White Polymer Light-emitting Diodes Research Articles

Silver Nanowire Percolation Network Soldered with Graphene Oxide at Room Temperature and Its Application for Fully Stretchable Polymer Light-Emitting Diodes

Transparent conductive electrodes with high surface conductivity, high transmittance in the visible wavelength range, and mechanical compliance are one of the major challenges in the fabrication of stretchable optoelectronic devices. We report the preparation of a transparent conductive electrode (TCE) based on a silver nanowire (AgNW) percolation network modified with graphene oxide (GO). The monatomic thickness, mechanical flexibility, and strong bonding with AgNWs enable the GO sheets to wrap around and solder the AgNW junctions and thus dramatically reduce the inter-nanowire contact resistance without heat treatment or high force pressing. The GO-soldered AgNW network has a figure-of-merit sheet resistance of 14 ohm/sq with 88% transmittance at 550 nm. Its storage stability is improved compared to a conventional high-temperature annealed AgNW network. The GO-soldered AgNW network on polyethylene terephthalate films was processed from solutions using a drawdown machine at room temperature. When bent to 4 mm radius, its sheet resistance was increased by only 2-3% after 12,000 bending cycles. GO solder can also improve the stretchability of the AgNW network. Composite TCE fabricated by inlaying a GO-soldered AgNW network in the surface layer of polyurethane acrylate films is stretchable, by greater than 100% linear strain without losing electrical conductivity. Fully stretchable white polymer light-emitting diodes (PLEDs) were fabricated for the first time, employing the stretchable TCE as both the anode and cathode. The PLED can survive after 100 stretching cycles between 0 and 40% strain and can be stretched up to 130% linear strain at room temperature.

Offsetting the problem of charge trapping in white polymer light-emitting diodes using a fluorenone-based luminogen

AA Fluorenone-based AIEE luminogen is used a covalent dopant to design a white light emitting copolymer with minimized charge trapping.

全湿法制备聚合物白光器件

全湿法制备聚合物白光器件

Synthesis and optoelectronic properties of a novel dinuclear cyclometalated platinum(II) complex containing triphenylamine-substituted indolo[3,2-b]carbazole derivative in the single-emissive-layer WPLEDs

A novel bi-picolinic acid derivative of H2dipic-BTICz containing binary triphenylamine-substituted indolo[3,2-b]carbazole (BTICz) unit and its dinuclear platinum(II) complex of (dfppy)2Pt2(dipic-BTICz) were synthesized as a single-component emitter used in the white polymer light-emitting diodes (WPLEDs), where dfppy is 2-(2,4-difluorophenyl)pyridine and dipic-BTICz is an anion of H2dipic-BTICz. The photophysical and electrochemical properties of (dfppy)2Pt2(dipic-BTICz) were investigated. Compared with the reported mononuclear platinum complex of (dfppy)Pt(pic), (dfppy)2Pt2(dipic-BTICz) exhibited a red-shifted photoluminescent peak at 434 nm in dilute dichloromethane (10−5 M), but a weakened and red-shifted aggregation emission peak at 640 nm besides its intrinsic emission at 445 nm in its neat films. Stable pure white emissions with CIE coordinates of (0.325±0.005, 0.345±0.015) and a maximum brightness of 208 cd/m2 were observed in the (dfppy)2Pt2(dipic-BTICz)-doped single-emissive-layer (SEL) PLEDs using a blend of poly(vinylcarbazole) and 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole as a host matrix at 1 wt % dopant concentrations under applied voltages from 9 to 14 V. It indicates that the intrinsic and aggregation emissions of this dinuclear platinum complex were effectively tuned by inserting a new BTICz fluorophore in the dual picolinic acid derivative. Therefore, it is a promising single-component emitter to get white emission in SEL PLEDs.

Recent design tactics for high performance white polymer light-emitting diodes

White polymer light-emitting diodes (WPLEDs) represent an intense research subject towards their potential applications in full-color displays, next-generation solid-state lighting sources and back-lighting of liquid-crystal displays due to their merits including low-cost fabrication, flexibility, large area and ease of construction etc. Unfortunately, WPLEDs generally show much poorer EL performance with respect to those made by the vacuum deposition strategy owing to the inherent disadvantages associated with the materials used, device structures and device fabrication processes etc., which has seriously restricted their practical applications. However, the performances of WPLEDs have been improved greatly in recent years, and can even realize some practical devices. In this review, the critical design tactics employed to achieve this goal are presented, which include developing high performance functional light emitters, maintaining a good charge injection/transport balance, introducing new functional layer, surface morphology engineering and employing novel device construction processes etc. In addition, the ongoing challenges and future perspectives of this research frontier are also highlighted.

