Effects of packing structure on the optoelectronic and charge transport properties in poly(9,9-di-n-octylfluorene-alt-benzothiadiazole)

Spin-coated poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) films of different molecular weights (Mn= 9-255 kg/mol), both in the pristine and annealed state, were studied in an effort to elucidate changes in the polymer packing structure and the effects this structure has on the optoelectronic and charge transport properties of these films. A model based on quantum chemical calculations, wide-angle X-ray scattering, atomic force microscopy, Raman spectroscopy, photoluminescence, and electron mobility measurements was developed to describe the restructuring of the polymer film as a function of polymer chain length and annealing. In pristine high molecular weight films, the polymer chains exhibit a significant torsion angle between the F8 and BT units, and the BT units in neighboring chains are close to one another. Annealing films to sufficiently high transition temperatures allows the polymers to adopt a lower energy configuration in which the BT units in one polymer chain are adjacent to F8 units in a neighboring chain ("alternating structure"), and the torsion angle between F8 and BT units is reduced. This restructuring, dictated by the strong dipole on the BT unit, subsequently affects the efficiencies of interchain electron transfer and exciton migration. Films exhibiting the alternating structure show significantly lower electron mobilities than those of the pristine high molecular weight films, due to a decrease in the efficiency of interchain electron transport in this structure. In addition, interchain exciton migration to low energy weakly emissive states is also reduced for these alternating structure films, as observed in their photoluminescence spectra and efficiencies.     

Optoelectronic and charge transport properties at organic-organic semiconductor interfaces: comparison between polyfluorene-based polymer blend and copolymer

We report detailed studies of optoelectronic and charge transport properties at the organic-organic semiconductor interfaces formed between polymer chains (interchain) and within a polymer chain (intrachain). These interfaces are fabricated using poly(9,9-di-n-octylfluorene-alt-N-(4-butylphenyl)diphenylamine) (TFB [f8-tfb]) (electron-donor) and poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT [f8-bt]) (electron-acceptor) conjugated polymers, by blending them together or by covalently attaching them via a main polymer backbone (copolymer). For optoelectronic properties, when a bulky and twisted tfb molecule is incorporated into a rigid F8BT conjugated backbone, it disturbs the conjugation of F8BT polymer, leading to a blue-shift in the lowest absorption transition. However, by acting as an effective electron donor, it assists the formation of an intrachain singlet exciton that has a strong charge-transfer character, leading to a red-shifted and longer-lived emission than that of F8BT. An extremely efficient and fast energy transfer from tfb donor to bt acceptor is observed in the copolymer (<1 ps) compared to transfer from TFB to F8BT in the blend (tens of ps). This efficient energy transfer in the copolymer is found to be associated with its low fluorescence efficiency (40-45% vs 60-65% for blend) because of the migration of radiative singlet excitons to low-energy states such as triplet and exciplex states that are nonemissive or weakly emissive. The presence of molecular-scale tfb-f8-bt interfaces in the copolymer, however, does not hinder an efficient transport of charge carriers at high drive voltages. Instead, it provides a better balance of charge carriers inside the device, which leads to slower decay of the device efficiency and thus more stable light-emitting diodes with increasing voltage than the blend devices. These distinctive optoelectronic and charge transport properties observed at different organic-organic semiconductor interfaces will provide useful input for the design rules of conjugated polymers required for improved molecular electronics.     

新型电磷光咔唑-co-吡啶共聚物的合成与发光性能

用Suzuki缩聚反应制备了一系列新型咔唑-吡啶共轭主链上含有金属铱配合物单元的共聚物. 共聚物发光器件结构为ITO/PEDOT:PSS/polymer＋PBD (w＝30%)/CsF/Al [氧化铟锡/苯磺酸掺杂聚乙烯基二氧噻吩/聚合物＋2,4-二苯-5-4-叔丁基苯-1,3,4-恶二唑(w＝30%)/氟化铯/铝]. 基于共聚物PCzPyIrMppy2的器件在电流密度为20.8 mA/cm2时, 最大外量子效率和流明效率分别达到4.1%和6.1 cd/A. 在电流密度200 mA/cm2时, 器件的最大外量子效率和流明效率仍分别达到3.2%和4.8 cd/A, 器件的发光的最大波长位于570 nm, 最大亮度达到13251 cd/m2. 研究结果表明, 在共轭聚合物主链上引入螯合金属铱配合单元是实现高效、稳定的电致磷光器件的有效方法之一.     

