Blend Thin Films Research Articles

I-V characteristics of Ag/OD-MO/Ag surface-type diode fabricated by deposition of organic film from solution under the effect of an electric field

In this study, Ag/OD-MO/Ag surface-type diode was fabricated by drop-casting orange dye and methyl orange blend thin film from 1.0 wt.% aqueous solution, under the effect of an electric field, on a glass substrate with preliminary deposited silver electrodes. Geometrical length and width of the semiconducting channel between metallic electrodes were equal to 40 μm and 21 mm, respectively. The dark I-V characteristics exhibited strong rectification behaviour though both electrodes were from the same metal. The average value of rectification ratio (RR) was 27 and it was found that RR varies with applied voltage in the range of 1–40. The dark I-V characteristics were simulated by modified Shockley equation and space-charge limited currents (SCLC) approach. Different diode parameters were calculated from current-voltage characteristics. The diode showed non-ideal I-V behaviour with a quality factor greater than unity. Energy band diagram is proposed for the Ag/OD-MO/Ag surface-type diode.

Plasmon-Enhanced Charge Carrier Generation in Organic Photovoltaic Films Using Silver Nanoprisms

We use photoinduced absorption spectroscopy to measure long-lived photogenerated charge carriers in optically thin donor/acceptor conjugated polymer blend films near plasmon-resonant silver nanoprisms. We measure up to 3 times more charge generation, as judged by the magnitude of the polaron absorption signal, in 35 nm thin blend films of poly(3-hexylthiophene)/phenyl-C(61)-butyric acid methyl ester on top of films of silver nanoprisms (approximately 40-100 nm edge length). We find that the polaron yields increase linearly with the total sample extinction. These excitation enhancements could in principle be used to increase photocurrents in thin organic solar cells.

Structure of Phase‐Separated Ferroelectric/Semiconducting Polymer Blends for Organic Non‐volatile Memories

The phase-separated structure of blends of the ferroelectric polymer P(VDF-TrFE) and the semiconducting polymer P3HT used in organic non-volatile memories is revealed with soft X-ray spectromicroscopy. These thin-film blends show a columnar morphology, with P3HT-rich columns enclosed in a continuous, essentially pure P(VDF-TrFE) phase favorable for data storage operation.

Directed Growth of 1D Assemblies of Perylene Diimide from a Conjugated Polymer

The templated growth of 1D assemblies of perylene diimides (PDI) formed within a conjugated polymer is described. The formation of this unique morphology is accomplished through the synthesis of a PDI-functionalized conjugated polymer and its solution casting in the presence of PDI. The strategy is demonstrated with poly(3-hexylthiophene) (P3HT), partially functionalized at the 4-position with N-(1-hexylheptyl)-N′-(12-carboxylicdodecyl)perylene-3,4,9,10-tetracarboxylbisimide (PP3HT). The 1D assemblies of PDI embedded in the optoelectronically active polymer are hundreds of nanometers wide, 10−20 nm thick, several micrometers in length, and run parallel to the surface of the substrate. Surrounding the 1D structures is a heterogeneous PP3HT-PDI blend, comprising PP3HT domains less than 10 nm in size. Excitation energy in thin film blends of PP3HT:PDI is quenched to a much greater extent than simple molecular blends of P3HT and PDI. These systems represent structures of multiple hierarchy, wherein nanosized domains of a π-conjugated polymer and an n-type material are in intimate contact and which encapsulate 1D assemblies of nanocrystallites that run parallel to the films surface. They offer an alternative route for the fabrication of innovative supramolecular structures for optoelectronic applications.

A hybrid organic semiconductor/silicon photodiode for efficient ultraviolet photodetection

A method employing conjugated polymer thin film blends is shown to provide a simple and convenient way of greatly enhancing the ultraviolet response of silicon photodetectors. Hybrid organic semiconductor/silicon photodetectors are demonstrated using fluorene copolymers and give a quantum efficiency of 60% at 200 nm. The quantum efficiency is greater than 34% over the entire 200-620 nm range. These devices show promise for use in high sensitivity, low cost UV-visible photodetection and imaging applications.

