Blend Thin Films Research Articles

Study on peculiar carbon pattern formation from polymer blend thin films under electric fields

In this study, we demonstrate the formation of a peculiar carbon pattern from polyacrylonitrile/poly(methyl methacrylate) (PAN/PMMA) blend thin films under external electric fields. When a spin-coated PAN/PMMA blend thin film was exposed to an electric field, the surface of the thin film facing air layer would fluctuate due to the electrohydrodynamic instability phenomenon induced by an electrostatic force. At the same time, the interface between PAN and PMMA would also become unstable because of the difference in the dielectric constants of PAN and PMMA. The microstructure of the PAN/PMMA thin film obtained after field exposure was influenced by the combined effect of the two phenomena. Subsequent controlled thermal treatment produced an unprecedented and peculiar carbon pattern, which remained intact during carbonization. The effect of experimental parameters such as field intensity, exposure time, and film thickness on the microstructure of the polymer precursor and resulting carbon pattern was investigated. This work provides a strategy to obtain carbon structures showing specific surface texture and certain properties that have been inaccessible by any other means.

Capillary Force Lithography Pattern-Directed Self-Assembly (CFL-PDSA) of Phase-Separating Polymer Blend Thin Films.

We report capillary force lithography pattern-directed self-assembly (CFL-PDSA), a facile technique for patterning immiscible polymer blend films of polystyrene (PS)/poly(methyl methacrylate) (PMMA), resulting in a highly ordered phase-separated morphology. The pattern replication is achieved by capillary force lithography (CFL), by annealing the film beyond the glass transition temperature of both the constituent polymers, while confining it between a patterned cross-linked poly(dimethyl siloxane) (PDMS) stamp and the silicon substrate. As the pattern replication takes place because of rise of the polymer meniscus along the confining stamp walls, higher affinity of PMMA toward the oxide-coated silicon substrate and of PS toward cross-linked PDMS leads to well-controlled vertically patterned phase separation of the two constituent polymers during thermal annealing. Although a perfect negative replica of the stamp pattern is obtained in all cases, the phase-separated morphology of the films under pattern confinement is strongly influenced by the blend composition and annealing time. The phase-separated domains coarsen with time because of migration of the two components into specific areas, PS into an elevated mesa region and PMMA toward the substrate, because of preferential wetting. We show that a well-controlled, phase-separated morphology is achieved when the blend ratio matches the volume ratio of the elevated region to the base region in the patterned films. The proposed top-down imprint patterning of blends can be easily made roll-to-roll-compatible for industrial adoption.

The efficient n-doping of [6,6]-phenyl C61-butyric acid methyl ester by leuco-crystal violet to enhance the performance of inverted organic solar cells

Inverted organic solar cells (OSCs) have been fabricated using the photoactive blend thin films based on regioregular poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61-butyric acid methyl ester (PCBM), and leuco-crystal violet (LCV). It was found that the LCV as an efficient n-dopant could significantly increase intrinsic electron concentration of PCBM zone. The electron mobility of P3HT:PCBM:LCV blend thin film was measured 1.75 times as high as that of P3HT:PCBM blend thin film, as a result of LCV-induced trap filling in the bandgap of PCBM. The power conversion efficiency for the inverted device using the photoactive layer of P3HT:PCBM:LCV could be 1.22 times as high as that for the inverted device using the conventional photoactive layer of P3HT:PCBM, mostly because (1) the higher electron mobility could enhance the exciton dissociation and thereby short-circuit current density in the former relative to the latter; (2) the increase in the electron concentration of PCBM zone in P3HT:PCBM:LCV blend thin film may help blocking holes diffusion towards cathode, improving the hole collection efficiency and thereby fill factor of device. We provide a new insight on optimizing the electron-conducting property of bulk-heterojunction photoactive thin film, useful for pushing forward inverted OSCs towards the cost-effective commercialization.

