Spin-cast Films Research Articles

Conductivity and touch-sensor application for atomic layer deposition ZnO and Al:ZnO on nylon nonwoven fiber mats

Flexible electronics and wearable technology represent a novel and growing market for next generation devices. In this work, the authors deposit conductive zinc oxide films by atomic layer deposition onto nylon-6 nonwoven fiber mats and spun-cast films, and quantify the impact that deposition temperature, coating thickness, and aluminum doping have on the conductivity of the coated substrates. The authors produce aluminum doped zinc oxide (AZO) coated fibers with conductivity of 230 S/cm, which is ∼6× more conductive than ZnO coated fibers. Furthermore, the authors demonstrate AZO coated fibers maintain 62% of their conductivity after being bent around a 3 mm radius cylinder. As an example application, the authors fabricate an “all-fiber” pressure sensor using AZO coated nylon-6 electrodes. The sensor signal scales exponentially under small applied force (<50 g/cm2), yielding a ∼106× current change under 200 g/cm2. This lightweight, flexible, and breathable touch/force sensor could function, for example, as an electronically active nonwoven for personal or engineered system analysis and diagnostics.

Highly conductive Cu2-xS nanoparticle films through room-temperature processing and an order of magnitude enhancement of conductivity via electrophoretic deposition.

A facile room-temperature method for assembling colloidal copper sulfide (Cu2-xS) nanoparticles into highly electrically conducting films is presented. Ammonium sulfide is utilized for connecting the nanoparticles via ligand removal, which transforms the as-deposited insulating films into highly conducting films. Electronic properties of the treated films are characterized with a combination of Hall effect measurements, field-effect transistor measurements, temperature-dependent conductivity measurements, and capacitance-voltage measurements, revealing their highly doped p-type semiconducting nature. The spin-cast nanoparticle films have carrier concentration of ∼ 10(19) cm(-3), Hall mobilities of ∼ 3 to 4 cm(2) V(-1) s(-1), and electrical conductivities of ∼ 5 to 6 S · cm(-1). Our films have hole mobilities that are 1-4 orders of magnitude higher than hole mobilities previously reported for heat-treated nanoparticle films of HgTe, InSb, PbS, PbTe, and PbSe. We show that electrophoretic deposition (EPD) as a method for nanoparticle film assembly leads to an order of magnitude enhancement in film conductivity (∼ 75 S · cm(-1)) over conventional spin-casting, creating copper sulfide nanoparticle films with conductivities comparable to bulk films formed through physical deposition methods. The X-ray diffraction patterns of the Cu2-xS films, with and without ligand removal, match the Djurleite phase (Cu(1.94)S) of copper sulfide and show that the nanoparticles maintain finite size after the ammonium sulfide processing. The high conductivities reported are attributed to better interparticle coupling through the ammonium sulfide treatment. This approach presents a scalable room-temperature route for fabricating highly conducting nanoparticle assemblies for large-area electronic and optoelectronic applications.

Glass transition temperature of thin polycarbonate films measured by flash differential scanning calorimetry

Flash differential scanning calorimetry was used to study the glass transition temperature Tg of polycarbonate ultrathin films. The investigation was made as a function of film thickness from 22 to 350 nm and over a range of cooling rates from 0.1 to 1000 K/s. Polycarbonate spin cast films were floated on a layer of grease on the calorimetric chip. The results show a greatly reduced glass temperature for the thinnest films relative to the macroscopic value. We also observed that the magnitude of the glass temperature reduction decreases as the cooling rate increases with the highest cooling rates showing little thickness dependence of the Tg. Dynamic fragility and activation energy at Tg were found to decrease with decreasing film thickness. The results are discussed in the context of literature reports for supported and freely standing polycarbonate films. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1462–1468

Direct laser writing of air-stable p-n junctions in graphene.

Photo-oxidation of spin-cast films of 6,13-bis(triisopropylsilylethynyl) pentacene has been exploited to develop a novel means of spatially modulating doping in graphene. The degree of n-doping of initially p-type graphene can be varied by laser irradiation time or intensity with carrier density change up to ∼7 × 10(12) cm(-2). This n-doping approach is demonstrated as an effective means of creating p-n junctions in graphene. The ability to direct-write arbitrary shapes and patterns of n-doped regions in graphene simply by scanning a laser source should facilitate the exploitation of p-n junctions for a variety of electronic and optoelectronic device applications.

Direct observation of doping sites in temperature-controlled, p-doped P3HT thin films by conducting atomic force microscopy.

The distribution of dopant sites in doped poly(3-hexylthiophene) (P3HT) thin films is characterized using optical absorption, grazing-incidence X-ray diffraction, and conducting atomic force microscopy (c-AFM). It is shown that dopant sites can be directly observed using c-AFM and that the solution temperature dramatically impacts phase separation and conductivity in spin-cast films.

