Phosphorescent Polymer Light-emitting Diodes Research Articles

Novel Pyrimidine-Based Iridium Complexes with Bulky Charge-Carrier Groups as Dopants for Highly Efficient Green Polymer Light-Emitting Diodes.

Two new pyrimidine-based iridium complexes with triphenylamine and tetraphenylsilane, namely (TPAPr)2IrAcac and (TPSPr)2IrAcac, were fully synthesized and characterized. Both of the targeted iridium complexes exhibit excellent thermal stability and high photoluminescence quantum yields. Compared to (TPAPr)2IrAcac, (TPSPr)2IrAcac achieved its highest PLQY and current efficiency (CE) at higher dopant concentration probably because of its bulky tetraphenylsilane group, which can effectively suppress the concentration quenching. However, according to DFT studies, (TPSPr)2IrAcac shows faster non-radiative transitions due to the presence of more excited-state distortions than (TPAPr)2IrAcac. As a result, Green phosphorescent polymer light-emitting diodes (PLEDs) containing (TPAPr)2IrAcac and (TPSPr)2IrAcac as dopants exhibit exceptional device performance with peak CE values of 38.24 cd A-1 and 36.06 cd A-1, respectively. (TPAPr)2IrAcac exhibited a superior efficiency than (TPSPr)2IrAcac because of its high Фp, low RMSD value, and efficient energy transfer from the host to the guest. More importantly, the PLEDs based on (TPAPr)2IrAcac and (TPSPr)2IrAcac show stable phosphorescent emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.313, 0.497) and (0.299, 0.483), respectively. This work points out a viable method for creating phosphorescent iridium complexes based on pyrimidine for high-efficiency organic light-emitting diodes (OLEDs).

High triplet, bipolar polymeric hosts for highly efficient solution-processed blue phosphorescent polymer light-emitting diodes

Two polymeric hosts PCzTPP and PCzTPPO with twisted geometrical configurations for blue phosphorescent polymer light-emitting diodes (PhPLEDs) were designed and synthesized by incorporating electron-accepting carbazole units with electron-donating TPP/TPPO groups. This molecular design endows PCzTPP and PCzTPPO with high glass transition temperatures of 204 °C and 215 °C, high triplet energies of 2.72 eV and bipolar features. In addition, the HOMO and LUMO of these polymers matched well with the HOMO of the hole-transport layer and the Fermi level of cathode compared with PVK, which facilitated the injection of holes and electrons. PCzTPP- and PCzTPPO-based single-emissive-layer blue PhPLEDs were fabricated with simplified device configuration by solution process using FIrpic as a dopant. These devices exhibited lower turn on voltages (<8 V) than PVK-based devices (12 V). The maximum luminances of PCzTPP- and PCzTPPO-based devices were twofold and threefold that of PVK-based devices, and the maximum current efficiencies were nearly threefold and ninefold, respectively. Moreover, PCzTPPO-based solution processed blue PhPLEDs with improved configuration showed maximum current efficiency and external quantum efficiency of 14.5 cd/A and 6.6%, respectively.

Synthesis and device performance of an oxygen‐interrupted bipolar polymer host with carbazole and triphenylphosphine oxide in the main chain

ABSTRACTA novel bipolar polymer host PC10CzPO, carrying hole‐transporting carbazole and electron‐transporting triphenylphosphine oxide units in the oxygen‐interrupted main chain, was synthesized and characterized. In addition to its excellent thermal stability and miscibility with phosphors, PC10CzPO is also reported to have a triplet energy (ET) as high as 2.83 eV due to oxygen‐interrupted π‐conjugation, ensuring that PC10CzPO can be used as a suitable host material. The PC10CzPO‐based phosphorescent devices were investigated and compared, while doping with typical blue phosphor {iridium(III)[bis(4,6‐difluorophenyl)pyridinato‐N, C2]picolinate, FIrpic)}, green phosphor {tris[2‐(p‐tolyl)pyridine]iridium(III), Ir(mppy)3}, and red phosphor [bis(1‐phenyl‐isoquinoline‐C2,N)(acetylacetonato)iridium(III), Ir(piq)2acac]. As a result, the FIrpic‐based blue devices showed better device performances than those of red and green devices, which was ascribed to more effective energy transfer. This indicates that the choice of proper host and dopant emitters to fabricate phosphorescent polymer light emitting diodes (PhPLED) is a simple and effective approach to optimize device performances. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44461.

