As-cast Films Research Articles

Improving uniformity and nanostructure of solution-processed thin films using ultrasonic substrate vibration post treatment (SVPT)

The main goal of this paper is to introduce a novel mechanical method herein terms as substrate vibration post treatment (SVPT) technique, powered by ultrasonic vibration imposed on the substrate to enhance the characteristics and functionality of spun-on thin films or thin films made by similar casting techniques, such as drop and dip coating. In this technique, the as-casted wet films are placed on a substrate vibrated by an ultrasonic transducer with controlled power and duration to improve the film characteristics, such as uniformity and nanostructure. The performance of this technique is examined on spun-on PEDOT: PSS thin films used in polymer and perovskite solar cells and unprecedented results are presented. We first explore the influence of the vibration duration time on the characteristics of the films made by pristine PEDOT: PSS solution, where it is found that the optimized vibration duration for the pristine PEDOT: PSS film is about 10s, resulting in significant increase in the film electrical conductivity and lowered thickness and roughness. In order to further test the generality and merit of the method, thin films made using PEDOT: PSS solution modified with various types of surfactants and cured by the SVPT are studied. The results show that the application of the SVPT method combined with surfactant modification leads to an impressive twelve-fold increase in the conductivity of the PEDOT: PSS thin films compared with that of the pristine non-vibrated PEDOT: PSS thin films. The sole effect of the SVPT is a four-fold increase in the conductivity of pristine PEDOT: PSS film compared with that of the non-vibrated film. This remarkable enhancement in conductivity is further explained by the AFM phase images of PEDOT: PSS films, showing that the ultrasonic energy could loosen the Coulomb forces between PEDOT and PSS chains, resulting in phase separation and localized reordering of the conducting PEDOT chains leading to an increase in the electrical conductivity of the film. Highly conductive PEDOT: PSS thin film is a viable candidate as electrodes in emerging solution-processed solar cells.

Improvement of dielectric properties of cyanoethyl cellulose via esterification and film stretching

This study offers a concept of material functionalization for cellulose derivatives via chemical modification followed by orientation control. Namely, we improved the dielectric properties of cyanoethyl cellulose (CyEC) by a combination of esterification and uniaxial heat-stretching. To this end, we prepared CyEC ester derivatives (Cm-CyEC: m = 3, 4, 5, 8, 12 and 18; m is the carbon number of the side chain) with almost no residual hydroxyl groups from CyEC (degree of cyanoethyl substitution = 2.54) in a homogeneous system. The introduction of higher fatty acids (m ≥ 8) improved the thermal processability of the derivatives, because of diminishing crystallinity. Cast films were stretched at temperatures close to the glass transition temperature (T g) (152–166 °C), and different manners of orientation between the original CyEC and C18-CyEC were confirmed by IR dichroism, wide-angle X-ray diffraction and birefringence measurements. The dielectric constant for as-cast films monotonically decreased with an increase in m. The constants of the drawn CyEC films also decreased with increasing percent elongation (γ), but those of the C18-CyEC films increased with increasing γ. Such contradictory behaviour may be attributed to a local segmental orientation, which greatly increased the sum of the dipole moments of carbonyl and cyano moieties for C18-CyEC. Consequently, the dielectric constant of the stretched film of C18-CyEC increased up to ~23 (at 102 Hz), a 2.5-fold increase beyond the maximum dielectric constant of the unstretched C18-CyEC film, while the dielectric loss remained ≤0.05 at 102–105 Hz and 25 °C.

Soluble acetylenic molecular glasses based on dithienyldiketopyrrolopyrrole for organic solar cells

We present the synthesis and photovoltaic response of soluble glass-forming molecules containing a dithienyldiketopyrrolopyrrole core and difluorenylarylamine endgroups, linked through an acetylene bond. Introducing the 2,6-naphthylene moiety vs. 1,4-phenylene leads to an increased glass transition temperature (Tg) and hole mobility. Thus, Tg = 107.7 °C, μh = 7.95 × 10−5 cm2 V−1 s−1 for the 1,4- phenylene compound, while Tg = 119.4 °C, μh = 9.17 × 10−5 cm2 V−1 s−1 for the 2,6-naphthylene compound. Thermal annealing the as-cast films of both compounds below Tg resulted in little change in optical absorption. The preliminary characterization of the bulk hetero-junction solar cells showed that the 2,6-naphthylene compound generally provides a higher power conversion efficiency than the 1,4-phenylene compound, due probably to its enhanced hole transport. The photoluminescence experiments show a weak exciplex emission around 880 nm for the blends containing the new dyes and PC61BM. The present work may be useful for further efforts toward high-performance cost-effective acetylenic organic semiconductors for organic solar cells with thermally stable morphology.

