On‐Chain Fluorenone Defect Emission from Single Polyfluorene Molecules in the Absence of Intermolecular Interactions

The emission of semiconducting polyfluorenes is often accompanied by an undesired feature in the green spectral region. Whereas a number of previous investigations have argued in favor of a monomolecular origin of the emission species based on ketonic defects, recent experimental results suggested the necessity of excimer formation between individual fluorenone units. We provide a range of new evidence supporting the monomolecular origin of green band emission in polyfluorenes. Most importantly, we succeed in performing single‐molecule spectroscopy on fluorenone‐containing polyfluorene model compounds. Whereas most fluorenone‐containing molecules exhibit both blue backbone and green fluorenone emission independent of fluorenone concentration, it is the relative intensities of the two species which correlate strongly with the fluorenone concentration on the single‐molecule level. Furthermore, we consider a novel model compound with a bifacial arrangement of two fluorenone units. This compound does not provide any signatures of enhanced intramolecular excimer formation but does strongly indicate that concentration quenching effects occur once fluorenone units can interact electronically. The ability to detect on‐chain defect emission in a single polymer molecule demonstrates that photochemical reactions in conjugated polymers can be monitored by fluorescence spectroscopy down to the level of a few atoms, constituting an unprecedented degree of materials characterization.     

Stabilized Blue Emission from Polymer–Dielectric Nanolayer Nanocomposites

Blue‐light‐emitting polymer (polyfluorene)/dielectric nanolayer nanocomposites were prepared by the solution intercalation method and employed in an electroluminescent (EL) device. Their photoluminescence (PL) and electroluminescence characteristics demonstrates that the interruption of interchain interaction in intercalated organic/inorganic hybrid systems reduces the low‐energy emission that results from keto‐defects. By reducing the probability that the excitons initially generated on the polyfluorenes will find keto‐defects, both the color purity and the luminescence stability were improved. Furthermore, the dielectric nanolayers have an aspect ratio of about five hundred, and therefore act as efficient exciton blocking layers and barriers to oxygen diffusion, producing a dramatic increase in the device stability. A nanocomposite device with a Li:Al alloy cathode gave a quantum efficiency of 1.0 %(ph/el), which corresponds to an approximate five times enhancement compared to the neat polymer device. The nanocomposite emitting layer is considered to have a pseudo‐multiple quantum well structure.     

Suppression of the Keto‐Emission in Polyfluorene Light‐Emitting Diodes: Experiments and Models

The spectral characteristics of polyfluorene (PF)‐based light‐emitting diodes (LEDs) containing a defined low concentration of either keto‐defects or of the polymer poly(9,9‐octylfluorene‐co‐benzothiadiazole) (F8BT) are presented. Both types of blend layers were tested in different device configurations with respect to the relative and absolute intensities of green and blue emission components. It is shown that blending hole‐transporting molecules into the emission layer at low concentration or incorporation of a suitable hole‐transporting layer reduces the green emission contribution in the electroluminescence (EL) spectrum of the PF:F8BT blend, which is similar to what is observed for the keto‐containing PF layer. We conclude that the keto‐defects in PF homopolymer layers mainly constitute weakly emissive electron traps, in agreement with the results of quantum‐mechanical calculations.     

White Electroluminescence from a Single‐Polymer System with Simultaneous Two‐Color Emission: Polyfluorene Blue Host and Side‐Chain‐Located Orange Dopant

Four single polymers with two kinds of attachment of orange chromophore to blue polymer host for white electroluminescence (EL) were designed. The effect of the side‐chain attachment and main‐chain attachment on the EL efficiencies of the resulting polymers was compared. The side‐chain‐type single polymers are found to exhibit more efficient white EL than that of the main‐chain‐type single polymers. Based on the side‐chain‐type white single polymer with 4‐(4‐alkyloxy‐phenyl)‐7‐(4‐diphenylamino‐phenyl)‐2,1,3‐benzothiadiazoles as the orange‐dopant unit and polyfluorene as the blue polymer host, white EL with simultaneous orange (λ_max = 545 nm) and blue emission (λ_max = 432 nm/460 nm) is realised. A single‐layer device (indium tin oxide/poly(3,4‐ethylenedioxythiophene)/polymer/Ca/Al) made of these polymers emits white light with the Commission Internationale de l'Éclairage coordinates of (0.30,0.40), possesses a turn‐on voltage of 3.5 V, luminous efficiency of 10.66 cd A^–1, power efficiency of 6.68 lm W^–1, and a maximum brightness of 21 240 cd m^–2.     