Time‐Resolved Studies of Energy Transfer in Thin Films of Green and Red Fluorescent Proteins

AbstractBiologically derived fluorescent proteins are attractive candidates for lasing and sensing due to their excellent optical properties, including their high quantum yield, spectral tunability, and robustness against concentration quenching. Here, a time‐resolved study of the fluorescence dynamics of protein thin films is reported for the enhanced green fluorescent protein (EGFP), the red‐emitting tandem‐dimer protein tdTomato, and blends of EGFP and tdTomato. The exciton dynamics are characterized by using spectrally and time‐resolved measurements of fluorescence and a threefold reduction in lifetime is observed when going from solution to thin film, down to 1 and 0.6 ns for EGFP and tdTomato, respectively. This finding is attributed to a dipole–dipole nonradiative Förster resonant energy transfer (FRET) in solid state. The temporal characteristics of FRET in blended thin films are also studied and increased nonradiative transfer rates are found. Finally, efficient sensitization of a semiconductor surface with a protein thin film is reported. Such a configuration may have important implications for energy harvesting in hybrid organic–inorganic solar cells and other hybrid optoelectronic devices.

Elongated phase separation domains in spin-cast polymer blend thin films characterized using a panoramic image.

Polymer thin films with micro/nano-structures can be prepared by a solvent evaporation induced phase separation process via spin-casting a polymer blend, where the elongated phase separation domains are always inevitable. The striation defect, as a thickness nonunifomity in spin-cast films, is generally coexistent with the elongated domains. Herein, the morphologies of polymer blend thin films are recorded from the spin-cast center to the edge in a panoramic view. The elongated domains are inclined to appear at the ridge regions of striations with increasing radial distance and align radially, exhibiting a coupling between the phase separation morphology and the striation defect that may exist. We demonstrate that the formation of elongated domains is not attributed to shape deformation, but is accomplished in situ. A possible model to describe the initiation and evolution of the polymer blend phase separation morphology during spin-casting is proposed.

Interplay between phase separation and dewetting in PS/PVME thin films: effect of temperature

We studied the effects of temperature on the interplay between dewetting and phase separation at shallow and deep depths at two-phase temperatures in PS/PVME polymer blend thin films. Optical microscopy, AFM measurements, and ellipsometry analysis were performed to investigate the dewetting behavior of the films. At the deep quench depth (phase separation temperature of 115 °C), a two-layer film formed, consisting of a thin PVME layer directly on the surface of a silicon wafer (as the wetting layer) and a bulk layer which was the upper layer. In the bulk layer, the phase separation mechanism was controlled by an apparent nucleation and growth mechanism, which was driven by entropic and anisotropic limitations rather than thermodynamic preferences. After about 106 min of annealing, liquid-liquid dewetting occurred in the interface of the formed layers, triggered by Laplace pressure differences. However, at the shallow quench depth (phase separation temperature of 95 °C), a tri-layered structure formed in the thin films and concentration fluctuations at the interfaces of the formed layers triggered surface fluctuations and instabilities (dewetting phenomenon).

Patterning Conjugated Polymers by Laser: Synergy of Nanostructure Formation in the All-Polymer Heterojunction P3HT/PCDTBT.

