Efficient Polymer Light-emitting Diodes Research Articles

Optimizing luminous efficiency in polymer light-emitting diodes through anode oxygen-plasma treatment under diverse pressure conditions

This study investigates the impact of oxygen-plasma (O2-plasma) treatment on the microscale electrical properties of indium tin oxide (ITO) surfaces for enhancement of the luminous efficiency of polymer light-emitting diodes (PLEDs). Significant effects on conductivity distribution were observed when treatment was performed at pressures exceeding 0.4 Torr. ITO-6, the ITO treated with O2-plasma at 0.6 Torr, emerged as the preferred anode substrate for PLEDs, displaying a 72.5 % coverage of stable-conductive regions. When utilized as the anode in PLED, it exhibited a remarkable 209 % increase in average current efficiency and a 184 % enhancement in electroluminescent intensity compared to the reference PLED. These findings provide valuable insights for optimizing PLEDs through surface modification of ITO.

Elastic-Plastic Fully π-Conjugated Polymer with Excellent Energy Dissipation Capacity for Ultra-Deep-Blue Flexible Polymer Light-Emitting Diodes with CIEy = 0.04.

Emerging intrinsically flexible fully π-conjugated polymers (FπCPs) are a promising functional material for flexible optoelectronics, attributed to their potential interchain interpenetration and entanglement. However, the challenge remains in obtaining elastic-plastic FπCPs with intrinsic robust optoelectronic property and excellent long-term and cycling deformation stability simultaneously for applications in deep-blue flexible polymer light-emitting diodes (PLEDs). This study, demonstrates a series of elastic-plastic FπCPs (P1-P4) with an excellent energy dissipation capacity via side-chain internal plasticization for the ultra-deep-blue flexible PLEDs. First, the freestanding P1 film exhibited a maximum fracture strain of 34.6%. More interestingly, the elastic behavior is observed with a low strain (≤10%), and the stretched film with a high deformation (>10%) attributed to plastic processing revealed the robust capacity to realize energy absorption and release. The elastic-plastic P1 film exhibits outstanding ultra-deep-blue emission, with an efficiency of 56.38%. Subsequently, efficient PLEDs are fabricated with an ultra-deep-blue emission of CIE (0.16, 0.04) and a maximum external quantum efficiency of 1.73%. Finally, stable and efficient ultra-deep-blue electroluminescence are obtained from PLEDs based on stretchable films with different strains and cycling deformations, suggesting excellent elastic-plastic behavior and deformation stability for flexible electronics.

Thermally Induced Phase Separation of the PEDOT:PSS Layer for Highly Efficient Laminated Polymer Light-Emitting Diodes.

Among various conductive polymers, the poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) film has been studied as a promising material for use as a transparent electrode and a hole-injecting layer in organic optoelectronic devices. Due to the increasing demand for the low-cost fabrication of organic light-emitting diodes (OLEDs), PEDOT:PSS has been employed as the top electrode by using the coating or lamination method. Herein, a facile method is reported for the fabrication of highly efficient polymer light-emitting diodes (PLEDs) based on a laminated transparent electrode (LTE) consisting of successive PEDOT:PSS and silver-nanowire (AgNW) layers. In particular, thermally induced phase separation (TIPS) of the PEDOT:PSS film is found to depend on the annealing temperature (Tanneal) during preparation of the LTE. At Tanneal close to the glass transition temperature of the PSS chains, a PSS-rich phase with a large number of PSS- molecules enhances the work function of the PEDOT:PSS on the glass-side surface relative to the air side. By using the optimized LTEs, bidirectional laminated PLEDs are obtained with a total external quantum efficiency of 2.9% and a turn-on voltage of 2.6 V, giving a comparable performance to that of the reference bottom-emitting PLED based on a costly evaporated metal electrode. In addition, an analysis of the angular characteristics, including the variation in the electroluminescence spectra and the change in luminance according to the emission angle, indicates that the laminated PLED with the LTE provides a more uniform angular distribution regardless of the direction of emission. Detailed optical and electrical analyses are also performed to evaluate the suitability of LTEs for the low-cost fabrication of efficient PLEDs.

Efficiency of Polymer Light-Emitting Diodes: A Perspective.