Analysis of damage in commercial natural rubber through NMR spectroscopy

Damage to natural rubber (NR) during processing was investigated through liquid state NMR spectroscopy. Various commercial NR were used in the present study such as Pale Crepe (PC), Rubber Smoked Sheet (RSS3), Technically Specified Sheet (TSS8®), Standard Thai Rubber (STR5L), and Standard Malaysian Rubber (SMR20). As-cast films of the rubbers were prepared by solution casting method and followed by purification by acetone extraction. Chemical structures of the rubbers were elucidated through 1H, 13C-NMR, and DEPT spectroscopy. The 13C-NMR signals at 39.8 and 40.0 ppm were assigned to the trans-1,4-isoprene unit in the trans-trans and cis-trans sequences, respectively, while the signals at 60.8 and 64.5 ppm were assigned to C2 and C3 of an cis epoxy group. The cis epoxy groups were identified for PC, STR5L, RSS3, TSS8®, and SMR20, whereas the trans-1,4-isoprene units from cis-trans isomerization were detected for TSS8® and SMR20. The amount of these groups in TSS8® and SMR20 were higher than those in PC, STR5L, and RSS3. Mechanical properties of TSS8® and SMR20 were found to be inferior to those of PC, STR5L and RSS3. The mechanical properties depended mainly upon the damage of the samples during processing.

Face-On and Edge-On Orientation Transition and Self-Epitaxial Crystallization of All-Conjugated Diblock Copolymer

The orientation transition and self-epitaxial crystallization of all-conjugated diblock copolymers poly(p-phenylene)-block-(3-hexylthiophene) (PPP-b-P3HT) were systematically investigated by in situ temperature-resolved two-dimensional grazing incidence X-ray diffraction (2D GIXD) in step-by-step heating and cooling process. B39T18 was selected; the results of 2D GIXD showed that the PPP block crystal adopted a face-on orientation while the crystallization of P3HT block was hindered in as-casted films. Three different molecular orientations transition were obtained in self-epitaxial crystallization circles. First, P3HT crystallize with edge-on during the heating process and induced the PPP blocks crystallized with edge-on during the cooling process. Then, the as-casted film was heated in the melting temperature region of PPP blocks and isothermally crystallized. The partial melting of PPP blocks promoted the P3HT blocks crystallize in a face-on due to the steric limitation effect; PPP blocks crystallized ...

Latent Alignment in Pathway-Dependent Ordering of Block Copolymer Thin Films.

Block copolymers spontaneously form well-defined nanoscale morphologies during thermal annealing. Yet, the structures one obtains can be influenced by nonequilibrium effects, including processing history or pathway-dependent assembly. Here, we explore various pathways for ordering of block copolymer thin films, using oven-annealing, as well as newly disclosed methods for rapid photothermal annealing and photothermal shearing. We report the discovery of an efficient pathway for ordering self-assembled films: ultrarapid shearing of as-cast films induces "latent alignment" in the disordered morphology. Subsequent thermal processing can then develop this directly into a uniaxially aligned morphology with low defect density. This deeper understanding of pathway-dependence may have broad implications in self-assembly.

