Acridan-Grafted Poly(biphenyl germanium) with High Triplet Energy, Low Polarizability, and an External Heavy-Atom Effect for Highly Efficient Sky-Blue TADF Electroluminescence.

We propose the novel σ-π conjugated polymer poly(biphenyl germanium) grafted with two electron-donating acridan moieties on the Ge atom for use as the host material in a polymer light-emitting diode (PLED) with the sky-blue-emitting thermally activated delayed fluorescence (TADF) material DMAC-TRZ as the guest. Its high triplet energy (ET ) of 2.86 eV is significantly higher than those of conventional π-π conjugated polymers (ET =2.65 eV as the limit) and this guest emitter (ET =2.77 eV). The TADF emitter emits bluer emission than in other host materials owing to the low orientation polarizability of the germanium-based polymer host. The Ge atom also provides an external heavy-atom effect, which increases the rate of reverse intersystem crossing in this TADF guest, so that more triplet excitons are harvested for light emission. The sky-blue TADF electroluminescence with this host/guest pair gave a record-high external quantum efficiency of 24.1 % at maximum and 22.8 % at 500 cd m-2 .

Mesoscale aggregation properties of C60 in toluene and chlorobenzene.

The mesoscale aggregation properties of C60 in two distinct aromatic solvents (toluene and chlorobenzene) and a practical range of concentrations (c = 1-2 and c = 1-5 mg mL(-1), respectively) were systematically explored by static/dynamic light scattering (SLS/DLS), small angle X-ray scattering (SAXS), depolarized dynamic light scattering (DDLS), and cryogenic transmission electron microscopy (cryo-TEM) analyses. The central observations were as follows: (1) aggregate species of sizes in the range of several hundred nanometers have been independently revealed by SLS, DLS, and DDLS analyses for both solvent systems. (2) DDLS and cryo-TEM measurements further revealed that while C60 clusters are notably anisotropic (rod-like) in chlorobenzene, they are basically isotropic (spherical) in toluene. (3) Detailed analyses of combined SLS and SAXS profiles suggested that varied, yet self-similar, solvent-induced aggregate units were responsible for the distinct (mesoscale) aggregation features noted above. (4) From a dynamic perspective, specially commissioned DLS measurements ubiquitously displayed two relaxation modes (fast and slow mode), with the second (slow) mode being q (wave vector) independent. While the fast mode in both solvent systems was basically diffusive by nature and leads to geometrical features in good agreement with the above static analyses, the slow mode was analyzed and tentatively suggested to reflect the effect of mutual confinement. (5) Micron-scale aggregate morphology of drop-cast thin films displays similar contrasting features for the two solvent media used. Overall, this study suggests that solvent-induced, nanoscale, aggregate units may be a promising factor to control a hierarchy of microscopic aggregation properties of C60 solutions and thin films.

Hierarchical self-assembly of nanoparticles in polymer matrix and the nature of the interparticle interaction.

Using small angle X-ray scattering (SAXS), we elucidated the spatial organization of palladium (Pd) nanoparticles (NPs) in the polymer matrix of poly(2-vinylpyridine) (P2VP) and the nature of inter-nanoparticle interactions, where the NPs were synthesized in the presence of P2VP by the reduction of palladium acetylacetonate (Pd(acac)2). The experimental SAXS profiles were analysed on the basis of a hierarchical structure model considering the following two types of interparticle potential: (i) hard-core repulsion only (i.e., the hard-sphere interaction) and (ii) hard-core repulsion together with an attractive potential well (i.e., the sticky hard-sphere interaction). The corresponding theoretical scattering functions, which were used for analysing the experimental SAXS profiles, were obtained within the context of the Percus-Yevick closure and the Ornstein-Zernike equation in the fundamental liquid theory. The analyses revealed that existence of the attractive potential well is indispensable to account for the experimental SAXS profiles. Moreover, the morphology of the hybrids was found to be characterized by a hierarchical structure with three levels, where about six primary NPs with the diameter of ca. 1.8 nm (level one) formed local clusters (level two), and these clusters aggregated to build up a large-scale mass-fractal structure (level three) with the fractal dimension of ca. 2.3. The scattering function developed here is of general use for quantitatively characterizing the morphological structures of polymer/NP hybrids and, in particular, for exploring the interaction potential of the NPs on the basis of the fundamental liquid theory.

