Organic crystal fibers aligned into oriented bundles with polarized emission

We demonstrate a simple method for growing organic crystal fiber bundles as long as centimeters by controlling the shape and dimension of the evaporation channel of the chloroform solution of N,N'-bis(1-ethylpropyl)-3,4,9,10-perylenebis(dicarboximide) (EPPTC). The capillary effect induces a thin solution film (capillary film) on the wall of the evaporation channel, and fast evaporation of the solvent gives rise to a concentration gradient along the channel. Thus, the strong pi-stacking between the EPPTC molecules in the capillary film results in formation of crystal fibers. Nearly linearly polarized emission centered around 620 nm has been detected from these crystal fibers under optical excitation. Measurements of the electron diffraction pattern and optical microscopic properties show well-defined stacking of the molecules in the crystal fibers with excellent alignment.     

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     

Control of morphology in efficient photovoltaic diodes from discotic liquid crystals

We report on the role of morphology in photovoltaic diodes with blend active layers composed of perylene tetracarboxdiimide (EPPTC) and hexabenzocoronene (HBC) derivatives as electron and hole acceptors. Controlled annealing of HBC:EPPTC films while in conformal contact with a flat elastomeric stamp improves photovoltaic response, leading to an external quantum efficiency of 29.5% at 460 nm and an open-circuit voltage of 0.70 V. The improved performance is attributed in part to larger crystalline domains following annealing. The elastomeric stamp restricts the top surface of the thin film during annealing, leading to low surface roughness, while also allowing for greater vertical stratification of the components in the bulk. Blended HBC:EPPTC films also exhibit an unique optical absorption feature near 590 nm, which we attribute to an altered crystalline packing of EPPTC in the presence of HBC. The significance of the local structure at molecular heterojunctions in blended active layer photovoltaic diodes is discussed.     

Light‐emitting copolymers based on fluorene and selenophene—Comparative studies with its sulfur analogue: Poly(fluorene‐co‐thiophene)

A series of soluble conjugated copolymers derived from 9,9‐dioctylfluorene (FO) and selenophene (SeH) was synthesized by a palladium‐catalyzed Suzuki coupling reaction with various feed ratios of SeH to FO less than or equal to 50%. The efficient energy transfer from fluorene segments to narrow band‐gap selenophene sites was observed. In comparison with the very well studied copolymer poly(fluorene‐co‐thiophene), poly(9,9‐dioctylfluorene‐co‐selenophene) (PFO‐SeH) shows redshifted photoluminescence (PL) and electroluminescence (EL) emission. PL spectra of the PFO‐SeH copolymers show a significant redshift along with increasing selenophene content in the copolymers and also with increasing polymer concentration in solution. PL quantum efficiency of the selenophene‐containing PFO copolymer is much lower than that of corresponding PFO‐thiophene (Th) copolymers. All these features of PFO‐SeH copolymers can be explained by the difference in aromaticity of selenophene and thiophene heterocycles and the heavy atom effect of Se in comparison with S‐atoms. The device fabricated with PFO‐SeH15 as the emissive layer exhibited high external quantum efficiency (0.51%) at a luminance of 1570 cd/m². Device performance is limited by electron injection and the strong quenching effect of Se atoms. Devices with PFO‐SeH copolymers blended into PFO homopolymers show significant improvement in device performance. External quantum efficiency as high as 1.7% can be obtained for PFO‐SeH30/PFO blend devices. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 823–836, 2005     

Full color light‐emitting electrospun nanofibers prepared from PFO/MEH‐PPV/PMMA ternary blends