White Electroluminescence from a Single‐Polymer System with Simultaneous Two‐Color Emission: Polyfluorene as the Blue Host and a 2,1,3‐Benzothiadiazole Derivative as the Orange Dopant

New single‐polymer electroluminescent systems containing two individual emission species—polyfluorenes as a blue host and 2,1,3‐benzothiadiazole derivative units as an orange dopant on the main chain—have been designed and synthesized. The resulting single polymers are found to have highly efficient white electroluminescence with simultaneous blue (λ_max = 421 nm/445 nm) and orange emission (λ_max = 564 nm) from the corresponding emitting species. The influence of the photoluminescence (PL) efficiencies of both the blue and orange species on the electroluminescence (EL) efficiencies of white polymer light‐emitting diodes (PLEDs) based on the single‐polymer systems has been investigated. The introduction of the highly efficient 4,7‐bis(4‐(N‐phenyl‐N‐(4‐methylphenyl)amino)phenyl)‐2,1,3‐benzothiadiazole unit to the main chain of polyfluorene provides significant improvement in EL efficiency. For a single‐layer device fabricated in air (indium tin oxide/poly(3,4‐ethylenedioxythiophene): poly(styrene sulfonic acid/polymer/Ca/Al), pure‐white electroluminescence with Commission Internationale de l'Eclairage (CIE) coordinates of (0.35,0.32), maximum brightness of 12 300 cd m^–2, luminance efficiency of 7.30 cd A^–1, and power efficiency of 3.34 lm W^–1 can be obtained. This device is approximately two times more efficient than that utilizing a single polyfluorene containing 1,8‐naphthalimide moieties, and shows remarkable improvement over the corresponding blend systems in terms of efficiency and color stability. Thermal treatment of the single‐layer device before cathode deposition leads to the further improvement of the device performance, with CIE coordinates of (0.35,0.34), turn‐on voltage of 3.5 V, luminance efficiency of 8.99 cd A^–1, power efficiency of 5.75 lm W^–1, external quantum efficiency of 3.8 %, and maximum brightness of 12 680 cd m^–2. This performance is roughly comparable to that of white organic light‐emitting diodes (WOLEDs) with multilayer device structures and complicated fabrication processes.     

Understanding the Origin of the 535 nm Emission Band in Oxidized Poly(9,9‐dioctylfluorene): The Essential Role of Inter‐Chain/Inter‐Segment Interactions

We present a careful study of the effects of photo‐oxidation on the emissive properties of poly(9,9‐dioctylfluorene) (PFO) that addresses important issues raised by a recent flurry of publications concerning the degradation of blue light‐emitting, fluorene‐based homo‐ and copolymers. The photoluminescence (PL) spectra of thin PFO films oxidized at room temperature comprise two major components, namely a vibronically structured blue band and a green, structureless component, referred to hereafter as the ‘g‐band’. These are common features in a wide range of poly(fluorene)s (PFs) and whilst the former is uniformly accepted to be the result of intra‐chain, fluorene‐based, singlet‐exciton emission, the origin of the ‘g‐band’ is subject to increasing debate. Our studies, described in detail below, support the proposed formation of oxidation‐induced fluorenone defects that quench intra‐chain, singlet‐exciton emission and activate the g‐band emission. However, whilst these fluorenone defects are concluded to be necessary for the g‐band emission to be observed, they are considered not to be, alone, sufficient. We show that inter‐chain/inter‐segment interactions are required for the appearance of the g‐band in the PL spectra of PFO and propose that the g‐band is attributable to emission from fluorenone‐based excimers rather than from localized fluorenone π–π* transitions as recently suggested.     