Phase-Separated Thin Film Structures for Efficient Polymer Blend Light-Emitting Diodes

We report laterally and vertically phase-separated thin film structures in conjugated polymer blends created by polymer molecular weight variation. We find that micrometer-scale lateral phase separation is critical in achieving high initial device efficiency of light-emitting diodes, whereas improved balance of charge carrier mobilities and film thickness uniformity are important in maintaining high efficiency at high voltages. The optoelectronic properties of these blend thin films and devices are strongly influenced by the polymer chain order/disorder and the interface state formed at polymer/polymer heterojunctions.

Phase separation in polymer blend thin films studied by differential AC chip calorimetry

AC chip calorimetry is used to study the phase separation behavior of 100 nm thin poly(vinyl methyl ether)/poly(styrene) (PVME/PS) blend films. Using the on-chip heaters, very short (10 ms–10 s) temperature jumps into the temperature window of phase separation are applied, simulating laser heating induced patterning. These temperature pulses produce a measurable shift in the glass transition temperature, evidencing phase separation. The effect of pulse length and height on phase separation can be studied. The thus phase separated PVME/PS thin films remix rapidly, in contrast with measurements in bulk. AC chip calorimetry seems to be a more sensitive technique than atomic force microscopy to detect the early stages of phase separation in polymer blend thin films.

Phase equilibria in polymer blend thin films: A Hamiltonian approach

We propose a Hamiltonian formulation of the Flory-Huggins-de Gennes theory describing a polymer blend thin film. We then focus on the case of 50:50 polymer blends confined between antisymmetric walls. The different phases of the system and the transitions between them, including finite-size effects, are systematically studied through their relation with the geometry of the Hamiltonian flow in phase space. This method provides an easy and efficient way, with strong graphical insight, to infer the qualitative physical behavior of polymer blend thin films.

In vitro degradation and drug release from polymer blends based on poly(dl-lactide), poly(l-lactide-glycolide) and poly(ε-caprolactone)

Bioresorbable materials are extensively used for a wide range of biomedical applications. Accurately modifying and evaluating the degradation rate of these materials is critical to their performance and the controlled release of bioactive agents. The aim of this work was to modify the physical properties, degradation rate and drug delivery characteristics of thin films for medical applications by blending poly(dl-lactic acid) (PDLLA), poly(l-lactide-co-glycolide) (PLGA) and poly(e-caprolactone) (PCL). The thin films were prepared using solvent casting and compression moulding and the in vitro degradation study was performed by immersing the films in a phosphate-buffered saline at elevated temperature for a period of 4 weeks. The degradation rate of the materials was analysed by differential scanning calorimetry, tensile testing and weight loss studies. The thermal analysis of the blends indicated that the presence of PLGA or PDLLA in the film resulted in increased degradation of the amorphous regions of PCL. It was observed that the samples consisting of PDLLA with PCL demonstrated the greatest weight loss. The decrease in mechanical properties observed for both sets of polymer blends proved to be similar. The solvent cast technique was selected as the most appropriate for the formation of the polymer/drug matrices, due to the potentially adverse thermal processing effects associated with compression moulding. It was found that modulation of drug release was achievable by altering the ratio of PCL to PDLLA or PLGA in the thin film blends.

Organic ferroelectric field-effect transistor with P(VDF-TrFE)/PMMA blend thin films for non-volatile memory applications

Among the many ferroelectrics polymers, poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)] has drawn a great attention with its promising memory properties such as large remanent polarization, good fatigue and retention properties. Since the ferroelectric layer plays a critical role in the operation of memory device, it is important to control the structure of ferroelectric thin film. In order to improve the performance of a ferroelectric device, in this contribution we employed P(VDF-TrFE) layers blended with various compositions of an amorphous poly(methyl methacrylate) (PMMA) from 0 wt.% to 20 wt.%. In metal/ferroelectric/metal capacitor structure, we observed the decrease of remanent polarization from 9.13 μC/cm 2 to 4.7 μC/cm 2 and increase of coercive voltage from 9.5 V to 15.2 V with PMMA. Polarization switching time of the ferroelectric blend films estimated from the switched polarization vs. time curves increases with the amount of PMMA. Furthermore, ferroelectric field-effect transistors (FeFETs) based on ferroelectric P(VDF-TrFE) and PMMA blend films with single-crystalline tri-isopropylsilylethynyl pentacene (TIPS-PEN) channels show that both ON and OFF currents of the transistors are maintained with the PMMA contents despite the reduction of remanent polarization of P(VDF-TrFE)/PMMA films.