Creation of Dual Thermally Activated Delayed-Fluorescence Exciplexes in a Bulk Emitting Layer and Its Interface with an Electron Transport Layer for Promoting the Performance of Thermally Activated Delayed-Fluorescence Organic Light-Emitting Diodes Fabricated by a Solution Process.

An exciplex, which is composed of electron donor and acceptor molecules and formed by intermolecular charge transfer, is an excited-state species that is able to emit light or transfer its energy to a lower-energy emitter. In reported exciplex-based organic light-emitting diodes (OLEDs), their working mechanism is to generate exciplexes either in the bulk emitting layer (bulk exciplex) or at its interface with an electron transport layer (interface exciplex); both types give promising device performance. Here, we propose a novel strategy of creating both types of exciplexes simultaneously (dual exciplexes) for the generation of more exciplexes for better device performance as indicated in the improved photoluminescence quantum yield (PLQY). Impressively, the dual exciplex-based device with blue thermally activated delayed fluorescence (TADF) emitter 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine (DMAC-TRZ) exhibits a record-high maximum external quantum efficiency (EQEmax) of 26.7% among the solution-processed TADF blue OLEDs. By further doping with the red-emitting phosphor emitter into the EML, the white device also gives a record-high EQEmax of 24.1% among the solution-processed TADF-phosphor hybrid white OLEDs (T-P WOLEDs) with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.34, 0.42), color rendering index of 70, and correlated color temperature of 5198 K. Furthermore, both blue and white devices show an ultralow efficiency roll-off with external quantum efficiencies at a practical brightness value of 1000 cd m-2 (EQE1000) of 25.1 and 23.9%, respectively. This is the first report of employing a dual exciplex-based OLED with excellent device performance.

Multiple functionalities of polyfluorene grafted with metal ion-intercalated crown ether as an electron transport layer for bulk-heterojunction polymer solar cells: optical interference, hole blocking, interfacial dipole, and electron conduction.

We present a novel electron transport (ET) polymer composed of polyfluorene grafted with a K(+)-intercalated crown ether involving six oxygen atoms (PFCn6:K(+)) for bulk-heterojunction polymer solar cells (PSCs) with regioregular poly(3-hexylthiophene) (P3HT) as the donor and indene-C(60) bisadduct (ICBA) or indene-[6,6]-phenyl-C(61)-butyric acid methyl ester (IPCBM) as the acceptor in the active layer and with Al or Ca/Al as the cathode. A remarkable improvement in the power conversion efficiency (PCE) (measured in air) was observed upon insertion of this ET layer, which increased the PCE from 5.78 to 7.5% for a PSC with ICBA and Ca/Al (5.53 to 6.63% with IPCBM) and from 3.87 to 6.88% for a PSC with ICBA and Al (3.06 to 6.21% with IPCBM). This ET layer provides multiple functionalities: (1) it generates an optical interference effect for redistribution of light intensity as an optical spacer; (2) it blocks electron-hole recombination at the interface with the cathode; (3) it forms an interfacial dipole that promotes the vacuum level of the cathode metal; and (4) it enhances electron conduction, as evidenced by (1) the increase in total absorption of 1:1 w/w P3HT:ICBA by a factor of 1.3; (2) the reduction in the hole-only current density profile by a factor of 3.3 at 2.0 × 10(5) V/cm; (3) the decrease of 0.81 eV in the work function of Al from 4.28 to 3.47 eV, as determined by UV photoelectron spectroscopy; and (4) the decrease in the series resistance of PSCs with ICBA and Al by a factor of 4.5, as determined by the current-voltage characteristic under dark conditions; respectively. The PSC of 7.5% is the highest among the reported values for PSC systems with the simplest donor polymer, P3HT.

