Influence of Dendronization on Spectral Diffusion and Aggregation in Conjugated Polymers

Dendronization has previously been suggested as a means of controlling the level of intermolecular interactions in macromolecular and polymeric compounds. We therefore studied the spectral relaxation dynamics of dendronized and non‐dendronized polyfluorenes at different temperatures. Dendronization strongly affects inhomogeneous spectral broadening, resulting in a blue‐shift and broadening of the otherwise narrow vibronic progression of polyfluorene at low temperature. Using gated spectroscopy we are able to identify emissive keto defects on isolated chains of both dendronized and non‐dendronized polymers, along with a strong aggregation band in the solid state. Surprisingly, the emission from chemical and structural defects is found to be stronger in the case of the dendronized material. The observation of aggregate emission in dendronized polyfluorenes suggests that aggregation in these materials does not occur through linear π‐stacking, but rather through electronic interactions at point contacts between chains introduced by structural kinks along the backbone.     

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.     

The Origin of Green Emission in Polyfluorene‐Based Conjugated Polymers: On‐Chain Defect Fluorescence

The low emission band at 2.2–2.3 eV in polyfluorene‐based conjugated materials is studied by various spectroscopic methods on defined copolymers of 9–9′‐difarnesyl‐fluorene with 9‐fluorenone, which can be seen as a model compound for degraded polyfluorenes. Absorption, electroluminescence, and photoluminescence in the film (temperature‐dependent) and solution (room temperature) reveal the optical properties of this low‐energy emission band emerging in polyfluorene‐type polymers upon degradation. All the experimental evidence presented yield direct evidence against excimer or aggregate formation as the primary source of the low‐energy emission band. Instead keto defect sites can be shown to be responsible for the emissive defect.     

聚芴材料低能级发射现象及其机理研究现状

聚芴是一类重要的蓝光发射材料,有高亮度、高效率等优点;然而聚芴类材料在发光器件中使用一段时间之后,光致发光光谱在530～550 nm、电致发光光谱在2.2～2.3e V的区域出现新的发射带,使得器件的发射变成绿光或蓝绿光,在色度失纯的同时,器件的发光效率也迅速降低.本文概述了目前提出的有关聚芴材料低能级发射形成的原因的假说、试验依据、理论计算、以及基于假说所提出的消除低能级发射、改善材料发射稳定性的方法等研究现状.     

High‐Efficiency and Color Stable Blue‐Light‐Emitting Polymers and Devices

Novel fluorene‐based blue‐light‐emitting copolymers with an ultraviolet‐blue‐light (UV‐blue‐light) emitting host and a blue‐light emitting component, 4‐N,N‐diphenylaminostilbene (DPS) have been designed and synthesized by using the palladium‐ catalyzed Suzuki coupling reaction. It was found that both copolymers poly [2,7‐(9,9‐dioctylfluorene)‐alt‐1,3‐(5‐carbazolphenylene)] (PFCz) DPS1 and PFCz‐DPS1‐OXD show pure blue‐light emission even with only 1 % DPS units because of the efficient energy transfer from the UV‐blue‐light emitting PFCz segments to the blue‐light‐emitting DPS units. Moreover, because of the efficient energy transfer/charge trapping in these copolymers, PFCz‐DPS1 and PFCz‐DPS1‐OXD show excellent device performance with a very stable pure blue‐light emission. By using a neutral surfactant poly[9,9‐bis(6'‐(diethanolamino)hexyl)‐fluorene] (PFN‐OH) as the electron injection layer, the device based on PFCz‐DPS1‐OXD5 with the configuration of ITO/PEDOT:PSS/PVK/polymer/PFN‐OH/Al showed a maximum quantum efficiency of 2.83 % and a maximum luminous efficiency of 2.50 cd A^–1. Its CIE 1931 chromaticity coordinates of (0.156, 0.080) match very well with the NTSC standard blue pixel coordinates of (0.14, 0.08). These results indicate that this kind of dopant/host copolymer could be a promising candidate for blue‐light‐emitting polymers with high efficiency, good color purity, and excellent color stability.     

Stable and Efficient White Electroluminescent Devices Based on a Single Emitting Layer of Polymer Blends

