Optical order of the polymer phase within polymer/fullerene blend films

This study reports on how the degree of polymer order within a polymer/fullerene blend can be investigated by spectroscopic methods. Non‐annealed blend compositions with 0–80 wt % fullerene content were analyzed using temperature dependent photoluminescence (PL) and room temperature spectroscopic ellipsometry (SE) measurements. To evaluate the SE data with respect to the optical order, an optical model was developed, including a lower and higher ordered polymer phase within a fullerene matrix. This was done using an effective medium approach describing the polymer by combining lower and higher ordered polymer properties (polymer‐EMA). The polymer/fullerene blend was then evaluated using another EMA consisting of the polymer‐EMA and the dielectric function of the disordered fullerene. The degree of optical order obtained by SE, was confirmed using another independent measurement, photoluminescence spectroscopy, according to the method of Francis C. Spano (2005). The volume fraction of the ordered polymer within the polymer‐EMA was found to be between 70 and 60 vol % for fullerene contents lower than 20 wt % in the polymer/fullerene blend. Above 20 wt % fullerene, the optical order of the polymer strongly decreases all the way down to 0 vol %. In contrast to the complementary performed X‐ray diffraction measurements, which address only the long‐range structural order of the blends, we give quantitative information on the optical order, including information on the composition, that is, volume fractions of the higher and lower ordered polymer. The gained information on the tilt of the polymer molecules with respect to the substrate is discussed comparing XRD results from the literature with those obtained by our SE model. Finally, the developed model is used to describe the influence of the P3HT molecular weight on the optical order. Results obtained with our model were compared to the structural data and mobility data in the literature. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Bulk heterojunction polymer solar cells based on binary and ternary blend systems

Polymer solar cells (PSCs) were fabricated using a ternary blend film consisting two conjugated polymers and a soluble fullerene derivative as the donor and acceptor materials, respectively. And, to compare ternary blend system, the single‐component copolymers consisting of the repeating units of each of the copolymers, used in ternary blend solar cells, were designed and synthesized for use as the electron donor materials in binary blend solar cells. We systematically investigated the field‐effect carrier mobilities and the optical, electrochemical, and photovoltaic properties of the copolymers. Under optimized conditions, the binary blend polymer systems showed power conversion efficiencies (PCEs) for the PSCs in the range 3.87–4.16% under AM 1.5 illumination (100 mW cm^?2). All polymers exhibited similar PCEs that did not depend on the ratio of repeating units. The binary blend solar cell containing a single‐component copolymer as the electron donor material performed better than the ternary blend solar cell in this work. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Birefringence control by hydrogen bonding on compatible polymer pair composed of hydrogenated ring‐opening polymer and modified polystyrene

A new polymer blend composed of a hydrogenated ring‐opening polymer (HROP) with an ester group and hydroxyl functionalized polystyrene (HFP) produced the excellent transparent materials which enabled a precise birefringence control in keeping with the other physical properties for optical film use. The blend with a composition from 0.28 to 0.35 for the HFP weight fraction showed an extraordinary wavelength dispersion, transmitting through a zero birefringence point at the critical fraction of 0.45, while each polymer showed an ordinary wavelength dispersion. The observed excellent transparency even above those of the glass transition temperature was attributed to a depressed phase separation that resulted from strong hydrogen bond between the ester and hydroxyl groups. An IR analysis of the film demonstrated a remarkable red‐shift in the carbonyl peak with an increase of the hydroxylated polystyrene content, indicating a strong hydrogen bond between those groups. This new polymer blend provides a useful design to achieve practical demands for film use, both optical and mechanical under the fabrication conditions using the melt extrusion technique. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3132–3143     

Characterization and Distribution of Poly(3‐hexylthiophene) Phases in an Annealed Blend Film

The characteristic absorption spectra of three kinds of phases, the isolated, ordered, and disordered phases, in a solvent‐vapor annealed poly(3‐hexylthiophene)/[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (P3HT/PCBM) blend film were studied by means of spectroelectrochemistry (SEC) and time‐resolved absorption spectroscopy (TAS). The results reveal that the content of three phases are 12 % isolated, 37 % ordered, and 51 % disordered for the annealed P3HT neat film, and 25 % isolated, 31 % ordered, and 44 % disordered for the annealed P3HT/PCBM blend film. The vertical distribution of the different phases in the blend film was studied by SEC, and the results show that the ordered and isolated phases are mainly distributed in the top and in the bottom of the annealed films, respectively, while the disordered phase is mainly distributed in the middle and the bottom of the films.     