Gelation behavior of poly(9,9-dioctylfluorene)/poly[9,9-di(2-ethylhexyl)-fluorenyl-2,7-diyl] blend in methylcyclohexane solutions.

We investigated the gelation behavior of the blend of two light-emitting polyfluorenes, i.e., poly(9,9-dioctylfluorene-2,7-diyl) (PF(8)) and poly[9,9-di(2-ethylhexyl)-fluorenyl-2,7-diyl] (PF(2/6)), in solutions with methylcyclohexane (MCH). Upon prolonged aging at room temperature, dynamic light scattering revealed that phase separation in the initially well-mixed solutions occurred via spinodal decomposition, yielding a PF(8)/PF(2/6)-enriched phase and an isotropic solvent-rich phase. Sheetlike aggregates in which a portion of the PF(8) chains adopted the ß conformation were formed in the polymer-enriched phase, as revealed by small-angle X-ray scattering and optical spectroscopy. Macroscopically, the phase separation transformed the original viscous liquid solution into a gel. Measurements of the gelation temperature (T(gel)), gel-to-sol transition temperature (T(g-s)), and spinodal decomposition temperature (T(SD)) indicated that gelation (and hence phase separation) became more difficult with increasing content of PF(2/6). The presence of PF(2/6) also hindered formation of the ß-phase of PF(8.) Therefore, the microstructure and photophysical properties of PF(8)/PF(2/6) in MCH solution can be tuned by the composition of the two polymers.

Poly(acridan-grafted biphenyl germanium) with High Triplet Energy as a Universal Host for High-Efficiency Thermally Activated Delayed Fluorescence Full-Color Devices and Their Hybrid with Phosphor for White Light Electroluminescence.

Developing an effective host for highly efficient full-color electroluminescence devices through a solution-process is still a challenge at present. Here, we use the σ-π conjugated polymer, poly(acridan grafted biphenyl germanium) P(DMAC-Ge), having the highest triplet energy (ET) 2.86 eV among conjugated polymers as the host in sky-blue phosphorescence, TADFs (blue (B), green (G), and red (R)), and hybrid white (W) PLEDs. Upon doping with a sky-blue phosphor-emitter (Firpic), the resulting device gives the high EQEmax 19.7% with Bmax 24,918 cd/m2. The Ge-containing polymer backbone can provide as a channel for electron transport and charge trap into the guest as manifested by the electroluminescence dynamics. Further introducing the bipolar material DCzPPy as cohost, the devices with a sky-blue phosphor (Firpic) and each of the TADF-guests─B (DMAC-TRZ), G (DACT-II), and R (TPA-DCPP) in the EML─achieve the high maximum EQEs as 19.7%, 19.4%, 21.5% and 3.82% with the emission peaks at 470, 485, 508, and 630 nm, respectively. As the three guests (DMAC-TRZ, Ir-O, Ir-R) are doped together into the emitting layer, we obtain a TADF-phosphor (T-P) hybrid white PLED giving a record-high EQE 22.5% among the solution processed hybrid OLED with CIE (0.34, 0.40) and Bmax 28,945 cd/m2. These results manifest that P(DMAC-Ge) is a potential polymer host for full-color TADF and hybrid white light PLEDs with high performance.

Creating a pseudometallic state of K+ by intercalation into 18-crown-6 grafted on polyfluorene as electron injection layer for high performance PLEDs with oxygen- and moisture-stable Al cathode.