Microstructure and dielectric response of BaSrTiO3/P(VDF-CTFE) nanocomposites

Ceramic–polymer composites made of nano-sized Ba0.5Sr0.5TiO3 (BST) powders as ceramic filler and P(VDF-CTFE) 88/12mol% copolymer as matrix were prepared by adding coupling agent, using solution casting and hot pressing process. Functionalized silane was used as agent to chemically treat the surface of BST powders. It is found that the silane coupling agent can significantly improve the connection between the BST powders and P(VDF-CTFE) copolymer matrix as reflected by a better distribution observed in the as-cast composites. The uniformity of the composite can be improved by hot pressing multilayers of the as-cast composite films. The dielectric constant around 77 with a dielectric loss of 0.1 was obtained at 1kHz for composite prepared by adding coupling agent and hot pressing process.

Hematite thin films with various nanoscopic morphologies through control of self-assembly structures.

Hematite (α-Fe2O3) thin films with various nanostructures were synthesized through self-assembly between iron oxide hydroxide particles, generated by hydrolysis and condensation of Fe(NO3)3 · 6H2O, and a Pluronic triblock copolymer (F127, (EO)106(PO)70(EO)106, EO = ethylene oxide, PO = propylene oxide), followed by calcination. The self-assembly structure can be tuned by introducing water in a controlled manner through the control of the humidity level in the surrounding of the as-cast films during aging stage. For the given Fe(NO3)3 · 6H2O:F127 ratio, there appear to be three different thermodynamically stable self-assembly structures depending on the water content in the film material, which correspond to mesoporous, spherical micellar, and rod-like micellar structures after removal of F127. Coupled with the thermodynamic driving forces, the kinetics of the irreversible reactions of coalescence of iron oxide hydroxide particles into larger ones induce diverse nanostructures of the resultant films. The length scale of so-obtained nanostructures ranges from 6 nm to a few hundred nanometers. In addition to water content, the effects of other experimental parameters such as aging temperature, spin rate during spin coating, type of substrate, and type of iron reagent were investigated.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-0936-x) contains supplementary material, which is available to authorized users.

Preparation and properties of natural rubber with filler nanomatrix structure

Natural rubber with filler nanomatrix structure was prepared by forming chemical linkages between natural rubber particles and filler nanoparticles. The filler nanomatrix structure was formed by graft copolymerization of vinyltriethoxysilane (VTES) onto natural rubber particles in the latex stage followed by casting of the latex to prepare an as-cast film. The silica nanoparticles were produced during the graft copolymerization through hydrolysis and condensation, i.e., sol–gel reaction; hence, they linked to the natural rubber particles. The nanomatrix structure was observed by transmission electron microscopy, in which the natural rubber particles of about 1 μm in diameter were well dispersed in the filler nanomatrix. Tensile properties were significantly improved by forming filler nanomatrix structure. The loss modulus and loss tangent of the natural rubber with the filler nanomatrix structure were almost independent of deformation frequency in the rubbery plateau region, which was explained to be due to the energetic elasticity and entropic elasticity characteristic of the nanomatrix structure.

Gas expanded polymer process to anneal nanoparticle dispersion in thin films

Organic photovoltaic (OPV) active layers made by spin-coating a solution comprising poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) nanoparticles have been shown to adopt a non-uniform concentration profile across the thin film dimension. This inhomogeneous distribution can reduce the efficiency of the device. In the present work a new process, gas expanded polymer (GXP) annealing, is applied to P3HT/PCBM thin film blends, enabling the distribution of the PCBM nanoparticles to be manipulated by varying the GXP processing conditions. Films of 50nm thickness (nominally) created by spin casting a blend of P3HT mixed with PCBM were annealed by oscillatory GXP and GXP at constant pressure using high pressure CO2. An increase in P3HT crystallinity (detected by X-ray diffraction and UV–vis spectroscopy) along with a more uniform distribution of PCBM nanoparticles in the thickness dimension, as interpreted from neutron reflectivity measurements, were observed after oscillatory GXP annealing. In addition, static water contact angles suggest that the film/air interface is enriched in PCBM relative to the as-cast film. These results demonstrate that GXP annealing, which is commercially scalable, can be successfully used to create a uniform distribution of PCBM nanoparticles across the thickness dimension in a P3HT thin film.

