Triplet level-dependent photoluminescence and photoconduction properties of π-conjugated polymer thin films doped by iridium complexes

Triplet energy level-dependent decay pathways of excitons populated on iridium (Ir) complexes within π-conjugated polymeric matrices were studied by means of photoluminescence (PL) and photoconduction action spectroscopy. We chose a set of matrices, poly(9-vinylcarbazole) (PVK), poly[9,9-bis(2-ethylhexyl)fluorene-2,7-diyl] (PF2/6), poly [2-(5′-cyano-5′-methyl-hexyloxy)-1,4-phenylene] (CNPPP), and poly [2-(5′-cyano-5′-methyl-hexyloxy)-1,4-phenylene-co-pridine] (CNPPP-py10 and CNPPP-Py20), having triplet energy levels ranging from 2.2 up to 3.0 eV. As Ir-complex dopants, we selected three phosphorescent emitters, iridium(III)bis(2-(2′-benzothienyl) pyridinato-N-acetylacetonate) (Ir(btp)₂acac), iridium(III)fac-tris(2-phenylpyridine) (Ir(ppy)₃), and iridium(III)bis[(4,6-fluorophenyl)-pyridinato-N,C^2′]picolinate (FIrpic), having triplet energy levels of 2.1, 2.5, and 2.7 eV, respectively. It was found that the triplet emission from the dopants, being populated via energy transfer from the matrices, was strongly dependent on the matching of triplet energy levels between matrix polymers and Ir-complexes. Photocurrent action spectra confirm effective exciton confinement at the dopants sites in the case of PVK matrix systems.     

Molecular design of efficient white‐light‐emitting fluorene‐based copolymers by mixing singlet and triplet emission

A new strategy to realize efficient white‐light emission from a binary fluorene‐based copolymer (PF‐Phq) with the fluorene segment as a blue emitter and the iridium complex, 9‐iridium(III)bis(2‐(2‐phenyl‐quinoline‐N,C³′)(11,13‐tetradecanedionate))‐3,6‐carbazole (Phq), as a red emitter has been proposed and demonstrated. The photo‐ and electroluminescence properties of the PF‐Phq copolymers were investigated. White‐light emission with two bands of blue and red was achieved from the binary copolymers. The efficiency increased with increasing concentration of iridium complex, which resulted from its efficient phosphorescence emission and the weak phosphorescent quenching due to its lower triplet energy level than that of polyfluorene. In comparison with the binary copolymer, the efficiency and color purity of the ternary copolymers (PF‐Phq‐BT) were improved by introducing fluorescent green benzothiadiazole (BT) unit into polyfluorene backbone. This was ascribed to the exciton confinement of the benzothiadiazole unit, which allowed efficient singlet energy transfer from fluorene segment to BT unit and avoided the triplet quenching resulted from the higher triplet energy levels of phosphorescent green emitters than that of polyfluorene. The phosphorescence quenching is a key factor in the design of white light‐emitting polyfluorene with triplet emitter. It is shown that using singlet green and triplet red emitters is an efficient approach to reduce and even avoid the phosphorescence quenching in the fluorene‐based copolymers. The strategy to incorporate singlet green emitter to polyfluorene backbone and to attach triplet red species to the side chain is promising for white polymer light‐emitting diodes. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 453–463, 2008     

Effective shielding of triplet energy transfer to conjugated polymer by its dense side chains from phosphor dopant for highly efficient electrophosphorescence

To examine the quenching of a triplet exciton by low triplet energy (E(T)) polymer hosts with different chain configurations for high E(T) phosphor guests, the quenching rate constant measurements were carried out and analyzed by the standard Stern-Volmer equation. We found that an effective shielding of triplet energy transfer from a high E(T) phosphor guest to a low E(T) polymer host is possible upon introducing dense side chains to the polymer to block direct contact from the guest such that the possibility of Dexter energy transfer between them is reduced to a minimum. Together with energy level matching to allow charge trapping on the guest, high device efficiency can be achieved. The extent of shielding for the systems of phenylene-based conjugated structures from iridium complexes follows the sequence di-substituted (octoxyl chain) in the para position (dC8OPPP) is greater than monosubstituted (mC8OPPP) and the PPPs with longer side chains are much higher than a phenylene tetramer (P4) with two short methyl groups. Further, capping the dialkoxyl-susbstituents with a carbazole (Cz) moiety (CzPPP) provides enhanced extent of shielding. Excellent device efficiency of 30 cd/A (8.25%) for a green electrophosphorescent device can be achieved with CzPPP as a host, which is higher than that of dC8OPPP as host (15 cd/A). The efficiency is higher than those of high E(T) conjugated polymers, poly(3,6-carbazole) derivatives, as hosts (23 cd/A). This observation suggests a new route for molecular design of electroluminescent polymers as a host for a phosphorescent dopant.     