Suppression of Green Emission in a New Class of Blue‐Emitting Polyfluorene Copolymers with Twisted Biphenyl Moieties

The color purity of polyfluorene‐based blue‐emitting polymers is often compromized by “long‐wavelength” green emission bands, attributed to polymer interchain species first and more recently to formation of emissive fluorenone defects. Here, we study the nature and the suppression of such bands via characterization of a new class of polyfluorene derivatives modified by insertion of functional groups at the bridging points (position C9), so as to increase inter‐ring torsion angles. We find that the solid‐state photoluminescence spectra of random copolymers of the modified polyfluorenes and the homopolymer display a progressive decrease of the long‐wavelength emission. Electroluminescence spectra also show efficient suppression of such bands in the copolymers with a concentration of ‘twisted' comonomer units of 40 % or greater. Quantum‐chemical calculations on model oligomers address the influence of the bridging unit on the torsion angles, and the resulting excited‐state properties; the impact on molecular packing is also explored with force‐field calculations. We conclude that increase of intra‐biphenyl torsion angles is a viable strategy for suppression of long‐wavelength emission bands in polyfluorenes.     

On the Origin of Green Emission Bands in Fluorene‐Based Conjugated Polymers

Blue‐light‐emitting diodes made of polyfluorenes have low stability and, under operation, rapidly degrade and produce undesirable low‐energy emission bands (green or g‐bands). A spectroelectrochemical study of the degradation process suffered by polyfluorenes is reported here. These polymers lose their electronic properties by electrochemical oxidation and reduction through σ‐bond breaking. In addition, upon electrochemical reduction, the development of a structured green emission band at 485 nm is observed. The position and shape of this band is different from the usual featureless band at 535 nm assigned to fluorenone defects. The green‐light‐emitting product is isolated and analyzed by Fourier‐transform IR spectroscopy; fluorenone formation is excluded. The isolated product is crosslinked; its green emission is probably related to the formation of an intramolecular excimer.     

Cover Picture: White Electroluminescence from a Single‐Polymer System with Simultaneous Two‐Color Emission: Polyfluorene as the Blue Host and a 2,1,3‐Benzothiadiazole Derivative as the Orange Dopant (Adv. Funct. Mater. 7/2006)

New single‐polymer electroluminescent systems containing two individual emission species—polyfluorenes as a blue host and 2,1,3‐benzothiadiazole derivative units as an orange dopant on the main chain—have been designed and synthesized by Wang and co‐workers on p. 957. The resulting single polymers are found to have highly efficient white electroluminescence with simultaneous blue and orange emission from the corresponding emitting species. A single‐layer device has been fabricated that has performance characteristics roughly comparable to those of organic white‐light‐emitting diodes with multilayer device structures. New single‐polymer electroluminescent systems containing two individual emission species—polyfluorenes as a blue host and 2,1,3‐benzothiadiazole derivative units as an orange dopant on the main chain—have been designed and synthesized. The resulting single polymers are found to have highly efficient white electroluminescence with simultaneous blue (λ_max = 421 nm/445 nm) and orange emission (λ_max = 564 nm) from the corresponding emitting species. The influence of the photoluminescence (PL) efficiencies of both the blue and orange species on the electroluminescence (EL) efficiencies of white polymer light‐emitting diodes (PLEDs) based on the single‐polymer systems has been investigated. The introduction of the highly efficient 4,7‐bis(4‐(N‐phenyl‐N‐(4‐methylphenyl)amino)phenyl)‐2,1,3‐benzothiadiazole unit to the main chain of polyfluorene provides significant improvement in EL efficiency. For a single‐layer device fabricated in air (indium tin oxide/poly(3,4‐ethylenedioxythiophene): poly(styrene sulfonic acid/polymer/Ca/Al), pure‐white electroluminescence with Commission Internationale de l'Eclairage (CIE) coordinates of (0.35,0.32), maximum brightness of 12 300 cd m^–2, luminance efficiency of 7.30 cd A^–1, and power efficiency of 3.34 lm W^–1 can be obtained. This device is approximately two times more efficient than that utilizing a single polyfluorene containing 1,8‐naphthalimide moieties, and shows remarkable improvement over the corresponding blend systems in terms of efficiency and color stability. Thermal treatment of the single‐layer device before cathode deposition leads to the further improvement of the device performance, with CIE coordinates of (0.35,0.34), turn‐on voltage of 3.5 V, luminance efficiency of 8.99 cd A^–1, power efficiency of 5.75 lm W^–1, external quantum efficiency of 3.8 %, and maximum brightness of 12 680 cd m^–2. This performance is roughly comparable to that of white organic light‐emitting diodes (WOLEDs) with multilayer device structures and complicated fabrication processes.     

