Blend Thin Films Research Articles

Surface morphology and optical properties of thin films of thiophene-based binary blends

We report on the relationship between optical and morphological properties of thin films of a blend of thiophene-based monomer and a diamine derivative. This system is of interest as it gives rise to white emission, due to the formation of exciplex energy levels in the solid state. The photoluminescence emission has been collected for different relative concentrations of the two blend components, and the film topography has been mapped correspondingly by means of atomic force microscopy. Along with a complex evolution of the film surface morphology, a significant variation of the emission properties has been observed upon change of the blend composition. In particular, the intensity of the broad low-energy exciplex band turns out to be affected by a balance of two factors, namely, the extension of homogeneous blend film areas, and their effective concentration.

Nanoscale localization of poly(vinylidene fluoride) in the lamellae of thin films of symmetric polystyrene–poly(methyl methacrylate) diblock copolymers

We employed thin film blends of diblock copolymers with functional homopolymers as a simple strategy to incorporate organic functional materials into nanodomains of diblock copolymers without serious synthesis. A blend pair of polystyrene–poly(methyl methacrylate) (PS–PMMA) diblock copolymers and poly(vinylidene fluoride) (PVDF) was selected as a model demonstration because PVDF is a well-known ferroelectric polymer and completely miscible with amorphous PMMA. Thin films of symmetric PS–PMMA copolymers provided the nanometer-sized PMMA lamellae, macroscopically parallel to the substrate, in which PVDF chains were dissolved. Thus, amorphous PVDF chains were effectively confined in the PMMA lamellae of thin film blends. The location of PVDF chains in the PMMA lamellae was investigated by the dependence of the lamellar period on the volume fraction of PVDF, from which we found that PVDF chains were localized in the middle of the PMMA lamellae. After the crystallization of PVDF, however, some of PVDF migrated to the surface of the film and formed small crystallites.

Correlation between Surface Photovoltage and Blend Morphology in Polyfluorene-Based Photodiodes

We present a microscopic study of photoinduced charge generation in polyfluorene-based photovoltaic structures. The sub-100 nm lateral resolution of scanning Kelvin probe microscopy allows characterizing the three-dimensional structure of thin films of blends of poly-(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly-(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine) (PFB). From the strong correlation between surface photovoltage and blend morphology, we propose a simple model for the lateral and vertical film structure identifying in particular those regions with the most efficient conduction pathway for the photocurrent.

Near-field spectroscopy of phase segregation in white-light-emitting blends based on low-mass molecules

We report on the direct observation of phase segregation occurring in thin-film blends of a thiophene monomer and an ammino compound, used in the fabrication of organic white-light-emitting diodes. In the homogeneous and uniform regions of the films, the interaction between the two molecular components gives rise to exciplex states responsible for a broad redshifted photoluminescence emission band, which disappears in the film zones where segregation occurs. This effect has been observed with submicrometer spatial resolution by means of local spectroscopic measurements performed in a scanning near-field optical microscope.

Monte Carlo simulations of phase separation in thin polymer blend films: scaling properties of morphological measures

Surface directed phase separation in thin polymer blend film has been studied with Monte Carlo simulations using a simple model based on reptation method. Time evolution of phase structure was characterized quantitatively by morphological measures (the Minkowski functionals) in addition to the inspection of concentration versus depth profiles. It was shown that the dynamical scaling hypothesis holds for the Minkowski functionals describing morphologies at the early stages of phase separation in thin films. Two time regimes with different scaling exponents (0.25, 0.33) were found for the growth of the characteristic length scale in the system corresponding to various transport mechanisms (diffusion along- and normally to the interface). Fast decrease in the morphological measures observed at the end of phase separation was attributed to the confinement of the thin film.

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Hole transporting polymer(poly［N-(p-diphenylamine)phenylmethacrylamide］, PDPMA) was doped with nile red dye at various concentrations to study the influence of doping on the energy transfer during light emitting processes. Organic LEDs composed of ITO/blend(PDPMA -nile red)/ Alq<TEX>$_3$</TEX>/Al as well as thin films of blend(PDPMA -nile red)/ Alq<TEX>$_3$</TEX> were manufactured for investigating photoluminescence, electroluminescence, and current-voltage characteristics. Atomic Force Microscopy was also used to observe surface morphology of the blend films. It was found that such doping. significantly influences the efficiency of the energy transfer from the Alq<TEX>$_3$</TEX> layer to blended layer and the optical/electrical properties could be optimized by choosing the right concentration of the dye molecule. The results also showed a interesting correlation with the morphological aspect, i.e. the optimum luminescence at the concentration with the least surface roughness. When the concentration of nile red was 0.8 wt%, the maximum energy transfer could be achieved.

