Blend Thin Films Research Articles

Annealing effect on performance and morphology of photovoltaic devices based on poly(3‐hexylthiophene)‐b‐poly(ethylene oxide)

AbstractNovel block copolymers, poly(3‐hexylthiophene)‐b‐poly(ethylene oxide) (P3HT‐b‐PEO) were synthesized via Suzuki coupling reaction of P3HT and PEO homopolymers. The copolymers were characterized by NMR, gel permeation chromatography, differential scanning calorimeter, and UV–vis measurements. A series of devices based on the block copolymers with a fullerene derivative were evaluated after thermal or solvent annealing. The device using P3HT‐b‐PEO showed higher efficiency than using P3HT blend after thermal annealing. Phase‐separated structures in the thin films of block copolymer blends were investigated by atomic force microscopy to clarify the relationship between morphologies constructed by annealing and the device performance. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Investigating Morphology and Stability of Fac‐tris (2‐phenylpyridyl)iridium(III) Films for OLEDs

AbstractStable film morphology is critical for long‐term high performance organic light‐emitting diodes (OLEDs). Neutron reflectometry (NR) is used to study the out‐of‐plane structure of blended thin films and multilayer structures comprising evaporated small molecules. It is found that as‐prepared blended films of fac‐tris(2‐phenylpyridyl)iridium(III) [Ir(ppy)3] in 4,4′‐bis(N‐carbazolyl)biphenyl (CBP) are uniformly mixed, but the occurrence of phase separation upon thermal annealing is dependent on the blend ratio. Films comprised of the ratio of 6 wt% of Ir(ppy)3 in CBP typically used in OLEDs are found to phase separate with moderate heating while a higher weight percent mixture (12 wt%) is found to be stable. Furthermore, it is found that thermal annealing of a multilayer film comprised of typical layers found in efficient devices ([tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA)/Ir(ppy)3:CBP/bathocuproine (BCP)]) causes the BCP layer to become mixed with the emissive blend layer, whereas the TCTA interface remains unchanged. This significant structural change causes no appreciable difference in the photo­luminescence of the stack although such a change would have a dramatic effect on the charge transport through the device, leading to changes in performance. These results demonstrate the effect of thermal stress on the delicate interplay between the chemical composition and morphology of OLED films.

High-Resolution Chemical Identification of Polymer Blend Thin Films Using Tip-Enhanced Raman Mapping

Nanoscale chemical identification is required for the analysis of functional materials with fine structures. For the first time, highresolution tip-enhanced Raman mapping (TERM) was applied to a polymer system: poly(methyl methacrylate) (PMMA)/ poly(styrene-co-acrylonitrile) (SAN) thin films. Before TERM measurements were performed, the linear enhancement of tipenhancedRamanspectroscopy(TERS)wasoptimizedintermsofmaximumRamanintensity.Upto15timesoflinearenhancement wasobtained,ascomparedtothe(conventional)confocalRamanintensity.TheenhancementfactorofTERSisgreaterthan1500if taking the 100 times smaller probing area into account. As a result, a short exposure time was sufficient for high-resolution TERM measurements. Using TERM, the phase separation behavior of PMMA/SAN thin films was monitored by chemical recognition of local composition. The interface width (∼200 nm) at the early stage of phase evolution was visualized. The comparison of TERM images at different stages of the phase separation process revealed an unexpected transition of PMMA from the dispersed phase to the continuous phase. This morphology transition of PMMA/SAN is briefly discussed in terms of the glass transition temperature, interface, and surface tension.

Self-assembly behavior and photoluminescence property of bispyrenyl-POSS nanoparticle hybrid

Two flexible ether bonds were designed to connect two pyrene rings on a polyhedral oligomeric silsesquioxane (BPy-POSS) to enrich the fraction of “intrinsic intramolecular pyrene-dimer” on the surface of crystal isobutyl-POSS (iBu-POSS) thin-films. Compared to the monomer emission of 1-pyrenemethanol (Py-OH), the emission spectra of BPy-POSS in dichloromethane show the large proportion of intramolecular and intermolecular excimers due to the formation of pyrenyl dimers or aggregates via the easy rotation of two adjacent ether bonds and the π–π interaction of pyrene rings, respectively. By blending inert iBu-POSS, the fluorescent dimers or aggregates of 5 wt.% and 20 wt.% BPy-POSS are distributed on the surface of iBu-POSS crystal fractal pattern as shown by confocal photoluminescence microscopy. Upon exposure to the vapors of nitrobenzene, the 5 wt.% BPy-POSS blend shows the similar quenching efficiency as 100 wt.% BPy-POSS blends, indicating the better excimer dispersion for vapor permeability of blend thin-films.