Electrogenerated Chemiluminescence of Pure Polymer Films and Polymer Blends

We recently reported the discovery of soliton‐like electrogenerated chemiluminescence (ECL) waves from pure conjugated polymer films and gold‐nanoparticles‐doped polymer films. In addition to a more detailed study of these polymer systems by changing the film thickness and the distribution of local leaks, we also apply the ECL wave phenomenon to polymer blends of conjugated and nonconjugated polymers. Poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) is used as the active material that is oxidized and produces ECL with the presence of a co‐reactant, tri‐n‐propylamine (TPA). Several factors such as film thickness, artificial leaks, and solubility of doped polymers are examined for their effects on the ECL behavior. When polystyrene (PS), less soluble in the electrolyte, is blended with F8BT, dotted ECL signals are observed and transported as waves. When poly(methyl methacrylate) (PMMA), more soluble in the electrolyte, is blended with F8BT, PMMA serves as local scratches and ECL waves are triggered simultaneously from the whole film.

     

Electroluminescence and Photoluminescence of Conjugated Polymer Films Prepared by Plasma Enhanced Chemical Vapor Deposition of Naphthalene

Polymer light-emitting devices were fabricated utilizing plasma polymerized thin films as emissive layers. These conjugated polymer films were prepared by RF plasma enhanced chemical vapor deposition using naphthalene as monomer. The effect of different applied powers on the chemical structure and optical properties of the conjugated polymers was investigated. Fourier transform infrared (FTIR) and Raman spectroscopies confirmed that a conjugated polymer film with a 3-D cross-linked network was developed. By increasing the power, products tended to form as highly cross-linked polymer films. The fabricated devices showed broadband Electroluminescence (EL) emission peaks with center at 535–550 nm. Photoluminescence (PL) spectra of plasma polymers showed different excimeric emissions, resulted from crosslinked architecture. As the plasma power increased, the optical properties showed two different domains; up to 200 W, EL, PL and UV–Vis spectra red-shifted and broadened significantly. At higher powers, a reverse behavior was observed. Also, the relation between the film structure and plasma species was investigated using optical emission spectroscopy.     

Quantifying the efficiency of förster‐assisted optical gain in semiconducting polymer blends by excitation wavelength selective amplified spontaneous emission

We study the correlation between Förster resonance energy transfer (FRET) and optical gain properties in conjugated polymer blends based on regioregular poly(3‐hexylthiophene) (P3HT) and poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT). First, FRET dynamics are investigated with femtosecond transient absorption spectroscopy observing a sub‐picosecond energy transfer from F8BT to P3HT (550 fs) at medium doping levels (40% wt P3HT in F8BT). Amplified spontaneous emission (ASE) is then characterized in blends upon exciting predominantly the host and guest polymers, respectively. The corresponding density of absorbed photons at threshold conditions is compared upon host or guest photoexcitation as a method to quantitatively determine the FRET‐assisted ASE efficiencies. We observe a reduction in ASE efficiency upon host photoexcitation of 20%, in the best case, respect to guest photoexcitation. Our results confirm that even in strongly coupled host:guest mixtures delayed exciton population by energy transfer is subtle to losses ascribed to exciton–exciton annihilation. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2311–2317.     

On the angular dependence of the optical polarization anisotropy in ladder-type polymers

Two ladder-type polymers have been examined using polarized optical absorption and fluorescence spectroscopy in solution and solid state. The fluorescence anisotropy excitation spectra of ladder-type methyl-substituted poly(para-phenylene) and poly(naphthylene-phenylene) follow their absorption profiles, contrary to the typical behavior in other conjugated polymers, which show excitation wavelength dependent anisotropy until the onset of the typical red edge effect. Using Raman microscopy and linear dichroism of aligned stretched polyethylene films containing the ladder polymer, it has been found that the orientation of the absorption transition dipole moment is dependent on the conjugation length. The transition dipole of the shorter conjugation lengths of the distribution is localized approximately 7 degrees off axis, and that of the longer conjugation lengths is delocalized along the chain.     

