Preparation and characterization of thin films of the poly(p‐phenylene vinylene) semiconducting polymer

Conjugated polymers have been the subject of many studies because of their widespread applications in electronic and optoelectronic devices. Poly(p‐phenylene vinylene) is a leading semiconducting polymer in optical applications. This work is focused on the development of thin films of poly(p‐phenylene vinylene) by spin coating and their characterization with Fourier transform infrared spectroscopy, X‐ray diffraction, and scanning electron microscopy to understand their changes. An empirical model has been developed to show the effect of the variables—the spin speed, polymer concentration, and spin time—on the film thickness. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Preparation of siloxane–silsesquioxane hybrid thin films for large‐scale‐integration interlayer dielectrics with excellent mechanical properties and low dielectric constants

Several kinds of homogeneous organic–inorganic hybrid polymer thin films were designed with improved mechanical properties and low dielectric constants (<3.0). Novel soluble siloxane–silsesquioxane hybrid polymers were synthesized with cyclic and/or cage silane monomers, which had triorganosiloxy (R₃Si_1/2), diorganosiloxane (R₂SiO_2/2), and organosilsesquioxane (RSiO_3/2) moieties with ethylene bridges at the molecular level, by the hydrolysis and condensation of 2,4,6,8‐tetramethyl‐2,4,6,8‐tetra(trimethoxysilylethyl)cyclotetrasiloxane (a cyclic monomer). The electrical properties of these films, including the dielectric constant (～2.51), leakage current (6.4 × 10^?11 A/cm² at 0.5 MV/cm), and breakdown voltage (～5.4 MV/cm) were fairly good. Moreover, the mechanical properties of the hybrid films, including the hardness (～7 GPa), modulus (～1.2 GPa), and crack‐free thickness (<2 μm), were excellent in comparison with those of previous spin‐on‐glass materials with low dielectric constants. The excellent mechanical properties were proposed to be due to the high contents of Si? OH groups (>30%) and the existence of ethylene bridge and siloxane moieties in the hybrid polymer precursors. In addition, the mechanical properties of the hybrid films were affected by the contents of the cagelike structures. The more cagelike structures a hybrid film contained, the worse its mechanical properties were. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 626–634, 2003     

High refractive index thermally stable phenoxyphenyl and phenylthiophenyl silicones for light‐emitting diode applications

Creating high refractive index (RI) thermally stable polymers for encapsulating high‐brightness light‐emitting diodes (LEDs) remains a challenge and is an opportunity for improving LED efficiencies. The best previously reported RI for a 200°C heat stable encapsulant for LEDs is 1.56. Here, we report the use of novel phenoxyphenyl and phenylthiophenyl silicone monomers to give fully formulated encapsulants with RIs above 1.60. These liquid dispensed encapsulants are highly heat stable, showing little change in optical properties after heat aging at 200°C in air for seven weeks, and were also little changed after cycling between ?10°C to 85°C over 6 months. Phenoxyphenyl(phenyl) dimethoxysilane and phenylthiophenyl(phenyl) dimethoxysilane monomers were prepared via Grignard reactions. The resulting monomers were copolymerized with commercial silicone monomers and incorporated into hydrosilation‐based thermosets designed for use as LED encapsulants. RIs for the cured polymers were 1.60 at 633 nm (1.62 at 450 nm) for the phenoxyphenyl ether system and 1.62 at 633 nm (1.65 at 450 nm) for the phenylthiophenyl ether system. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39824.     

Excimer and electromer suppression of tetraphenylsilane‐based blue‐light‐emitting polymers

Excimer and electromer suppression of tetraphenylsilane‐derivative‐based blue polymer light‐emitting devices (PLEDs) was investigated. Tetraphenylsilane with a rigid bulky structure certainly but not completely suppressed excimer formation among polymer‐chain segments. A poor solvent, toluene, resulted in excimer formation in the solid film during the spin‐coating process, which could not be suppressed by the incorporation of a bulky moiety onto the polymer backbone. In addition, electromers or electroplexes formed by the strong interaction between the oxadiazole and diphenyl(4‐tolyl)amine groups could not be prevented by the tetraphenylsilane moiety. The influences of the bulky moiety, bipolar unit, and device fabrication conditions on the suppression of excimers or electromers in PLEDs are discussed in detail. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Uniaxially aligned poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐bithiophene] thin films characterized by the X‐ray diffraction pole figure technique