A Blue‐Light‐Emitting Polymer with a Rigid Backbone for Enhanced Color Stability

Despite the promising expectations of poly(fluorene) (PF)‐type materials as efficient blue‐light‐emitting polymers, the devices based on these materials are not yet fully utilized. Under prolonged operation of the devices, the PF‐type materials undergo degradation with the appearance of a long‐range emission around 2.2–2.3 eV. As a consequence, the emissive color changes from blue to green with a decrease in the device efficiency. Here, an innovative approach that leads to a new blue‐emitting polymer with remarkable color stability is reported. By modifying the chemical structure of PF to inhibit the formation of keto defects, it is demonstrated that the devices exhibit excellent color stability. This new blue‐emitting polymer, poly(2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta‐[def]phenanthrene)) (PCPP), emits a stabilized, efficient blue electroluminescence without exhibiting any peak in the long‐wavelength region even after prolonged operation of the devices in air.     

Highly Efficient Pure‐White‐Light‐Emitting Diodes from a Single Polymer: Polyfluorene with Naphthalimide Moieties

Light‐emitting diodes exhibiting efficient pure‐white‐light electroluminescence have been successfully developed by using a single polymer: polyfluorene derivatives with 1,8‐naphthalimide chromophores chemically doped onto the polyfluorene backbones. By adjusting the emission wavelength of the 1,8‐naphthalimide components and optimizing the relative content of 1,8‐naphthalimide derivatives in the resulting polymers, white‐light electroluminescence from a single polymer, as opposed to a polymer blend, has been obtained in a device with a configuration of indium tin oxide/poly(3,4‐ethylenedioxythiophene)(50 nm)/polymer(80 nm)/Ca(10 nm)/Al(100 nm). The device exhibits Commission Internationale de l'Eclairage coordinates of (0.32,0.36), a maximum brightness of 11 900 cd m^–2, a current efficiency of 3.8 cd A^–1, a power efficiency of 2.0 lm W^–1, an external quantum efficiency of 1.50 %, and quite stable color coordinates at different driving voltages, even at high luminances of over 5000 cd m^–2.     

Electron‐Enhanced Hole Injection in Blue Polyfluorene‐Based Polymer Light‐Emitting Diodes

It has recently been reported that, after electrical conditioning, an ohmic hole contact is formed in poly(9,9‐dioctylfluorene) (PFO)‐based polymer light‐emitting diodes (PLED), despite the large hole‐injection barrier obtained with a poly(styrene sulfonic acid)‐doped poly(3,4‐ethylenedioxythiophene) (PEDOT:PSS) anode. We demonstrate that the initial current at low voltages in a PEDOT:PSS/PFO‐based PLED is electron dominated. The voltage at which the hole injection is enhanced strongly depends on the electron‐transport properties of the device, which can be modified by the replacement of reactive end groups by monomers in the synthesis. Our measurements reveal that the switching voltage of the PLED is governed by the electron concentration at the PEDOT:PSS/PFO contact. The switching effect in PFO is only observed for a PEDOT:PSS hole contact and not for other anodes such as indium tin oxide or Ag.     