Polymer light-emitting diodes (PLEDs) suffer from inadequate lifetimes because of the use of environmentally sensitive metals as the cathodes. We present the use of water/methanol-soluble polyfluorene grafted with 18-crown-6 chelating to K(+) as the electron-injection layer (EIL) for deep-blue-emission PLEDs, allowing the use of environmentally stable Al as the cathode since electron donation from the 18-crown-6 can reduce K(+) to a stable "pseudometallic state", enabling it to act as an intermediate step for electron injection. Furthermore, when poly(ethylene oxide) was blended into the EIL to provide hole blocking (HB), the device exhibited the highest performance reported to date for a deep-blue-emission PLED based on a conjugated polymer as the emitting layer, with a brightness of 54,800 cd/m(2) and an external quantum efficiency of 5.42%. The use of such an EI-HB layer opens a broad avenue leading toward industrialization of PLEDs.

Nanoscale ordered structure distribution in thin solid film of conjugated polymers: its significance in charge transport across the film and in performance of electroluminescent device.

We use ultrahigh-vacuum conducting atomic force microscopy to probe the local current distributions in spin-cast thin films from the solutions of poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and poly(9,9-di-n-octyl-2,7-fluorene) (PFO). We found that spatially homogeneous distribution of the ordered structures (well-packed chains and/or aggregates) in MEH-PPV can be controlled by the selection of solvent or mixed solvent, by which effects of spatial charge transport distribution in MEH-PPV thin films on the performance of polymer light-emitting diodes (PLEDs) are unambiguously clarified. For PFO thin film, after the treatment by immersing in the mixed nonsolvent composed of a solvent and nonsolvent, the ordered structures (beta-phase) are generated; its excess content can result in highly conducting regions. However, the device efficiency can be promoted significantly by optimizing the content of beta-phase.

Hole mobility on isolated chains of poly(3-hexylthiophene) by microwave conductivity measurement.

We demonstrate a facile method to investigate intrinsic charge mobility on isolated chains of conjugated polymers by use of microwave absorption method. Hole carriers are generated on conjugated polymer chains in dilute solution by doping with the p-type dopants NOSbF(6), instead of using excitation sources of pulsed laser and electron beam as reported in the literature. The number of hole carriers can be easily estimated by doping level. Measurements on poly(3-hexylthiophene) in benzene with various doping levels from 0.1% to 3% indicate that the hole mobility can be divided into two ranges. In the doping level 0.1%-0.3%, the hole mobility maintains at the constant level 0.03 cm(2)/V s, which can be regarded as that on an isolated chain since the average number of hole carriers per chain is only around one. As the doping level is higher than 0.3%, a presence of multiple hole carriers on a chain occurs, which results in a repulsion of hole carriers and leads to a reduced hole mobility.

Highly Efficient Solution-Processed Thermally Activated Delayed Fluorescence Bluish-Green and Hybrid White Organic Light-Emitting Diodes Using Novel Bipolar Host Materials.

Two pyridine-containing bipolar host materials with high triplet energy, 9,10-dihydro-9,9-dimethyl-10-(3-(6-(3-(9,9-dimethylacridin-10(9H)-yl)phenyl)pyridin-2-yl)phenyl acridin (DDMACPy) and N-(3-(6-(3-(diphenyl amino)phenyl)pyridin-2-yl)phenyl)-N-phenylbenzenamine (DTPAPy), are synthesized from the modification of the commonly adapted host material 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (DCzPPy). The highest occupied molecular orbital levels of DDMACPy (5.50 eV) and DTPAPy (5.60 eV) are found to be shallower than that of DCzPPy (5.90 eV) that leads to the improvement in hole injection from the hole transport layer PEDOT:PSS (WF = 5.10 eV). These host materials are used in the emitting layer of bluish-green organic light-emitting diode (OLED) with the thermally activated delayed fluorescence (TADF) emitter, 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine, as the guest. The DDMACPy-based device shows the highest performance among them, with the maximum external quantum efficiency (EQEmax), current efficiency (CEmax), and power efficiency (PEmax) of 21.0%, 53.1 cd A-1, and 44.0 lm W-1 at CIE (0.17, 0.42), respectively. By further doping with the red-emitting phosphor iridium(III) bis(2-phenylquinoline)(2,2,6,6-tetramethylheptane-3,5-ionate) [Ir(dpm)PQ2] and yellow-emitting phosphor iridium(III) bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C20)acetylacetonate (PO-01-TB) emitters into the bluish-green emitting layer, a TADF-phosphor hybrid white OLED (T-P WOLED) is obtained with excellent EQEmax, CEmax, and PEmax of 17.4%, 48.7 cd A-1, and 44.5 lm W-1 at CIE (0.35, 0.44), respectively. Moreover, both the bluish-green and WOLED show a low efficiency roll-off with external quantum efficiencies at the brightness of 1000 cd m-2 (EQE1000) of 18.7 and 16.2%, respectively, which are the highest performance records among the solution-processed TADF bluish-green and T-P WOLEDs.