In this study, luminescence electrospun (ES) nanofibers based on ternary blends of poly(9,9‐dioctylfluoreny‐2,7‐diyl) (PFO)/poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV)/poly(methyl methacrylate) (PMMA) were prepared from chloroform solutions using a single capillary spinneret. Effects of PFO/MEH‐PPV ratio on the morphology and photophysical properties were studied while the PMMA weight percentage was fixed at 90 wt %. The morphologies of the prepared ES fibers were characterized by FE‐SEM and fluorescence microscopy. The obtained fibers had diameters around a few hundred nm and pore sizes in the range of 30–35 nm. The emission colors of the PFO/MEH‐PPV/PMMA blend ES fibers changed from blue, white, yellowish‐green, greenish‐yellow, orange, to yellow, as the MEH‐PPV composition increased. In contrast, the emission colors of the corresponding spin‐coated films were blue, orange, pink‐red, red, and deep‐red. Based on the values of solubility parameters, the PFO and MEH‐PPV are miscible to each other and trapped in the PMMA matrix. Hence, energy transfer between these two polymers is possible. The smaller aggregated domains in the ES fiber compared to those of spin‐coated films possibly reduce the efficiency of energy transfer, leading to different emission colors. Also, the prepared ES fibers had higher photoluminescence efficiencies than those of the spin‐coated films. Pure white light‐emitting fibers prepared from the PFO/MEH‐PPV/PMMA blend ratio of 9.5/0.5/90 had the Commission Internationale de L'Eclairage (CIE) coordinate of (0.33, 0.31). Our results showed that different color light‐emitting ES fibers were produced through optimizing the composition of semiconducting polymer in the transparent polymer matrix. This type of ES fibers could have potential applications as new light sources or sensory materials for smart textiles. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 463–470, 2009     

Noncrystalline phases in poly(9,9-di-n-octyl-2,7-fluorene)

Selective formation of amorphous, nematic (N), and beta phases in poly(9,9-di-n-octyl-2,7-fluorene) (PFO) films was achieved via judicious choice of process parameters. Phase structure and film morphology were carefully examined by means of X-ray diffraction as well as electron microscopy. "Amorphous" thin films were obtained by quick evaporation of solvent. Slow solvent removal during film formation or extended treatment of the amorphous film with solvent vapor resulted in predominantly the beta phase, which corresponds to a frozen (due to decreased segmental mobility upon solvent removal) and intrinsically metastable state of transformation midway between a solvent-induced clathrate phase and the equilibrium crystalline order in the undiluted state. The frozen transformation process is reactivated upon an increase in temperature beyond 100 degrees C. Compared to the amorphous film, extended backbone conjugation in the beta phase is evidenced from the emergence of a characteristic absorption peak around 430 nm near the absorption edge. For films of frozen nematic order (obtained by quenching from the nematic state), the conjugation length is also greater than the amorphous films as revealed by an absorption shoulder around 420 nm. Well-behaved single-chromophore emission with single-mode phonon coupling was observed for the beta phase; in the case of nematic films, dual-mode phonon coupling must exist if single-chromophore emission is assumed. In comparison, the emission spectrum of the amorphous film of generally shorter conjugation lengths exhibited mixed characteristics of nematic and beta phases, implying the presence of minor populations of extended conjugation similar to those in nematic and beta phases, which are of biased weightings in the emission spectra. All these films consist of nanograins (ca. 10 nm in size) of collapsed chains; the films are therefore inherently inhomogeneous in this length scale. In combination with previous observations on the crystalline (alpha and alpha') forms, the phase behavior of PFO is then generally summarized in terms of relative thermodynamic stability.     

The role of segregation in the polarized emission from polyfluorene embedded in a liquid crystal

Polyfluorene (PFO) embedded in a nematic liquid crystal (LC) matrix is investigated. For low PFO weight contents, a homogeneous dispersion is obtained which displays a strong fluorescence anisotropy along the LC director, indicating a significant alignment of the polymeric chains along this direction. Besides, for relatively high PFO weight contents, phase separation takes place. Under these conditions, the sample is composed of micrometer‐sized domains, where the two species are in solution, enclosed by segregated polymeric boundaries. By polarized‐photoluminescence imaging and spectroscopy, it is found that most of the light emission originates from these boundaries and gets strongly pinned along their orientation. Since boundaries are mainly oriented orthogonal to the LC chains, this morphological alignment results in a system in which the orientation of the polarization emission can be predicted and possibly controlled. Conversely, in the homogeneous sample one can obtain a homogeneous emission polarization by controlling the alignment of the LC. These features are potentially relevant for the development of flexible polarization‐sensitive optoelectronic devices. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1558–1563     

Liquid phase demixing in ferroelectric/semiconducting polymer blends: An experimental and theoretical study

