Blend Thin Films Research Articles

Controllable electrical performance of spray-coated semiconducting small molecule/insulating polymer blend thin film for organic field effect transistors application

A systematic study on solution content and solvent effect of spray-coated 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN)/poly(α-methylstyrene) (PαMS) semiconducting blends for organic field effect transistors (OFETs) application was investigated based on different concentrations (10, 15 and 20mgml−1), different compositions (TIPS-PEN/PαMS of 7/3, 5/5 and 3/7) and three different solvents, toluene (TOL), chlorobenzene (CB), and 1,2,3,4-tetrahydronaphthalene (THN). Polarized optical microscopy (POM), atomic force microscopy (AFM) and grazing-incidence X-ray diffraction (GIXRD) were used to evaluate the film morphology and chain orientation. The blend film can contribute to large coverage of TIPS-PEN carrier pathway across the OFETs channel if higher TIPS-PEN composition with suitable total blend concentration was introduced in the blend. Besides, the blend film processed from high boiling point THN can reduce the rate of evaporation which provides more time for TIPS-PEN migration to form a large grain of TIPS-PEN crystal. The resulting OFETs based on spray-coated from TIPS/PαMS (7/3) blend solution in THN exhibit maximum mobility of 3.23×10−1cm2V−1s−1 and ON/OFF ratio of >104.

Ambipolar Carrier Mobility in Binary Blend Thin Film of Non-Peripheral Alkylphthalocyanines

Charge carrier mobility in blend films of two types of soluble phthalocyanine derivatives with different substituent length, 1, 4, 8, 11, 15, 18, 22, 25-octapentyl-phthalocyanine (C5PcH2) and 1, 4, 8, 11, 15, 18, 22, 25-octahexyl-phthalocyanine (C6PcH2) has been investigated. The charge carrier mobility was measured by the time-of-flight technique. In the case of C5PcH2 blend ratio below 25 mol%, the high mobility, such as 0.8-1.1 cm2V-1s-1 for hole and 0.6 cm2V-1s-1 for electron, were obtained at room temperature. In the thin films with C5PcH2 above 30 mol%, the charge carrier mobility decreased by one order of magnitude and had the different temperature dependence from that below 25 mol%. The marked change of the charge carrier mobility depending on the blend ratio of phthalocyanine derivatives has been discussed by taking the miscibility and the molecular packing structure into consideration.

Rheological behavior of Japanese cedar-based biobinder as partial replacement for bituminous binder

Currently, road and airport pavement construction uses approximately 90% asphalt, most of which is mainly derived from fossil fuels. However, with the improvement in oil refinery technology, the quality of asphalt binders is degrading gradually. Moreover, from the environment standpoint, fossil fuels are the main source of greenhouse gases, which lead to global warming. Consequently, there is an urgent need to develop a more sustainable binding material from renewable or other alternative resources to replace or modify the existing petroleum-based asphalt binder for the highway industry. The main objective of this study was to investigate the potential use of a bio-oil byproduct as a partial replacement for a bitumen-based binder. The properties of various blending combinations of a biobinder and asphalt binder were compared with those of an unmodified asphalt binder through several physical and rheological tests. The specific gravity of the biobinder was found to be higher than that of the petroleum-based binder. In general, biobinder blends have a higher viscosity than that of the unmodified binder at temperatures higher than 60°C. Binders containing the biobinder at 25% and 50% have lower temperature susceptibility than that of the unmodified binder. Adding the biobinder at up to 25% does not significantly change the complex shear modulus of unaged binders, whereas rolling thin-film oven-aged biobinder blends show a higher complex modulus and greater viscosity than those of the unmodified binder. Moreover, the biobinder blends show higher G∗/sinδ, lower nonrecoverable compliance and greater recovery ability than those of conventional AC-20.