Enhancement of external quantum efficiency through steric hindrance of phenazine derivative for white polymer light-emitting diode materials

Copolymers introducing dithienyl-benzophenazine (<0.1mol%) as a dopant have been synthesized on a polyfluorene (PF) backbone based on the Suzuki coupling reaction. The UV–vis spectra of polymers showed similar behaviors in the solution and on the film. However, PL spectra were similar to that of PF in solution, but the peak around 615nm increased as the amounts of dithienyl-benzophenazine were increased in the casting film. Compared with PF-PDTQ05, PF-FDTQ series showed red-shift emission peak by increased conjugation length of dithienyl-benzophenazine. The emission intensity of PF-FDTQ03 and PF-FDTQ05 was more intense compared to that of PF-PDTQ05 due to strong steric-hindrance of FDTQ. In case of PF-FDTQ05, the luminous efficiency and external quantum efficiency were 0.78cd/A and 0.64%, respectively with a maximum brightness of 3112cd/m2.

Voltage independent white emission from all solution processed polymer light-emitting diode with dual emitting layers spaced by an alcohol soluble conjugated polymer as interlayer

Spatial control of recombination zone in multilayer white polymer light emitting diode (WPLED) is highly desirable for stable white-light emission. In this work, the utilization of 18-crown-6 (Cn6)-grafted polyfluorene (PFCn6) as an interlayer in between two emitting layers is demonstrated to control the recombination zones for the multilayer WPLED with β-phase and rubrene doped poly(9,9-di-n-octylfluorene) (PFO) as blue- and yellow-emitting layers, respectively. The device gives the maximum brightness of 15,695cd/m2 and maximum efficiency 5.43cd/A, accompanying with voltage-independent electroluminescence spectrum having invariant Commission Internationale de L’Eclairage (CIE) coordinates of (0.32, 0.36). The performance with the luminance efficiency 5.43cd/A and voltage independent white emission is the highest record among the reported multilayer WPLED.

Synthesis and characterization of white light-emitting polyfluorene-based copolymers containing new red iridium complex in the main chain

We report the synthesis of single chain white light-emitting polyfluorene (PF)-based copolymers containing a novel red iridium complex ((CVz-PhQ)2IrdbmBr), benzothiadiazole (BT), and 2,5-bisphenyl-1,3,4-oxadiazole (OXD) segments. The resulting copolymers were highly soluble in common organic solvents and could be easily spin-coated onto an indium-tin oxide (ITO)-coated glass substrate to obtain high quality optical thin films. The weight-average molecular weight (Mw) and polydispersity of the copolymers were (17.5–36.3)×104 and 1.54–1.65, respectively. We fabricated white polymer light-emitting diodes (WPLEDs) in ITO/PEDOT:PSS/PVK/copolymer/CsF/Ca/Al, both with and without the poly(9-vinylcarbazole) (PVK) interlayer. By carefully controlling the concentrations of the low-energy-emitting species in the resulting copolymers, white light-emission with contributions from each of the three primary colors was achieved. By incorporating 2mol% of an electron transporting material, OXD, as a comonomer into the copolymer backbone and inserting a PVK interlayer, the external quantum efficiency (EQE) and luminance efficiency (LE) of the WPLEDs were enhanced. The turn-on voltages of the copolymers were in the range of 5.5–7.5V, and the maximum EQE and LE were 1.79% and 1.89cd/A with CIE coordinates of (0.32, 0.34), which are close to the standard for white light emission.

Highly efficient blue and all-phosphorescent white polymer light-emitting devices based on polyfluorene host

We report efficient blue electrophosphorescent polymer light emitting devices with polyfluorene (PFO) as the host and iridium bis[2-(4,6-difluorophenyl)-pyridinato-N,C2] picolinate (FIrpic) as the dopant. Despite the low-lying triplet energy level of the polyfluorene polymer host, phosphorescent quenching can be suppressed by using poly(N-vinylcarbazole) (PVK) as anode buffer layer, resulting in a high luminous efficiency of 26.4cdA−1, which is one of the best results in the literature based on conjugated polymer reported to date. The reduced phosphorescent quenching is found to be associated with the exciton formation and charge carrier recombination within the PVK layer and the PVK/PFO interface due to the accumulation of holes. As compared with the devices based on non-conjugated host polymer PVK, the devices based on PFO showed a lower turn-on voltage (3.6V vs. 4.4V) and higher power efficiency (17lmW−1 vs. 8.3lmW−1) due to the higher mobility of PFO. When doubly doped with a newly synthesized yellow-emitting metallophosphor, white polymer light-emitting devices with superior device performance (a peak device efficiency of 40.9cdA−1, a CIE coordinates of (0.32,0.48), and a power efficiency of 31.4lmW−1) was achieved. These findings can broaden our selection in polymer hosts for highly efficient phosphorescent blue emitting devices and can find potential applications in full color displays and solid-state lighting applications in the future.