This article describes a combined experimental and theoretical study on nanophase structure development as a result of liquid phase demixing in solution‐cast blends of the organic semiconductor poly(9,9′‐dioctyl fluorene) (PFO) and the ferroelectric polymer poly(vinylidene fluoride‐co‐trifluoroethylene) (P(VDF‐TrFE)). Blend layers (200 nm) are prepared by spin coating a 1:9 (w/w) PFO:P(VDF‐TrFE) blend solution in a common solvent on a poly(ethylenedioxy thiophene)/poly(styrene sulfonate) substrate. Owing to the pronounced incompatibility between the two polymers, a strong phase‐separated morphology is obtained, characterized by disk‐like nanodomains of PFO embedded in a P(VDF‐TrFE) matrix, as revealed by scanning electron microscopy. By varying the processing conditions, we find the average domain size and standard deviation to increase with spinning time. The considerable increase in domain size suggests the coarsening process not to be impeded by a steep rise in viscosity. This indicates solvent evaporation to be only moderate within the experimental time frame. The evolution of the observed phase morphology is modeled using ternary diffuse interface theory integrated with a modified Flory–Huggins (FH) treatment of the homogeneous (bulk) free energy of mixing, to account for significant molecular differences between the active blend components. Using calculated FH interaction parameters, the model confirms the phase separation to occur via spinodal decomposition of the blend solution during spin coating, as suggested by experimental observations. The simulated phase morphologies as well as the modeled trends in domain growth and standard deviation compare favorably with the experimental data. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1255–1262, 2011     

Fluorene copolymers containing bithiophene/2,5‐ or 2,6‐pyridine units: A study of their optical,electrochemical, and electroluminescence properties

Two alternating copolymers, poly[(2,5‐di(2‐thienyl)‐pyridine‐5,5′‐diyl)‐alt‐(9,9‐dioctylfluorene‐2,7‐diyl)], PFO‐TPy25T, and poly[(2,6‐di(2‐thienyl)‐pyridine‐5,5′‐diyl)‐alt‐(9,9‐dioctylfluorene‐2,7‐diyl)], PFO‐TPy26T, were synthesized by the Pd‐catalyzed Suzuki polymerization method. The pyridine units are present as trimeric monomers in these copolymers and have different connectivities to their two neighboring thiophenes, para‐ and meta‐linkages. We investigated the variations in the optical and electrochemical properties of the copolymers that arise from these different connectivities. The two polymers exhibit 5% weight loss above 410 °C and high glass transition temperatures (T_g: 113 °C for PFO‐TPy25T, 142 °C for PFO‐TPy26T). The UV–vis absorption maximum peaks of PFO‐TPy25T and PFO‐TPy26T in the solid state were found to be 449 and 398 nm respectively, with photoluminescence maximum peaks in the solid state of 573 and 490 nm respectively. Using cyclic voltammetry, we determined their energy band gaps: 3.08 eV for PFO‐TPy25T and 3.49 eV for PFO‐TPy25T. The cyclic voltammetry study of these polymers revealed that there are some differences. The electroluminescence (EL) properties of the copolymers were measured for the device configuration of ITO/PEDOT/polymers/Ca/Al. The device fabricated with the polymer containing 2,5‐pyridine exhibits pale orange emission, whereas the device fabricated with the polymer containing 2,6‐pyridine exhibits pale blue emission. The EL device fabricated with PFO‐TPy25T has a higher brightness (2010 cd/m²) and external quantum efficiency (0.1%) than the PFO‐TPy26T device (260 cd/m², 0.008%), because it has a smaller energy barrier to the injection of charges from PEDOT and Ca into the HOMO and LUMO levels. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4611–4620, 2006     

Competition between the charge transfer state and the singlet states of donor or acceptor limiting the efficiency in polymer:fullerene solar cells