Control of self‐organized low‐dimensional morphology in poly(styrene‐b‐4vinylpyridine)/polystyrene blend thin films

AbstractThe surface morphologies of poly(styrene‐b‐4vinylpyridine) (PS‐b‐P4VP) diblock copolymer and homopolystyrene (hPS) binary blend thin films were investigated by atomic force microscopy as a function of total volume fraction of PS (ϕPS) in the mixture. It was found that when hPS was added into symmetric PS‐b‐P4VP diblock copolymers, the surface morphology of this diblock copolymer was changed to a certain degree. With ϕPS increasing at first, hPS was solubilized into the corresponding domains of block copolymer and formed cylinders. Moreover, the more solubilized the hPS, the more cylinders exist. However, when the limit was reached, excessive hPS tended to separate from the domains independently instead of solubilizing into the corresponding domains any longer, that is, a macrophase separation occurred. A model describing transitions of these morphologies with an increase in ϕPS is proposed. The effect of composition on the phase morphology of blend films when graphite is used as a substrate is also investigated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3496–3504, 2004

Morphology of Immiscible Polymer Blend Thin Films Prepared by Spin-Coating

The morphologies of spin-coated thin films of the immiscible polymer blend of polystyrene and poly(tert-butyl acrylate) (PS/PtBA) are characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and scanning force microscopy (SFM) combined with selective dissolution. PS and PtBA are found to phase separate laterally in the thin films after spin-coating. The overall trend with blend composition is for PS islands to form when the PS:PtBA ratio is less than unity and for PtBA holes to form when the PS:PtBA ratio is larger than unity. The thickness of PS phases always exceeds that of the PtBA phases with both thicknesses increasing linearly with concentration of the spin-coating solution. The addition of P(S-b-tBA) diblock copolymer decreases the overall domain dimensions. After long annealing periods, PtBA is found to segregate preferentially to both the air−polymer interface and the polymer−substrate interface, effectively encapsulating PS droplet domains. A simple free energy model developed for the annealed specimens predicts a local free energy minimum as a function of PS droplet diameter, accounting for the observed stability of PS droplet phases and their very slow growth upon annealing. The surface tension of PtBA and the interfacial tension between PS and PtBA are shown to be critical parameters that affect the domain morphology in annealed PS/PtBA thin films.

ToF-SIMS molecular characterization and nano-SIMS imaging of submicron domain formation at the surface of PS/PMMA blend and copolymer thin films

Thin films of polystyrene (PS)/poly(methyl methacrylate) (PMMA) blends and block copolymers were characterized by two different SIMS instruments. ToF-SIMS was used to obtain the molecular composition of the upper most surface while nano-SIMS allowed us to visualize the surface morphology with submicrometer lateral resolution. The blends and copolymers showed different effects on annealing. ToF-SIMS spectra of annealed blends showed a decrease in the concentration of PS and an increase in the concentration of PMMA as compared to the pristine sample. By contrast, the spectra of annealed copolymers showed a decrease in the concentration of PMMA and an increase in the concentration of PS. The result obtained from the nano-SIMS imaging of the blends and copolymers revealed the formation of submicron domains at the surface on annealing. In case of blends, the images showed a strong oxygen signal on the surface suggesting a higher PMMA concentration, while for copolymers, the oxygen signal was quite low. ToF-SIMS spectra combined with nano-SIMS images help to interpret the surface topographical changes that the PS/PMMA blend and copolymer thin films undergo on annealing.

Near-field microscopy and lithography of light-emitting polymers.

We describe the application of scanning near-field optical microscopy (SNOM) to the study of the photophysical and self-organization properties of thin films of blends of conjugated polymers, and also to the lateral nanoscale patterning of conjugated-polymer structures. Such thin-film plastic semiconductor nanostructures offer significant potential for use in opto-electronic devices. The implementation of SNOM we employ is the most established form in which a probe with a sub-wavelength aperture is scanned in close proximity to the sample surface. We consider the nature of the near-field optical distribution, which decays within the first ca. 100 nm of these semiconductor materials, and address the identification of topographic artefacts in near-field optical images. While the topographic information obtained simultaneously with optical data in any SNOM experiment enables an easy comparison with the higher-resolution tapping-mode atomic force microscopy, the spectroscopic contrast provided by fluorescence SNOM gives an unambiguous chemical identification of the different phases in a conjugated-polymer blend. Both fluorescence and photoconductivity SNOM indicate that intermixing of constituent polymers in a blend, or nanoscale phase separation, is responsible for the high efficiency of devices employing these materials as their active layer. We also demonstrate a scheme for nano-optical lithography with SNOM of conjugated-polymer structures, which has been employed successfully for the fabrication of poly(-phenylene vinylene) nanostructures with 160 nm feature sizes.