An efficient orange‐light‐emitting polymer (PFTO‐BSeD5) has been developed through the incorporation of low‐bandgap benzoselenadiazole (BSeD) moieties into the backbone of a blue‐light‐emitting polyfluorene copolymer (PFTO poly{[9,9‐bis(4‐(5‐(4‐tert‐butylphenyl)‐[1,3,4]‐oxadiazol‐2‐yl)phenyl)‐9′,9′‐di‐n‐octyl‐[2,2′]‐bifluoren‐7,7′‐diyl]‐stat‐[9,9‐bis(4‐(N,N‐di(4‐n‐butylphenyl)amino)phenyl)‐9′,9′‐di‐n‐octyl‐[2,2′]‐bifluoren‐7,7′‐diyl]}) that contains hole‐transporting triphenylamine and electron‐transporting oxadiazole pendent groups. A polymer light‐emitting device based on this copolymer exhibits a strong, bright‐orange emission with Commission Internationale de L'Eclairage (CIE) color coordinates (0.45,0.52). The maximum brightness is 13 716 cd m^–2 and the maximum luminance efficiency is 5.53 cd A^–1. The use of blends of PFTO‐BSeD5 in PFTO leads to efficient and stable white‐light‐emitting diodes—at a doping concentration of 9 wt %, the device reaches its maximum external quantum efficiency of 1.64 % (4.08 cd A^–1). The emission color remains almost unchanged under different bias conditions: the CIE coordinates are (0.32,0.33) at 11.0 V (2.54 mA cm^–2, 102 cd m^–2) and (0.31,0.33) at 21.0 V (281 mA cm^–2, 7328 cd m^–2). These values are very close to the ideal CIE chromaticity coordinates for a pure white color (0.33,0.33).     

Thiophenol‐Modified CdS Nanoparticles Enhance the Luminescence of Benzoxyl Dendron‐ Substituted Polyfluorene Copolymers

Highly luminescent dendron‐substituted copolyfluorenes that incorporate surface‐modified cadmium sulfide nanoparticles have been prepared. A small percentage of these nanoparticles can be incorporated into the dendritic structures upon tailoring the interfaces between the ligands on the nanoparticles and the dendritic structures in the copolyfluorene. Both the photoluminescence and electroluminescence efficiencies of the polymer nanocomposites are dramatically enhanced—sometimes more than doubled—relative to the values of the pure polymer.     

Threshold Reduction in Polymer Lasers Based on Poly(9,9‐dioctylfluorene) with Statistical Binaphthyl Units

We introduce novel statistical copolymers of poly(9,9‐dioctylfluorene), PFO, which contain various concentrations of 6,6′‐(2,2′‐octyloxy‐1,1′‐binaphthyl) spacer groups. We demonstrate that, owing to the large dihedral angle (> 60°) between neighboring naphthalene units, we could hinder the formation of the highly ordered β‐phase in thin films of the copolymers. In low‐temperature photoluminescence measurements, the typical signature of the PFO β‐phase at 442 nm is no longer observed for copolymers with a binaphthyl concentration of about 12 %. Moreover, the optical properties of the copolymers resembled those of the glassy α‐phase PFO. Second‐order distributed feedback (DFB) lasers based on thin films of the homopolymer PFO showed a minimum lasing threshold of 11.7 μJ cm^–2 (λ_max = 452 nm, excitation at λ = 337 nm with 500 ps pulses). With increasing binaphthyl concentration in the copolymer backbone, the lasing threshold steadily decreased to 3 μJ cm^–2 for a binaphthyl concentration of about 12 %. Therefore, our novel copolymers provide a vast improvement for PFO‐based optoelectronics.     

Microscopic Morphology of Polyfluorene–Poly(ethylene oxide) Block Copolymers: Influence of the Block Ratio

In this paper, we describe the synthesis and characterization of poly(9,9′‐dioctylfluorene)–poly(ethylene oxide) (PF‐PEO) block copolymers with different block ratio and molecular architectures (diblock or triblock copolymers). Tapping‐mode atomic force microscopy is used to investigate the relationship between the molecular structure and the microscopic morphology of thin deposits. Copolymers with a low average volume ratio of PEO (f_EO from 0.1 to 0.3) exhibit a well‐defined organization into nanoribbons. A model of chain packing is proposed; these structures arise from the interplay of π–π interactions between conjugated PF segments and the interactions of PEO with the mica substrate surface. For copolymers with higher average volume ratio of PEO (f_EO > 0.4), the organized structures disappear and lead to untextured aggregates, probably because long‐range, regular π–π stacking of the segments can no longer take place. We also observe that the nature of the solvent from which deposits are grown and the substrate polarity have a strong impact on the microscopic morphology.     

Photoaddressable Alignment Layers for Fluorescent Polymers in Polarized Electroluminescence Devices

Liquid‐crystalline (LC) polyfluorenes have been successfully aligned on photoaddressable polymers (PAPs). This is the first example of the alignment of a LC main chain polymer on a photoaligned layer. The degree of molecular alignment in the fluorescent polyfluorene layer on top of an ultra‐thin PAP layer is shown to depend strongly on the chemical nature of the PAP. Good alignment with dichroic ratios of more than 10 was only achieved with PAPs containing liquid‐crystalline side chains. Patterning with laterally structured alignment was realized in several ways, utilizing reorientation with orthogonally polarized light. Thin PAP layers have further been utilized as hole‐conducting alignment layers in polymer light‐emitting diodes (LEDs) with polarized emission. In order to facilitate hole transport through the alignment layer, different concentrations of a hole‐transporting molecule (HTM) have been mixed into the PAP layer. These hole‐conducting alignment layers retained their aligning abilities even at HTM concentrations of 20 wt.‐%. LEDs with photometric polarization ratios in emission of up to 14 at a brightness of up to 200 cd/m² and an efficiency of 0.3 cd/A could be realized.