We study the appearance and energy of the charge transfer (CT) state using measurements of electroluminescence (EL) and photoluminescence (PL) in blend films of high-performance polymers with fullerene acceptors. EL spectroscopy provides a direct probe of the energy of the interfacial states without the need to rely on the LUMO and HOMO energies as estimated in pristine materials. For each polymer, we use different fullerenes with varying LUMO levels as electron acceptors, in order to vary the energy of the CT state relative to the blend with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). As the energy of the CT state emission approaches the absorption onset of the blend component with the smaller optical bandgap, E(opt,min) ≡ min{E(opt,donor); E(opt,acceptor)}, we observe a transition in the EL spectrum from CT emission to singlet emission from the component with the smaller bandgap. The appearance of component singlet emission coincides with reduced photocurrent and fill factor. We conclude that the open circuit voltage V(OC) is limited by the smaller bandgap of the two blend components. From the losses of the studied materials, we derive an empirical limit for the open circuit voltage: V(OC) ? E(opt,min)/e - (0.66 ± 0.08)eV.     

壳聚糖-聚羟基丁酸酯共混膜的制备与性质

以乙酸为共溶剂,制备了壳聚糖-聚羟基丁酸酯(CTS-PHB)共混膜,利用红外光谱(FT-IR)、广角X粉末衍射(WAXD)、扫描电镜(SEM)及差热分析(DTA)表征了共混膜的化学组成、晶形、形貌和热稳定性能。研究表明:CTS和PHB可在体积百分数为62.5%的乙酸溶液中共混,形成表面光滑、不透明的膜,干态共混膜具备一定的力学强度。各比例的CTS-PHB共混膜有相同的热分解温度,共混膜的形貌特征随两组分质量配比变化,其中mPHB/mCTS=1/1的共混膜显示出良好的有序结构。     

Effect of side‐chain positions on morphology and photovoltaic properties of phenazine‐based donor–acceptor copolymers

Two phenazine donor–acceptor‐conjugated copolymers (P1 and P2) with the same polymer backbone but different anchoring positions of alkoxy chain on the phenazine unit were investigated to identify the effect of changing the position of alkoxy chains on their optical, electrochemical, blend film morphology, and photovoltaic properties. Although the optical absorption and frontier orbital energy levels were insensitive to the position of alkoxy chains, the film morphologies and photovoltaic performances changed significantly. P1/PC₇₁BM blend film showed the formation of phase separation with large coarse aggregates, whereas P2/PC₇₁BM blend film was homogeneous and smooth. Accordingly, power conversion efficiency (PCE) of photovoltaic devices increased from 1.50% for P1 to 2.54% for P2. In addition, the PCE of the polymer solar cell based on P2/PC₇₁BM blend film could be further improved to 3.49% by using solvent vapor annealing treatment. These results clearly revealed that tuning the side‐chain position could be an effective way to adjust the morphology of the active layer and the efficiency of the photovoltaic device. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2910–2918     

Photophysics and morphology of a polyfluorene donor–acceptor triblock copolymer for solar cells

We present a study of the optical, structural and device properties of a polyfluorene (PFM)‐based (PFM‐F8BT‐PFM) donor–acceptor triblock copolymer for use in an organic solar cell. Neutron reflectivity is employed to probe the vertical composition profile before and after thermal annealing while the crystallinity was examined using grazing incidence wide‐angle X‐ray. The absorption spectra and photoluminescence emission for the triblock and analogous blend of PFM with F8BT reveal a greater degree of intermixing in the triblock. However, the triblock copolymer exhibits exciplex emission, which necessitates a geminate polar pair; long‐lived exciplex states are detrimental in organic photovoltaic devices. The triplet yield in the triblock and the blend is estimated using photoinduced absorption, with the triblock copolymer generating a triplet population 20 times that of the blend. This is far from ideal as triplets are wasted states in organic photovoltaic devices and they can also act as scavengers of polarons reducing the efficiency even more. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1705–1718     

Role of solvent in the compatibility of a vinylchloride–vinylacetate–maleic acid terpolymer and nitrocellulose blends

The compatibility of cast films of a vinylchloride–vinylacetate–maleic acid terpolymer (VMCH) and nitrocellulose (NC) blends is influenced by solvents. Transparent films of VMCH/NC blends are obtained when cast from solvents such as tetrahydrofuran or cyclohexanone, whereas hazy films are obtained when cast from solvents such as acetone or ethylacetate. Visible spectroscopy and phase morphology were used to analyse the compatibility–incompatibility of the blend. Differential scanning calorimetry (DSC) studies demonstrate that the transparent film is compatible, but the hazy film is incompatible. Fourier transform infra-red (FTIR) studies establish that a greater interaction is observed between the polymer pair in case of the compatible blend than in the case of the incompatible blend. A solvent dependency of blend compatibility is reflected in this study. The conformational state of the polymers in solution, which is responsible for the compatibility phenomena, may depend on the donor number and/or Taft-β value of the solvent. The greater the donor number and/or the Taft-β value, the higher may be the level of interaction between the solvent and the polymer molecules, which in turn may give a compatible blend after removal of the solvent.     