A novel conjugated polymer containing alternating p‐ and n‐type moieties with balanced properties of conducting holes and electrons

A new electroluminescent conjugated polymer consisting of 2,2′‐bipyridylenevinylene (BPyV) and 2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene (MEH‐PV) moieties (BPy‐MEH‐PPV), was synthesized by incorporating an n‐dope type moiety, BPyV, and a p‐dope type moiety, MEH‐PV, into the polymer chain. This facile method provides a new approach to the synthesis of conjugated polymers with balanced ability of conducting electrons and holes. PBy‐MEH‐PPV exhibits tunable optical properties through protonation, and the emissive color can be progressively changed from orange to deep red depending on the degree of protonation.     

Growth of organic n-conductors on thin polymer films for use in organic field effect transistors

Thin films of different polymers - poly(styrene) (PS), poly(methylmethacrylate) (PMMA), poly(vinylcarbazole) (PVCz), poly(vinylchloride) (PVC) and poly(vinylidene fluoride) (PVDF) - were deposited by spin-coating or by vapor deposition. On these polymers, thin films of (hexadecafluorophthalocyaninato)-oxovanadium (F₁₆PcVO) were prepared by physical vapor deposition. The growth of these films was monitored in situ by optical spectroscopy. The optical absorbance spectra were analyzed based on the coupling of transition dipoles to obtain information on the intermolecular arrangement of chromophores in the films. In all of these samples, the molecules are oriented with their molecular plane preferentially perpendicular to the substrate surface. This gives the desired overlap of the π-systems for electric conductance parallel to the substrate. Differences in the interactions were detected when deposition temperatures below or above the glass transition temperature of a given polymer were compared. The morphology of the polymer films and the deposited semiconductors were investigated by atomic force microscopy and scanning electron microscopy. The influence of the chosen substrate on the film structure is determined. The optical and electric properties of the films could thereby be influenced and the applicability of such films as active layers in organic thin film transistors is discussed.     

基于氟代异靛蓝并[7,6-g]异靛蓝的共轭聚合物的合成及其半导体性质

采用Stille缩聚,合成了3个异靛蓝并[7,6-g]异靛蓝(DIID)和乙烯单元交替排列的共轭聚合物P0F、P2F和P4F,三者在DIID单元中分别含0、2和4个氟原子(F).3个聚合物均具有良好的平面性,前线分子轨道几乎在整个共轭骨架上离域.它们均具有宽的吸收光谱,吸收范围在400~1000 nm,光学带隙约为1.25 eV;随着氟原子数目的增加,聚合物的最高占有分子轨道(HOMO)和最低空分子轨道(LUMO)能级依次下降0.1~0.2 eV.以这3个聚合物作为活性层,制备了顶栅-底接触型有机场效应晶体管器件,随着氟原子数目的增加,聚合物的传输性质由双极传输变为n型传输.P0F和P2F是双极传输型聚合物,空穴迁移率(μ_h)分别达到0.11和0.30 cm~2 V~(-1) s~(-1),电子迁移率(me)分别达到0.22和1.19 cm~2 V~(-1) s~(-1).P4F是n型聚合物,me达到0.18 cm~2 V~(-1) s~(-1).     

Micropatterning of functional conductive polymers with multiple surface chemistries in register

A versatile procedure is presented for fast and efficient micropatterning of multiple types of covalently bound surface chemistry in perfect register on and between conductive polymer microcircuits. The micropatterning principle is applied to several types of native and functionalized PEDOT (poly(3,4-ethylenedioxythiophene)) thin films. The method is based on contacting PEDOT-type thin films with a micropatterned agarose stamp containing an oxidant (aqueous hypochlorite) and applying a nonionic detergent. Where contacted, PEDOT not only loses its conductance but is entirely removed, thereby locally revealing the underlying substrate. Surface analysis showed that the substrate surface chemistry was fully exposed and not affected by the treatment. Click chemistry could thus be applied to selectively modify re-exposed alkyne and azide functional groups of functionalized polystyrene substrates. The versatility of the method is illustrated by micropatterning cell-binding RGD-functionalized PEDOT on low cell-binding PMOXA (poly(2-methyl-2-oxazoline)) to produce cell-capturing microelectrodes on a cell nonadhesive background in a few simple steps. The method should be applicable to a wide range of native and chemically functionalized conjugated polymer systems.     