Guided phase separation of polymer blend thin films on ion beam-induced pre-patterned substrates

A technique for fabricating pre-pattern induced phase-separated polymer blend films is presented. The gold layer coated silicon substrates were bombarded with focused ion beams (FIB) to sputter away Au grains in the irradiated regions and expose silicon oxide surface underneath, thereby creating pre-patterns of relatively hydrophobic (Au)/hydrophilic (silicon oxide) regions. In the pre-patterned regions, the spin-coated films consisting of immiscible PS (polystyrene) and PtBA (poly-tert-butyl acrylate) blend exhibited phase separation induced by the underlying pre-patterns and formed two distinct ordered morphologies with periodicities much smaller than that of the pre-patterns. The influence of varying periodicities of the pre-patterned structures, blending ratios and spin-coating parameters on the resulting morphologies are presented.

Time-resolved specular and off-specular neutron reflectivity measurements on deuterated polystyrene and poly(vinyl methyl ether) blend thin films during dewetting process

We performed time-resolved specular and off-specular neutron reflectivity measurements on blend thin films 42 and 98 nm thick of deuterated polystyrene and poly(vinyl methyl ether) during dewetting process induced by the phase separation in two phase region using a time-of-flight neutron reflectometer. In the specular measurements we found that the phase separation directed to the depth direction occurred near the air interface as well as near the Si substrate during the incubation period before dewetting. In addition we also found that the phase separation occurred asymmetrically at the two interfaces and inhomogeneously in the film plane, showing that the dewetting was induced by the composition fluctuation mechanism. Off-specular reflectivity was analyzed, for the first time, to evaluate kinetics of structure formation in the film plane during the dewetting process. We found in the analysis that the droplets formation in micrometer scale occurred in the late stage of dewetting.

Strategies for Increasing the Efficiency of Heterojunction Organic Solar Cells: Material Selection and Device Architecture

Thin-film blends or bilayers of donor- and acceptor-type organic semiconductors form the core of heterojunction organic photovoltaic cells. Researchers measure the quality of photovoltaic cells based on their power conversion efficiency, the ratio of the electrical power that can be generated versus the power of incident solar radiation. The efficiency of organic solar cells has increased steadily in the last decade, currently reaching up to 6%. Understanding and combating the various loss mechanisms that occur in processes from optical excitation to charge collection should lead to efficiencies on the order of 10% in the near future. In organic heterojunction solar cells, the generation of photocurrent is a cascade of four steps: generation of excitons (electrically neutral bound electron-hole pairs) by photon absorption, diffusion of excitons to the heterojunction, dissociation of the excitons into free charge carriers, and transport of these carriers to the contacts. In this Account, we review our recent contributions to the understanding of the mechanisms that govern these steps. Starting from archetype donor-acceptor systems of planar small-molecule heterojunctions and solution-processed bulk heterojunctions, we outline our search for alternative materials and device architectures. We show that non-planar phthalocynanines have appealing absorption characteristics but also have reduced charge carrier transport. As a result, the donor layer needs to be ultrathin, and all layers of the device have to be tuned to account for optical interference effects. Using these optimization techniques, we illustrate cells with 3.1% efficiency for the non-planar chloroboron subphthalocyanine donor. Molecules offering a better compromise between absorption and carrier mobility should allow for further improvements. We also propose a method for increasing the exciton diffusion length by converting singlet excitons into long-lived triplets. By doping a polymer with a phosphorescent molecule, we demonstrate an increase in the exciton diffusion length of a polymer from 4 to 9 nm. If researchers can identify suitable phosphorescent dopants, this method could be employed with other materials. The carrier transport from the junction to the contacts is markedly different for a bulk heterojunction cell than for planar junction cells. Unlike for bulk heterojunction cells, the open-circuit voltage of planar-junction cells is independent of the contact work functions, as a consequence of the balance of drift and diffusion currents in these systems. This understanding helps to guide the development of new materials (particularly donor materials) that can further boost the efficiency of single-junction cells to 10%. With multijunction architectures, we expect that efficiencies of 12-16% could be attained, at which point organic photovoltaic cells could become an important renewable energy source.