The Fabrication and Characterization of Single‐Component Polymeric White‐Light‐Emitting Diodes

By using Ni⁰‐mediated polymerization, we have systematically synthesized a series of fluorene‐based copolymers composed of blue‐, green‐, and red‐light‐emitting comonomers with a view to producing polymers with white‐light emission. 2,7‐Dibromo‐9,9‐dihexylfluorene, {4‐(2‐[2,5‐dibromo‐4‐{2‐(4‐diphenylamino‐phenyl)‐vinyl}‐phenyl]‐vinyl)‐phenyl}‐diphenylamine (DTPA), and 2‐{2‐(2‐[4‐{bis(4‐bromo‐phenyl)amino}‐phenyl]‐vinyl)‐6‐tert‐butyl‐pyran‐4‐ylidene}‐malononitrile (TPDCM) were used as the blue‐, green‐, and red‐light‐emitting comonomers, respectively. It was found that the emission spectra of the resulting copolymers could easily be tuned by varying their DTPA and TPDCM content. Thus with the appropriate red/green/blue (RGB) unit ratio, we were able to obtain white‐light emission from these copolymers. A white‐light‐emitting diode using the polyfluorene copolymer containing 3 % green‐emitting DTPA and 2 % red‐emitting TPDCM (PG3R2) with a structure of indium tin oxide/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid)/PG3R2/Ca/Al was found to exhibit a maximum brightness of 820 cd m^–2 at 11 V with Commission Internationale de L'Eclairage (CIE) coordinates of (0.33,0.35), which are close to the standard CIE coordinates for white‐light emission (0.33,0.33).     

White‐Light‐Emitting Diodes Based on Iridium Complexes via Efficient Energy Transfer from a Conjugated Polymer

Efficient white‐light‐emitting diodes (WLEDs) have been developed using a polyfluorene‐type blue‐emitting conjugated polymer doped with green and red phosphorescent dyes. The emission spectrum of the conjugated polymer, which has a very high luminescent efficiency, shows a large spectral overlap with the absorbance of green and red iridium complexes. Also, efficient energy transfer from the conjugated polymer to the iridium complexes is observed. Poly(N‐vinyl carbazole) is used to improve the miscibility between conjugated polymer and iridium complexes because of their poor chemical compatibility and phase separation. A white emission spectrum is easily obtained by varying the contents of the three materials and controlling the phase morphology. Moreover, these WLEDs show a voltage‐independent electroluminescence owing to the threshold and driving voltage of the three materials being similar as a result of energy transfer.     

A Molecular Glass for Deep‐Blue Organic Light‐Emitting Diodes Comprising a 9,9′‐Spirobifluorene Core and Peripheral Carbazole Groups

Novel fluorene‐based compounds, TCPC‐6 and TCPC‐4, with rigid central spirobifluorene cores and peripheral carbazole groups are synthesized using the Suzuki coupling reaction. The optical, electrochemical, and thermal properties of these compounds are characterized. The compounds show strong deep‐blue emission both in solution and as thin films. Both TCPC‐6 and TCPC‐4 exhibit amorphous morphologies in the solid state with high glass transition temperatures (T_g) of 108 and 143 °C, respectively. Atomic force microscopy (AFM) measurements indicate that high‐quality amorphous films of these novel compounds can be prepared by spin‐coating. The oxidation potentials of TCPC‐6 and TCPC‐4 are significant lower than that of model compounds without peripheral carbazole groups, which suggests that these compounds have relatively high highest occupied molecular orbital (HOMO) energy levels and better hole‐injection capabilities. Light‐emitting devices fabricated by spin‐coating films of these molecules exhibit deep‐blue emission with Commission Internationale de l'Eclairage (CIE) chromaticity coordinates (x, y) of (0.16, 0.05); the devices fabricated using spin‐coated TCPC‐6 and TCPC‐4 layers exhibit high luminance efficiencies of 1.35 and 0.90 cd A^–1 (with external quantum efficiencies of 3.72 and 2.47 %), respectively.     

