electrochemically deposited film as an interface layer to improve the performance of polymer light-emitting diodes

An insoluble electrochemically cross-linked thin film is successfully applied as an interface layer between PEDOT:PSS and the light emitting layer in solution-processed polymer light-emitting diodes for enhanced device performance.

Effect of PEDOT–PSS resistivity and work function on PLED performance

The effect of a commonly used hole injection layer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT–PSS), on polymer light-emitting diode (PLED) performance has been investigated. A series of four different types of commercial PEDOT–PSS, with varying resistivity and work function were examined in devices with the structure Indium Tin Oxide (ITO)/PEDOT–PSS/High Molecular Weight Poly(n-vinylcarbazole) (PVKH): 30% N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD)/Low molecular Weight Poly(n-vinylcarbazole) (PVKL): 40% 2-(4-Biphenyl)-5-(4-tert-butylphenyl)-1,2,4-oxadiazole (PBD): 8% Ir(ppy)3. It was found that the PEDOT–PSS with the highest work function and resistivity produced the devices with the highest efficiencies; this is due to the improved hole injection effect, the decrease in electron leakage current and the prevention of pixel crosstalk. A maximum device current efficiency of 33.4cdA−1 has been achieved for the most resistive PEDOT; this corresponded to an external quantum efficiency (E.Q.E.) of 11%. Increasing the work function of the PEDOT used resulted in a 60% increase in E.Q.E. and device efficiency for PEDOTs in the same resistivity range. Drift–diffusion simulations, carried out using SEmiconducting Thin Film Optics Simulation software (SETFOS) 3.2, produced J–V curves in good agreement with the experimentally observed results; this allowed us to extract qualitative values for the effective device mobility along with the PEDOT work function and resistivity.

Influence of the anion on the optoelectronic characteristics of triphenylsulfonium salts modified polymer light emitting devices

Triphenylsulfonium salts addition in the emitting layer of polymer light emitting diodes (PLEDs) has been shown to be beneficial for charge injection and transport due to both ionic effects and π-conjugation in the phenyl rings of the cation. In some cases the emission profile can be also modified through an electroplex formation. Herein we investigate the effect of four TPS-salts with different counter anions on the overall PLED performance upon blending each salt with the conjugated polymer poly[2-(6-cyano-6-methyl-heptyloxy)-1,4-phenylene] (CN-PPP). In particular, three perfluoroalkanesulfonate organic anions of increasing size (triflate, nonaflate, PFOS) and a perfluorinated inorganic anion (SbF6) are compared. It is shown that the anion size affects primarily the turn-on and operational voltage, whereas its chemical nature is crucial for achieving high luminance values. The counteranion exerts also a direct impact on the dispersion properties of the salt in the polymer matrix, and thus, the film morphology, which in turn influences the emission colour and efficiency of an electroplex that is proposed to be formed at the sulfonium salt/polymer interfaces in the bulk. This study highlights the importance of properly selecting the counterions of the salts added in the emitting layer of PLEDs, which, in addition to their various functionalities, significantly influence device performance.

Solvent treatment as an efficient anode modification method to improve device performance of polymer light-emitting diodes

Solvent treatment was discovered as an efficient anode modification approach to improve the performance of polymer light-emitting diodes. By simply spin-coating several drops of the polar solvent on top of the PEDOT:PSS hole injection layer, the maximum luminance efficiency was increased by as much as 83% without sacrificing the operation voltage. The combination effects of the reduced work function and the lowered resistivity of PEDOT:PSS film, decreased the hole leakage current, leading to a more balanced charge density inside the emission layer, thereby enhancing the device performance.