Understanding polyanhydride blend phase behavior using scattering, microscopy, and molecular simulations

The phase behavior of a biocompatible binary polyanhydride blend system composed of poly[1,6-bis( p-carboxyphenoxy)hexane] (poly(CPH)) and poly(sebacic acid) (poly(SA)) is described. The phase behavior is determined from the CPH-SA segmental interaction parameter, χ, obtained from in situ small angle X-ray scattering (SAXS) experiments. The predicted phase diagram has an upper critical solution temperature (UCST) with a critical point of 114 °C. The phase diagram is validated by optical microscopy (cloud point determination) of blend films. However, the full range of blend compositions is not accessible via cloud point measurements, because the melting point of poly(CPH) is above the critical point. Additionally, the poly(CPH) crystallinity interferes with cloud point determination because the length scale of the amorphous phase separation and that of the crystallinity are both near the limit of resolution of the optical microscope. The poly(CPH)-rich region of the phase diagram was investigated by ex situ atomic force microscopy on thin blend films. Finally, in order to validate the use of molecular simulations to study energetic and structural properties of this system, χ is also computed from molecular dynamics both above and below the critical point. Excellent agreement is obtained for all three experimental methods and the computational technique. The results are compared to a simple group contribution method for computing the solubility parameters of the polymers. This technique fails to accurately predict the phase diagram.

Frequency dependent conductivity of the thin film blend of electroluminescent poly( p-phenylene vinylene) with waterborne polyurethane as ionomer

The frequency dependent conductivity, σ( ω), of the blend containing two different charge carrying material, a conjugated polymer, poly( p-phenylene vinylene), PPV and an ionomer, waterborne polyurethane ionomer, WPUI, was investigated in the frequency ( ω) range from 0.1 to 1000 kHz by impedance spectroscopy and compared with the individual materials, PPV and WPUI, respectively. Clear differences could be seen in the dependence of ω on both real and imaginary parts of impedance between WPUI, PPV and the blend film. At high frequencies (>200 kHz), all the three materials, exhibit a power law dependence between σ( ω) and ω as σ( ω) ∝ω s . Simulation of s value through theoretical fitting reveals that the s value of the blend (1.108) is in between PPV (1.289) and WPUI (1.035) and signifies the mixed contribution of carrier transport of PPV and WPUI in the blend. The morphology of the blend as observed from atomic force microscopy (AFM) imaging reveals that WPUI connects the islanded parts of PPV and provides a path for carrier conduction. Results observed indicate that carrier transport operates through barrier hopping in these materials and the difference in the dependence of ω on σ in the blend is attributed from the influence of SO 3 − group in WPUI on the carrier transport of PPV.

Phase Separation in Polyfluorene-Based Conjugated Polymer Blends: Lateral and Vertical Analysis of Blend Spin-Cast Thin Films

We have studied phase separation in polyfluorene-based electroluminescent polymer blends consisting of poly(2,7-(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) and poly(2,7-(9,9-di-n-octylfluorene)-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene)) (TFB). We present detailed studies on spin-cast thin-film morphology of the blends, using micro-Raman spectroscopy and X-ray photoelectron spectroscopy. Micron-scale lateral phase separation is observed in these blend thin films. However, these phase-separated domains are not pure at the submicron length scale, and a nanoscale vertical phase segregation occurs with enrichment of the lower surface energy component (TFB) at both air and substrate interfaces. Imaging of the spatial uniformity of electroluminescence emission on the microscopic scale indicates spatially localised charge-carrier recombination in light-emitting diodes (LEDs) fabricated with these blends. On the basis of these studies, we propose a model for the development of thin-film...

Organic Memory Devices Using C60 and Insulating Polymer

ABSTRACTIn this paper, we describe an all organic molecular memory device that combines the advantages of molecular and organic electronics. We accomplish this by combining C60 molecules with poly(4-vinylphenol) (PVP) and co-dissolving them in iso-propanol. The current-voltage measurements show a large hysteresis in the blend devices, in contrast to pure PVP devices. The thin blend films have been thoroughly characterized using Raman spectroscopy, atomic force microscopy and scanning electron microscopy.

Oligobetapinene as barrier improver to CO2 of HDPE in thin films of binary blends

Quenched thin films (about 100 microns thick) of high-density polyethylene (HDPE) / oligobetapinene (OBP) blends were prepared by melt mixing with the amount of OBP varying from 0 to 40%. The results of CO 2 permeation, thermal behavior and morphology are reported. From -130 to -100 °C we observed two peaks in DSC measurements, the lower one being ascribed to γ-transition of HDPE. The upper one attributed to Tg which was shift at high contents of OBP. The OPB molecules also displayed another transition at higher temperatures. The normalized degree of crystallinity of HDPE remained constant while the overall crystallinity of the blends was reduced in all blends. We have hypothesized that three distinct phases coexist, viz. a HDPE-amorphous phase with some amount of OBP molecules, OBP-amorphous phase with polyolefin and HDPE-crystalline phases. The permeation test revealed a decrease in permeability to CO 2 independent of the amount of OBP in the blends. The reduction of gas permeation could be explained mainly by the rigidity of the OBP rich phase that has counterbalanced the decrease of overall crystallinity of the film blends.