Thiophene Fused Azacoronenes: Regioselective Synthesis, Self-Organization, Charge Transport and Its Incorporation in Conjugated Polymers

A regioselective synthesis of an azacoronene fused with two peripheral thiophene groups has been realized through a concise synthetic route. The resulting thienoazacoronene (TAC) derivatives show high degree of self-organization in solution, in single crystals, in the bulk, and in spuncast thin films. Spuncast thin film field-effect transistors of the TACs exhibited mobilities up to 0.028 cm2 V–1 S–1, which is among the top field effect mobilities for solution processed discotic materials. Organic photovoltaic devices using TAC-containing conjugated polymers as the donor material exhibited a high open-circuit voltage of 0.89 V, which was ascribable to TAC’s low-lying highest occupied molecular orbital energy level.

Detection of surface mobility of poly (2, 3, 4, 5, 6-pentafluorostyrene) films by in situ variable-temperature ToF-SIMS and contact angle measurements

Poly (2, 3, 4, 5, 6-pentafluorostyrene) (5FPS) was prepared by bulk radical polymerization. The spin-cast films of this polymer were analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS) at various temperatures ranging from room temperature to 120°C. Principal component analysis (PCA) of the ToF-SIMS data revealed a transition temperature (TT) at which the surface structure of 5FPS was rearranged. A comparison between the results of the PCA of ToF-SIMS spectra obtained on 5FPS and polystyrene (PS) indicate that the pendant groups of 5FPS and PS moved in exactly opposite directions as the temperature increased. More pendant groups of 5FPS and PS migrated from the bulk to the surface and verse versa, respectively, as the temperature increased. These results clearly support the view that the abrupt changes in the normalized principal component 1 value was caused by the surface reorientation of the polymers and not by a change in the ion fragmentation mechanism at temperatures above the TT. Contact angle measurement, which is another extremely surface sensitive technique, was used to monitor the change in the surface tension as a function of temperature. A clear TT was determined by the contact angle measurements. The TT values determined by contact angle measurements and ToF-SIMS were very similar.

Low Spin-Casting Solution Temperatures Enhance the Molecular Ordering in Polythiophene Films

High-crystallinity poly(3-hexylthiophene) (P3HT) thin films were prepared by aging the precursor solutions, prepared using a good solvent, chloroform, at low temperatures prior to spin-casting. Lower solution temperatures significantly improved the molecular ordering in the spin-cast P3HT films and, therefore, the electrical properties of field-effect transistors prepared using these films. Solution cooling enhanced the electrical properties by shifting the P3HT configuration equilibrium away from random coils and toward more ordered aggregates. At room temperature, the P3HT molecules were completely solvated in chloroform and adopted a random coil conformation. Upon cooling, however, the chloroform poorly solvated the P3HT molecules, favoring the formation of ordered P3HT aggregates, which then yielded more highly crystalline molecular ordering in the P3HT thin films produced from the solution.

Effect of Water Swelling on the Tribological Properties of PMMA Spin-Cast Film and Brush in Aqueous Environment

Due to their light weight, low corrosion and good tribological properties, polymer films have been widely studied in dry condition as well as recently in aqueous environment. Though the presence of water can further reduce the friction, it promotes the wear rate of the polymer films. As a remedy to decrease the wear rate of polymer films under aqueous condition, in this study, we used PMMA brush which is chemically anchored to a substrate and compared its friction and wear properties with those of conventional PMMA spin-cast film. Ellipsometry, contact angle measurements and atomic force microscopy are used to study the surface properties, e.g., wear mechanisms and wear depths of PMMA films. Under different sliding speeds and applied loads, PMMA brush showed lower friction than PMMA spin-cast film in aqueous. Moreover, it was shown that the swelling of water molecules is a dominant factor in determining the wear durability of PMMA films in which PMMA brush showed better wear performance than PMMA spin-cast film.

High performance polymer solar cells with as-prepared zirconium acetylacetonate film as cathode buffer layer.

Low-work-function active metals are commonly used as cathode in polymer solar cells (PSCs), but sensitivity of the active metals towards moisture and oxygen results in poor stability of the devices. Therefore, solution-proceessable and stable cathode buffer layer is of great importance for the application of PSCs. Here we demonstrate high performance PSCs by employing as-prepared zirconium acetylacetonate (a-ZrAcac) film spin-cast from its ethanol solution as cathode buffer layer. The PSCs based on a low bandgap polymer PBDTBDD as donor and PC60BM as acceptor with a-ZrAcac/Al cathode demonstrated an average power conversion efficiency (PCE) of 8.75% which is significantly improved than that of the devices with traditional Ca/Al cathode. The improved photovoltaic performance is benefitted from the decreased series resistance and enhanced light harvest of the PSCs with the a-ZrAcac/Al cathode. The results indicate that a-ZrAcac is a promising high performance cathode buffer layer for fabricating large area flexible PSCs.

