Hole Injection in a Model Fluorene–Triarylamine Copolymer

Recent developments in synthesis and purification have yielded conjugated polymers with hole mobilities exceeding 0.01 cm² V^?1 s^?1. Essential to harvesting the potential of these materials in organic light emitting diodes (OLEDs) is the identification of suitable ohmic contacts. Using a model fluorene copolymer that shows high‐mobility, non‐dispersive hole transport, it is demonstrated that electrodes commonly used as anodes in OLEDs are very poor hole injectors. Injection from Au and indium tin oxide anodes is limited by energy barriers of 0.75 and 0.65 eV, respectively, and the injected current is found to be temperature independent—a prediction that was not reproduced by the leading injection model for disordered organic semiconductors. Injection from a poly(3,4‐ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) anode, on the other hand, is found to become less efficient with electric field, a behavior which is currently not understood. In thinner poly[(9,9′‐dioctylfluorenyl‐2,7‐diyl)‐co‐(4,4′‐(N‐(4‐sec‐butyl))diphenylamine)] films, which are of relevance to OLEDs, ohmic losses on the PEDOT:PSS layer are found to limit the flow of current. These results illustrate the opportunity to further improve the performance of OLEDs as well as the challenge posed by high mobility conjugated polymers for the design of hole injection layers.     

Structural and Electronic Properties of the First Monolayers of Spin‐Cast Poly(fluorene)‐Based Conjugated‐ Polymer Films

This study is an extended investigation on the formation of the first few monolayers of conjugated poly(fluorene)‐based polymer films prepared from solution on defined polar and nonpolar surfaces. In particular, a symmetrical A–B–A triblock copolymer consisting of poly(2‐alkylaniline) as A blocks and poly(9,9‐dialkylfluorene) as B blocks and a poly(9,9‐dialkylfluorene) homopolymer is used for this study. The dependence on drying conditions by means of solvent selection, the influence of a subsequent heat treatment, and the influence of the substrate polarity are investigated for ultrathin films as well as the transition from the first monolayers to the bulk polymer. The study is performed with optical absorption and photoluminescence spectroscopy, and atomic force microscopy to obtain complementary information of optical properties and morphological details. We find that ultrathin films (ca. 1–2 nm) prepared on mica from various solvents form highly defined, flat monolayers at the interface without lateral regularities indicating a dipole–dipole interaction between conjugated‐polymer segments and mica surface dipoles. This is further confirmed by bathochromic photoluminescence shifts observed for the ultrathin layers compared to the bulk polymer. Complementary experiments on nonpolar surfaces, highly oriented pyrolytic graphite (HOPG), show a total absence of defined flat films supporting the assumption of a dipole–dipole assisted formation on mica. For increased film thickness on mica (5 nm and more) the homopolymer does not form any regular structures or ordered layers on top of the monolayer. In contrast, the triblock copolymer, shorter in length, revealed a tendency to form a less‐defined layer‐type growth (3–3.5 nm thick) above the monolayer that was of higher order for higher‐boiling‐point solvents, indicating that the polymers are found in a different conformation. Moreover, one finds that some solvents that show partial immiscibility with the polymer strongly alter the formation of the film. The observations made for the two different types of polymers allow for an assignment of film‐formation driving forces to individual polymer segments and allow for the formulation of a growth model that explains the observed results and indicates the importance of appropriate substrate selection for organic electronic/optoelectronic devices.     

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.     

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.     

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.     

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.     

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.     

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.     

Ellipsometric Characterization of the Optical Constants of Polyfluorene Gain Media

Variable‐angle spectroscopic ellipsometry (VASE) has been applied to five polyfluorene gain media. The ellipsometric data have been analyzed using an electronic model based on critical points of zero dimension, i.e., an exciton model. The optical constants of thin‐film samples on spectrosil B substrates have been deduced and are used to characterize the waveguiding conditions in these asymmetric slab structures. The exciton model that we have used leads to a small correlation of the parameters and accurate fits. The good match with normal‐incidence transmission spectrophotometry data and surface profilometry determinations of thickness gives further confidence in the suitability of this model. Based on our measurements, we calculate the cut‐off thicknesses for the fundamental TE‐guided modes in our silica–polymer–air structures to lie within the range of 40 nm to 70 nm (depending on the specific polymer), and demonstrate corresponding confinement factors (Γ) between 37 % and 63 %. The calculated thickness dependence of the cut‐off wavelength agrees well with experimental data for the thickness dependence of the peak amplified spontaneous emission (ASE) wavelength and is, therefore, consistent with previous explanations of the ASE spectral shifts.     

BLUE LIGHT-EMITTING POLY（FLUORENE-CO-BENZENE） CONTAINING AN OXADIAZOLE MOIETY

A new kind of polyfluorene containing oxadiazole as the side chain was synthesized. The introduction of oxadiazole moiety as more bulky group prevents the aggregation and reduces the crystallinity of the polymers. Efficient intramolecular energy transfer from oxadiazole moiety to the conjugated backbone has been realized, leading to 70% improvement of photoluminescence quantum efficiency of the designed polymers. Compared with PAF, the PFOXD exhibits significant improvement in electroluminescence properties, with luminous efficiency of 0.8 cd/A and maximum luminance of 1800cd/m^2,