Aligned thin films of the liquid‐crystalline polymer poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐bithiophene] were prepared, and the correlation between the optical anisotropy and the structural properties was shown. A series of samples with different thicknesses were prepared via a spin‐casting process on rubbed polyimide surfaces. The alignment of the polymer chains was obtained by a temperature treatment just below the clearing temperature. The degree of alignment was investigated with ultraviolet–visible absorption spectroscopy and in‐plane X‐ray diffraction. Independently, each technique revealed Hermans orientation functions in the range of 0.75–0.8. Surprisingly, a layer‐thickness dependence was not observed. In addition, the X‐ray diffraction pole figure technique revealed that the polymer chains were uniaxially aligned along the rubbing direction. The aligned films were in the nematic state, with the director elongated along the rubbing direction. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Microstructural,wetting, and dielectric properties of plasma polymerized polypyrrole thin films

Polypyrrole (PPy) thin films were synthesized by plasma polymerization technique and investigated the influence of discharge power on microstructural, optical, surface wettability, and dielectric properties of grown films. As deposited PPy films were characterized by X‐ray diffraction (XRD), Fourier transform Infrared spectroscopy (FTIR), Atomic force microscopy, UV‐VIS spectroscopy and dielectric spectroscopy. The broad XRD peak present at 2θ = 23.5° revealed the amorphous nature of grown PPy films. The FTIR spectra displayed characteristic peaks in the wavenumbers regions 3300–3400 cm^?1 and 1635–1700 cm^?1 and respective peaks intensities decreased slightly as a function of discharge powers. Significant modifications in surface morphology of the films were observed as a function of discharge powers and PPy films synthesized at higher discharge power of 50 W demonstrated characteristic surface morphology composed of characteristic vertical cone shaped clusters provided with rms roughness of 3.42 nm. The UV‐VIS absorption spectra evidenced that the optical density values varied as a function of discharge power. The evaluated band gap energies decreased with an increase of discharge power and found to be 2.53 eV for PPy films prepared at higher discharge power of 50 W. The surface wettability studies evidenced that as prepared PPy films were found to be hydrophilic in nature. The dielectric measurements were carried out for “ITO/polymer/ITO” structures in the frequency range 10 mHz to 100 kHz. As evidenced from dielectric spectroscopic measurements, PPy films synthesized at 50 W were demonstrated conductivity value of 6.0 × 10^?12 S/m. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43982.     

Sol‐gel derived TiNb2O7 dielectric thin films for transparent electronic applications

To reduce power consumption of transparent oxide‐semiconductor thin film transistors, a gate dielectric material with high dielectric constant and low leakage current density is favorable. According to previous study, the bulk TiNb₂O₇ with outstanding dielectric properties may have an interest in its thin‐film form. The optical, chemical states and surface morphology of sol‐gel derived TiNb₂O₇ (TNO) thin films are investigated the effect of postannealing temperature lower than 500°C, which is crucial to the glass transition temperature. All films possess a transmittance near 80% in the visible region. The existence of non‐lattice oxygen in the TNO film is proposed. The peak area ratio of non‐lattice oxygen plays an important role in the control of leakage current density of MIM capacitors. Also, the capacitance density and dissipation factor were affected by the indium tin oxide (ITO) sheet resistance at high frequencies. The sample after postannealing at 300°C and electrode‐annealing at 150°C possesses a high dielectric constant (>30 at 1 MHz) and a low leakage current density (<1 × 10^?6 A/cm² at 1 V), which makes it a very promising gate dielectric material for transparent oxide‐semiconductor thin film transistors.     

Characterization of cross‐linking depth for thin polymeric films using atomic force microscopy

Thin polymeric films made with various elastomers, like polyisoprene, and elastomer composites were prepared for characterization of cross‐linking depth in this study. Various cross‐linking methods have been applied to get mechanically stronger, more thermally stable and chemically resistant polymer coatings. However, there is no existing approach that could effectively characterize the degree or depth of cross‐linking for thin polymer films. The objective of this work is to use atomic force microscopy to characterize cross‐linking depth in a precise way. Hyperthermal hydrogen bombardment‐induced cross‐linking was employed as a cross‐linking method and the depth of cross‐linking was estimated via local change of the elastic modulus along the sample cross‐section with precise force measurement and high spatial resolution. It is found that the cross‐linking depth is closely related to the chemical composition of thin films. Understanding the depth of cross‐linking is vital for a broad range of applications. It is believed that the developed technique is also applicable for studying other cross‐linkable materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41493.     