In this work we report a broad scenario for the patterning of semiconducting polymers by laser-induced periodic surface structures (LIPSS). Based on the LIPSS formation in the semicrystalline poly(3-hexylthiophene) (P3HT), we have extended the LIPSS fabrication to an essentially amorphous semiconducting polymer like poly[N-90-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (PCDTBT). This polymer shows a good quality and well-ordered nanostructures not only at the 532 nm laser wavelength, as in the case of P3HT, but also at 266 nm providing gratings with smaller pitch. In addition, we have proven the feasibility of fabricating LIPSS in the P3HT/PCDTBT (1:1) blend, which can be considered as a model bulk-heterojunction for all-polymer solar cells. In spite of the heterogeneous roughness, due to phase separation in the blend, both P3HT and PCDTBT domains present well-defined LIPSS as well as a synergy for both components in the blend when irradiating at wavelengths of 532 and 266 nm. Both, P3HT and PCDTBT in the blend require lower fluence and less pulses in order to optimize LIPSS morphology than in the case of irradiating the homopolymers separately. Near edge X-ray absorption fine structure and Raman spectroscopy reveal a good chemical stability of both components in the blend thin films during LIPSS formation. In addition, scanning transmission X-ray spectro-microscopy shows that the mechanisms of LIPSS formation do not induce a further phase segregation neither a mixture of the components. Conducting atomic force microscopy reveals a heterogeneous electrical conductivity for the irradiated homopolymer and for the blend thin films, showing higher electrical conduction in the trenches than in the ridge regions of the LIPSS.

How Bound and Free Fatty Acids in Cellulose Films Impact Nonspecific Protein Adsorption.

The effect of fatty acids and fatty acid esters to impair nonspecific protein adsorption on cellulose thin films is investigated. Thin films are prepared by blending trimethylsilyl cellulose solutions with either cellulose stearoyl ester or stearic acid at various ratios. After film formation by spin coating, the trimethylsilyl cellulose fraction of the films is converted to cellulose by exposure to HCl vapors. The morphologies and surface roughness of the blends were examined by atomic force microscopy revealing different feature shapes and sizes depending on the blend ratios. Nonspecific protein adsorption at the example of bovine serum albumin toward the blend thin films was tested by means of surface plasmon resonance spectroscopy in real-time. Incorporation of stearic acid into the cellulose leads to highly protein repellent surfaces regardless of the amount added. The stearic acid acts as a sacrificial compound that builds a complex with bovine serum albumin thereby inhibiting protein adsorption. For the blends where stearoyl ester is added to the cellulose films, the cellulose:cellulose stearoyl ester ratios of 3:1 and 1:1 lead to much lower nonspecific protein adsorption compared to pure cellulose, whereas for the other ratios, adsorption increases. Supplementary results were obtained from atomic force microscopy experiments performed in liquid during exposure to protein solution and surface free energy determinations.

Unraveling the Dynamics of Nanoscopically Confined PVME in Thin Films of a Miscible PVME/PS Blend

Broadband dielectric spectroscopy (BDS) was employed to investigate the glassy dynamics of thin films (7-200 nm) of a poly(vinyl methyl ether) (PVME)/polystyrene (PS) blend (50:50 wt %). For BDS measurements, nanostructured capacitors (NSCs) were employed, where films are allowed a free surface. This method was applied for film thicknesses up to 36 nm. For thicker films, samples were prepared between crossed electrode capacitors (CECs). The relaxation spectra of the films showed multiple processes. The first process was assigned to the α-relaxation of a bulklike layer. For films measured by NSCs, the rates of α-relaxation were higher compared to those of the bulk blend. This behavior was related to the PVME-rich free surface layer at the polymer/air interface. The second process was observed for all films measured by CECs (process X) and the 36 nm film measured by NSCs (process X2). This process was assigned to fluctuations of constraint PVME segments by PS. Its activation energy was found to be thickness-dependent because of the evidenced thickness dependency of the compositional heterogeneity. Finally, a third process with an activated temperature dependence was observed for all films measured by NSCs (process X1). It resembled the molecular fluctuations in an adsorbed layer found for thin films of pure PVME, and thus, it is assigned accordingly. This process undergoes an extra confinement because of frozen adsorbed PS segments at the polymer/substrate interface. To our knowledge, this is the first example where confinement-induced changes were observed by BDS for blend thin films.

Unexpected Tribological Synergy in Polymer Blend Coatings: Leveraging Phase Separation to Isolate Domain Size Effects and Reduce Friction.