Evolution of Structure, Optoelectronic Properties, and Device Performance of Polythiophene:Fullerene Solar Cells During Thermal Annealing

AbstractWe use spectroscopic ellipsometry to study the evolution of structure and optoelectronic properties of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) photovoltaic thin film blends upon thermal annealing. Four distinct processes are identified: the evaporation of residual solvent above the glass transition temperature of the blend, the relaxation of non‐equilibrium molecular conformation formed through spin‐casting, the crystallization of both P3HT and PCBM components, and the phase separation of the P3HT and PCBM domains. Devices annealed at 150 °C for between 10 and 60 min exhibit an average power conversion efficiency of around 4.0%. We find that the rate at which the P3HT/PCBM is returned to room temperature is more important in determining device efficiency than the duration of the isothermal annealing process. We conclude that the rapid quenching of a film from the annealing temperature to room temperature hampers the crystallization of the P3HT and can trap non‐equilibrium morphological states. Such states apparently impact on device short circuit current, fill factor and, thus, operational efficiency.

Blue emitting amino-DPQ with PS and PMMA phosphors for organic light emitting diode

New conjugated polymer, 2(4′-aminophenyl)-4-phenylquinoline i.e., amino-DPQ have been synthesized by Friedlander condensation method at 140 °C. The Optical, structural and thermal properties of the amino-DPQ has been studied. The blended thin films of amino-DPQ with PMMA and PS at different wt% concentrations have been prepared. The UV–vis absorption spectra could provide a good deal of information on the electronic structures of the polymeric compound. When powder form of amino-DPQ is excited at 385 nm, it emits intense blue light of wavelength 486 nm. The blended thin films of amino-DPQ with PMMA and PS at different wt% concentrations show the emission peak in the range of 415–450 nm. Hence, the blue emitting 2(4′-aminophenyl)-4-phenylquinoline (amino-DPQ) has generated considerable interest owing to their good photoluminescence efficiencies.

Continuous-wave photoinduced absorption studies in polythiophene and fullerene blended thin films

The generation and recombination dynamics of photocarriers (polarons) in poly(3-hexylthiophene) and $[6,6]$-phenyl-${\mathrm{C}}_{61}$-butyric acid methyl ester blended thin films have been investigated by means of continuous-wave photoinduced absorption spectroscopy. Continuous-wave photoinduced absorption spectroscopy has the advantage of being used to investigate long-lived excited species with lifetimes that range from microseconds to milliseconds. In this time range, three kinds of excited species are recognized in the blends: trapped, mobile, and delocalized polarons. From the modulation-frequency dependence, the mean lifetimes of the three types of polarons at low temperature were estimated. It is known that polarons are not generated directly by the pump beam but rather through Coulombically bound radical pairs. We have found that this two-step model for polaron generation can be recognized on a microsecond time scale by continuous-wave photoinduced absorption measurements.

Electroluminescence Enhancement by Modifying Poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl

We report on the fabrication and characterization of light emitting diodes based on pristine or modified poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl]. The modified polyfluorene was formed by blending with the hole transport material poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(N,N′-bis(4-butylphenyl-1,1′-biphenylene-4,4′-diamine))], or end-capping with dimethylphenyl. The addition of the hole-transporting polymer to the host polymer or the attachment of the end-capper to the polymer chain improved device properties. Phase-separated domains in blend thin film morphology were observed in scanning electron microscope pictures. These domains may play a role in the improved device performance. The introduction of the hole-transporting polymer or the end-capper also improved blue emission.

Solvent Annealing Induced Perpendicular Orientation of Cylindrical Microdomains in Polystyrene-b-poly(4-hydroxyl styrene)/PEG Oligomer Blend Thin Film Made by Spin-coating from Selective Solvent

The microphase separated structure of PS-b-PHS/PEG blend thin film with thickness of 500 ~ 600 nm was investigated by grazing incidence small angle X-ray scattering. The thin film was obtained by two different solutions; one was THF which was common good solvent for all components of polymers used here. The other is toluene which was selective solvent for PS and poor-solvent for PHS and PEG. The equilibrium morphology of the block copolymer and blend sample was hexagonally packed cylinder in the bulk and thin film. The structure in the thin film obtained by spin cast from toluene solution was non-equilibrium. After THF vopar annealing of the thin film (cast from toluene), the highly ordered and perpendicular oriented cylindrical structure was obtained. Perpendicular orientation was failure when the thin film sample made by spin cast from THF solution and subsequent THF vapor annealing. The perpendicular nano-holes were fabricated after removing PEG oligomer by washing with water.