Red, Green, and Blue Light‐Emitting Polyfluorenes Containing a Dibenzothiophene‐S,S‐Dioxide Unit and Efficient High‐Color‐Rendering‐Index White‐Light‐Emitting Diodes Made Therefrom

AbstractA series of blue (B), green (G) and red (R) light‐emitting, 9,9‐bis(4‐(2‐ethyl‐hexyloxy)phenyl)fluorene (PPF) based polymers containing a dibenzothiophene‐S,S‐dioxide (SO) unit (PPF‐SO polymer), with an additional benzothiadiazole (BT) unit (PPF‐SO‐BT polymer) or a 4,7‐di(4‐hexylthien‐2‐yl)‐benzothiadiazole (DHTBT) unit (PPF‐SO‐DHTBT polymer) are synthesized. These polymers exhibit high fluorescence yields and good thermal stability. Light‐emitting diodes (LEDs) using PPF‐SO25, PPF‐SO15‐BT1, and PPF‐SO15‐DHTBT1 as emission polymers have maximum efficiencies LEmax = 7.0, 17.6 and 6.1 cd A−1 with CIE coordinates (0.15, 0.17), (0.37, 0.56) and (0.62, 0.36), respectively. 1D distributed feedback lasers using PPF‐SO30 as the gain medium are demonstrated, with a wavelength tuning range 467 to 487 nm and low pump energy thresholds (≥18 nJ). Blending different ratios of B (PPF‐SO), G (PPF‐SO‐BT) and R (PPF‐SO‐DHTBT) polymers allows highly efficient white polymer light‐emitting diodes (WPLEDs) to be realized. The optimized devices have an attractive color temperature close to 4700 K and an excellent color rendering index (CRI) ≥90. They are relatively stable, with the emission color remaining almost unchanged when the current densities increase from 20 to 260 mA cm−2. The use of these polymers enables WPLEDs with a superior trade‐off between device efficiency, CRI, and color stability.

White polymer light-emitting diode materials with efficient electron injection backbone containing polyfluorene, oxadiazole and quinoxaline derivatives

Copolymer including dithienylquinoxaline (TQ) (<0.1mol%) as a dopant and oxadiazole derivative as a side chain has been synthesized on a Polyfluorene (PF) backbone based on the Suzuki coupling reaction. The UV–vis spectra of polymers showed similar behaviors in the solution and on the film. However, PL spectra were similar to that of PF in solution, but the peak around 524nm increased as the amounts of TQ were increased in the casting film. With increase in TQ, effective Forster energy transfer (from PF and PFOxd15 to TQ) was observed. According to CV measurement, HOMO and LUMO levels declined as the amount of oxadiazole derivative increased. In case of PFOxd15TQ10, the luminous efficiency and power efficiency were 1.23cd/A and 0.64lm/W, respectively with a maximum brightness of 6940cd/m2. CIE coordinates were close to pure white (0.33, 0.39).

Novel cyclometalated platinum (II) complex containing carrier-transporting groups: Synthesis, luminescence and application in single dopant white PLEDs

Abstract In order to investigate the influence of carrier-transporting groups on the luminescent properties, especially on excimer/aggregation emission, and to obtain single emissive dopants used in white polymer light-emitting diodes with a single emissive-layer structure, two novel platinum complexes derived from a platinum(II) (2-(4′,6′-difluorophenyl)pyridinato-N,C2′)(picolinate) unit containing carrier-transporting groups were synthesized and characterized, where a triphenylamino or an oxadiazole-triphenylamino functionalized unit was pending into the picolinate by non-conjugated linkage, respectively. Their optophysical, electrochemical and electroluminescent properties were investigated. Compared to their parent platinum complex, both functionalized platinum complexes exhibited a significantly red-shifted photoluminescent profile in neat film and higher photoluminescent quantum efficiency in dichloromethane at room temperature. Furthermore, the platinum complex with oxadiazole-triphenylamino ambipolar transporting group exhibited better photoluminescent properties and offered potential application as a single dopant in a single emissive-layer-based white polymer light-emitting diode. More stable white light emissions were observed in this platinum complex-doped devices at dopant concentrations of 4–8 wt%. The maximum luminous efficiency of 1.01 cd A−1 and the maximum brightness of 2177 cd m−2 were obtained in the polyfluorene-hosted devices.