We employed a systematic processing approach to control phase separation in polymer blend thin films and significantly reduce dynamic friction coefficients (μ)s. We leveraged this modulation of phase separation to generate composite surfaces with dynamic friction coefficients that were substantially lower than expected on the basis of simple mixing rules, and in several cases, these friction coefficients were lower than those of both pure components. Using a model polyisoprene [PI]/polystyrene [PS] composite system, a minimum μ was found in films with PS mass fractions between 0.60 and 0.80 (μblend = 0.11 ± 0.03); that value was significantly lower than the friction coefficient of PS (μPS = 0.52 ± 0.01) or PI (μPI = 1.3 ± 0.09) homopolymers and was comparable to the friction coefficient of poly(tetrafluoroethylene) [PTFE] (μPTFE = 0.09 ± 0.01) measured under similar conditions. Additionally, through experiments in which the domain size was systematically varied at constant composition (through an annealing process), we demonstrated that μ decreased with decreasing characteristic domain size. Thus, the tribological synergy between PS and PI domains (discrete size, physical domain isolation, and overall film composition) was shown to play an integral role in the friction and wear of these PS/PI composites. Overall, our results suggest that even high friction polymers can be used to create low friction polymer blends by following appropriate design rules and demonstrate that engineering microstructure is critical for controlling the friction and adhesion properties of composite films for tribologically relevant coatings.

Markedly different molecular formation in DPP-based small-molecule solar cells probed by grazing-incidence wide-angle X-ray scattering.

This study comprehensively explores the nanostructural properties of two diketopyrrolo[3,4-c]pyrrole-1,4-dione (DPP)-based small molecules with different alkyl side groups and their blends with the fullerene derivative PC71BM, using grazing-incidence wide-angle X-ray scattering synchrotron techniques. Preferentially relative face-on orientation within the larger and more ordered stacking phase of SM1 with its shorter side group (ethylhexyl) was observed in the majority of both pristine and blend thin films, whereas SM2 crystals showed strictly perpendicular orientation. These contrasting crystalline characteristics led to significant differences in the results, from which crystalline structure-performance property correlations are proposed. Thus, the results not only demonstrate important scientific insights into the relationship between molecular structure and crystalline formation but also provide molecular design directions that will facilitate further improvement to the morphology and performance of DPP-based small-molecule solar cells.

High efficiency organic solar cells based on amorphous electron-donating polymer and modified fullerene acceptor

The current manuscript examines the structure and property relationship of amorphous conjugated polymer based bulk heterojunction solar cells. Solvent vapor annealing (SVA) is used to optimize the morphology. It is seen that the structure order of the blended thin film does not change but phase separation becomes prominent. The space charge limited current and admittance spectroscopy reveals that the charge transport ability of the SVA-treated device is significantly strengthened. Furthermore, both the current-voltage analysis and impedance spectroscopy illustrates that SVA can effectively promote the fill factor (FF) and short circuit current density (Jsc) due to the enhanced charge transport properties. However, the increased mobility after the SVA treatment causes non-geminate charge carrier recombination and reduces geminate recombinations. In particular, the reduction in open circuit voltage (Voc) of the solvent vapor annealed devices is found to originate from the charge recombination induced by the quasi-Fermi level variation between the donor and acceptor materials.

Using Femtosecond Laser Irradiation to Enhance the Vertical Electrical Properties and Tailor the Morphology of a Conducting Polymer Blend Film.

We report femtosecond infrared laser-induced selective tailoring of carrier transport as well as surface morphology on a conducting polymer blend thin film. Maximal 2.4 times enhancement on vertical current transport in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester, was achieved by this irradiation. The laser irradiation induced a photo expansion without deteriorating its molecular structure and the film morphology could be customized in the micron scale by adjusting the laser writing parameters. In the photoexpanded region, the face-on populations were about 2.2 times larger in comparison with the pristine region, which was a major contributor to the enhanced carrier transport.