Morphology dependent amplified spontaneous emission in π-conjugated polymer

Abstract The amplified spontaneous emission (ASE) spectra of a π-conjugated polymer Poly [2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) are studied in as-cast film, annealed film, 1 mg/ml solution and 2.5 mg/ml solution, respectively, using a 10 ns pulsed laser as an excitation source. We found that for annealed film (420 K), the ASE is hardly achieved compared to the as-cast film, which is consistent with the formation of the aggregation; whereas the film's temperature had much less effect on its ASE threshold. In solution, the ASE spectra show both 0–0 peak and 0–1 peak in 1 mg/ml solution, but only 0–1 peak in 2.5 mg/ml one. When the temperature of solution increases slightly from 300 K to 330 K, the ASE threshold increases dramatically in 1 mg/ml solution but slightly in 2.5 mg/ml one. Our results show the important role the morphology played in the ASE spectra in both film and solution. Therefore, controlling the interchain interaction in PCPs may be the key factor for performance of the organic lasers.

A Solution-Processable Dithienyldiketopyrrolopyrrole Dye Molecule with Acetylene as a π-Linkage for Organic Solar Cells

We present the efficient synthesis and photovoltaic response of a linearly conjugated molecule DT-DPP (AAnAT)2, which contains a dithienyldiketopyrrolopyrrole core symmetrically connected through a 9,10-anthracenyl unit to 3-alkylthiophene via an acetylene linkage. The compound is crystalline with a Tm of 204 °C. The as-cast film has a main broad absorption at 650 nm with a shoulder at 737 nm, tailing until ca. 800 nm. The HOMO level was estimated as −5.26 eV, derived from the first reversible electron oxidation wave at ca. 0.93 V vs. Ag/AgCl reference electrode. The space charge limited current hole mobility of a pristine film is ca. 2.90×10−4 cm2 V−1 s−1. The preliminary characterization of simple bulk heterojunction solar cells provides a power conversion efficiency (PCE) of ca. 4.0 % with VOC=0.77 V, JSC=10.0 mA cm−2, and FF=52 %, based on the as-cast active layer without a processing additive or solvent vapor annealing. Thermal annealing the active layer at a mild temperature of 50 °C increased the PCE to 4.39 % due mainly to the improvement of the FF.

Synthesis and photovoltaic response of a solution-processable dithienyldiketopyrrolopyrrole-based molecular semiconductor with thienylvinylthienyl endgroups

We describe the synthesis and preliminary photovoltaic performance of a solution-processable organic small-molecule electron donor DT-DPP(TVT)2 that consists of dithienyldiketopyrrolopyrrole (DT-DPP) as the core and thienylvinylthiophene (TVT) as the endgroups. The new compound is a crystalline solid with a Tm of approximately 216°C. Cyclic voltammetry indicates that DT-DPP(TVT)2 exhibits two quasi-reversible one-electron oxidation waves at ca. 0.68 and 0.90 V versus an Ag/AgCl reference electrode, respectively, leading to an estimated highest occupied molecular orbital (HOMO) level of about −5.08 eV. Introducing the branched 2-hexyldecyl side chain provides DT-DPP(TVT)2 with a high solubility in chloroform up to ca. 36 mg mL−1 at room temperature. Thermal annealing increases the crystallinity of the as-cast film from chloroform solution, thereby rendering slightly red-shifted charge-transfer absorption maxima. Fitting the space-charge-limited current characteristics of the thermally annealed thin film yields an improved hole mobility of ∼2.14×10−4 cm2 V−1 s−1 at low voltages versus ∼1.46×10−4 cm2 V−1 s−1 of the as-cast film. A first characterization of the solar cell [ITO/PEDOT: PSS/DT-DPP(TVT)2: PC61BM/Al] produces a power conversion efficiency of ∼3% with VOC≈0.78 V, JSC≈7.91 mA cm−2, and FF≈48.7%, under simulated AM1.5G with an illumination intensity of 100 mW cm−2. It should be noted that the thermal effect on the thin film absorption of DT-DPP(TVT)2 does not seem to be completely similar to the molecular donor DPP(TFNa)2 reported earlier, which bears 6-fluoronaphthyl endgroups.

Reversible optical control of conjugated polymer solubility with sub-micrometer resolution.