Pyroelectric and dielectric properties of spin‐coated thin films of vinylidene fluoride–trifluoroethylene copolymers

This work emphasizes the use of vinylidene fluoride and trifluoroethylene copolymer P(VDF‐TrFE) as a pyroelectric sensor. The pyroelectric and dielectric properties of the copolymer have been investigated in the temperature interval 150–350 K. The samples were prepared by using a spin‐coating technique with 70/30 mol% VDF/TrFE copolymer. The final film thickness of the samples, which is mainly determined by the concentration of the copolymer, spinning rate and spin time, was measured with a surface profiler. The samples were annealed at 150 °C for 10 min to improve the crystallinity of the copolymer. The crystallinity of the annealed and non‐annealed samples was compared by IR spectroscopy. The most effective process by which to improve the pyroelectric response of the material is to pole the sample with huge poling field‐strengths at elevated temperatures. Both pyroelectric and dielectric activities of the samples were measured after each successful poling process. It was observed that while the pyroelectric activity of the material increases, the dielectric activity decreases, so the figure‐of‐merit of the material, which shows the sensor capability of the material, was increased by a significant amount. It was found that the pyroelectric coefficient of VDF/TrFE (70/30 mol%) copolymer is 68.7 µC m^?2 K^?1 at 300 K. © 2001 Society of Chemical Industry     

Electrical properties of poly(p‐phenylene vinylene) films with an incorporation of platinum metal nanoparticles

The effects of platinum metal nanoparticles on a conjugated polymer were investigated by monitoring the electronic structures and measuring the electrical properties of poly(p‐phenylene vinylene) (PPV) and PPV/Pt nanocomposites films. Enhanced current density in PPV/Pt nanocomposite films was obtained by the incorporation of Pt nanoparticles into the conjugated polymer PPV. This result agrees well with our observation of an increase in the electron affinity and an increase in roughness with increasing Pt nanoparticle content. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synthesis and characterization of a kind of poly(3‐butylthiophene methine) with azo side groups

To enhance the third nonlinear optical properties of poly(thiophene methine), we synthesized a new kind of poly(3‐butylthiophene methine) with azo side groups: poly[(3‐butylthiophene‐2,5‐diyl)‐p‐(N,N‐dimethylamino)azobenzylidenequinomethane] (PBTDMABQ). PBTDMABQ and its intermediate product were characterized with IR, ultraviolet–visible, and ¹H‐NMR spectroscopy. The band gaps of PBTDMABQ were calculated to be 1.94 and 2.06 eV with two different models. The thermal stability, determined by thermogravimetric analysis, indicated that PBTDMABQ decomposed above 345°C. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1261–1265, 2005     

New fluorene–pyrazino[2,3‐g]quinoxaline‐conjugated copolymers: Synthesis,optoelectronic properties,and electroluminescence characteristics

New donor–acceptor conjugated copolymers called poly}2,7‐(9,9′‐dihexylfluorene)‐co‐5,10‐[pyrazino(2,3‐g)quinoxaline]{s or PFPQs [where F represents the 2,7‐(9,9′‐dihexylfluorene) moiety and PQ represents the 5,10‐(pyrazino[2,3‐g]quinoxaline) moiety], synthesized by the palladium‐catalyzed Suzuki coupling reaction, are reported. The PQ contents in the PFPQ copolymers were 0.3, 1, 5, and 50 mol %, and the resulting copolymers were named PFPQ0.3, PFPQ01, PFPQ05, and PFPQ50, respectively. Absorption spectra showed a progressive redshift as the PQ acceptor content increased. The relatively small optical band gap of 2.08 eV for PFPQ50 suggested strong intramolecular charge transfer (ICT) between the F and PQ moieties. The photoluminescence emission peaks of the PFPQ copolymer films also exhibited a large redshift with enhanced PQ contents, ranging from 551 nm for PFPQ0.3 to 592 nm for PFPQ50. However, the PFPQ copolymer based electroluminescence (EL) devices showed poor device performances probably due to the strong confinement of the electrons in the PQ moiety or significant ICT. This problem was resolved with a binary blend of poly[2,7‐(9,9‐dihexylfluorene)] (PF) and PFPQ with a volume ratio of 95/5 (BPQ05). Multiple emission peaks were observed at 421, 444, 480, 516, and 567 nm in the BPQ05‐based EL devices because the low PQ content led to incomplete energy transfer. The Commission Internationale de L'Eclairage 1931 coordinates of the BPQ05‐based EL device were (0.31, 0.32), which were very close to the standard white emission of (0.33, 0.33). Furthermore, the maximum luminescence intensity and luminescence yield were 524 cd/m² and 0.33 cd/A, respectively. This study suggested that a pure white light emission was achieved with the PFPQ copolymers or PF/PFPQ blends through the control of the energy transfer between F and PQ. Such PFPQ copolymers or PF/PFPQ blends would be interesting for electronic and optoelectronic devices. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Preparation and properties of cardanol‐based polybenzoxazine/SiO2 hybrids by sol‐gel technique