High-efficiency red-light emission from polyfluorenes grafted with cyclometalated iridium complexes and charge transport moiety

We report a new route for the design of electroluminescent polymers by grafting high-efficiency phosphorescent organometallic complexes as dopants and charge transport moieties onto alky side chains of fully conjugated polymers for polymer light-emitting diodes (PLED) with single layer/single polymers. The polymer system studied involves polyfluorene (PF) as the base conjugated polymer, carbazole (Cz) as the charge transport moiety and a source for green emission by forming an electroplex with the PF main chain, and cyclometalated iridium (Ir) complexes as the phosphorescent dopant. Energy transfer from the green Ir complex or an electroplex formed between the fluorene main chain and side-chain carbazole moieties, in addition to that from the PF main chain, to the red Ir complex can significantly enhance the device performance, and a red light-emitting device with the high efficiency 2.8 cd/A at 7 V and 65 cd/m2, comparable to that of the same Ir complex-based OLED, and a broad-band light-emitting device containing blue, green, and red peaks (2.16 cd/A at 9 V) are obtained.     

磷光客体三重态能级对基于PVK主体的电致发光器件的影响

将磷光客体掺入PVK:PBD主体材料中是获得高效率磷光电致发光器件 (PhPLED)的有效途径.然而,分别掺杂Ir(pppy)₃、Ir(F-pppy)₃和Ir(F₂-pppy)₃三种磷光客体的PhPLED器件性能件却显著不同.我们分别模拟主-客体间能量转移和客体直接捕获电子空穴对两种机制,研究了三种客体材料的磷光发射及其衰减过程.研究表明:平衡的载流子注入和客体高效率的载流子捕获是实现Ir-配合物掺杂的PhPLED器件性能优良的主要机制,PVK较低的三重态能级使其无法成为蓝光磷光电致发光器件的主体,在PVK中掺入PBD的方法,则进一步降低了主体材料的三重态能级,这是蓝光磷光电致发光器件运行效率较低的主要原因.本文的研究结果可为高分子磷光电致发光器件的制备技术和新型磷光主体的设计提供参考.     

Synthesis and characterization of a polyfluorene containing carbazole and oxadiazole dipolar pendent groups and its application to electroluminescent devices

We have synthesized a blue‐light‐emitting polyfluorene (PF) derivative ( PF‐CBZ‐OXD ) that presents bulky hole‐transporting carbazole and electron‐transporting oxadiazole pendent groups functionalized at the C‐9 positions of alternating fluorene units. The results from photoluminescence and electrochemical measurements indicate that both the side chains and the PF main chain retain their own electronic characteristics in the copolymer. An electroluminescent device incorporating this polymer as the emitting layer was turned on at 4.5 V; it exhibited a stable blue emission with a maximum external quantum efficiency of 1.1%. Moreover, we doped PF‐CBZ‐OXD and its analogue PF‐TPA‐OXD with a red‐light‐emitting iridium phosphor for use as components of phosphorescent red‐light emitters to investigate the effect of the host's HOMO energy level on the degree of charge trapping and on the electrophosphorescent efficiency. We found that spectral overlap and individual energy level matching between the host and guest were both crucial features affecting the performance of the electroluminescence devices. Atomic force microscopy measurements indicated that the dipolar nature of PF‐CBZ‐OXD , in contrast to the general nonpolarity of polydialkylfluorenes, provided a stabilizing environment that allowed homogeneous dispersion of the polar iridium triplet dopant. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2925–2937, 2007     

White electroluminescent single‐polymer achieved by incorporating three polyfluorene blue arms into a star‐shaped orange core

A series of star‐like dopant/host single‐polymer systems with a D‐A type star‐shaped orange core and three blue polyfluorene arms were designed and synthesized. Through tuning the doping concentration of the orange core and thermal annealing treatment of white polymer light‐emitting diodes based on them, highly efficient white electroluminescence has been achieved. A typical single‐layer device (ITO/PEDOT:PSS/polymer/Ca/Al) realized pure white emission with a luminous efficiency of 16.62 cd A^?1, an external quantum efficiency of 6.28% and CIE coordinates of (0.33, 0.36) for S‐WP‐002TPB3 containing 0.02 mol % orange core. The high efficiency of the devices could be mainly attributed to the suppressed concentration quenching of the dopant units, more efficient energy transfer from polymer host to orange dopant and thermal annealing‐induced α‐phase polyfluorene (PF) self‐dopant in amorphous PF host. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