Organic electronics promise to provide flexible, large-area circuitry such as photovoltaics, displays, and light emitting diodes that can be fabricated inexpensively from solutions. A major obstacle to this vision is that most conjugated organic materials are miscible, making solution-based fabrication of multilayer or micro- to nanoscale patterned films problematic. Here we demonstrate that the solubility of prototypical conductive polymer poly(3-hexylthiophene) (P3HT) can be reversibly "switched off" using high electron affinity molecular dopants, then later recovered with light or a suitable dedoping solution. Using this technique, we are able to stack mutually soluble materials and laterally pattern polymer films by evaporation or with light, achieving sub-micrometer, optically limited feature sizes. After forming these structures, the films can be dedoped without disrupting the patterned features; dedoped films have identical optical characteristics, charge carrier mobilities, and NMR spectra as as-cast P3HT films. This method greatly simplifies solution-based device fabrication, is easily adaptable to current manufacturing workflows, and is potentially generalizable to other classes of materials.

Residual stress in spin-cast polyurethane thin films

Residual stress is inevitable during spin-casting. Herein, we report a straightforward method to evaluate the residual stress in as-cast polyurethane thin films using area shrinkage measurement of films in floating state, which shows that the residual stress is independent of radial location on the substrate and decreased with decreasing film thickness below a critical value. We demonstrate that the residual stress is developed due to the solvent evaporation after vitrification during spin-casting and the polymer chains in thin films may undergo vitrification at an increased concentration. The buildup of residual stress in spin-cast polymer films provides an insight into the size effects on the nature of polymer thin films.

Gas permeation in thin films of “high free-volume” glassy perfluoropolymers: Part II. CO2 plasticization and sorption

Carbon dioxide (CO2) plasticization and sorption effects in both thick and thin films of “high free-volume” glassy perfluoropolymers were studied by monitoring CO2 permeability and by observing changes in the film thickness and refractive index with ellipsometry measurements. The film thickness, aging time, thermal history and CO2 exposure protocols have significant effect on the absolute CO2 permeability and plasticization behavior of both thick and thin films. The extent of CO2 plasticization increases as film thickness decreases and as the aging time is increased. The as-cast films showed higher plasticization compared to films which were annealed above Tg; however, the CO2 permeability of both the as-cast and annealed films continuously decreased during the depressurization step unlike other glassy polymers. In general, the various CO2 exposure protocols revealed lower CO2 plasticization for perfluoropolymers compared to other reported glassy polymers. The extent of CO2 sorption obtained from the ellipsometry measurements was found to decrease with the decrease in the excess volume and increase in the aging time for perfluoropolymers; in addition, the structural differences among the various glassy polymers resulting in different polymer–gas interactions also affects the overall sorption characteristics. The lower plasticization in perfluoropolymers compared to Matrimid was also confirmed from the smaller percent increase observed for the experimental diffusion coefficient compared to the theoretically predicted diffusion coefficient from the dual sorption-mobility model. The Langmuir sorption parameter, CH′, and solubility at infinite dilution, S0, obtained from fitting dual sorption-mobility model to sorption data, showed an excellent linear correlation with (Tg-35) °C. The CO2 diffusivity and permeability data obtained for thin films of various glassy polymers also showed a strong correlation with free volume. The somewhat unusual behavior of thin films of AF 2400 in comparison to other glassy polymers studied to date is believed to be related to the low cohesive energy density expected of perfluorinated structures and its high free volume resulting from the bulky dioxole comonomer.

Thermo-mechanical properties, microstructure and biocompatibility in poly-β-hydroxybutyrates (PHB) produced by OP and OPN strains of Azotobacter vinelandii