The various contributions to the external quantum efficiency (EQE) of polymer light-emitting diodes (PLEDs) are discussed. The EQE of an organic light-emitting diode is governed by a number of parameters, such as the electrical efficiency, the photoluminescence quantum yield (PLQY), the optical outcoupling efficiency and the spin statistics for singlet exciton generation. In the last decade, the electrical efficiency has been determined from a numerical PLED device model. More recently, an optical model to simulate the fraction of photons outcoupled to air for PLEDs with a broad recombination zone has been developed. Together with the directly measured PLQY, the EQE of a PLED can then be estimated. However, it has been observed that the measured EQEs of fluorescent PLEDs, including the model system super-yellow poly(p-phenylene vinylene) (SY-PPV) often exceed the expected values. To solve this discrepancy, it is demonstrate that the electrical PLED model has to be expanded by the inclusion of triplet-triplet annihilation (TTA), which is shown to be responsible for a substantial EQE enhancement. Experimentally, it is obtained that TTA contributes to a singlet-exciton generation efficiency of ≈40% in SY-PPV PLEDs, giving rise to an EQE of ≈4% instead of the expected value of 2.5%.

Efficient polyfluorene derivatives for blue light-emitting diodes enabled by tuning conjugation length of bulky chromophores

In this work, a series of blue chromophores containing PAH core with different conjugation lengths and end-capped with diphenylamine were developed and introduced into PPF derivatives. Since the blue chromophores can enhance the thermal stability and suppress the interchain interactions efficiently, all copolymers present outstanding thermal decomposition temperatures up to 425 °C, high photoluminescence quantum yields (PLQYs) up to 48% and smooth surface morphology. Furthermore, the corresponding single-layer blue polymer light-emitting devices (PLEDs) based on these blue copolymers show good blue light emission peaking at 456–468 nm, and excellent stability of electroluminescence (EL) spectra. In particular, the best PLED with a maximum luminance of 8938 cd m−2, a maximum luminous efficiency (LEmax) of 3.73 cd A−1, Commission International de L'Eclairage (CIE) coordinates of (0.14, 0.16) is achieved based on PPF-F. These results demonstrate that tuning the conjugation length of the bulky chromophores in blue light-emitting polymers is a novel and effective molecular design strategy toward highly stable and efficient blue polymer light-emitting diodes.

An Electroactive Pure Organic Room‐Temperature Phosphorescence Polymer Based on a Donor‐Oxygen‐Acceptor Geometry

AbstractAn electroactive room‐temperature phosphorescence (RTP) polymer has been demonstrated based on a characteristic donor‐oxygen‐acceptor geometry. Compared with the donor–acceptor reference, the inserted oxygen atom between donor and acceptor can not only decrease hole‐electron orbital overlap to suppress the charge transfer fluorescence, but also strengthen spin‐orbital coupling effect to facilitate the intersystem crossing and subsequent phosphorescence channels. As a result, a significant RTP is observed in solid states under photo excitation. Most noticeably, the corresponding polymer light‐emitting diodes (PLEDs) reveal a dominant electrophosphorescence with a record‐high external quantum efficiency of 9.7 %. The performance goes well beyond the 5 % theoretical limit for typical fluors, opening a new door to the development of pure organic RTP polymers towards efficient PLEDs.

An Electroactive Pure Organic Room-Temperature Phosphorescence Polymer Based on a Donor-Oxygen-Acceptor Geometry.

An electroactive room-temperature phosphorescence (RTP) polymer has been demonstrated based on a characteristic donor-oxygen-acceptor geometry. Compared with the donor-acceptor reference, the inserted oxygen atom between donor and acceptor can not only decrease hole-electron orbital overlap to suppress the charge transfer fluorescence, but also strengthen spin-orbital coupling effect to facilitate the intersystem crossing and subsequent phosphorescence channels. As a result, a significant RTP is observed in solid states under photo excitation. Most noticeably, the corresponding polymer light-emitting diodes (PLEDs) reveal a dominant electrophosphorescence with a record-high external quantum efficiency of 9.7 %. The performance goes well beyond the 5 % theoretical limit for typical fluors, opening a new door to the development of pure organic RTP polymers towards efficient PLEDs.

Efficient, stable and high color rendering index white polymer light-emitting diodes by restraining the electron trapping

Abstract Superior solid-state light source should possess not only a high device efficiency but also high color rendering index (CRI) and excellent color stability. Here, we obtained efficient, stable and high CRI white polymer light-emitting diodes (WPLEDs) through applying the well-chosen trichromatic polymers and restraining the electron trapping. The trichromatic polymers all possess the similar mainchain units, their EL spectra shows an equidistant distribution. Detailed studies revealed that the part of green and red emission in the WPLED derive from the electron trapping which led to the current-dependent spectra. Applied proper electron transporting layer can enhance the direct electron injection which resulted in the electron trapping being restrained and improving the color stability. The optimized WPLED got a LEmax of 10.17 cd A−1 and CRI of 92, the CIE coordinates (0.346, 0.345) almost keep the same with the operating current density increasing from 10.53 mA cm−2 to 263.16 mA cm−2.