Phase behaviour of liquid-crystalline polymer/fullerene organic photovoltaic blends: thermal stability and miscibility

The thermal behaviour of an organic photovoltaic (OPV) binary system comprised of a liquidcrystalline fluorene-based polymer and a fullerene derivative is investigated. We employ variabletemperature ellipsometry complemented by photo- and electroluminescence spectroscopy as well as optical microscopy and scanning force nanoscopy to explore phase transitions of blend thin films. The high glass transition temperature correlates with the good thermal stability of solar cells based on these materials. Furthermore, we observe partial miscibility of the donor and acceptor together with the tendency of excess fullerene derivative to segregate into exceedingly large domains. Thus, for charge generation less adequate bulk-heterojunction nanostructures are poised to develop if this mixture is exposed to more elevated temperatures. Gratifyingly, the solubility of the fullerene derivative in the polymer phase is found to decrease if a higher molecular-weight polymer fraction is employed, which offers routes towards improving the photovoltaic performance of non-crystalline OPV blends.

3Pa2-4 Evaluation of Viscoelasticity of Polymer Blend Thin Films in High-Temperature Region by a QCM-D Method(Poster Session)

3Pa2-4 Evaluation of Viscoelasticity of Polymer Blend Thin Films in High-Temperature Region by a QCM-D Method(Poster Session)

Structure of thin film polymer/nanoparticle systems: polystyrene (PS) coated-Au nanoparticle/tetramethyl bisphenol-A polycarbonate mixtures (TMPC)

The morphologies of thin film blends of polystyrene (PS)-brush coated Au nanoparticles with tetramethyl bisphenol-A polycarbonate (TMPC) were investigated. Our results reveal that entropic effects, associated with the brush/host chain interactions, nanoparticle diameter, D, and asymmetries in monomer sizes of the host and grafted chains, can play a more important role than the favorable PS/TMPC enthalpic interactions toward determining the phase miscibility of the system. A diagram of states is constructed to show the phase separated and dispersed regimes as a function of D, N, the degree of polymerization of the grafted chains, and P, the degree of polymerization of the host chains, at a constant grafting density. These results have important implications on the design of brush coated nanoparticle/homopolymer mixtures for various applications.

Formation of Well‐Ordered Heterojunctions in Polymer:PCBM Photovoltaic Devices

AbstractThe nanoscale morphology in polymer:PCBM based photovoltaic devices is a major contributor to overall device performance. The disordered nature of the phase‐separated structure, in combination with the small length scales involved and the inherent difficulty of reproducing the exact morphologies when spin‐coating and annealing thin blend films, have greatly hampered the development of a detailed understanding of how morphology impacts photo­voltaic device functioning. In this paper we demonstrate a double nano­imprinting process that allows the formation of nanostructured polymer:PCBM heterojunctions of composition and morphology that can be selected independently. We fabricated photovoltaic (PV) devices with extremely high densities (1014 mm−2) of interpenetrating nanoscale columnar features (as small as 25 nm; at or below the exciton diffusion length) in the active layer. By comparing device results of different feature sizes and two different polymer:PCBM combinations, we demonstrate how double imprinting can be a powerful tool to systematically study different parameters in polymer photovoltaic devices.

Shape control and density multiplication of cylinder-forming ternary block copolymer-homopolymer blend thin films on chemical patterns

The effect of the chemical pattern spot size, the spacing on the size, and the shape of the cylindrical domains in thin films of a ternary block copolymer/homopolymer/homopolymer blend was investigated over a range of homopolymer volume fractions. Cylinder-forming ternary blends were composed of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA), and the corresponding PS and PMMA homopolymers were directed to assemble on chemical patterns that had density multiplication ratios ranging from 1:1 to 4:1. By increasing the homopolymer fraction in the blends, the dimensions of the domains were expanded. When the size of the spots on the chemical pattern was not matched with the size of the domain of the blend in the bulk, the dimensions of the domains at the free surface of the assembled films differed from those at the interface with the chemical pattern.

Observation of the missing mode effect in a poly-phenylenevinylene derivative: Effect of solvent, chain packing, and composition

Optical emission spectra of poly[2-methoxy-5-[3('),7(')-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) in dilute solutions exhibit a vibronic progression interval (∼1225 cm(-1)) that does not correspond to any ground state vibrational mode frequency. This phenomenon is assigned as the missing mode effect (MIME) in which five key displaced polymer backbone vibrational modes in the range of 800-1600 cm(-1) contribute to the MIME interval. Emission spectra are calculated by analytically solving the time-dependent Schrödinger equation using estimates of mode-specific vibrational displacements determined independently from preresonance Raman intensities. Emission spectra of MDMO-PPV thin films and nanoparticles are measured and lineshapes show an increase of the MIME frequency to ∼1340 cm(-1) in addition to changes in vibronic intensity distributions and energies. Composite blend thin films consisting of MDMO-PPV and a fullerene derivative (1:1 w/w) exhibit a substantially larger MIME interval (∼1450 cm(-1)) that arises from an increase in polymer chain planarity. This structural change is most apparent from large decreases of the excited state displacement of an out-of-plane C-H bending mode (961 cm(-1)) that becomes forbidden in the planar structure.