Stabilized Blue Emission from Polyfluorene‐Based Light‐Emitting Diodes: Elimination of Fluorenone Defects

Polyfluorene (PF)‐based light‐emitting diodes (LEDs) typically exhibit device degradation under operation with the emergence of a strong low‐energy emission band (at ～ 2.2–2.4 eV). This longer wavelength band converts the desired blue emission to blue–green or even yellow. We have studied both the photoluminescence (PL) and electroluminescence (EL) of PFs with different molecular structures and found that the low‐energy emission band originates from fluorenone defects which are introduced by photo‐oxidization, thermal oxidation, or during device fabrication. X‐ray photo‐emission spectroscopy (XPS) results show that the oxidation of PF is strongly catalyzed by the presence of calcium. The fluorenone defects generate a stronger contribution to the EL than to the PL. By utilization of a novel electron‐transporting material as a buffer layer between the emissive PF and the Ca/Ag (Ba/Ag) cathode, the blue EL emission from the PF was stabilized.     

The Quest for High‐Performance Host Materials for Electrophosphorescent Blue Dopants

We report a systematic comparison study of 3,5‐di(N‐carbazolyl)tetraphenylsilane (SimCP) and N,N′‐dicarbazolyl‐3,5‐benzene (mCP), which are used as the host materials for phosphorescent blue dopants in organic light‐emitting diodes (OLEDs). On the basis of photoexcitation emission spectroscopy, thermal stability analysis, photoelectron analysis, charge transport measurements, and molecular dynamics (MD) simulations, we conclude that the non‐π‐conjugated meta‐substituted triphenylsilyl moiety of SimCP exerts a unique hindering effect on the molecular packing characteristics in the condensed phase. The chemical origin of the superior performance of SimCP over mCP is revealed, and is expected to be helpful for the molecular design of effective host materials for enhancing the performance of blue phosphorescent OLEDs.     

Versatile,Benzimidazole/Amine‐Based Ambipolar Compounds for Electroluminescent Applications: Single‐Layer,Blue, Fluorescent OLEDs,Hosts for Single‐Layer,Phosphorescent OLEDs

A series of compounds containing arylamine and 1,2‐diphenyl‐1H‐benz[d]imidazole moieties are developed as ambipolar, blue‐emitting materials with tunable blue‐emitting wavelengths, tunable ambipolar carrier‐transport properties and tunable triplet energy gaps. These compounds possess several novel properties: (1) they emit in the blue region with high quantum yields; (2) they have high morphological stability and thermal stability; (3) they are capable of ambipolar carrier transport; (4) they possess tunable triplet energy gaps, suitable as hosts for yellow‐orange to green phosphors. The electron and hole mobilities of these compounds lie in the range of 0.68–144 × 10^?6 and 0.34–147 × 10^?6 cm² V^?1 s^?1, respectively. High‐performance, single‐layer, blue‐emitting, fluorescent organic light‐emitting diodes (OLEDs) are achieved with these ambipolar materials. High‐performance, single‐layer, phosphorescent OLEDs with yellow‐orange to green emission are also been demonstrated using these ambipolar materials, which have different triplet energy gaps as the host for yellow‐orange‐emitting to green‐emitting iridium complexes. When these ambipolar, blue‐emitting materials are lightly doped with a yellow‐orange‐emitting iridium complex, white organic light‐emitting diodes (WOLEDs) can be achieved, as well by the use of the incomplete energy transfer between the host and the dopant.     

Towards General Guidelines for Aligned,Nanoscale Assemblies of Hairy‐Rod Polyfluorene

This account highlights recent progress towards understanding the complex hierarchical levels of solid‐state structure in a prototypical helical hairy‐rod polyfluorene, poly[9,9‐bis(2‐ethylhexyl)fluorene‐2,7‐diyl] (or PF2/6). This branched‐side‐chain containing polyfluorene undergoes a systematic intermolecular self‐assembly and liquid‐crystalline phase behavior in combination with uniaxial and biaxial alignment. The latter processes yield full three‐dimensional orientation of the crystallites and polymer chains. Also reviewed are the impact of the molecular structure and phase behavior on surface morphology, anisotropic film formation, and, ultimately, the overall impact of these physical attributes on optical constants. This particular polyfluorene also represents a model system for demonstrating the applicability of mean‐field theory in detailing the self‐organization of aligned hairy‐rod block‐copolymer systems. These results of PF2/6 are compared to those of other archetypical π‐conjugated hairy‐rod polymers. General guidelines of how molecular weight influences nanostructure, phase behavior, alignment, and surface morphology are given.