Nanoscale investigations on interchain organization in thin films of polymer-liquid crystal blend

Optimized nanomorphology in organic thin active layers is crucial for good performance in organic solar cells. However, the relation between morphology and electronic properties at nanoscale remains not completely understood. Here, we study the effect of film thickness and temperature annealing on the ordering of poly(3-hexylthiophene) chains when the polymer is blended with a columnar liquid crystalline molecule. Electronic absorption, atomic force microscopy measurements, and Raman spectroscopy show that morphology and chain ordering of the blend depend on the film thickness. We highlight the benefit of using a liquid crystal in organic blends, opening the way to use simple processing methods for the fabrication of organic electronic devices.

Effect of PCBM additive on morphology and optoelectronic properties of P3HT-b-PS films

Thin blend films of the diblock copolymer poly(3-hexylthiophene-2,5-diyl)-block-polystyrene (P3HT-b-PS) and phenyl-C61-butyric acid methyl ester (PCBM) are prepared and investigated. The morphology of films with 9, 25, 33, 50, 58 and 67% PCBM weight fraction is determined. The surface topography is probed with optical microscopy and atomic force microscopy whereas the lateral inner structures are determined using grazing incidence small angle neutron scattering (GISANS). Grazing incidence X-ray diffraction (GI-XRD) shows a blend ratio dependence of the P3HT crystallites orientation within the block copolymer films. The thermodynamically favoured P3HT edge-on orientation is suppressed upon increasing the PCBM content from 9% till 33% while it recovers in samples with a PCBM weight fraction above 50%. PCBM tends to interact with the PS block at low PCBM fractions and forms segregated domains at a PCBM weight fraction higher than 50%. The addition of PCBM decreases the degree of lamellar structure order and well-ordered hexagonal packed cylindrical structures emerge in at a PCBM weight fraction of 67%. Absorbance and photoluminescence measurements show the relationship between morphology transformation and optoelectronic properties. The phenomena reported here could provide an alternative view for potential nanotechnology applications.

Alginate/Lignosulfonate Blends with Photoprotective and Antioxidant Properties for Active Packaging Applications

Sodium alginate/ammonium lignosulfonate blend solutions and thin films with different compositions have been prepared and studied. The rheology of the film forming solutions was comparatively evaluated and was established the effect of lignosulfonate incorporation onto sodium alginate solution properties and partial compatibility of components was found. The shear-thinning behaviour of the ALG/LS solutions was better evidenced at high ALG content, suggesting that the time required for the reorganization of the network structure is longer for the samples containing predominantly ALG, because its structure is damaged by increasing shear rate. The structural, morphological, optical and antioxidant properties of the crosslinked ALG/LS blend films were investigated with the aim to identify their potential as new active materials with photoprotective and antioxidant properties. The ATR-FTIR spectroscopy revealed the hydrogen bonding interactions between the components and scanning electron microscopy showed the modified microstructure of the blend films depending on composition. The surface topography changes have been evidenced by atomic force microscopy, showing an increased average surface roughness when the increasing lignosulfonate amount was incorporated into ALG matrix. The optical properties measurements revealed the light barrier properties of the blend films and the antioxidant activity evaluation showed the enhanced radical scavenging activity of the films with higher lignosulfonate content.

Electron-transporting small molecule/o-xylene hybrid additives to boost the performance of simplified inverted polymer solar cells

Electron-transporting small molecule bathophenanthroline (Bphen) together with o-xylene has been used as hybrid additives to improve the performance of simplified inverted polymer solar cells employing ITO alone as cathode and photoactive layer based on polymer [[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b] dithiophene] [3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-Th) and (He et al., Nat. Photon. 9:174, 2015)-phenyl C71-butyric acid methyl ester (PC71BM). Because the Bphen additive could spontaneously aggregate onto the indium tin oxide (ITO) cathode during spin coating, the mixed solution containing PTB7-Th, PC71BM, and Bphen, thereby forming a thin cathode-modifying layer, the ternary blend PTB7-Th:PC71BM:Bphen functioned similar to a layered polyethylenimine ethoxylated (PEIE, a conventional cathode-modifying material)/binary blend PTB7-Th:PC71BM combination. The o-xylene additive was confirmed to enrich the distribution of PTB7-Th donor towards the bottom area of photoactive layer, thereby benefiting the charge collection in inverted structure. With the o-xylene optimization, the PTB7-Th:PC71BM:Bphen blend thin film showed a power conversion efficiency (PCE) of 5.87% in simplified inverted structure of ITO/photoactive layer/MoO3/Ag, much higher than that (4.21%) of the PTB7-Th:PC71BM blend thin film. The current research can be considered as a useful attempt towards fabricating low-cost photovoltaic devices without marked loss in PCE.