Tuning color-correlated temperature and color rendering index of phosphorescent white polymer light-emitting diodes: Towards healthy solid-state lighting

We report on efficient solution-processed phosphorescent white polymer light-emitting diodes (WPLEDs) with tunable color-correlated temperature (CCT) and color rendering index (CRI), through rationally controlling the composition of the emission layer (EML) based on a near-infrared (NIR)-emitting dinuclear cyclometalated platinum (II) complex bridged with NˆS anionic ligand, named (niq)2Pt2(μ-C8PhOXT)2 (Pt-1, in which PhOXT is 5-(phenyl-1,3,4-oxadiazole)-2-thiol, niq is 1-naphthylisoquinolinato), a sky-blue emitter iridium (III) bis[(4,6-di-fluorophenyl)-pyridinato-N,C2] (picolinate) (FIrpic), and a yellow emitter bis[2-(thieno[3,2-c]pyridin-4-yl)phenyl]iridium(III)(acetylacetonato) (PO-01). One of the best three-color WPLEDs shows a CCT of 3246 K as well as an excellent high CRI of 87, which are greatly beneficial in reducing deep-blue light damage and simultaneously meet the requirement for good color reproduction. Meanwhile, the relevant WPLED also achieves a maximum current efficiency of 12.1 cd/A, corresponding to an external quantum efficiency of 10.6%. This work presents an effective approach through rational combination of sky-blue, yellow, and NIR emitters towards high-performance solution-processable WPLEDs with a physiologically-friendly CCT and a high CRI.

Novel bipolar fluorescent polymers bearing N+P–O− resonance structures for fluorescent–phosphorescent (F–P) hybrid white polymer light-emitting diodes (WPLEDs)

Two novel bipolar fluorescent polymers, PPOPAF and PPOCzF, were developed as blue emitters and hosts for an orange-red phosphor to realize fluorescent–phosphorescent (F–P) hybrid white polymer light-emitting diodes (WPLEDs).

Solution‐processed high‐performance orange phosphorescent and white PLEDs with a high color‐rendering index from an unprecedented π‐stacked and π‐conjugated host material

ABSTRACTA blue fluorescent polymer based on poly(vinyl carbazole) (PVK) and terfluorene, combined to make a chemical hybrid at the carbazole unit (PVK‐TF), is fully characterized in this study. PVK‐TF shows useful emission features, such as peaks at 400, 420, 437, 460, and 496 nm, depending on the processing conditions. It possesses a relatively high triplet energy level (2.23 eV), electrochemical stability, good film‐forming ability, and morphological stability. Based on this blue fluorescent material, highly efficient orange phosphorescent polymer light‐emitting diodes (PLEDs) were fabricated with a maximum efficiency of 21.99 cd A−1, and a maximum luminance of 19552.3 cd m−2. Single‐layer hybrid white PLEDs were developed, with a high color rendering index of 81.9 that emitted across the whole visible spectrum from 380 to 780 nm, corresponding to the Commission International de L'Eclairage coordinates x, y values of around (0.38, 0.40) and CCT = 3774, with a maximum current efficiency of 10.69 cd A−1, and a maximum brightness of 15723.3 cd m−2. © 2014 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014, 52, 587–595

Efficient π-conjugated interrupted host polymer by metal-free polymerization for blue/green phosphorescent light-emitting diodes

A new aromatic host polymer poly{[1,4-bis(9-decylcarbazole-3-yl)-2,3,5,6-tetrafluorobenzene-3,3-diyl]-alt-[N-methylisatin-2-one-3,3-diyl]} (PICzFB) containing carbazole-tetrafluorinebeneze-carbazole moiety in the -conjugated interrupted polymer backbone was synthesized by superacid-catalyzed metal-free polyhydroxyalkylation. The resulted copolymer PICzFB showed a comparatively wide band gap up to 3.32 eV and high triplet energy (E-T) of 2.73 eV due to confined conjugation by the C bond interrupted polymer backbone. Blue and green light-emitting devices with PICzFB as host, FIrpic and Ir(mppy)(3) as phosphorescent dopants showed the maximum luminous efficiencies of 5.0 and 27.6 cd/A, respectively. The results suggested that the strategy of incorporating bipolar unit into the -conjugated interrupted polymer backbone can be a promising approach to obtain host polymer with high triplet level for solution-processed blue and green phosphorescent polymer light-emitting diodes. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1037-1046

Post-functionalized poly(N-vinylcarbazole)s as effective low turn-on-voltage phosphorescent-hosts in polymeric light emitting devices