A high-resolution near-edge x-ray absorption fine structure investigation of the molecular orientation in the pentacene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) pentacene/system

We present x-ray photoemission spectroscopy and highly resolved near-edge x-ray absorption fine structure spectroscopy measurements taken on pentacene thin films of different thicknesses deposited on a spin coated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) substrate. Thin films of pentacene were prepared by using organic molecular beam deposition in situ using strictly controlled evaporation conditions. Our investigations show that pentacene thin films on PEDOT:PSS are characterized by upright standing molecules. Due to the strong dichroic behavior, the calculated values of the molecular orientation give a clear indication not only of the real molecular arrangement in the films but also of a high orientational order. This high degree of molecular orientation order is a characteristic already of the first layer. The films show the tendency to grow on the PEDOT:PSS substrate following an island-fashion mode, with a relatively narrow intermixing zone at the interface between the pentacene and the polymer blend. The peculiarity of the growth of pentacene on PEDOT:PSS is due to the fact that the substrate does not offer any template for the nucleated films and thus exerts a lateral order toward the crystal structure arrangement. Under these conditions, the upright orientation of the molecules in the films minimizes the energy required for the system stability.     

Synthesis and characterization of fluorene‐based copolymers containing benzothiadiazole derivative for light‐emitting diodes applications

Five new thermally robust electroluminescent fluorene‐based conjugated copolymers, including poly[2,7‐(9,9‐dioctylfluorene)‐co‐4,7‐{5,6‐bis(3,7‐dimethyloctyloxymethyl)‐2,1,3‐(benzothiadiazole)}] ( PFO‐P2C₁₀BT ) were synthesized and used to fabricate the efficient polymer light‐emitting diodes (PLEDs). The glass transition temperatures of the polymers were found to be higher than that of poly(9,9‐dialkylfluorenes) and are in the range 113–165 °C. We fabricated PLEDs in indium‐tin oxide/PEDOT/light‐emitting polymer/cathode configurations using either double‐layer LiF/Al or triple‐layer Alq₃/LiF/Al cathode structures. The new copolymers were found to have emission colors that vary from greenish blue (491 nm) to green (543 nm) depending on the copolymer composition. The maximum brightness and luminance efficiency of these PLEDs were found to be up to 5347 cd/m² and 1.51 cd/A at 10 V, respectively. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6762–6769, 2008     

Highly emissive conjugated polymer excimers

Conjugated polymers often display a decrease of fluorescence efficiency upon aggregation due in large part to enhanced interpolymer interactions that produce weakly emissive species generally described as having excimer-like character. We have found that poly(phenylene ethynylene)s with fused pendant [2.2.2] ring structures having alkene bridges substituted with two ester groups function to give highly emissive, broad, and red-shifted emission spectra in the solid state. To best understand the origin of this new solid-state emissive species, we have performed photophysical studies of a series of different materials in solution, spin-coated thin films, solid solutions, and Langmuir films. We conclude that the new, red-shifted, emissive species originate from excimers produced by interchain interactions being mediated by the particular [2.2.2] ring system employed. The ability to design structures that can reliably produce highly emissive conjugated polymer excimers offers new opportunities in the emission tailoring of electroluminescence and sensory devices.     

新型超支化聚(芴-co-吡啶并[3,2-b]吡嗪)共聚物的合成与发光性能

合成了新型三溴代的含有吡啶并[3,2-b]吡嗪的化合物, 通过Suzuki缩聚反应合成了以吡啶并[3,2-b]吡嗪为核, 以9,9-二辛基芴为枝的超支化共聚物. 该共聚物在甲苯和四氢呋喃等有机溶剂中具有较好的溶解性能. 采用共聚物制备了结构为ITO/PEDOT/polymer/Ca/Al的发光器件, 其中基于共聚物PFPQ-2的器件在电流密度为3.8 mA/cm²时得到蓝光发射, 最大外量子效率为1.29%, 最大亮度为1173 cd/m², 色坐标为(0.16, 0.11).     

共轭高分子光电功能材料的多尺度性能研究——聚合物电子学领域中的新机遇

共轭高分子材料特异的金属或半导体的电子特性兼有质轻、价廉、易于加工的优点使其在有机场效应晶体管、有机太阳能电池和有机发光二极管等领域显示了重要的应用前景.然而,尽管经过几十年的不断研究,共轭高分子材料种类及其相关器件性能均已得到显著发展,但是共轭高分子材料的本征电荷传输特性仍不清楚,其研究面临巨大挑战,这主要是由共轭高分子材料本身分子量分布弥散、分子间相互缠结以及在常规旋涂薄膜器件中分子高度无序等特性所决定的.从调控共轭高分子聚集态结构的角度出发,不断提高共轭高分子的结构有序性及减小电荷传输过程中的晶界及缺陷密度,是实现共轭高分子材料本征性能认识的有效途径之一.本文首先简单归纳总结了研究者在共轭高分子多尺度聚集态结构调控及性能研究方面的初步结果,进一步结合国内外相关研究进展,重点对共轭高分子晶体方面的工作展开详细介绍,最后对该领域未来发展的挑战及机遇进行了简单评述.     