The thermo-mechanical properties, microstructure, and biocompatibility of a molecular weight series of poly-β-hydroxybutyrate (PHB) produced by Azotobacter vinelandii wild-type strain (OP) and mutant derivatives strains OPN, SOP1 and OPNA were determined. A broad molecular weight range was investigated, spanning from 160kDa to 2030kDa. Thermal properties were determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The results showed that the melting, crystallization and degradation temperatures of the PHBs initially increased with increasing the molecular weight. However, the highest molecular weight PHBs exhibited cold crystallization and reduction in melting temperature and melting enthalpy, suggesting difficulty to crystallize. Polarized optical microscopy (POM) showed a spherulitic morphology for all PHBs, with a ringed internal structure. Uniaxial tensile tests showed that the mechanical Young’s modulus is an increasing function of molecular weight, whereas the strain at fracture decreased, denoting brittleness as molecular weight increased. Interestingly, the physical properties of these bacterial PHBs were independent of the strain and aeration conditions used in its production. Finally, the PHBs obtained by this novel route did not exhibit any cytotoxic effect and allowed the growth of Human Embryonic Kidney 293 cells (HEK293) on as-cast films indicating the biocompatibility as a support for cell systems.

Controlling the thermomechanical behavior of nanoparticle/polymer films.

We show that the mesoscale (∼200 nm) thermomechanical properties of polymer nanocomposites formed from silica nanoparticles (NPs) and poly(2-vinylpyridine) (P2VP) critically depend on their interfacial structure, which can be controlled by the casting solvent. The composite films are solvent cast from either pyridine (PYR) or methylethylketone (MEK), with uniform NP spatial distribution obtained in both cases. In the films cast from MEK, our previous work has shown that a bound layer of P2VP is formed at the NP surfaces, while no such bound layer is formed when PYR is used as the casting solvent. In PYR as-cast films, Brillouin light scattering reveals a single acoustic phonon with its longitudinal sound velocity increasing with NP loading. This implies a homogeneous mixture of the NP and the polymer on the mesoscopic scales for all compositions examined. However, in the MEK as-cast films, two longitudinal and two transverse acoustic phonons are observed at NP loadings above ∼20 wt % (or ∼11 vol %), reminiscent of two metastable microscopic phases. The dense microphase is attributed to the bridging of NPs by P2VP chains, whereas for the softer medium, we conjecture that there exists an interfacial lower density P2VP layer whose longitudinal sound velocity barely changes with NP loading. These solvent-induced differences in the (elastic) mechanical behavior disappear upon thermal annealing, suggesting that these nanocomposite interfacial structures in the as-cast state (far from equilibrium) locally approach equilibrium (i.e., near equilibrium after annealing). Consistent with these conclusions, the abrupt decrease of the longitudinal sound velocity with temperature occurs at a single glass transition temperature for the annealed nanocomposites irrespective of the casting solvent used, which assumes only a slightly higher (∼5 K at 45 wt % or ∼29 vol %) value than that in bulk P2VP. The results emphasize the important role of solvent in determining the interfacial structure of nanocomposites, which can be used to tailor their thermomechanical behavior.

Photopatternable Interfaces for Block Copolymer Lithography

Directly photopatternable interfaces are introduced that facilitate two-dimensional spatial control of block copolymer (BCP) orientation in thin films. Copolymers containing an acid labile monomer were synthesized, formulated with a photoacid generator (PAG), and coated to create grafted surface treatments (GSTs). These as-cast GST films are either inherently neutral or preferential (but not both) to lamella-forming poly(styrene-block-trimethylsilylstyrene) (PS-b-PTMSS). Subsequent contact printing and baking produced GSTs with submicron chemically patterned gratings. The catalytic reaction of the photoacid generated in the UV-exposed regions of the GSTs changed the interfacial interactions between the BCP and the GST in one of two ways: from neutral to preferential ("N2P") or preferential to neutral ("P2N"). When PS-b-PTMSS was thermally annealed between a chemically patterned GST and a top coat, alternating regions of perpendicular and parallel BCP lamellae were formed.

Top‐Down Approach for Nanophase Reconstruction in Bulk Heterojunction Solar Cells

"Top-Down" nanophase reconstruction via a post-additive soaking process is first presented with various BHJ binary composites. By simply rinsing as-cast BHJ films with a solvent mixture containing a few traces of a nanophase-control reagent such as 1,8-diiodooctane, oversized fullerene-rich clusters (>100 nm in dia-meter) in the BHJ film are instataneously disassembled and entirely reorganized into finely intermixed donor/acceptor nanophases (ca. 10 nm) with a 3D compositional homogeneity, without surface segregation.