DTAB-PVKs are prepared by post-functionalization of poly(N-vinylcarbazole) (PVK) with di(4-tolyl)aminobenzyl acetate (DTABOAc) as the pendant groups via the Friedel–Crafts reaction. Introduction of the methyl substituents at the para-position of DTABOAc is essential for suppression of its self-condensation. The details of the reactions mechanisms are elucidated by 1H NMR study. The Fermi level of DTAB-PVKs could be tuned by controlling the amounts of DTABOAc content in the polymer and DTAB-PVKs are found to be good hosts for phosphorescent polymer light-emitting diodes (PLEDs). In a device of PLED comprising PEDOT: PSS/DTAB-PVK-Ir(ppy)3-PBD/Mg–Ag, low turn on voltage at 6.5V is found with a reasonably high current efficiency of 11.5cd/A at 13.0V. The power efficiency reaches 2.7lm/W at 12.0V. More importantly, the device performance related to the Fermi level of DTAB-PVKs.

Fabrication and characterization of green light emitting diode

Phosphorescent polymer light emitting diodes (PLEDs) have been fabricated and characterized. A PLED was configured in an ITO/PEDOT:PSS/TPD:PBD:PVK:Ir(mppy)_3/LiF/Al device structure. Thicknesses of the active layer were optimized for an efficient phosphorescent organic light-emitting diode (OLED) device. The uniform mixing of the active layer was varied with different thicknesses. A hole transport layer of PEDOT:PSS was deposited in a thickness of 35 nm and an emissive layer of TPD:PBD:PVK:Ir(mppy)_3 was deposited in thicknesses of 90 nm, 56 nm, 40 nm, and 35 nm. The 56 nm thickness of the active layer was determined as the proper thickness according to results of current density, luminance, and voltage characteristics of the PLED. The processed PLED device exhibited a turn-on voltage of 3.6 V and a maximum luminance of 575.5 cd m^{-2} at 2.8 mA.

Novel 9,9′-(1,3-phenylene)bis-9H-carbazole-containing copolymers as hole-transporting and host materials for blue phosphorescent polymer light-emitting diodes

Novel photo-crosslinkable hole-transport and host materials incorporated into multilayer blue phosphorescent polymer light-emitting diodes (Ph-PLEDs) were demonstrated in this study. The oxetane-containing copolymers, which function as hole-transport layers (HTL), could be cured by UV irradiation in the presence of a cationic photoinitiator. The composition of the two monomers was varied to yield three different hole-transporting copolymers, [Poly(9,9′-(5-(((4-(7-(4-(((3-methyloxetan-3-yl)methoxy)methyl)phenyl)octan-3-yl)benzyl)oxy)methyl)−1,3-phenylene)bis(9H-carbazole)) (P(mCP-Ox)-I, -II, and -III)]. In addition, monomer 1 was copolymerized with styrene to produce copolymer P(mCP-Ph) as a host material for bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic), a blue-emitting dopant. All mCP-based copolymers displayed high glass transition temperatures (Tg) of up to 130–140 °C and triplet energies of up to 3.00 eV. The blue Ph-PLEDs exhibited a maximum external quantum efficiency of 2.55%, in addition to a luminous efficiency of 8.75 cd A−1 when using the device configuration of indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/P(mCP-OX)-III/P(mCP-Ph):FIrpic(15 wt %)/3,3′-[5′-[3-(3-pyridinyl)phenyl][1,1′:3′,1′′-terphenyl]-3,3′′-diyl]bispyridine/LiF/Al. The device bearing P(mCP-Ox)-III HTL, containing the highest composition of mCP unit, exhibited better performance than the other devices, which is attributed to induction of more balanced charge carriers and carrier recombination in the emissive layer. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 707–718

A Newly Designed Polyfluorene as an Efficient Host Material for Phosphorescent-Dye-Doped Polymer Light-Emitting Diodes

A newly designed polyfluorene derivative, poly[2,7-(9,9-bis(5-cyanopentyl fluorene)-co-alt)-2,5-dimethyl-phenylene] (CNPFX), was synthesized for use as a host material for a phosphorescent dye, fac-tris(2-phenylpyridine) [Ir(ppy)3], in phosphorescent polymer light-emitting diodes. Efficient energy transfer to Ir(ppy)3 was achieved as a result of improved chemical compatibility via introduction of a polar unit, as well as increased spectrum overlap due to a blue-shift in the emission spectrum. Photo- and electro-luminescent spectra of Ir(ppy)3-doped CNPFX film showed clear green emission from Ir(ppy)3 due to efficient energy transfer, whereas those of Ir(ppy)3-doped poly(9,9-dihexylfluorene) (PF6) film showed blue emission from PF6. The CNPFX:Ir(ppy)3 (8 wt %) single layer device showed significantly improved performance.