The molecular regioregularity induced morphological evolution of polymer blend thin films

The polymer regioregularity has a dramatic effect on device performance of organic photovoltaics in terms of material crystallinity and phase separated morphology. In this study, we investigated the molecular regioregularity induced morphological evolution of P3HT and PF12TBT blend thin films processed by solvents with different solubility and boiling point. After cast by good solvents chloroform (CF) and o-dichlorobenzene (oDCB), the regioregular (RR) and regiorandom (RA)-P3HT/PF12TBT blend films showed the similar phase separated morphology, namely fast solidification induced unsharp morphology for CF blend films and long-range demixing via spinodal decomposition within extending drying process for oDCB blend films, while the domain size for RA blend films were larger than those of RR blend films which could be attributed to easily disentanglement and diffusion of RA-P3HT molecules facilitated by smaller aggregation size in RA blend solutions. In addition, the morphology of RR blend films exhibited micro change while the RA blend films developed into long-range phase separation after thermal annealing for CF blend films which also confirmed the molecular diffusion induced morphological evolution. For marginal solvents p-xylene (XY), the RR P3HT showed obvious molecular aggregation in XY solution due to the intensive solvent–polymer interaction (Δδ) and high regioregularity and therefore the value of aggregation size (Ra) for RR-P3HT is as three times as that of RA-P3HT. After cast from XY solution, the RA blend films displayed droplet and bicontinuous morphology based on various blend ratios via spinodal decomposition. Meanwhile, the nanofiber morphology could be observed when the weight fraction of RR-P3HT beyond 10% in RR blend films, which resulting from the continuous growth of P3HT aggregations formed anterior of the phase separation in the film solidification process.

Microstructural behavior and failure mechanisms of organic semicrystalline thin film blends

Organic thin film blends of P3HT semiconducting polymers and PCBM fullerenes have enabled large-scale semiconductor fabrication pertaining to flexible and stretchable electronics. However, molecular packing and film morphologies can significantly alter mechanical stability and failure behavior. To further understand and identify the fundamental mechanisms affecting failure, a multiphase microstructurally based formulation and nonlinear finite-element fracture methodology were used to investigate the heterogeneous deformation and failure modes of organic semicrystalline thin film blends. The multiphase formulation accounts for the crystalline and amorphous behavior, polymer tie-chains, and the PCBM aggregates. Face-on packing orientations resulted in extensive inelastic deformation and crystalline rotation, and this was characterized by ductile failure modes and interfacial delamination. For edge-on packing orientations, brittle failure modes and film cracking were due to lower inelastic deformation and higher film stress in comparison with the face-on orientations. The higher crystallinity of P3HT and larger PCBM aggregates associated with larger domain sizes, strengthened the film and resulted in extensive film cracking. These predictions of ductile and brittle failure are consistent with experimental observations for P3HT:PCBM films. The proposed predictive framework can be used to improve organic film ductility and strength through the control of molecular packing orientations and microstructural mechanisms. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 896–907

Enhancing the photovoltaic performance of binary acceptor-based conjugated polymers incorporating methyl units

“Binary acceptors” conjugated polymers with methyl side units were found to provide high photocurrent and efficiency in solar cells due to the small crystal domain of polymers in blended thin films.

Boundary-induced segregation in nanoscale thin films of athermal polymer blends.

The surface segregation of binary athermal polymer blends confined in a nanoscale thin film was investigated by dissipative particle dynamics. The polymer blend included linear/linear, star/linear, bottlebrush/linear, and rod-like/linear polymer systems. The segregation was driven by purely entropic effects and two different mechanisms were found. For the linear/linear and star/linear polymer blends, the smaller sized polymers were preferentially segregated to the boundary because their excluded volumes were smaller than those of the matrix polymers. For the bottlebrush/linear and rod-like/linear polymer blends, the polymers with a larger persistent length were preferentially segregated to the boundary because they favored staying in the depletion zone by alignment with the wall. Our simulation outcome was consistent with experimental results and also agreed with theoretical predictions - that is, a surface excess dictated by the chain ends for the branch/linear system. These consequences are of great importance in controlling the homogeneity and surface properties of polymer blend thin films.

Soft-shear induced phase-separated nanoparticle string-structures in polymer thin films.

Application of shear stress has been shown to unidirectionally orient the microstructures of block copolymers and polymer blends. In the present work, we study the phase separation of a novel nanoparticle (NP)-polymer blend thin film system under shear using a soft-shear dynamic zone annealing (DZA-SS) method. The nanoparticles are densely grafted with polymer chains of chemically dissimilar composition from the matrix polymer, which induces phase separation upon thermal annealing into concentrated nanoparticle domains. We systematically examine the influence of DZA-SS translation speed and thus the effective shear rate on nanoparticle domain elongation and compare this with the counterpart binary polymer blend behavior. Unidirectionally aligned nanoparticle string-domains are fabricated in the presence of soft-shear in confined thin film geometry. We expect this DZA-SS method to be applicable to various NP-polymer blends towards unidirectionally aligned nanoparticle structures, which are important to functional nanoparticle structure fabrication.