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 π‐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 δ? 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 π‐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. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1037–1046     

Benzimidazobenzothiazole‐Based Bipolar Hosts to Harvest Nearly All of the Excitons from Blue Delayed Fluorescence and Phosphorescent Organic Light‐Emitting Diodes

Much effort has been devoted to developing highly efficient organic light‐emitting diodes (OLEDs) that function through phosphorescence or thermally activated delayed fluorescence (TADF). However, efficient host materials for blue TADF and phosphorescent guest emitters are limited because of their requirement of high triplet energy levels. Herein, we report the rigid acceptor unit benzimidazobenzothiazole (BID‐BT), which is suitable for use in bipolar hosts in blue OLEDs. The designed host materials, based on BID‐BT, possess high triplet energy and bipolar carrier transport ability. Both blue TADF and phosphorescent OLEDs containing BID‐BT‐based derivatives exhibit external quantum efficiencies as high as 20 %, indicating that these hosts allow efficient triplet exciton confinement appropriate for blue TADF and phosphorescent guest emitters.     

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).     

Carbazole compounds as host materials for triplet emitters in organic light-emitting diodes: polymer hosts for high-efficiency light-emitting diodes

A carbazole homopolymer and carbazole copolymers based on 9,9'-dialkyl-[3,3']-bicarbazolyl, 2,5-diphenyl-[1,3,4]-oxadiazole and 9,9-bis(4-[3,7-dimethyloctyloxy]phenyl)fluorene were synthesized and their electrical and photophysical properties were characterized with respect to their application as host in phosphorescent polymer light-emitting diodes. It is shown that the triplet energy of a polymer depends on the specific connections between its building blocks. Without changing the composition of the polymer, its triplet energy can be increased from 2.3 to 2.6 eV by changing the way in which the different building blocks are coupled together. For poly(9-vinylcarbazole) (PVK), a carbazole polymer often used as host for high-energy triplet emitters in polymer light-emitting diodes, a large hole-injection barrier of about 1 eV exists due to the low-lying HOMO level of PVK. For all carbazole polymers presented here, the HOMO levels are much closer to the Fermi level of a commonly used anode such as ITO and/or a commonly used hole-injection layer such as PEDOT:PSS. This makes high current densities and consequently high luminance levels possible at moderate applied voltages in polymer light-emitting diodes. A double-layer polymer light-emitting diode is constructed comprising a PEDOT:PSS layer as hole-injection layer and a carbazole-oxadiazole copolymer doped with a green triplet emitter as emissive layer that shows an efficacy of 23 cd/A independent of current density and light output.     

Triplet harvesting in poly(9‐vinylcarbazole) and poly(9‐(2,3‐epoxypropyl)carbazole) doped with CdSe/ZnS quantum dots encapsulated with 16‐(N‐carbazolyl) hexadecanoic acid ligands

Semiconductor quantum dots (QDs) can be used as alternative for transition metal complexes to harvest the nonemissive triplet excitons in organic light‐emitting diodes (OLEDs). In search for a QD‐based OLED material generating blue emission, poly(9‐vinylcarbazole) (PVK) and poly(9‐(2,3‐epoxypropyl) carbazole) (PEPK) are chosen as host for blue‐emitting CdSe/ZnS core/shell QDs. The QDs are encapsulated with 16‐(N‐carbazolyl) hexadecanoic acid (C16), a ligand terminated by a carbazole moiety. As alternative for PVK, PEPK, where the lower molecular weight and less extensive excimer formation could promise a better film formation and more extensive exciton hopping, is explored. The efficiencies of singlet ( ) and triplet ( ) energy transfer to the C16 capped QDs are estimated by combining stationary photoluminescence spectra and fluorescence decays of pristine polymer films with those of polymer films doped with the QDs. At a loading of 30 wt % of the QDs, increases from 12 ± 1% in PVK to 41 ± 2% in PEPK while increases from 37 ± 22% in PVK to 72 ± 48% in PEPK. The investigation of the film morphology by atomic force microscopy confirms that the main factor limiting the triplet transfer efficiency in the PVK matrix is the clustering of the C16 capped QDs. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 539–551     