Optoelectronic Properties of High Triplet σ-π-Conjugated Poly[(biphenyl group IV-A atom (C, Si, Ge, Sn)] Backbones.

A series of σ-π-conjugated polymers composed of biphenyl and X atom as backbone repeat unit (where X is the group IV-A atom: carbon, silicon, germanium, or tin) grafted with two alkoxy-substituted biphenyls at the X atom as side chains are synthesized and their optoelectronic properties are studied systematically. We choose biphenyl rather than alkyl as the side chain because its frontier molecular orbital distributions are close to those of our previously reported σ-π-conjugated polymer grafted with transport moieties. The present σ-π polymers with various X atoms show significant differences in triplet energy (ET) ranging from 2.58 to 2.83 eV with the sequence Ge > Si > C > Sn and in charge mobilities from 10-9 to 10-7 cm2/(V s) with the sequence Si > Ge > Sn > C, indicating that the properties of the σ-π polymers are largely affected by their X atoms. The Ge- and Sn-based σ-π-conjugated polymers show the highest and lowest ET values, respectively, due to their different levels of π-electron delocalization caused by size effects and (d-p)π orbital interaction. For their charge transport properties, the Si-based conjugated σ-π polymer gives the highest hole and electron mobilities due to the stronger σ-π conjugation and shorter Si-C bond length between the attached carbon atom in biphenyl and Si. On the contrary, the C-based σ-π-conjugated polymer gives the lowest charge mobilities due to a lack of d orbital in the C atom leading to a poor σ-π conjugation characteristic. These σ-π polymers with different ET levels and charge transport properties show a significant effect on their electroluminescence characteristics. Among them, the Ge-based σ-π-conjugated polymer when used as host shows the best device performance due to its higher ET and reasonable charge mobility. Such findings of different optoelectronic properties of these σ-π-conjugated polymers provide useful guidelines for the selection of backbone for designing σ-π-conjugated polymer host grafted with charge transport moieties.

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.

Development of a Highly Efficient Hybrid White Organic-Light-Emitting Diode with a Single Emission Layer by Solution Processing.

We use a mixed host, 2,6-bis[3-(carbazol-9-yl)phenyl]pyridine blended with 20 wt % tris(4-carbazoyl-9-ylphenyl)amine, to lower the hole-injection barrier, along with the bipolar and high-photoluminescence-quantum-yield (Φp= 84%), blue thermally activated delay fluorescence (TADF) material of 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine (DMAC-TRZ) as a blue dopant to compose the emission layer for the fabrication of a TADF blue organic-light-emitting diode (BOLED). The device is highly efficient with the following performance parameters: maximum brightness (Bmax) = 57586 cd/m2, maximum current efficiency (CEmax) = 35.3 cd/A, maximum power efficiency (PEmax) = 21.4 lm/W, maximum external quantum efficiency (EQEmax) = 14.1%, and CIE coordinates (0.18, 0.42). This device has the best performance recorded among the reported solution-processed TADF BOLEDs and has a low efficiency roll-off: at brightness values of 1000 and 5000 cd/m2, its CEs are close, being 35.1 and 30.1 cd/A, respectively. Upon further doping of the red phosphor Ir(dpm)PQ2 (emission peak λmax = 595 nm) into the blue emission layer, we obtained a TADF-phosphor hybrid white organic-light-emitting diode (T-P hybrid WOLED) with high performance: Bmax = 43594 cd/m2, CEmax = 28.8 cd/A, PEmax = 18.1 lm/W, and CIE coordinates (0.38, 0.44). This Bmax = 43594 cd/m2 is better than that of the vacuum-deposited WOLED with a blue TADF emitter, 10000 cd/m2. This is also the first report on a T-P hybrid WOLED with a solution-processed emitting layer.