Thin-film solid-state proton NMR measurements using a synthetic mica substrate: Polymer blends

Solid-state proton nuclear magnetic resonance (NMR) measurements are performed successfully on polymer blend thin films through the use of synthetic mica as a substrate. When used as a substrate, synthetic fluorophlogopite mica with its proton-free, diamagnetic character, allows for adequate measurement sensitivity while minimally perturbing the proton thin-film spectra, especially relative to more commonly available natural micas. Specifically, we use multiple-pulse techniques in the presence of magic-angle spinning to measure the degree of mixing in two different polymer blend thin films, polystyrene/poly(xylylene ether) and poly(1-methyladamantyl methacrylate) (PMAdMA)/triphenylsulfonium perfluorobutanesulfonate (TPS-PFBS), spin-coated onto mica substrates. Our earlier studies had focused on bulk systems where NMR signals are stronger, but may not be representative of thin films of the same systems that are relevant to many applications such as photoresist formulations in the electronics industry. The superiority of synthetic over natural paramagnetic mica is demonstrated by the maintenance of resolution and spinning sideband intensities (relative to bulk samples) for the synthetic mica samples. In contrast, degraded resolution and large spinning sidebands are shown to typify spectra of the natural mica samples. This approach can be applied to many other proton measurements of solid thin films, thereby greatly extending the types of systems to be investigated. Magnetic susceptibility measurements are also reported for all micas used.

SCFT Simulations of Thin Film Blends of Block Copolymer and Homopolymer Laterally Confined in a Square Well

Recently there has been significant interest in manipulating the self-assembly behavior of block copolymers to obtain structures that are not observed in the bulk. Here we explore the conditions for which self-assembly in laterally confined thin block copolymer films results in tetragonal square arrays of standing up cylinders. More specifically, we used self-consistent-field theory (SCFT) to study the equilibrium phase behavior of thin films composed of a blend of AB block copolymer and A homopolymer laterally confined in square wells. By using suitable homopolymer additives and appropriately sized wells, we observed square lattices of upright B cylinders that are not stable in pure AB block copolymer systems. We further investigated the optimal conditions and parameters that lead to defect-free, in-plane tetragonal ordering. Considering the potential application of such films in block copolymer lithography, we also conducted numerical SCFT simulations of the role of line edge roughness at the periphery ...

High-content profiling of cell responsiveness to graded substrates based on combinyatorially variant polymers.

We have developed a novel approach combining high information and high throughput analysis to characterize cell adhesive responses to biomaterial substrates possessing gradients in surface topography. These gradients were fabricated by subjecting thin film blends of tyrosine-derived polycarbonates, i.e. poly(DTE carbonate) and poly(DTO carbonate) to a gradient temperature annealing protocol. Saos-2 cells engineered with a green fluorescent protein (GFP) reporter for farnesylation (GFP-f) were cultured on the gradient substrates to assess the effects of nanoscale surface topology and roughness that arise during the phase separation process on cell attachment and adhesion strength. The high throughput imaging approach allowed us to rapidly identify the "global" and "high content" structure-property relationships between cell adhesion and biomaterial properties such as polymer chemistry and topography. This study found that cell attachment and spreading increased monotonically with DTE content and were significantly elevated at the position with intermediate regions corresponding to the highest "gradient" of surface roughness, while GFP-f farnesylation intensity descriptors were sensitively altered by surface roughness, even in cells with comparable levels of spreading.

Impact of Polymer-Bound Iodine on Fibronectin Adsorption and Osteoblast Cell Morphology in Radiopaque Medical Polymers: Tyrosine-Derived Polycarbonate Blends as a Model System

Imaging of polymer implants during surgical implantations is challenging in that most materials lack sufficient X-ray contrast. Synthetic derivatization with iodine serves to increase the scattering contrast but results in distinct physicochemical properties in the material which influence subsequent protein adsorption and cell morphology behavior. Herein we report the impact of increasing iodine inclusion on the cell morphology (cell area and shape) of MC3T3-E1 osteoblasts on a series of homopolymers and discrete blend thin films of poly(desaminotyrosyl tyrosine ethyl ester carbonate), poly(DTE carbonate), and an iodinated analogue poly(I(2)-DTE carbonate). Cell morphology is correlated to film chemical composition via measuring fibronectin (FN) adhesion protein adsorption profile on these films. FN exhibits up to 2-fold greater adsorption affinity for poly(I(2)-DTE carbonate) than (poly(DTE carbonate)). A correlation was established between cell area, roundness, and the measured FN adsorption profile on the blend films up to 75% by mass poly(I(2)-DTE carbonate). Data suggest that incorporation of iodine within the polymer backbone has a distinct impact on the way FN proteins adsorb to the surface and within the studied blend systems; the effect is composition dependent.