Simultaneous Surface and Bulk Imaging of Polymer Blends with X‐ray Spectromicroscopy

We demonstrate the utility of soft X-ray spectromicroscopy to simultaneously image the surface and bulk composition of polymer blend thin films. In addition to conventional scanning transmission X-ray microscopy that employs a scintillator and photomultiplier tube to measure the transmitted X-ray flux, channeltron detection of near-surface photoelectrons is employed to provide information of the composition of the first few nanometers of the film. Laterally phase-separated blends of two polyfluorene co-polymers are studied, with the structure of both wetting and capping layers clearly imaged. This new information provides insight into the connectivity of bulk and surface structures that is of particular relevance to the operation of such blends in optoelectronic devices.

Charge transport and microstructure in PFO:MEH-PPV polymer blend thin films

Current density–voltage characteristics of poly(9,9′-dihexyl fluorenyl-2,7-diyl) (PFO) thin films (∼120 nm) have been studied in hole only device configuration at different temperatures (100–290 K) in pure form and with blending (∼0.25–50 wt%) of poly(2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene) (MEH-PPV). It has been found that in the case of pure PFO the charge transport at low fields show an ohmic region which is followed by space charge limited conduction region. Blending of PFO with MEH-PPV at low ratio (up to 2 wt%) increases the current density. As the ratio is increased further, the current density decreases. Analysis show that there is a change in conduction mechanism up to 6 V from SCLC to thermally activated ohmic conduction upon blending indicating the creation of new energy level near the transport states of PFO. Morphology of the polymer blended thin films was obtained by atomic force microscopy (AFM) technique. It has been found that the surface roughness of the investigated films is significantly increased upon blending indicating aggregation as well as phase separation at high blending ratios. The decrease in conductivity at high blending ratio can be related to the change in morphology of the films.

Photoluminescent energy transfer from poly(phenyleneethynylene)s to near-infrared emitting fluorophores

Photoluminescent energy transfer was investigated in conjugated polymer-fluorophore blended thin films. A pentiptycene-containing poly(phenyleneethynylene) was used as the energy donor, and 13 fluorophores were used as energy acceptors. The efficiency of energy transfer was measured by monitoring both the quenching of the polymer emission and the enhancement of the fluorophore emission. Near-infrared emitting squaraines and terrylenes were identified as excellent energy acceptors. These results, where a new fluorescent signal occurs in the near-infrared region on a completely dark background, offer substantial possibilities for designing highly sensitive turn-on sensors. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3382–3391, 2010

Physical and Mechanical Properties of Blends Based on Poly (dl-lactide), Poly (l-lactide-glycolide) and Poly (ϵ-caprolactone)

Bioresorbable materials are extensively used for a wide range of biomedical applications. In this study, the common industrial processes of compression moulding and solvent casting were utilised for initial preparation of thin film blends based on Poly (dl-lactide), Poly (l-lactide-glycolide) and Poly (ϵ-caprolactone). Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), differential scanning calorimetry (DSC), phosphate buffered saline adsorption, tensile testing and contact angle measurement were used as a means of investigating the physical and mechanical properties of the blends.

ChemInform Abstract: Formation of Mesoporous Silica Nanotubes.

[29] M. A. van Dijk, R. van den Berg, Macromolecules 1995, 28, 6773.[30] From the morphology of the polymer blend thin film, it was alreadypointed out that there is no clear affinity for one of the two hydropho-bic phases to preferentially wet the hydrophilic silicon substrate.[31] V. Z.-H.Chan,E. L. Thomas, R. G. H.Lammertink, M. A. Hempenius,G. J. Vancso, H. Wang,R. J. Composto, unpublished.[32] Auger electron spectroscopy (AES) data obtained for oxygen etchedPFS homopolymer are consistent with the XPS results. Depth profiling(argon ion sputtering) indicated an approximately 10 nm thin layerrich of iron, silicon, and oxide at the surface of the oxygen etchedpolymer film.[33] A. B. Fischer, J. B. Kinney, R. H. Staley, M. S. Wrighton, J. Am.Chem. Soc. 1979, 101, 6501.