Phase-sensitive acoustic microscopy of polymer thin films

The three-dimensional images obtained by scanning acoustic microscopy with vector contrast (PSAM), contain significant qualitative and quantitative information that is not easily obtainable by other methods. We employ this technique to examine homopolymer and polymer blend thin films. The complex V( z) functions derived from the images, and the results obtained by image processing and meticulous analysis are employed to render the morphology, composition and micro-mechanical properties of the polymer films. In addition, ways by which the information inherent in the phase images can be extracted are examined. This is highly desirable, as the phase images contain very useful additional information.

Solid state solvation in amorphous organic thin films.

The photoluminescence (PL) of the red laser dye DCM2, doped into blended thin films of polystyrene (PS) and the polar small molecule camphoric anhydride (CA), redshifts as the CA concentration increases. The DCM2 PL peaks at 2.20 eV (lambda=563 nm) for pure PS films and shifts to 2.05 eV (lambda=605 nm) for films with 24.5% CA (by mass). The capacitively measured electronic permittivity also increases from epsilon=2.4 to epsilon=5.6 with CA concentration. These results are consistent with the theory of solvatochromism developed for organic molecules in liquid solvents. To our knowledge, this work is the first application of a quantitative theory of solvation to organic molecules in amorphous thin films with continuously controllable permittivity, and demonstrates that "solid state solvation" can be used to predictably tune exciton energies in organic thin film structures.

Charge Carrier Mobility in Blends of Poly(9,9-dioctylfluorene) and Poly(3-hexylthiophene)

A series of 10 binary blends of poly(9,9-dioctylfluorene) and regioregular poly(3-hexylthiophene) were investigated and found to be phase separated and to exhibit high field-effect mobility of holes. The mobility of holes determined from blend thin film transistors was in the range of 2.0 × 10-4−1 × 10-3 cm2/(V s) in blends of 10−80 wt % poly(9,9-dioctylfluorene). A concave-upward dependence of hole mobility on blend composition was found with a minimum in mobility around the symmetric blend composition (50 wt %). The field-effect mobility of holes in poly(9,9-dioctylfluorene) homopolymer could not be determined directly because of the barrier to charge injection from gold electrodes. However, from the composition-dependent field-effect mobility of holes in the series of binary blends, we estimated the hole mobility in poly(9,9-dioctylfluorene) homopolymer to be (6−10) × 10-4 cm2/(V s), which is in good agreement with previous results obtained by the time-of-flight technique. These results demonstrate that relatively high field-effect mobility of charge carriers can be realized in blends of conjugated polymers of current interest for various plastic electronic applications.

The influence of spatial disorder of the ion distribution on the surface morphology in thin films of blend based organic mixed ionic-electronic conductors

We report on investigations of the morphology of spin-coated thin films of an organic mixed ionic-electronic conductor consisting of the conjugated polymer methyl substituted ladder type poly(para- phenylene) (mLPPP) blended with a solid state electrolyte (Dicyclohexano18crown6 (DCH18C6), complexed with lithium trifluoromethanesulfonate (LiCF 3SO 3)). This blend system was successfully applied as active layer in light-emitting electrochemical cells (LECs). While thin films blends of the conjugated polymer and the pristine crown ether show a very smooth surface, the addition of LiCF 3SO 3 causes a pronounced surface roughening. Since such a distinct surface roughness can be the reason for a device failure mechanism that limits the device lifetime, this attitude is investigated by tapping mode atomic force microscopy (AFM). These studies are complemented with X-ray analysis by means of energy dispersive X-ray spectrometry (EDXS), in the scanning electron microscope (SEM), in order to get a better insight into the ion distribution within the blend layer and its influence on the surface roughness formed.

Surface Morphology of Annealed Polystyrene and Poly(methyl methacrylate) Thin Film Blends and Bilayers

Thin films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) were spun-cast onto silicon substrates, annealed, and analyzed by atomic force microscopy (AFM), total electron yield (TEY), and partial electron yield (PEY) near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in order to resolve conflicting prior literature regarding the tendency of PS to form a wetting layer or overlayer on top of PMMA. From the comparison of the three methods of analysis and on the basis of the extraordinary surface sensitivity of PEY NEXAFS, we conclude that PS does not form an overlayer in samples with morphologies near thermodynamic equilibrium. The PS forms droplets of a large size range on top of a PMMA layer that wets the hydrophilic SiOx substrate. From our results, the maximum thickness of a continuous PS wetting layer would be about 0.25 nm. This is in contrast to recent experiments that imply an equivalent PS wetting layer of about 5−10 nm is forming during annealing.