Solution‐crystallization and related phenomena in 9,9‐dialkyl‐fluorene polymers. I. Crystalline polymer‐solvent compound formation for poly(9,9‐dioctylfluorene)

Polymer‐solvent compound formation, occurring via co‐crystallization of polymer chains and selected small‐molecular species, is demonstrated for the conjugated polymer poly(9,9‐dioctylfluorene) (PFO) and a range of organic solvents. The resulting crystallization and gelation processes in PFO solutions are studied by differential scanning calorimetry, with X‐ray diffraction providing additional information on the resulting microstructure. It is shown that PFO‐solvent compounds comprise an ultra‐regular molecular‐level arrangement of the semiconducting polymer host and small‐molecular solvent guest. Crystals form following adoption of the planar‐zigzag β‐phase chain conformation, which, due to its geometry, creates periodic cavities that accommodate the ordered inclusion of solvent molecules of matching volume. The findings are formalized in terms of nonequilibrium temperature–composition phase diagrams. The potential applications of these compounds and the new functionalities that they might enable are also discussed. © 2015 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1481–1491     

Birefringence control by hydrogen bonding on compatible polymer pair composed of hydrogenated ring‐opening polymer and modified polystyrene

A new polymer blend composed of a hydrogenated ring‐opening polymer (HROP) with an ester group and hydroxyl functionalized polystyrene (HFP) produced the excellent transparent materials which enabled a precise birefringence control in keeping with the other physical properties for optical film use. The blend with a composition from 0.28 to 0.35 for the HFP weight fraction showed an extraordinary wavelength dispersion, transmitting through a zero birefringence point at the critical fraction of 0.45, while each polymer showed an ordinary wavelength dispersion. The observed excellent transparency even above those of the glass transition temperature was attributed to a depressed phase separation that resulted from strong hydrogen bond between the ester and hydroxyl groups. An IR analysis of the film demonstrated a remarkable red‐shift in the carbonyl peak with an increase of the hydroxylated polystyrene content, indicating a strong hydrogen bond between those groups. This new polymer blend provides a useful design to achieve practical demands for film use, both optical and mechanical under the fabrication conditions using the melt extrusion technique. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3132–3143     

Spectroscopic properties of poly(9,9‐dioctylfluorene) thin films possessing varied fractions of β‐phase chain segments: enhanced photoluminescence efficiency via conformation structuring

Poly(9,9‐dioctylfluorene) (PFO) is a widely studied blue‐emitting conjugated polymer, the optoelectronic properties of which are strongly affected by the presence of a well‐defined chain‐extended “β‐phase” conformational isomer. In this study, optical and Raman spectroscopy are used to systematically investigate the properties of PFO thin films featuring a varied fraction of β‐phase chain segments. Results show that the photoluminescence quantum efficiency (PLQE) of PFO films is highly sensitive to both the β‐phase fraction and the method by which it was induced. Notably, a PLQE of ～69% is measured for PFO films possessing a ～6% β‐phase fraction induced by immersion in solvent/nonsolvent mixtures; this value is substantially higher than the average PLQE of ～55% recorded for other β‐phase films. Furthermore, a linear relationship is observed between the intensity ratios of selected Raman peaks and the β‐phase fraction determined by commonly used absorption calibrations, suggesting that Raman spectroscopy can be used as an alternative means to quantify the β‐phase fraction. As a specific example, spatial Raman mapping is used to image a mm‐scale β‐phase stripe patterned in a glassy PFO film, with the extracted β‐phase fraction showing excellent agreement with the results of optical spectroscopy. © 2016 The Authors. Journal of Polymer Science Part B: Polymer Physics Published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1995–2006     