Electric-Field-Induced Excimer Formation at the Interface of Deep-Blue Emission Poly(9,9-dioctyl-2,7-fluorene) with Polyelectrolyte or Its Precursor as Electron-Injection Layer in Polymer Light-Emitting Diode and Its Prevention for Stable Emission and Higher Performance.

Conjugated polyelectrolytes and their precursors as electron-injection layer (EIL) in polymer light-emitting diode have attracted extensive attention because they allow the use of environmentally stable high work function metals as cathode with efficient electron injection. Here, for the first time, we find that an undesirable green emission component (470-650 nm) in the electroluminescence spectra is observed during continuous operation of deep-blue emission ß-phase poly(9,9-dioctyl-2,7-fluorene) (ß-PFO) device upon introducing polyelectrolyte poly[9,9-bis(6'-(18-crown-6)methoxy)hexyl fluorene] chelating to potassium ion (PFCn6:K+) as EIL. This phenomenon also happens to nonchelating PFCn6, poly[(9,9-bis(3'-( N, N-dimethylamino)propyl)-2,7-fluorene)- alt-2,7-(9,9-dioctylfluorene)], or even nonemissive poly[4-((18-crown-6)methoxy)methyl styrene] chelating to K+ (PSCn6:K+). It can be ascribed to electric-field induction accompanied by thermal motion of a highly polar side chain in the polyelectrolyte leading to local segmental alignment of PFO main chains at the emitting layer (EML)/EIL interface and thus formation of green emission excimer, which is supported by the following observations: appearance of green emission component using nonemissive PSCn6:K+ as EIL, absence of green emission component as the device is operated at low-temperature (78 K) at which molecular thermal motion are frozen, and absence of green emission upon introducing 2,2',2″-(1,3,5-phenylbenzenetriyl)tris[1-phenyl-1 H-benzimidazole] as buffer layer in between EML and EIL for the prevention of direct contact of EML with polyelectrolyte or its precursor EIL.

Effect of conjugation and aromaticity of 3,6 di-substituted carbazoles on triplet energy and the implication of triplet energy in multiple-cyclic aromatic compounds.

It is well-known that short conjugation is needed to obtain a high triplet energy. Carbazole has 3 fused rings and yet it has a high triplet energy. In order to illuminate the reason behind this, we synthesized a range of carbazole derivatives with substitution at the 3,6-positions. All carbazoles with phenyl moieties substituted at the 3,6-positions exhibit a lower triplet energy than that of carbazole itself. We also quantified the aromaticity of carbazole using the nucleus-independent chemical shift tensor. We discovered that the five-membered heterocyclic aromatic ring in carbazole has reduced aromaticity. This results in a reduced conjugation effect between the five-membered heterocyclic aromatic ring and the neighboring benzene rings. Inspired by this finding, the triplet energies of compounds with up to seven benzene units separated by heterocycles (furan, pyrrole, thiophene, silole, and phosphole) and cyclopentadiene were calculated using time-dependent density functional theory. A high triplet energy (>3 eV) can be obtained by alternating high aromaticity and reduced aromaticity in highly extended fused π systems containing furan and pyrrole. In tricyclic aromatic compounds (dibenzofuran, carbazole, fluorene, dibenzothiophene, 5H-benzo[b]phosphinedole and 9H-9-silafluorene) and their extended fused π systems that we have examined so far, the triplet energy is related to the electronegativity of the oxygen, nitrogen, carbon, sulfur, phosphorous and silicon atoms. These findings provide new intuitive insight related to the structures of molecules and the triplet energies, which could be useful in organic optoelectronics.