A new cardanol‐based benzoxazine monomer containing a hydroxyl group (CBZ) was synthesized and characterized by FT‐IR and ¹H NMR spectroscopy. The formation mechanism of cardanol‐based polybenzoxazine/SiO₂ hybrids from CBZ and tetraethoxysilane (TEOS) by sol‐gel technique was invested by FT‐IR analysis. FT‐IR and SEM confirmed that there were many SiO₂ particles formed and embedded in the cardanol‐based polybenzoxazine. A phase separation occurred when the ratio (wt) of TEOS to CBZ was not less than 3 : 5. TGA results illuminated the thermal stability and char yield of cardanol‐based polybenzoxazine could be enhanced due to the formation of SiO₂ particles in the polybenzoxazine matrix. Incorporation of SiO₂ could improve the surface de‐wetting and anti‐ultraviolet properties of the films. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Ultrathin buffer layer at organic/organic interface for managing the recombination profile in organic light‐emitting diodes: Metal versus dielectric buffer

We report on the utilization of an ultrathin buffer layer at the organic/organic (O/O) interface to enhance device efficiency in organic light‐emitting diodes. Two different kinds of buffer layers are examined: metal and dielectric. It is shown that employment of an ultrathin Ag layer with a thickness of 1–2 nm enhances the device performance, while a MgF₂ dielectric buffer cannot affect the device properties considerably. In particular, the turn‐on voltage of the device with an appropriate buffer layer is reduced about 3 V, its current efficiency increases by a factor of more than three, and the power efficiency increases by a factor of more than five in comparison to the control device when a Ag buffer layer is introduced at the O/O interface. By employment of the buffer layer at the interface, an accumulation of current carriers appears within the device that redistribute the recombination profile toward the interior part of the emissive layer. Also, morphological examinations reveal that distinguishable phase segregation occurs in the blend of the hole‐transport layer. In particular, the polymer component remains at the surface and facilitates the hole transport into the successive layers. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43894.     

Studies on the electronic transport and optical properties of some new chelate modified polysulfones in thin films

The studied polymers (chelate modified polysulfones) have been prepared by the polycondensation reaction between chloro‐end‐capped polysulfones and bis(2,4‐dihydroxybenzaldehyde)Cu²⁺ in the dymethyl sulfoxide/dichlormethane system, in the presence of an aqueous sodium hydroxide solution. The temperature dependence of electrical conductivity and Seebeck coefficient of the respective polymers was investigated using thin‐film samples, deposited from chloroform solutions onto glass substrates. The polymers under study have typical semiconducting properties. The values of some characteristic parameters of the investigated polymers (for example, activation energy of electrical conduction, ratio of carrier mobilities, etc.) have been determined. The nature of the electrical conduction mechanism is discussed. The values of the optical bandgap energy are determined from the absorption spectra. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 100–106, 2006     

Nanomechanical study of polymer‐polymer thin film interface under applied service conditions

Single layer and multilayer polymer thin film coating on polymer substrate are gaining significant importance in different industries. The quantitative and qualitative estimation of interface response for thin film coating under different service conditions is significantly important from the perspective of modeling and designing novel materials. However, to characterize an interface between the soft polymer layer and soft polymer substrate is challenging because of the confinement effect, surface roughness, the viscoelastic nature of the polymers involved, and most importantly, the comparable mechanical properties of soft polymeric film and polymer substrate. Nanoindentation technique was applied in this work to find out the mechanical response of thin film PMMA (100–200 nm) and Epoxy interfaces of different interfacial strengths. Interfaces of different strengths were obtained by exposing the film‐substrate system to different service conditions. It has been observed from this study that pile‐up plays a major role in finding out the mechanical response of the interfaces of different strengths. The hardness was observed to increase as the interfacial strength reduces. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43532.     