Investigation of Donor/acceptor Composition Ratio and Annealing Effects on Performances of Organic Solar Cells based on P3HT:C<sub>60</sub>

The optimization of organic photovoltaic (OPV) devices based on a thin-film blend of the polymer poly(3-hexyl thiophene) [P3HT] and the fullerene [C60] was explored. To achieve an efficient photo induced charge transfer, the following aspects have been studied: (1) Determination of the donor/acceptor composition ratio that yields good film interface and high photon absorption; (2) Selection of suitable Nanoparticle to obtain optimal absorption spectrum and good efficiency; (3) Thermal annealing process to enhance the photon absorption, improve the short circuit current and the filling factor, and therefore the efficiency of the devices. In particular, the preparation of the composition and thermal treatment of the active layer along with the effect of Nanoparticle's presence are addressed. In addition, the use of Nanoparticle (WO3) into the P3HT/C60 layer to improve the power conversion efficiency (PCE) of the device is investigated. WO3 Nanoparticle's doped poly(3-hexylthiophene) and C60 blends (P3HT:C60: WO3) have been produced as the photoactive layer of organic photovoltaic devices (OPVs). The 40 ppm WO3 doping OPVs exhibit higher power conversion efficiency (PCE) than other OPVs.

Characterization and Application of a Hybrid Optical Microscopy/Laser Ablation Liquid Vortex Capture/Electrospray Ionization System for Mass Spectrometry Imaging with Sub-micrometer Spatial Resolution.

A commercial optical microscope, laser microdissection instrument was coupled with an electrospray ionization mass spectrometer via a low profile liquid vortex capture probe to yield a hybrid optical microscopy/mass spectrometry imaging system. The instrument has bright-field and fluorescence microscopy capabilities in addition to a highly focused UV laser beam that is utilized for laser ablation of samples. With this system, material laser ablated from a sample using the microscope was caught by a liquid vortex capture probe and transported in solution for analysis by electrospray ionization mass spectrometry. Both lane scanning and spot sampling mass spectral imaging modes were used. The smallest area the system was able to ablate was ∼0.544 μm × ∼0.544 μm, achieved by oversampling of the smallest laser ablation spot size that could be obtained (∼1.9 μm). With use of a model photoresist surface, known features as small as ∼1.5 μm were resolved. The capabilities of the system with real world samples were demonstrated first with a blended polymer thin film containing poly(2-vinylpyridine) and poly(N-vinylcarbazole). Using spot sampling imaging, sub-micrometer sized features (0.62, 0.86, and 0.98 μm) visible by optical microscopy were clearly distinguished in the mass spectral images. A second real world example showed the imaging of trace amounts of cocaine in mouse brain thin tissue sections. With use of a lane scanning mode with ∼6 μm × ∼6 μm data pixels, features in the tissue as small as 15 μm in size could be distinguished in both the mass spectral and optical images.

The effect of residual palladium catalyst on the performance and stability of PCDTBT:PC70BM organic solar cells

Palladium (Pd) is commonly used as a catalyst in the polymerisation of conjugated polymers such as poly[N-9′-heptadecanyl-2,7-carbozole-alt-5,5-(4′,7′-di-2-thenyl-2′,1′,3′-benzothiadiazole)] (PCDTBT). Here we explore the effect of residual catalyst on the performance of organic photovoltaic devices (OPVs) based on a PCDTBT:fullerene thin-film blend. We find that as the relative concentration of Pd increases, the power conversion efficiency of the PV is reduced, dropping from 4.55% to 2.42% as the Pd concentration was increased to 2570 ppm (relative to that of the PCDTBT). This reduction in efficiency resulted primarily from a reduction in PV fill factor and shunt-resistance, indicating the presence of current-shunts within the device. Using optical microscopy, laser beam induced current mapping and scanning electron microscopy, we are able to demonstrate that such current shunts are associated with micron-sized aggregates of Pd-containing nanoparticles. We show that the presence of high concentrations of Pd within a PCDTBT OPV contribute to a larger drop in efficiency during the initial ‘burn-in’ period.