Interplay between solid state microstructure and photophysics for poly(9,9‐dioctylfluorene) within oriented polyethylene hosts

We present a study of isotropic and uniaxially oriented binary blend films comprising ≤1 wt % of the conjugated polymer poly(9,9‐dioctylfluorene) (PFO) dispersed in both ultra‐high molecular weight (UHMW) and linear‐low‐density (LLD) polyethylene (PE). Polarized absorption, fluorescence and Raman spectroscopy, scanning electron microscopy, and X‐ray diffraction are used to characterize the samples before and after tensile deformation. Results show that blend films can be prepared with PFO chains adopting a combination of several distinct molecular conformations, namely glassy, crystalline, and the so‐called β‐phase, which directly influences the resulting optical properties. Both PFO concentration and drawing temperature strongly affect the alignment of PFO chains during the tensile drawing of the blend films. In both PE hosts, crystallization of PFO takes place during drawing; the resulting ordered chains show optimal optical anisotropy. Our results clarify the PFO microstructure in oriented blends with PE and the processing conditions required for achieving the maximal optical anisotropy. © 2014 The Authors. Journal of Polymer Science Part B: Polymer Physics Published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 22–38     

Characterization of tectoRNA assembly with cationic conjugated polymers

Association between RNAs with preprogrammed molecular recognition units can be quantified by using cationic, water-soluble conjugated polymers. The method uses a fluorophore-labeled probe RNA (RNA-F*), which is treated with a target structure (RNA-T). Heterodimer formation, (RNA-T/RNA-F*), increases the total negative charge on the F*-bearing macromolecule and reduces the number of negatively charged molecules (relative to unbound RNA-T+ RNA-F*). On the basis of electrostatic interactions, we anticipated more effective binding between CCP and (RNAT/RNA-F*), a reduction of the average CCP- - -F* distance, and more effective FRET upon excitation of the conjugated polymer. The resulting signals benefit from the optical amplification characteristic of emissive conjugated polymers. Solution dissociation constants can be determined by monitoring F* intensity changes as a function of [RNA-F*] and the ratio: [I(T) - I(NB)]/I(NB), where I(T) and I(NB) are the F* intensities in the presence of the target RNA (RNA-T) and a nonbinding RNA (RNA-NB), respectively, while keeping the concentration of the conjugated polymer constant. By focusing on [I(T) - I(NB)]/I(NB) as a function of RNA concentration, one can detect the concentration range wherein increased fluorescence is the result of dimerization.     

Application of thin films of conjugated polymers in novel light-emitting devices and liquid crystal “light-valves”

Light-emitting electroluminescent diodes and electrochemically-driven electroluminescent devices involving conjugated polymers are described. The effect on the properties of polypyrrole films (deposited from aqueous polymerizing solutions of pyrrole) caused by the hydrophilicity/hydrophobicity of the substrate surface is utilized by a “microcontact printing” technique to form patterned liquid crystal display devices.     

苯基取代聚苯撑乙烯的合成及其电致发光性能

聚苯撑乙烯(PPV)类聚合物是优异的发光材料, 有望作为全色显示中三基色的材料之一得到应用. 我们采用2-溴-1,4-亚二甲苯二乙酯为原料, 合成了商品名为Supper Yellow PPV (SY PPV)的苯基取代PPV. 中间体、单体和聚合物的结构都通过核磁共振、元素分析进行了表征. SY PPV的吸收峰在434 nm, 吸收边在510 nm, 带隙2.44 eV. 光致发光峰值和电致发光峰值分别在516和552 nm. SY PPV的器件性能为: 启动电压为2.4 V, 最大亮度大于49000 cd·m^-2, 最大流明效率为21 cd·A^-1, 显著优于采用老方法合成SY PPV的最大流明效率(16-18 cd·A^-1).