Enhancement of the electrical properties of poly(p‐phenylene vinylene) by the incorporation of silicon dioxide nanoparticles

The electrical properties of a poly(p‐phenylene vinylene) (PPV) conjugated polymer using silver (Ag) as a cathode were improved by the incorporation of silicon dioxide (SiO₂) nanoparticles. The current density of the Ag–PPV/SiO₂ nanocomposite system was higher than that of Ag–PPV. A lower level of interfacial oxidation was found in the Ag–PPV/SiO₂ nanocomposite than in Ag–PPV, confirming that a more complete elimination of residue occurred in the nanocomposite. This was due to the relatively large surface area of the PPV/SiO₂ nanocomposite film and the hydrophilic surface of the SiO₂ nanoparticles. The lower level of oxidation contributed to an improvement in the material's current–voltage characteristics. Morphology‐dependent current–voltage characteristics were enhanced by a large variation in the thickness of the Ag–PPV/SiO₂ nanocomposite film because an increased effective field strength could be induced in the thinner regions of the film. The incorporation of SiO₂ nanoparticles altered the effective film thickness and the amount of residue in the interior of the PPV without disrupting the structure of the conjugated polymer. The Ag cathode created a stable interface with the PPV film layer without causing the formation of an organic–metal complex, which would have obstructed electron injection. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Amorphous Ge‐Sb‐Se thin films fabricated by co‐sputtering: Properties and photosensitivity

Amorphous Ge–Sb–Se thin films were fabricated by a rf‐magnetron co‐sputtering technique employing the following cathodes: GeSe₂, Sb₂Se₃, and Ge₂₈Sb₁₂Se₆₀. The influence of the composition, determined by energy‐dispersive X‐ray spectroscopy, on the optical properties was studied. Optical properties were analyzed based on variable angle spectroscopic ellipsometry and UV‐Vis‐NIR spectrophotometry. The results show that the optical bandgap range 1.35‐2.08 eV with corresponding refractive index ranging from 3.33 to 2.36 can be reliably covered. Furthermore, morphological and topographical properties of selenide‐sputtered films studied by scanning electron microscopy and atomic force microscopy showed a good quality of fabricated films. In addition, structure of the films was controlled using Raman scattering spectroscopy. Finally, irreversible photoinduced changes by means of change in optical bandgap energy and refractive index of co‐sputtered films were studied revealing the photobleaching effect in Ge‐rich films when irradiated by near‐bandgap light under Ar atmosphere. The photobleaching effect tends to decrease with increasing antimony content.     

Investigation of plasma polymerized benzene and furan thin films for application in opto‐electronic devices

Conjugated polymers are among the most promising organic materials for opto‐electronic devices. In such applications, the main fabrication problem is to get uniform, defect‐free, and reproducible thin films of these materials. In this investigation, an RF plasma reactor was used to produce cross‐linked organic thin films from benzene and furan precursors. Uniform thin films of desired thickness were fabricated using this plasma polymerization technique. The composition of the plasma‐polymerized films was determined with X‐ray photoelectron spectroscopy. Fourier transform infrared spectra of the monomers and plasma‐polymerized thin films prepared were compared to analyze the chemical structure of the films. Ultraviolet–visible absorption spectroscopy shows a red shift of 45 nm in λ_max for the case of plasma‐polymerized benzene films and 52 nm in the case of plasma‐polymerized furan films when compared to their respective monomer spectra. Photo luminescence spectra of these films show a blue emission with a broad peak at 460 nm for the plasma‐polymerized benzene films and 445 nm for the plasma‐polymerized furan films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 523–528, 2003     

Electrical properties of dye‐doped colour tunable organic light emitting diode

In this work, we report on the electrical properties of dye‐doped colour tunable organic light‐emitting diode (OLED). The device structure is glass substrate/indium tin oxide/N,N′‐di(naphthalen‐1‐yl)‐N,N′‐diphenyl‐benzidine (NPB) 30 nm/Alq₃:DCM 50 nm/Aluminum (Al) 150 nm where NPB is the hole transport layer. Alq₃:DCM is the emitting layer which made of tris(8‐hydroxyquinoline) aluminium (Alq₃) doped with 4‐(Dicyanomethylene)‐2‐methyl‐6‐(4‐dimethyl‐aminostyryl)‐4H‐pyran (DCM) organic dye. The influence of doping concentration has been investigated by current density–voltage measurement, luminance intensity–voltage characteristic, electroluminescence (EL) and impedance spectroscopy, respectively. The EL spectrum exhibits the shifted of peak position from green to red region. The threshold voltage of the device decreased at the low DCM doping concentration (1 wt.%), in contrast, when the increase in the doping concentrations then the threshold voltage will be increased. The highest luminance intensity and lowest turn‐on voltage of OLED can be observed at doping concentration about of 1 wt.% of DCM. The impedance characteristics of the dye‐doped OLED can be modelled by simply adopting the conventional equivalent circuit with the simple combination of resistors and capacitors network. © 2012 Canadian Society for Chemical Engineering