Improving the out-coupling efficiency of polymer light-emitting diodes with soft nanoimprinted random corrugated structures

A simple method is demonstrated in this study by applying random corrugated structures into the polymer emitting layer via soft nanoimprint technology to enhance the light out-coupling efficiency of polymer light-emitting diodes (PLEDs). The device with the random corrugated structures presents high efficiency and stable emission close to the Lambertian pattern. Compared with the flat device, about 52% efficiency enhancement is achieved in the PLEDs with the random corrugated structures, which can be understood by extraction of waveguide mode and surface plasmon polariton mode losses simultaneously. With these advantages, the simple random corrugated structures have the potential to stimulate the development of low-cost and large-area flexible PLEDs with high efficiency in the applications of solid-state light sources and full-color displays.

Correction: Efficient polymer light-emitting diodes (PLEDs) based on chiral [Pt(C^N)(N^O)] complexes with near-infrared (NIR) luminescence and circularly polarized (CP) light

Correction for ‘Efficient polymer light-emitting diodes (PLEDs) based on chiral [Pt(C^N)(N^O)] complexes with near-infrared (NIR) luminescence and circularly polarized (CP) light’ by Guorui Fu et al., J. Mater. Chem. C, 2019, 7, 13743–13747.

Efficient white polymer light-emitting diodes (WPLEDs) based on covalent-grafting of [Zn2(MP)3(OAc)] into PVK.

Thanks to the straightforward white light of single grafting-type polymers based on earth-abundant Zn(ii)-complexes, producing cost-effective flexible WOLEDs/WPLEDs with good device performance remains a formidable challenge. Herein, by using the polymer Poly(NVK-co-[Zn2(MP)3(OAc)]) with excellent physical properties for single-layer WPLEDs, record-high efficiencies (ηMaxc = 13.0 cd A−1, ηMaxp = 6.1 lm W−1 and ηMaxEQE = 9.2%) and low (ca. 25%) efficiency roll-off compared to previous organo-Zn2+-based WOLEDs/WPLEDs are realized. This finding renders single Zn(ii)-complex-grafted polymers a new route to low-cost and large-area flexible WPLEDs for potential full-colour flat displays.

Pyrene-based hyperbranched porous polymers with doped Ir(piq)2(acac) red emitter for highly efficient white polymer light-emitting diodes

Here, we designed and prepared a series of hyperbranched porous polymers constructed using fluorene branches and pyrene core, and all hyperbranched porous polymers exhibit intense blue fluorescence, good morphological stability, and high thermal stability. Further, it is found that the aperture sizes for hyperbranched porous polymers can be adjusted by simply changing the content of pyrene and fluorene in the synthesis process. When the feed ratios of pyrene in the total polymers is 15 mol%, the optimized aperture size was obtained, which is slightly larger than the maxlength of complementary red emitter Bis(1-phenylisoquinoline)(acetylacetonate)iridium (III) (Ir(piq)2acac), indicating the Ir(piq)2acac can well distributed in the apertures of hyperbranched porous polymers in co-doped film of Ir(piq)2acac and hyperbranched porous polymers. The fabricated polymer-light-emitting diode (PLED) with such co-doped film as light-emitting layer realizes good white emission with almost equal blue and red emission intensity from hyperbranched porous polymers and Ir(piq)2acac. The balanced electroluminescent (EL) spectra contribute to ideal Commission Internationale de l’Eclairage (CIE) coordinate of (0.326, 0.374) located at white light zone. In addition, the device also achieves high device performance with maximum luminance and current efficiency reaching 5369 cd/m2 and 8.35 cd/A, respectively. We believe that such porous-structure polymers have huge potential applications in the development of highly efficient white PLEDs with reducing production cost.

Facile brush-coated β-phase poly(9,9-dioctylfluorene) films for efficient and stable pure-blue polymer light-emitting diodes

Abstract β-Phase poly(9,9-dioctylfluorene) (PFO) films have been constructed in one step by direct brush-coating PFO solution onto substrates. The alignment of PFO chains was manipulated under directional stress, resulting in coplanar orientation of adjacent fluorene units. Both additional absorption peak and red-shifted photoluminescence (PL) spectra suggest the formation of β-phase for the brush-coated PFO films. β-Phase PFO device shows pure-blue emission with corresponding CIE coordinates of (0.170, 0.112), achieving a high current efficiency of 3.37 cd/A, which is twice to that of the device with spin-coated amorphous-phase PFO film. In addition, the β-phase devices show much more stable EL spectra as compared with those of the amorphous-phase devices. The results suggest that brush-coating can be used as a simple and efficient methodology to orient the polymer semiconductor films for applications in constructing high-performance optoelectronic devices.