Effect of Polymer-Substrate Interactions on the Surface Morphology of Polymer Blend Thin Films: Comparison between Simulation and Experiment

Microphase and macrophase separation phenomena can simultaneously appear in ABA/C copolymer blend systems due to the immiscibility among monomers A, B, and C. In this work, the surface morphologies and compositions of ABA/C blend thin films confined between two walls, which were used to mimic SEBS/PMMA films, have been simulated by a lattice Monte Carlo (MC) method. The effect of the polymer-wall interaction on the surface morphologies and compositions of thin films was investigated as a function of blend composition and film thickness. It is shown that the simulated surface morphologies of thin films resulting from the macrophase separation between copolymer ABA and homopolymer C and the microphase separation between block A and block B in ABA copolymer are similar to the experimental surface morphology of SEBS/PMMA polymer blend films observed by atomic force microscope (AFM). The effect of substrate on the surface morphologies by MC simulation is qualitatively consistent with the experimental results. The composition profiles of thin films are given to characterize the micro- and macrophase separation in thin films. It is indicated that the surface energy of the substrate (substrate/air) plays a crucial role on the surface composition. For a fixed surface, the adsorptions of polymer on the substrate and film thickness are also important.

Resonance Chemical Imaging of Polythiophene/Fullerene Photovoltaic Thin Films: Mapping Morphology-Dependent Aggregated and Unaggregated C═C Species

Resonance Raman spectroscopic imaging is introduced as a physical probe to identify and spatially map morphology-dependent variations of intra- and interchain interactions and order in poly-3-hexylthiophene (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) photovoltaic blend thin films. Absorption spectra and C=C symmetric stretching Raman modes of P3HT/PCBM blend films show contributions from two distinct species that are assigned as aggregated and unaggregated P3HT chains with characteristic Raman frequencies of approximately 1450 (I(C=C)(agg)) and approximately 1470 cm(-1) (I(C=C)(un)), respectively. Upon thermal annealing of blend films, the relative concentrations of I(C=C)(agg) and I(C=C)(un) species (R = I(C=C)(agg)/I(C=C)(un)) changes on average from 0.79 +/- 0.20 (as-cast) to 2.45 +/- 0.77 (annealed). It is proposed that R values report on the relative densities of states (DOS) of aggregated and unaggregated species, and resonance Raman imaging is then used to spatially map morphology-dependent variations of R values and uncover subclassifications of these species. From both R and frequency dispersion resonance Raman images of I(C=C)(agg) and I(C=C)(un) species, four distinct types of P3HT chains are identified and mapped in annealed P3HT/PCBM blend thin films: (i) highly aggregated/crystalline, (ii) partially aggregated, (iii) interfacial, and (iv) unaggregated/PCBM rich. The change in aggregation upon annealing is attributed to an increase in planarity of the P3HT chains that is determined from the ratios of C=C/C-C symmetric stretching mode intensities.

Dimensional Scaling of Cylinders in Thin Films of Block Copolymer−Homopolymer Ternary Blends

The cylindrical domain diameter (d), spacing (D), and uniformity in thin films of ternary blends of a cylinder-forming polystyrene-block-poly(methyl methacrylate) block copolymer and the corresponding homopolymers were investigated as a function of the molecular weight (Mn) and concentration (φH) of the homopolymers. Thin films (20−65 nm) of the blends were deposited on silicon wafers that had been modified with random copolymer brushes such that the cylindrical domains of the annealed blends were oriented perpendicular to the substrate. The best uniformity of d, at a given φH, was achieved in the blends with lower homopolymer Mns. For blends made with homopolymers that had Mn values of the same order of magnitude as those of the corresponding blocks of the copolymer, D increased with increasing φH as D = D0(1 − φH)−β, where D0 is the domain spacing of the block copolymer and β is a parameter that depends on the ratio of the homopolymer and block copolymer molecular weights. The cylindrical domains of the blends could be swollen up to 150% of the original diameter while maintaining hexagonal ordering. For most of the blends on the random copolymer brushes, uniform arrays of perpendicular cylinders were formed most frequently at a film thickness near D0. A ternary blend with homopolymers with very low Mns produced markedly different results: D decreased in size with increasing φH, measurable up to φH = 0.4, and produced uniform patterns in films with thicknesses ranging from 20 to 37 nm.