Structure—Property relationships for exciton transfer in conjugated polymers

The ability of conjugated polymers to function as electronic materials is dependent on the efficient transport of excitons along the polymer chain. Generally, the photophysics of the chromophore monomer dictate the excited state behavior of the corresponding conjugated polymers. Different molecular structures are examined to study the role of excited state lifetimes and molecular conformations on energy transfer. The incorporation of rigid, three‐dimensional scaffolds, such as iptycenes and cyclophanes, can encourage an oblique packing of the chromophore units of a conjugated polymer, thus allowing the formation of electronically‐coupled aggregates that retain high quantum yields of emission. Rigid iptycene scaffolds also act as excellent structural directors that encourage complete solvation of PPEs in a liquid crystal (LC) solvent. LC‐PPE mixtures display both an enhanced conformational alignment of polymer chains and extended effective conjugation lengths relative to isotropic solutions, which leads to enhanced energy transfer. Facile exciton migration in poly(p‐phenylene ethynylene)s (PPEs) allows energy absorbed over large areas to be funneled into traps created by the binding of analytes, resulting in signal amplification in sensory devices. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Singlet-triplet and triplet-triplet interactions in conjugated polymer single molecules

Single molecule fluorescence correlation spectroscopy has been used to investigate the photodynamics of isolated single multichromophoric polymer chains of the conjugated polymers MEH-PPV and F8BT on the microsecond to millisecond time scale. The experimental results (and associated kinetic modeling) demonstrate that (i) triplet exciton pairs undergo efficient triplet-triplet annihilation on the <30 micros time scale, (ii) triplet-triplet annihilation is the dominant mechanism for triplet decay at incident excitation powers > or =50 W/cm(2), and (iii) singlet excitons are quenched by triplet excitons with an efficiency on the order of (1)/(2). The high efficiency of this latter process ensures that single molecule fluorescence spectroscopy can be effectively used to indirectly monitor triplet exciton population dynamics in conjugated polymers. Finally, correlation spectroscopy of MEH-PPV molecules in a multilayer device environment reveals that triplet excitons are efficiently quenched by hole polarons.     

Sensitive triplet exciton detection in polyfluorene using Pd-coordinated porphyrin

We developed a sensitive spectroscopic method to probe triplet concentration in thin films of polyfluorene (PF) at room temperature. The energy of photoexcited triplet excitons is transferred to the guest metal-organic complex, meso-tetratolylporphyrin-Pd (PdTPP), and detected as phosphorescent emission. The phosphorescence intensity of PdTPP-PF blends is proportional to the independently measured triplet concentration using photoinduced absorption experiments. The high sensitivity of this method allows room temperature detection of triplet excitons in spin-coated polymer films as thin as 10 nm. We found that the triplet lifetime is independent of PdTPP concentration and therefore this method is nearly non-perturbing for the triplet population.     

Photodegradation‐induced photoluminescence behaviors of π‐conjugated polymers upon the doping of organometallic triplet emitters

The different effects on the photodegradation‐induced photoluminescence (PL) of π‐conjugated polymeric thin films upon the doping of Ir(III) containing triplet emitters in ambient conditions at room temperature were investigated. In this study, we prepared spin‐coated thin films using three different polymer matrices including poly(9‐vinylcarbazole) (PVK), poly[9,9‐bis(2‐ethylhexyl)fluorene‐2,7‐diyl] (PF2/6), and poly[2‐(5′‐cyano‐5′‐methyl‐hexyloxy)‐1,4‐phenylene] (CNPPP) derivatives doped with Ir(III) containing triplet emitters: Ir(III) bis[(4,6‐fluorophenyl)‐pyridinato‐N,C²′] picolinate (FIrpic), or Ir(III)fac‐tris(2‐phenylpyridine) (Ir(ppy)₃), or Ir(III)bis(2‐(2′‐benzothienyl) pyridinato‐N‐acetylacetonate) (Ir(btp)₂acac). Using the doped films, and their neat films, on quartz substrates, the UV‐Visible absorption (UV‐Vis) and PL spectra were recorded under continuous illumination with the excitation wavelengths at the absorption maxima of the corresponding matrix polymers. The dopant effects on the photodegradation‐induced PL were extracted from the kinetic data obtained from the doped films by subtracting the mutual degradation kinetics of their corresponding neat films. The obtained dopant effects show a strong correlation between the photo‐induced PL degradation and the exciton migration behaviors. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2395–2403, 2008     

Soluble polynorbornenes with pendant carbazole derivatives as host materials for highly efficient blue phosphorescent organic light‐emitting diodes