Highly circularly polarized white light using a combination of white polymer light-emitting diode and wideband cholesteric liquid crystal reflector

We present a simple and intriguing device that generates highly circularly polarized white light. It comprises white polymer light-emitting diodes (WPLEDs) attached to a wideband cholesteric liquid crystal (CLC) reflector with a wide photonic bandgap (PBG) covering the visible range. The degree of circular polarization realized is very high over the visible range. The wide PBG was realized by introducing a gradient in pitch of the cholesteric helix by controlling the twisting power within the CLC medium. WPLEDs fabricated using a ternary (red, green, and blue) fluorescent polymer blend with the same moiety showed a low turn-on voltage, high brightness, high efficiency, and good color stability.

Preparation and Characterization of White Polymer Light Emitting Diodes Using PVK:PFO:MDMO-PPV Emission Layer

White polymer light emitting diodes (WPLEDs) with a glass/ITO/PEDOT:PSS/PVK: PFO:MDMO-PPV/TPBI/LiF/Al structure were fabricated to investigate the effects of the doping concentration of MDMO-PPV emission materials. The PVK, PFO and MDMO-PPV conjugated polymers as host and guest emission materials were spin coated at various concentrations of MDMO-PPV ranging from 15.0 to 30.0 vol%. As the concentration of MDMO-PPV increased from 15.0 to 30.0 vol%, the luminance and current efficiency values were decreased clearly, which are attributable to the quenching effect at a high doping concentration. The maximum luminance and current efficiency were 2280 cd/m2 and 3.83 cd/A, respectively for a WPLED with 15.0 vol% of MDMO-PPV. The average CIE color coordinates were about x = 0.33, y = 0.37 at 15 V, showing a white emission color.

High efficient plastic substrate polymer white light emitting diode

White polymer light emitting diode (PLED) has attracted the interest of researchers by the advantage of having low cost, flexible light sources. One of the major advantages of PLED is that it can be able to fabricate in flexible plastic substrate instead of glass substrate. Generally PLED’s requires a substrate of high refractive index to enhance the amount of trapped light in the device, but the refractive index of flexible plastic substrate is low (n < 1.6). In this paper, we present a white PLED on a flexible plastic substrate with a new enhancement method. In which the semi-transparent gold layer is sandwiched between the layers of tantalum oxide and molybdenum oxide which does not require a high refractive index substrate. Using this design, the extraction efficiency of the device is increased from 1.5 to 2.1 cw compared to that of the device using glass substrate.

Highly Efficient White Polymer Light-Emitting Diodes with a Conjugated Polyelectrolyte as Host Polymer

In this presentation, efficient white polymer-light-emitting diodes have been fabricated with high work-function cathode. The device structure consists of ITO/poly(ehtlenedioxythiophene) : poly(styrene sulfonic acid) (PEDOT : PSS) / poly(N-vinylcarbazole) (PVK) / poly[(9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt- 2,7-(9,9-dioctylfluorene)] (PFN): bis(2-(4,6-difluorophenyl)-pyridinato-N,C2') picolinate (FIrpic): poly[2,7-(9,9-dioc-tylfluorene)-co-(4,7-dithien-2-yl)-2,1,3-benzothiadiazole] (PFODBT15)/Al. By tuning the weight ratio of FIrpic to PFO-DBT15 in PFN, white light was obtained. The 1931 CIE coordinates of the white lights obtained were (0.37, 0.41) and (0.36, 0.42). The maximum luminous efficiency was 3 cd/A.