Physical mechanisms responsible for the water‐induced degradation of PC61BM P3HT photovoltaic thin films

ABSTRACTWe show that [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC61BM) at the surface of thin film blends of poly(3‐hexylthiophene) (P3HT):PC61BM can be patterned by water. Using a series of heating and cooling steps, water droplets condense onto the blend film surface. This is possible due to the liquid‐like, water swollen layer of poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate. Breath pattern water deformation and subsequent drying on the film surface results in isolated PC61BM structures, showing that migration of PC61BM takes place. This was confirmed by selective wavelength illumination to spatially map the photoluminescence from the P3HT and PC61BM. Within a device, redistribution of the surface PC61BM into aggregates would be catastrophic, as it would markedly alter device performance. We also postulate that repeated volume change of the poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate layer by water swelling may be, in part, responsible for the delamination failure mechanism in thin film solar cells devices. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 141–146

Effect of Halogenation in Isoindigo-Based Polymers on the Phase Separation and Molecular Orientation of Bulk Heterojunction Solar Cells

Fluorinated and chlorinated isoindigo-based polymers, F-IIDT and Cl-IIDT, were developed as the donor materials for photovoltaic cells. Although showing similar frontier orbital energy levels, both polymers exhibited distinct solar cell device performance, for example, power conversion efficiency of 4.60% for Cl-IIDT and 1.19% for F-IIDT. The investigation of photophysical properties demonstrated that the strong preaggregation of F-IIDT was formed in solution due to increased noncovalent interactions between the backbones of the polymer. Therefore, large crystal domains in blended thin films with no preferred crystallographic orientation were obtained. On the other hand, the introduction of chlorine atoms increased the torsional angle of the polymer backbone, resulting in reduced crystallization tendency. Films with less crystallinity and face-on polymer orientation contributed to better device performance of Cl-IIDT. Our research proved that the effects of halogenation are far more than the tuning of fro...

Phase Separated Morphology of Ferroelectric–Semiconductor Polymer Blends Probed by Synchrotron X-ray Methods

Control of the domain size and morphology of ferroelectric-semiconductor polymer blend thin films is essential for producing working organic ferroelectric resistive switches that can be used for low-cost, flexible memory applications. However, improvements in characterization techniques that can selectively probe these polymers are still needed. The unique core-level absorption profiles of these polymers make synchrotron based soft X-ray techniques ideal to achieve contrast and chemical sensitivity between polymers and characterize thin film morphology. Transmission soft X-ray microscopy and scattering reveal that a phase separated structure exists through the bulk for a blend of a semicrystalline semiconducting polythiophene with a functionalized side chain and a well-studied ferroelectric polymer. Surface sensitive soft X-ray spectroscopy and wide-angle X-ray scattering suggest a potential enhancement of polythiophene at the film surface, and that the surface layer is more amorphous in character. This w...

Optical, Structural and Morphological Properties of Ternary Thin Film Blend of P3HT:PCBM:ZnO Nanoparticles

Ternary blend film of conjugated polymer, fullerene and inorganic nanoparticles has intensively studied as active material for high power conversion efficiency (PCE) of hybrid organic-inorganic solar cells. The incorporation of two electron acceptor materials of organic fullerene and inorganic nanoparticles into hybrid with electron donor conjugated polymer is strongly believed can improve the PCE of solar cells by increasing exciton dissociation efficiency due to an increase of interface area between donor and acceptor materials where the positive and negative charges dissociated. We studied optical, structural and morphological properties of ternary thin film containing blend of conjugated polymer poly(3-hexylthiophene (P3HT):fullerene derivative PCBM:Zinc oxide nanoparticles (ZnO-NP) by measuring its optical absorption, crystal structure and film surface morphology. Zinc oxide nanoparticle was prepared by sol-gel method. It has optical absorption below 370 nm and average particle size 40 nm as shown by TEM picture. Ternary thin blend films of P3HT:PCBM:ZnO-NP were prepared by use of spin-coating method. The UV-Vis spectrum of thin film contains absorption peaks originated from contribution of P3HT at wavelengths 520 nm, 550 nm and 600 nm, from contribution of PCBM at 260 nm and 330 nm and from ZnO-NP at wavelengths below 370 nm which confirms that these three materials were well mixed in the films. Its XRD pattern also contains the peaks from each of these three-materials. In this report, we compare surface morphology of thin films of pure P3HT, pure ZnO-NP, blend of P3HT:PCBM, blend of P3HT:ZnO-NP and ternary blend of P3HT:PCBM:ZnO-NP.