Particle plasmon‐induced charge trapping at heterointerfaces in PCDTBT:PC70BM blends

We investigate the influence of particle plasmons on exciton and charge generation and recombination processes in the blend of poly (9‐(1‐octylnonyl)‐9H‐carbazole‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl) (PCDTBT) and [6,6]‐phenyl‐C₇₀butyric acid methyl ester (PC₇₀BM). The particle plasmons are generated from gold nanoparticles, which are embedded into PCDTBT:PC₇₀BM blend. For the blend with gold nanoparticles, we observe enhance light harvesting. Despite the enhanced light collection, we find that the quasi‐steady‐state charge generation has not been influenced by the particle plasmons. However, the generation and recombination of long‐lived (sub‐millisecond) polaron paris have been significantly enhanced: from untrapped state in the pristine blend to the trapped state in the gold nanoparticle‐embedded blend. This result implies that the plasmon‐influenced polarons are trapped at the broadband geminate polaron pair (GPP) state. This state acts as an intermediate state, which either leads to the formation of charge transfer excitons (CTXs) or free charge carriers. In our case, the particle plasmon‐influenced polarons are trapped in the GPP state, which leads to the formation of CTXs. For this reason, we do not observe the enhanced charge generation in PCDTBT:PC₇₀BM blend with particle plasmon resonance. Finally, we revealed that the long‐lived polarons mainly resulted from the localization by particle plasmons. The macroscopic modification in the blend film made negligible contributions to this influence. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 940–947     

Long range energy transfer in conjugated polymer sequential bilayers

Steady-state and time-resolved photoluminescence have been used to investigate the optical properties of bilayer and blend films made from poly(9,9-dioctyl-fluorene-2,7-diyl) (PFO) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH PPV). Energy transfer has been observed in both systems. From steady-state photoluminescence measurements, the energy transfer was characterized by the effective enhancement of the MEH PPV emission intensity after exciting the donor states. Relatively faster decays for the PFO donor emission have been observed in the blends as well as in the bilayer structures, confirming effective energy transfer in both structures. In contrast to the bilayers, the time decay of the acceptor emission in the blends presents a long decay component, which was assigned to the exciplex formation in these samples. For the blends the acceptor emission is in fact a composition of exciplex and MEH PPV emissions, the later being due to Fo?rster energy transfer from PFO. In the bilayers, the exciplex is not observed and temperature dependence photoluminescence measurements show that exciton migration has no significant contribution to the energy transfer. The efficiency and very long range of the energy transfer in the bilayers is explained assuming a surface-surface interaction geometry where the donor/acceptor distances involved are much longer than the common Fo?rster radius.     

Synergistic Effects of External Electric Field and Solvent Vapor Annealing with Different Polarities to Enhance β-Phase and Carrier Mobility of the Poly(9,9-dioctylfluorene) Films

In this work, the synergistic effects of external electric field(EEF) and solvent vapor annealing to enhance β-phase and carrier mobility of poly(9,9-dioctylfluorene)(PFO) films were investigated. It is found that EEF can promote the PFO β-phase conformation transition and orientate the PFO chains along the EEF direction with the assistance of polar solvent vapor annealing. PFO chain orderness is closely related to the solvent polarity. In particular, the β-phase content in the annealed film of strong polar chloroform vapor increases from 18.7% to 34.9% after EEF treatment. Meanwhile a characteristic needle-like crystal is formed in the film, as a result, the hole mobility is enhanced by an order of magnitude. The mechanism can be attributed to the fast polarization of solvent dipole under the action of EEF, thus forming a driving force that greatly facilitates the orientation of PFO dipole unit. Research also reveals that EEF driving of the PFO chains does not occur with an insoluble solvent vapor since the solvent molecules cannot swell the film, thus there is insufficient free volume for PFO chains to adjust their conformation. This research enriches the understanding of the relationship between solvent vapor annealing and EEF in orientation polymers, and this method is simple and controlled, and capable of integrating into large-area thin film process, which provides new insights to manufacture low-cost and highly ordered polymer films, and is of great significance to enhance carrier mobility and efficiency of photoelectric devices based on polymer condensed matter physics.