Efficient polymer light-emitting diodes (PLEDs) based on chiral [Pt(C^N)(N^O)] complexes with near-infrared (NIR) luminescence and circularly polarized (CP) light

Chiral NIR-light emitters [Pt(iqbt)(S-Ln)] (1–4) were used as dopants to produce CP-NIR-PLEDs displaying λem of 732 nm, ηMaxEQE of 0.87–0.93%, and gEL of up to 10−3.

Efficient white polymer light-emitting diodes (WPLEDs) based on double emitting layers of a PVK:Eu(iii)-complex and Alq3

Elaborated from the dEML-configured structure for WPLEDs with low turn-on voltage (6.0–7.5 V) and weak efficiency-roll-off, their efficiencies (ηc = 0.32–0.39 cd A−1, ηEQE = 0.54–0.56% and ηp = 0.99–1.10 lm W−1) represent the record values of organo-Eu3+-incorporated WPLEDs.

Efficient tandem polymer light-emitting diodes with PTPA-P/ZnO as the charge generation layer

By using the ether-soluble PTPA-P and alcohol-soluble ZnO as the charge generation layer, the solution-processed tandem PLEDs realized a double efficiency value compared with that of the relevant single-emitting-layer device.

Microwave-assisted synthesis of colloidal ZnO nanocrystals and their utilization in improving polymer light emitting diodes efficiency

Zinc oxide nanoparticles were synthesized via microwave-assisted reactions from a zinc acetate dihydrate. The reactions were performed in diethylene glycol (DEG) at 220 °C and 250 °C while using oleic acid as the surfactant. The ZnO precipitates obtained were washed with methanol and dried or kept as colloidal solutions redispersed in toluene. The size of the nanoparticles ranged from ca. 5 to 12 nm and they could be modified by the concentration of Zn precursor. Different morphologies due to the function of oleic acid concentration were observed. The resulting ZnO nanocolloids were mixed with the poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) polymer to obtain MEH-PPV/ZnO nanocomposites. The nanocomposite layers exhibited optoelectronic properties which were found to be beneficial for being applied as the active layer in the polymer light emitting diodes. The intensity of the electroluminescence of the prepared PLED devices was enhanced by one order of magnitude keeping all other parameters constant and powered as the same operation voltage. As additional step the effect of active layer thickness on optoelectronic performance of PLED devices was investigated as well.

Hole-Buffer Material Derived from Pyrene, Schiff Base and Tris to Enhance Emission Efficiency of Polymer Light-Emitting Diodes

Inserting a hole-buffer layer is an effective way to enhance emission efficiency of electroluminescence devices. We have successfully synthesized a new hole-buffer material PSB composed of pyrene, Schiff base and trihydroxy tert-butyl groups by the Suzuki-coupling reaction. The HOMO and LUMO lev-els were -6.33 eV and -2.55 eV, respectively, as estimated from cyclic volt-ammograms. In addition, homogeneous films (rms roughness ~2 nm) were readily obtained by spin-coating process. Multilayer polymer light-emitting diodes, ITO/PEDOT:PSS/PSB/SY/LiF/Al, have been fabricated using PSB as hole-buffer layer (HBL). Inserting PSB as HBL significantly enhances the per-formance (maximum luminance: 26,439 cd/m2, maximum current efficiency: 7.03 cd/A), compared with the one without PSB (9802 cd/m2, 2.43 cd/A). It is also superior to the device with conventional BCP as hole-blocking layer (ITO/PEDOT:PSS/SY/BCP/LiF/Al: 15,496 cd/m2, 5.56 cd/A). Current results strongly indicate that the PSB is a potential hole-buffer material for electrolu-minescent devices.

Polyazomethines based on oxadiazolyl or 1,2,4-triazolyl groups: synthesis and hole-buffering application in polymer light-emitting diodes

Balance in charge carriers is a prerequisite for obtaining high emission efficiency in polymer light-emitting diodes (PLEDs). Polyazomethines P1 and P2 buffer holes effectively to balance carriers and enhance emission efficiency.

Copper thiocyanate/copper iodide based hole transport composites with balanced properties for efficient polymer light-emitting diodes

A hole transport composite with balanced properties is achieved by merging inorganic CuSCN and CuI, enhancing the overall performances of PLEDs.