Optoelectronic characteristics of MEH-PPV + BT blend thin films in polymer light emitting diodes

Due to the unique optical and electronic properties of conjugated polymers, much research has been conducted to study the effect of the incorporation of electron-transporting materials on the polymer blends’ compatibility and their capability for use in optoelectronic devices. In this work, to characterize the optoelectronic properties of blend thin films of poly [2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with benzothiadiazole (BT), polymer light- emitting diodes (PLEDs) with single-emission layers of MEH-PPV + BT blends have been fabricated. The influence of MEH-PPV + BT blend weight ratios over ITO/PEDOT:PSS/MEH-PPV + BT/Al PLEDs performances, e.g., lifetime, turn-on voltage, and current density-voltage (J-V) characteristics, has been studied. According to the obtained results, the turn-on voltage of the devices successfully decreased with the addition of the BT as an electronic transportation material. At an optimum condition, we obtained a turn-on voltage as low as 5 V and a lifetime of about 190 h for a device incorporating 65% BT. The logarithmic plots of the J-V characteristics of the fabricated devices showed a power law behavior (J ∝ Vk+1) with three distinct regions. The J-V characteristics have been explained by the Fowler–Nordheim (FN) tunneling model. It was found that the hole-injection barrier height decreases with increasing BT content in the range of 0–65%. According to the obtained results, in all of our investigations, the electroluminescence (EL) originated exclusively from the MEH-PPV material, even for the high BT contents.

Periodic nanopatterns from polymer blends via directional solidification and subsequent epitaxial crystallization

Periodic lamellar nanostructures have been created in polymer blend thin films composed of poly(L-lactide) (PLLA) and poly(vinyl phenol) (PVPh) via directional solidification and epitaxial crystallization. The obtained morphology was strongly dependent on annealing condition and blending ratio. These processing parameters closely correlate to the strength of the hydrogen bonding between PLLA and PVPh, hence it was highlighted that the noncovalent interaction between polymers affects the pattern formation.

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.

Morphological study of F8BT:PFB thin film blends

We have studied the thin film morphology of a semiconducting polymer photovoltaic blend comprising an electron acceptor poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and the donor poly(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine) (PFB). The molecular weight and blend weight ratio of the constituent polymers were used to modify the morphology. The average chemical composition of the bulk of F8BT:PFB blend in thin films was mapped using Raman microscopy at different depths from the air-film interface through controlled successive etching from the upper surface layer using an oxygen plasma. Correlating the lateral to the vertical Raman analysis of the phase separation of the film (blend weight ratio of 50:50) reveals that the μm scale de-mixed lateral phase structure seen on the free surface is present throughout most of the film thickness, though there is also some F8BT content within the PFB-rich wetting layer on the glass substrate, which we consider is due to the F8BT-rich interface at the surface to the substrate. The dependence of photovoltaic performance on morphology is discussed.

Enhanced mobility and effective control of threshold voltage in P3HT-based field-effect transistors via inclusion of oligothiophenes.

Improved organic field-effect transistor (OFET) performance through a polymer-oligomer semiconductor blend approach is demonstrated. Incorporation of 2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene (BTTT) into poly(3-hexylthiophene) (P3HT) thin films leads to approximately a 5-fold increase in charge carrier mobility, a 10-fold increase in current on-off ratio, and concomitantly, a decreased threshold voltage to as low as 1.7 V in comparison to single component thin films. The blend approach required no pre- and/or post treatments, and processing was conducted under ambient conditions. The correlation of crystallinity, surface morphology and photophysical properties of the blend thin films was systematically investigated via X-ray diffraction, atomic force microscopy and optical absorption measurements respectively, as a function of blend composition. The dependence of thin-film morphology on the blend composition is illustrated for the P3HT:BTTT system. The blend approach provides an alternative avenue to combine the advantageous properties of conjugated polymers and oligomers for optimized semiconductor performance.