Highly efficient red phosphorescent organic light-emitting diodes based on solution processed emissive layer

Highly efficient red phosphorescent organic polymer light-emitting diodes (PhOLEDs) were fabricated based on a solution-processed small-molecule host 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) by doping an iridium complex, tris(1-(2,6-dimethylphenoxy)-4-(4-chlorophenyl)phthalazine)iridium (III) (Ir(MPCPPZ)3). A hole blocking layer 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBI) with a function of electron transport was thermally deposited onto the top of CBP layer. The diode with the structure of ITO/PEDOT:PSS (50nm)/CBP:Ir(MPCPPZ)3 (55nm)/TPBI (30nm)/Ba (4nm)/Al (120nm) showed an external quantum efficiency (QEext) of 19.3% and luminous efficiency (LE) of 18.3cd/A at a current density of 0.16mA/cm2, and Commission International de I'Eclairage (CIE) coordinates of (0.607, 0.375). It was suggested that the diodes using TPBI layer exhibited nearly 100% internal quantum efficiency and one order magnitude enhanced LE or QEext efficiencies.

Carbazole-based polysiloxane hosts for highly efficient solution-processed blue electrophosphorescent devices

The efficient host materials containing carbazole moieties linked to the backbone of polysiloxane through a phenyl bridge have been synthesized and characterized. They exhibit good film forming ability, high thermal decomposition temperatures and suitable glass transition temperatures, so as to form stable amorphous states. Moreover, the silicon–oxygen linkage disrupts their conjugation and results in a sufficiently high triplet energy level (3.0 eV). Iridium bis(4,6-difluorophenyl)pyridinato-N,C2 picolinate (FIrpic)-based devices using them as hosts show good overall performance with low efficiency roll-off. The device using PCzMSi as the host demonstrates the best performance with a maximum current efficiency of 22.8 cd A−1, a maximum power efficiency of 9.4 lm W−1 and a maximum external quantum efficiency of 11.9% at a practical luminance of 1165 cd m−2. Even at a brightness of 5000 cd m−2 level, the external quantum efficiency (EQE) still remains as high as 10%, suggesting a gentle roll-off of device efficiency at high current density. In addition, typically, the host PCzMSi film displays good mechanical performance by a nanoindentation technique to meet the practical application. These results demonstrate that the design of polysiloxane-based host materials is a promising approach to realize high performance solution-processed blue phosphorescent polymer light emitting diodes (PhPLEDs).

ChemInform Abstract: Efficient Phosphorescent Polymer Light‐Emitting Diodes by Suppressing Triplet Energy Back Transfer

AbstractReview: 57 refs.

Efficient phosphorescent polymer light-emitting diodes by suppressing triplet energy back transfer

Phosphorescent polymer light-emitting diodes (PhPLEDs) are promising devices in flat panel displays and solid state lighting sources since they can combine the advantages of the high efficiency of electrophosphorescence and low-cost, large-scale manufacture by using a solution process. However, their efficiencies are generally much lower than those of small-molecule-based devices fabricated by using a thermal deposition approach. One of the major reasons for their low efficiency is that energy is lost by back transfer to a polymer host. This tutorial review gives a brief introduction to the fundamentals of PhPLEDs, and then highlights recent progress in the main approaches to suppress triplet energy back transfer from the phosphor to the polymer host towards realizing highly efficient PhPLEDs. The suppressing mechanisms are discussed, and the achievement of high device efficiencies are demonstrated. Emphasis is placed on the relationships between molecular structure, the extent of suppressing triplet energy back transfer, and device performance.

Highly efficient blue phosphorescent polymer light-emitting diodes by using interfacial modification

Solution processed highly efficient blue electrophosphorescent polymer light-emitting diodes have been fabricated containing an oligo(ethylene oxide) (PEO-DME) additive to enhance the electroluminescence efficiency. The addition of PEO-DME at 5–10 wt % effectively lowers both the electron and hole injection barriers. The maximum current efficiency obtained was 26.5 cd/A at an emission brightness of 2500 cd/m2. The high performance is due to (i) improved charge carrier injection at the interface resulting from specific interfacial interactions between PEO-DME and aluminum and (ii) improved charge carrier transporting ability and high electric field resulting from PEO-DME doping.