We report novel host polymers for a high‐efficiency polymer‐based solution‐processed phosphorescent organic light‐emitting diode with typical blue‐emitting dopant bis(4,6‐difluorophenylpyridinato‐N,C²)iridium(III) picolinate (FIrpic). The host polymers, soluble polynorbornenes with pendant carbazole derivatives, N‐phenyl‐9H‐carbazole ( P1 ), N‐biphenyl‐9H‐carbazole ( P2 ), and 9,9′‐(1,3‐phenylene)bis‐9H‐carbazole (mCP) ( P3 ) are efficiently synthesized by vinyl addition polymerization of norbornene monomers using Pd(II) catalyst in combination with 1‐octene chain transfer agent. The polymers exhibit high thermal stability with high decomposition (T_d5 > 410 °C) and glass transition temperatures (T_g ≈ 268 °C). The HOMO (ca. ?5.5 to ?5.7 eV) and LUMO (ca. ?2.0 to ?2.1 eV) levels with the high triplet energy of about 2.7–3.0 eV suggest that the polymers are suitable for a host material for blue emitters. Among the solution‐processed devices that were fabricated based on the emissive layers containing the P1 ? P3 host doped with various concentrations of FIrpic (7–13 wt %), the best device with P3 host exhibits power efficiency of 3.0 lm W^?1 and external quantum efficiency of 4.0% at a luminance of 1000 cd m^?2 that is outstanding among the polymeric rivals. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Phosphorescent organic light‐emitting diodes using an iridium complex polymer as the solution‐processible host material

We prepared an iridium polymer complex having 2‐phenylpyridine as a η²‐cyclometallated ligand, a new OLED containing a solution‐processible iridium polymer as a host, and a phosphorescent iridium complex, [Ir(piq‐^tBu)₃] as a guest. This is the first example to apply a phosphorescent iridium complex polymer to a host material in a phosphorescent OLED. A phosphine copolymer ligand made from methyl methacrylate (MMA) and 4‐styryldiphenylphosphine can be used as an anchor polymer, which coordinates to luminescent iridium units to form a host metallopolymer easily. The OLED containing the host iridium‐complex polymer film, in which the guest, 2 wt % Ir(piq‐^tBu)₃, was doped, showed red electroluminescence as a result of efficient energy transfer from the iridium polymer host to the iridium guest. The maximum current efficiency of the device was 1.00, suggesting that a soluble iridium complex polymer can be used as a solution‐processible polymer host in EL devices. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4358–4365, 2009     

Highly efficient red-electrophosphorescent devices based on polyfluorene copolymers containing charge-transporting pendant units

We have systematically examined the photoluminescence (PL) and electroluminescence (EL) behavior of blends comprising two efficient red phosphors doped, respectively, into the blue-emitting polyfluorene derivatives PF-TPA-OXD and PF-OXD. The host polymers, which contain both hole- and electron-transporting or merely electron-transporting side chains, are capable of facilitating charge injection and transport. After determining the HOMO and LUMO energy levels of these materials, we were able to match the dopant with its most suitable host to achieve the direct formation and confinement of an exciton at the dopant. This configuration also leads to a reduction in the electrical excitation of the host polymer, which in turn decreases the degree of exciton loss arising from nonradiative decay of the host triplet. Using this approach, we were able to realize the production of high-performance red-electrophosphorescent devices. For Os(fppz)-doped devices, we obtain a balanced charge recombination in conjunction with higher current and luminance when using PF-TPA-OXD as the host matrix; this device reached a maximum external quantum efficiency of 8.37% with a peak brightness of 16 720 cd/m2. The absence of charge-transporting pendant units, i.e., the device fabricated from poly[9,9-dioctylfluorene-2,7-diyl] (POF), led, however, to relatively poor electroluminescence characteristics (5.81% and 2144 cd/m2).     

Pure blue electroluminescent poly(aryl ether)s with dopant–host systems

A series of novel arylene ether polymers (P5F‐BCzVFs) containing both pentafluorene (5F) and distyrylarylene derivative (BCzVF) units in the side chains for efficient pure blue light emission were prepared by a facile, metal‐free condensation polymerization. The emission spectra indicated that color tuning could be achieved through efficient Förster energy transfer from the deep‐blue 5F host to the pure‐blue BCzVF dopant. Single‐layer polymer light‐emitting diodes (PLEDs) based on P5F‐BCzVFs (ITO/(PEDOT:PSS)/polymer/Ca/Al) exhibited voltage‐independent and stable pure blue emission with a Commission International de L'Eclairage (CIE) coordinate of (0.15, 0.15), a maximum brightness of 3576 cd/m², and a maximum luminous efficiencies of 2.15 cd/A, respectively. As most polymers with dopant‐host systems, the luminous efficiencies of all P5F‐BCzVFs surpassed that of the host‐only polymer (P5F), due to the energy transfer and charge trapping from the host to the dopant. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011