Polymers of norbornenyl‐4‐phenol: Dissolution rate characteristics, positive tone photo‐patterning, and polymer properties

ABSTRACTVinyl addition norbornene polymers with phenol pendent groups are obtained by methanolysis of the 4‐acetoxyphenyl norbornene polymer. Thin films of this polymer blended with additives such as norbornene polymers with hexafluoroalcohol pendent groups and diazonaphthoquinone compounds result in linear dissolution in tetramethylammonium hydroxide developer. Good resolution, positive tone patterns are obtained from image‐wise exposure of thin films of blends of the diazonaphthoquinone additives with both phenol pendent norbornene homopolymer and with norbornene copolymers with both phenol and hexafluoroalcohol pendent groups. Properties such as transparency, dielectric constant, chemical, and thermal resistance were determined for cured copolymer compositions containing an epoxy crosslinker. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44952.

Solution-processed organic light-emitting diodes with emission from a doublet exciton; using (2,4,6-trichlorophenyl)methyl as emitter

Neutral-π radical based open shell molecules foster new potential in light emitting diodes because of their theoretical near-equity quantum efficiencies. In this study, we report organic light emitting diodes (OLEDs) based on a novel open shell molecule (2,4,6-trichlorophenyl)methyl (TTM) as the electroluminescent layer. The singly occupied molecular orbital (SOMO) level and optical bandgap of TTM was calculated using cyclic voltammetry and UV–visible absorption respectively. Thermogravimetric analysis showed a stable molecule capable of sublimation. Two decidedly different approaches, thermal evaporation and solution processing, were employed to deposit TTM:host blend thin films for OLED device fabrication. OLED devices fabricated via thermal evaporation and solution processing yielded external quantum efficiencies of 2.78% and 0.28% and luminances of 330 cdm-2 and 78 cdm-2 respectively. Further, the effect of doping ratios of the host materials on OLED device performance were investigated and optimal ratios were established. We report for the first time solution processed open shell organic molecules for light emitting diode applications. Our results elucidate the potential for low-cost and high efficiency optoelectronic devices.

Control of morphology and lamellae orientations in thin poly (ε-caprolactone) films by blending with different molar mass of polystyrene

The morphology and lamellar orientations in the thin blend films (∼30nm) of poly(ε-caprolactone) (PCL) and different molar mass of polystyrene(PS) were investigated by atomic force microscopy (AFM). The influence of PS molar mass on the PCL crystal structures was studied. The results show that both fibrous edge-on and flat-on microcrystal can be obtained when low molar mass PS (LPS with Mw=3.7k and 7.5k) is added into the thin PCL films, while only flat-on crystal is constructed for the films blending with high molar mass PS (HPS with Mw=15.5k and 54k and 106k). The different crystal morphology formation is explained based on the competition between phase separation and crystallization. For the LPS/PCL blend films, it is the homogeneous nucleation crystallization of PCL at the films surface that leads to the formation of fibrous edge-on together with several isolated and faceted flat-on crystals. While for the HPS/PCL blend films, it is the first vertical phase separation followed by the heterogeneous nucleation of PCL at polymer/substrate interface process that results in the morphology of flat-on PCL crystal decorated by PS domain island or dewetting hollows on the top.