Fabrication and Characterization of White Polymer Light Emitting Diodes Using PFO:MDMO-PPV

White polymer light emitting diodes (WPLEDs) with a glass/ITO/PEDOT:PSS/PFO:MDMO-PPV/ TPBI/LIF/Al structure were fabricated in order to investigate the optimum doping concentration of the emission materials. PEDOT:PSS was introduced as the hole transport material. The PFO and MDMO-PPV were used as the light emitting host and the guest materials, respectively. The PFO:MDMO-PPV mixed solution was spin-coated onto the PEDOT:PSS/ITO substrate. TPBI, LiF and Al were deposited by thermal evaporation as the hole blocking, electron injection, and cathode materials, respectively. As a result, the current density and luminance of the WPLED with the 20.0 wt% MDMO-PPV concentration in the PFO host material were found to be about 365 mA/cm2 and 4315 cd/m2, respectively. The maximum external quantum efficiency (EQE) of the same sample was found to be 11.26%, which may be ascribed to the efficient energy transfer from the PFO host to the MDMO-PPV guest material.

Efficient hybrid white polymer light-emitting devices with electroluminescence covered the entire visible range and reduced efficiency roll-off

We report efficient hybrid white polymer light emitting devices (WPLEDs) fabricated via simple solution-proceeded process from a newly synthesized wide band-gap fluorene-co-dibenzothiophene-S,S-dioxide copolymer, which dually function as fluorescent blue emitter and host material for electrophosphorescent sky-blue, yellow, and saturated-red dyes. The Commission Internationale d’Énclairage coordinates of the best devices are (0.356, 0.334), with electroluminescence covered the entire visible light spectrum from 400 to 780 nm, resulting in a high color rendering index of 90. Incorporation of a bilayer electrode consisting of water/alcohol-soluble conjugated polymer and Al as electron-injection cathode boosts an enhancement of 50% in device efficiency, leading to external quantum efficiency of 12.6%, and peak power efficiency of 21.4 l m W−1 as measured in an integrating sphere. Both the efficiency and the color quality of the obtained device are ranking among one of the highest values for WPLEDs reported to date. Furthermore, as compared with those all-phosphorescent WPLEDs, the hybrid WPLEDs studied here exhibit a significantly reduced efficiency roll-off due to the very low doping concentration.

Highly efficient single-layer white polymer light-emitting devices employing triphenylamine-based iridium dendritic complexes as orange emissive component

New triphenylamine-based iridium(III) dendritic complexes are demonstrated as active components for white polymer light-emitting diodes (WPLEDs). Initially the orange PLEDs are fabricated through dispersing the dendrimers into PVK in the presence of an electron-transport material 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD). High performance of orange PLEDs is acquired by use of the low-conductivity anode buffer layer P8000, with the maximum current/power/external quantum efficiencies of 52.4 cd A−1/21.6 lm W−1/21.0%. Based on the excellent performance of the orange PLEDs, single-layer white polymer light-emitting devices (WPLEDs) are fabricated by double-doping sky-blue emitter iridium(III) bis(2-(4,6-difluorophenyl)-pyridinato-N,C2)picolinate (FIrpic) and the orange iridium dendrimers. These WPLEDs achieve the maximum current/power/external quantum efficiencies of 37.0 cd A−1/19.4 lm W−1/18.5%, which are among the highest efficiencies for dual-color WPLEDs ever reported. This work presents a premiere attempt on applying dendritic phosphor in highly efficient all-phosphor WPLEDs and a step forward to panchromatic application of Ir(III) dendrimers.

Synthesis and optoelectronic properties of a heterobimetallic Pt(ii)–Ir(iii) complex used as a single-component emitter in white PLEDs

To tune aggregation/excimer emission and obtain a single active emitter for white polymer light-emitting devices (PLEDs), a heterobimetallic Pt(II)-Ir(III) complex of FIr(pic)-C(6)DBC(6)-(pic)PtF was designed and synthesized, in which C(6)DBC(6) is a di(phenyloxyhexyloxy) bridging group, FIr(pic) is an iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C(2)'] (picolinate) chromophore and FPt(pic) is a platinum(II) [(4,6-difluorophenyl)pyridinato-N,C(2)'] (picolinate) chromophore. Its physical and opto-electronic properties were investigated. Interestingly, the excimer emission was efficiently controlled by this heterobimetallic Pt(II)-Ir(III) complex compared to the PL profile of the mononuclear FPt(pic) complex in the solid state. Near-white emissions were obtained in the single emissive layer (SEL) PLEDs using this heterobimetallic Pt(II)-Ir(III) complex as a single dopant and poly(vinylcarbazole) as a host matrix at dopant concentrations from 0.5 wt% to 2 wt%. This work indicates that incorporating a non-planar iridium(III) complex into the planar platinum(II) complex can control aggregation/excimer emissions and a single phosphorescent emitter can be obtained to exhibit white emission in SEL devices.