Formation Mechanism of High-Density, Flattened Polymer Nanolayers Adsorbed on Planar Solids

Thermal annealing is one of the most indispensable polymer fabrication processes and plays essential roles in controlling morphologies and properties of polymeric materials. We here report that thermal annealing also facilitates polymer adsorption from the melt on planar silicon (Si) substrates, resulting in the formation of a high-density polymer nanolayer with flattened chain confirmations. Three different homopolymers (polystyrene, poly(2-vinylpyridine), and poly(methyl methacrylate)), which have similar inherent stiffness and bulk glass transition temperature (Tg), but have different affinities with Si substrates, were chosen as models. Spin-cast films (∼50 nm in thickness) with the three polymers were prepared on cleaned Si substrates and then placed in a vacuum oven set at a temperature far above the bulk Tg. In order to monitor the polymer adsorption process at the solid-polymer melt interface during thermal annealing, we used the protocol that combines vitrification of the annealed films (via rapi...

Cellulose nanofibril based graft conjugated polymer films act as a chemosensor for nitroaromatic

A cellulose nanofibril film is modified by chemical assembly of boronate-terminated conjugated polymer chains at its specific sites, C-6 carboxyl groups. The modified cellulose nanofibril film is used as a fluorescent sensor for nitroaromatic vapor. Thanks to the specific reactive sites, numerous loose cavities or pathways located in the film sensor's out-layer have been formed, and the fraction of easily accessible cavities of the novel fluorescent film sensor is up to 0.97, which could benefit the penetration and diffusion of analyte vapor. Therefore, the novel fluorescent film sensor exhibits high sensitivity toward nitroaromatic vapor with a fast response. The fluorescence quenching efficiency of the chemical-assembly film sensor is about 3 times larger than that of the spin-cast film sensor using the same conjugated polymer for 600s exposure to DNT vapor. In addition, the novel fluorescent film sensor shows good reversibility.

Solution-processed annealing-free ZnO nanoparticles for stable inverted organic solar cells

We report the development and application of high-quality zinc oxide nanoparticles (ZnO NPs) processed in air for stable inverted bulk heterojunction solar cells as an electron extraction layer (EEL). The ZnO NPs (average size ∼11nm) were dispersed in chloroform and stabilized by propylamine (PA). We demonstrated that the ZnO NP dispersion with 4vol.% of PA as stabilizer can be used in air directly and remains clear up to one month after preparation. Our inverted solar cells consisted of a blade-coated poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) (1: 4 by weight) active layer sandwiched between a ZnO electron extraction layer and a MoO3/Ag anode. All solar cells with ZnO films fabricated in air using PA-stabilized ZnO dispersions prepared within a time window of one month exhibited power conversion efficiencies (PCE) above 4%. In contrast, if the ZnO film was prepared in air using regular un-stabilized ZnO NP dispersion, the PCE would drop to 0.2% due to poor film quality. More interestingly, X-ray photoelectron spectroscopy and nuclear magnetic resonance measurements indicated that the PA ligands were not covalently bonded to ZnO NPs and did not exist in the deposited ZnO films. The spin-cast ZnO thin films (without any thermal treatment) are insoluble in organic solvents and can be directly used as an EEL in solar cells. This feature is beneficial for fabricating organic solar cells on flexible polymer substrates. More importantly, our non-encapsulated inverted solar cells are highly stable with their PCEs remaining unchanged after being stored in air for 50days.

Influence of casting solvent on phenyl ordering at the surface of spin cast polymer thin films

Controlling the surface molecular structure of spin cast polymer films is important for the rational design of surface properties. However, the relationship between spin casting parameters and film surface molecular structure is poorly understood. We report that the surface molecular structure of spin cast homopolymers which contain phenyl groups is influenced by the solvent aromaticity, investigated by a nonlinear optical spectroscopy, sum frequency generation (SFG) vibrational spectroscopy. When phenyl groups were located in a linear polymer backbone, spin casting with aromatic solvents enhanced the phenyl SFG signal relative to when a non-aromatic solvent was used which suggests that the aromatic solvent induced the surface phenyl groups to be more ordered and/or to lie more perpendicular to the film surface. In addition, when alkyl structures were believed to be present at the solvent/air interface, alkyl structures were observed at the film/air interface which suggests that molecular structure at the solvent/air interface was carried to the film surface. The effects of solvent aromaticity on phenyl ordering at spin cast film surfaces can be explained by different molecular structures of polymer chains at solvent/air interfaces, preferential solvation of functional groups during evaporation, and re-orientation of bulky side groups at the polymer film/air interface.