Investigating side chain mediated electroluminescence from carbazole-modified polyfluorene.

In molecular design of electroluminescent (EL) conjugated polymers, introducing a charge transport moiety on a side chain is found to be a promising method for balancing electron and hole fluxes in EL devices without changing the emitting color if there is no interaction between moiety and main chain. In the case of grafting a carbazole (Cz) moiety (hole transporting) on blue emitting polyfluorene, a green emission appears with intensity comparable to the blue emission, which was attributed to a possible interaction between main chain and Cz as previously reported by us. Here, a detailed study of its EL mechanism was carried out by means of time-resolved EL with the assistance of molecular simulation and thermally stimulated current measurements; exploration of how main chain segments interact with the transport moiety was performed. We found the Cz groups in Cz100PF play multiple roles: they act as (1) hole transporter to improve hole injection, (2) hole trapping site for efficient electron-hole recombination to yield blue-emitting excitons, and (3) source of green emission from electroplex formed via electric field-mediated interaction of the Cz/Cz radical cation with an electron in the nearby PF backbone. In combination, these observations suggest that integrated consideration for both intramolecular and intermolecular interactions provides a new route of molecular design of efficient EL polymers.

Singlet Exciton Fraction in Electroluminescence from Conjugated Polymer.

The efficiency of electrofluorescent polymer light-emitting diodes is determined by singlet exciton fraction (χS) formation and its value still remains controversial. In this work, χS in spiropolyfluorene (SPF) is determined by analyzing transient emission of phosphor-dopant probe. The χS is found to range from 50% to 76%, depending on applied voltage. Higher applied voltage gives larger χS. Besides, more rapid increment in χS with applied voltage is observed in the higher-molecular-weight polymer. The voltage or molecular weight dependence of χS suggests the probability of singlet exciton (SE) generation through triplet-triplet annihilation (TTA) is enhanced due to higher triplet exciton (TE) concentration at higher applied voltage or accommodation of more TEs in a polymer chain with high molecular weight, thereby increasing probability of TTA. At lower applied voltage, χS is contributed by charge recombination. Its value (χS ~50%) higher than the statistical limit 25% is in agreement with efficient interconversion between triplet and singlet polaron pairs (PP) and with larger formation rate of SE relative to that of TE.

High Brightness Fluorescent White Polymer Light-Emitting Diodes by Promoted Hole Injection via Reduced Barrier by Interfacial Dipole Imparted from Chlorinated Indium Tin Oxide to the Hole Injection Layer PEDOT:PSS.

We demonstrated that introducing poly(3,4-ethylenedioxythiophene) polystyrene sulfonate as a hole transport layer (HTL) on top of chlorinated indium tin oxide (Cl-ITO) anode can lead to a deeper highest occupied molecular orbital level of the HTL (promoting from 5.22 to 5.42 eV) due to the interfacial dipole imparted by the Cl-ITO, which allows barrier-free hole injection to the emitting layer with polyspirobifluorene doped with the yellow emitter rubrene and significantly prevents excitons quenching by residual chlorine radicals on the surface of Cl-ITO. By use of poly[9,9-bis(6'-(18-crown-6)methoxy)hexyl)fluorene] chelating to potassium ion (PFCn6:K+) as electron injection layer and air-stable high work function (EΦ) metal aluminum as the cathode, the performance of fluorescent white polymer light-emitting diode (WPLED) achieves the high maximum brightness (Bmax) of 61 523 cd/m2 and maximum luminance efficiency (ηL, max) of 10.3 cd/A. Replacing PFCn6:K+/Al cathode by CsF/Al, the Bmax and ηL, max are promoted to 87 615 cd/m2 (the record value in WPLED) and 11.1 cd/A, respectively.