Hole‐transporting host‐polymer series consisting of triphenylamine basic structures for phosphorescent polymer light‐emitting diodes

AbstractA series of novel styrene derived monomers with triphenylamine‐based units, and their polymers have been synthesized and compared with the well‐known structure of polymer ofN,N′‐bis(3‐methylphenyl)‐N,N′‐diphenylbenzidine with respect to their hole‐transporting behavior in phosphorescent polymer light‐emitting diodes (PLEDs). A vinyltriphenylamine structure was selected as a basic unit, functionalized at the para positions with the following side groups: diphenylamine, 3‐methylphenyl‐aniline, 1‐ and 2‐naphthylamine, carbazole, and phenothiazine. The polymers are used in PLEDs as host polymers for blend systems with the following device configuration: glass/indium–tin–oxide/PEDOT:PSS/polymer‐blend/CsF/Ca/Ag. In addition to the hole‐transporting host polymer, the polymer blend includes a phosphorescent dopant [Ir(Me‐ppy)3] and an electron‐transporting molecule (2‐(4‐biphenyl)‐5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole). We demonstrate that two polymers are excellent hole‐transporting matrix materials for these blend systems because of their good overall electroluminescent performances and their comparatively high glass transition temperatures. For the carbazole‐substituted polymer (Tg= 246 °C), a luminous efficiency of 35 cd A−1and a brightness of 6700 cd m−2at 10 V is accessible. The phenothiazine‐functionalized polymer (Tg= 220 °C) shows nearly the same outstanding PLED behavior. Hence, both these polymers outperform the well‐known polymer ofN,N′‐bis(3‐methylphenyl)‐N,N′‐diphenylbenzidine, showing only a luminous efficiency of 7.9 cd A−1and a brightness of 2500 cd m−2(10 V). © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3417–3430, 2010

Self-trapping mechanism in green phosphorescent dye-doped polymer light-emitting diodes

The mechanism for exciting electroluminescence (EL) in a green phosphorescent dye, iridium(III)tris(2-(4-tolyl)pyridinato-N,C2) (Ir(mppy)3), doped in a host blue-emitting conducting polymer, poly[9,9-di-n-hexyl-fluorenyl-2,7-diyl] (PFO), has been studied. Photoluminescence measurements have been made on PFO/Ir(mppy)3 (0–12%) composites to rule out the possibility of singlet exciton energy transfer from the host polymer to the green dye. EL measurements have also been made to study the behavior of the composites in the presence of dc bias. The dominant mechanism for energy transfer from PFO to Ir(mppy)3 is found to be self-trapping of the charge carriers in the dye molecules, due to the extremely low LUMO and high HOMO levels as compared with PFO, thereby producing EL in the green region.

Effect of Fabrication Parameters on Three-Dimensional Nanostructures and Device Efficiency of Polymer Light-Emitting Diodes

By using 10 kV C(60)(+) and 200 V Ar(+) ion co-sputtering, a crater was created on the light-emitting layer of phosphorescent polymer light-emitting diodes, which consisted of a poly(9-vinyl carbazole) (PVK) host doped with a 24 wt % iridium(III)bis[(4,6-difluorophenyl)pyridinato-N,C(2)] (FIrpic) guest. A force modulation microscope (FMM) was used to analyze the nanostructure at the flat slope near the edge of the crater. The three-dimensional distribution of PVK and FIrpic was determined based on the difference in their mechanical properties from FMM. It was found that significant phase separation occurred when the luminance layer was spin coated at 30 degrees C, and the phase-separated nanostructure provides a route for electron transportation using the guest-enriched phase. This does not generate excitons on the host, which would produce photons less effectively. On the other hand, a more homogeneous distribution of molecules was observed when the layer was spin coated at 60 degrees C. As a result, a 30% enhancement in device performance was observed.

Electroluminescence of poly(N-vinylcarbazole) films: fluorescence, phosphorescence and electromers

We have studied the electroluminescence (EL) properties of pure poly(N-vinylcarbazole) (PVK) films. Three types of light emission in the EL spectrum were observed, attributed to fluorescence, phosphorescence and electromers, respectively. The observation of electrophosphorescence from PVK films at room temperature is very meaningful, indicating that PVK can produce a large number of triplet excitons under an electric field at room temperature. Our results demonstrate clearly the reason why PVK is an excellent host material for phosphorescent polymer light-emitting diodes (PLEDs).