Organic Electronics: Charge Transport Over Multiple Length Scales in Supramolecular Fiber Transistors: Single Fiber Versus Ensemble Performance (Adv. Mater. 3/2014)

The transistor characteristics of self-assembled organic fibers are assessed both individually and at the ensemble level by Paolo Samorì and co-workers on page 430. Their work demonstrates the first evidence of band-like transport in a solution-processed n-type organic semiconductor, and an intrinsic electron mobility that is several orders of magnitude higher than in fiber ensembles or spin-cast films, highlighting the performance potential of self-assembled organic architectures.

A high molecular weight triisopropylsilylethynyl (TIPS)-benzodithiophene and diketopyrrolopyrrole-based copolymer for high performance organic photovoltaic cells

A high-molecular-weight conjugated polymer consisting of alternating TIPS-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) and diketopyrrolo[3,4-c]pyrrole (DPP) was synthesized by Suzuki cross-coupling and characterized for use in high performance OPVs. Good structural order, self-assembly in spun-cast films, and carrier mobility were observed. PTIPSBDT–DPP showed an OFET hole mobility of up to 0.12 cm2 V−1 s−1. Power conversion efficiencies of up to 8.0% were demonstrated for OPV devices.

Tunable surface plasmon resonance and enhanced electrical conductivity of In doped ZnO colloidal nanocrystals.

We report a new synthesis process of colloidal indium (In) doped zinc oxide (ZIO) nanocrystals by a hot injection technique. By fine tuning the synthesis we reached the same nucleation temperature for indium oxide and zinc oxide which helped us to study a dopant precursor dependent In incorporation into the ZnO matrix by using different In sources. The dopant induced shape evolution changes the hexagonal pyramid structured ZnO to a platelet like structure upon 8% In doping. The introduction of trivalent In(3+) into the ZnO lattice and consequent substitution of divalent Zn(2+) generates free electrons in the conduction band which produces a plasmonic resonance in the infrared region. The electron concentration controls plasmon frequency as well as the band gap of host ZnO. The variation of the band gap and the modification of the conduction band have been explained by the Burstein-Moss effect and Mie's theory respectively. The In dopant changes the defect chemistry of pure ZnO nanocrystals which has been studied by photoluminescence and other spectroscopic measurements. The nanocrystals are highly stable in the organic medium and can be deposited as a crack free thin film on different substrates. Careful ligand exchange and thermal annealing of the spin cast film lead to a good conductive film (720 Ω per square to 120 Ω per square) with stable inherent plasmonic absorption in the infrared and 90% transmittance in the visible region. A temperature induced metal-semiconductor transition was found for doped ZnO nanocrystals. The transition temperature shifts to a lower temperature with increase of the doping concentration.

Metal-Free, Single-Polymer Device Exhibits Resistive Memory Effect

All-polymer, write-once-read-many times resistive memory devices have been fabricated on flexible substrates using a single polymer, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). Spin-cast or inkjet-printed films of solvent-modified PEDOT:PSS are used as electrodes, while the unmodified or as-is PEDOT:PSS is used as the semiconducting active layer. The all-polymer devices exhibit an irreversible but stable transition from a low resistance state (ON) to a high resistance state (OFF) at low voltages caused by an electric-field-induced morphological rearrangement of PEDOT and PSS at the electrode interface. However, in the metal-PEDOT:PSS-metal devices, we have shown a metal filament formation switching the device from an initial high resistance state (OFF) to the low resistance state (ON). The all-PEDOT:PSS memory device has low write voltages (<3 V), high ON/OFF ratio (>10(3)), good retention characteristics (>10,000 s), and stability in ambient storage (>3 months).

Charge Transport Over Multiple Length Scales in Supramolecular Fiber Transistors: Single Fiber Versus Ensemble Performance

Self-assembled organic fibers combine facile solution processing with the performance benefits of single crystals. Here, the first evidence is shown of band-like transport in an n-type solution-processed small molecule system, a limited role of shallow traps, and a single fiber electron mobility that is several orders of magnitude higher than that measured in fiber ensembles or spin-cast films.

Improved Electrical Properties of Solution-Processed ZrO2 Gate Dielectric for Large-Area Flexible Electronics

Zirconium oxide (ZrO2), which has high dielectric constant, was investigated for application in flexible electronics. When the spun-cast film was annealed at a low temperature, the electrical properties were not encouraging because residual organic particles remained at the dielectric. To address this problem we used plasma annealing at a reasonably low temperature and achieved improved dielectric properties such as lower leakage current, higher dielectric constant, and better reliability. Auger depth profile spectroscopy results suggested reduction of carbon percentage at the dielectric. We demonstrated device application by fabricating transistor device with an organic channel layer. The transistor electrical properties were encouraging, exhibiting an electron mobility of 0.3 cm2/(V·s). The results were very promising and suggest that ZrO2 could be applied to all-printed electronic devices in the near future.