Influence of fullerene photodimerization on the PCBM crystallization in polymer: Fullerene bulk heterojunctions under thermal stress

For an increased lifetime of polymer:fullerene bulk heterojunction (BHJ) solar cells, an understanding of the chemical and morphological degradation phenomena taking place under operational conditions is crucial. Phase separation between polymer and fullerene induced by thermal stress has been pointed out as a major issue to overcome. While often the effect of thermal stress on the morphology of polymer:fullerene BHJ is investigated in the darkness, here we observe that light exposure slows down fullerene crystallization and phase separation induced at elevated temperatures. The observed photo‐stabilizing effect on active layer morphology is quite independent on the polymer and is attributed to light‐induced dimerization of the fullerene. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1209–1214     

Dual effects of fullerene doped to holographic polymer dispersed liquid crystals

Doping of conductive fullerene particles to the formulation of conventional holographic polymer dispersed liquid crystal‐induced dual effects of reducing both droplet coalescence and operating voltage. Fullerene induced an induction period which otherwise does not exist, followed by a gradual increase of diffraction efficiency to a saturation value being increased with increasing fullerene content. The increased diffraction efficiency was caused by the decreased droplet coalescence which was due to the hindered migration of LC by the fullerene particles. On the other hand, doped fullerene particles augmented the conductivity of polymer phase and hence the local electrical field imposed on LC droplet, which overcome the threshold for driving and reduced operating voltage and response times. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5590–5596, 2007     

Controlled self‐assembly of porphyrin/fullerene donor–acceptor complex in a polymer thin film

Thin films composed of polycyclohexane (PCHE), zinc(II)‐5,10,15,20‐tetra‐(2‐naphthyl)porphyrin (ZnTNpP), and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) blends are prepared to investigate their potential for the controlled self‐assembly of a porphyrin/fullerene donor–acceptor complex in a polymer thin film. The compatibilities of PCHE/PCBM (p), PCHE/ZnTNpP (q), and ZnTNpP/PCBM (r) in these blends have a significant effect on the dispersion of the ZnTNpP/PCBM donor–acceptor complex in the PCHE thin film. When the compatibilities are p << q, r, and q ≈ r, the ZnTNpP/PCBM donor–acceptor complex is formed between the PCHE and PCBM phases. This concept to form a controlled self‐assembly of the ZnTNpP/PCBM donor–acceptor complex may be applied to various combinations of porphyrin/fullerene systems in polymer thin film solar cells to achieve excellent performance. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 743–746     

3D‐morphology reconstruction of nanoscale phase‐separation in polymer memory blends

In many organic electronic devices functionality is achieved by blending two or more materials, typically polymers or molecules, with distinctly different optical or electrical properties in a single film. The local scale morphology of such blends is vital for the device performance. Here, a simple approach to study the full 3D morphology of phase‐separated blends, taking advantage of the possibility to selectively dissolve the different components is introduced. This method is applied in combination with AFM to investigate a blend of a semiconducting and ferroelectric polymer typically used as active layer in organic ferroelectric resistive switches. It is found that the blend consists of a ferroelectric matrix with three types of embedded semiconductor domains and a thin wetting layer at the bottom electrode. Statistical analysis of the obtained images excludes the presence of a fourth type of domains. The criteria for the applicability of the presented technique are discussed. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1231–1237     

Improved phase separation in polymer solar cells by solvent blending

The effect of solvent blending on the performance of an anthracene‐containing poly(p‐phenylene‐ethynylene)‐alt‐poly(p‐phenylene‐vinylene) backbone‐based donor polymer with asymmetrically substituted branched 2‐ethylhexyloxy and methyloxy side‐chains in bulk heterojunction solar cells is reported. This copolymer yields relatively high open‐circuit voltages with fullerene‐based electron acceptors. We systematically studied the thin‐film blend morphology and solar cell performance as a function of solvent composition (chlorobenzene to chloroform ratio) and polymer to [6,6]‐phenyl C61‐butyric acid methylester (PCBM) ratio. We combined photophysical investigations with atomic force microscopy and grazing incidence wide‐angle X‐ray scattering to elucidate the solid‐state morphology in thin films. In the investigated polymer system, the blend morphology becomes independent of the supporting solvent for high PCBM concentrations. Deposition from solvent blends rather than from pure chlorobenzene facilitates the beneficial phase separation between polymer and PCBM, leading to improved charge transport properties (short‐circuit currents) at lower PCBM concentrations. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013, 51, 868–874     

Highly polarized luminescence from optical quality films of a semiconducting polymer aligned within oriented mesoporous silica

In this Communication, we show that nanometer scale control of semiconducting polymer chain conformation is possible using host/guest chemistry in highly ordered and macroscopically oriented thin films of mesoporous silica. This control leads to a thin film composite material that is optically transparent, densely filled with polymer, and has highly polarized optical properties. Calculations of absorption and emission anisotropies further indicate full incorporation of the polymer into the nanoscale pore spaces. Such materials could serve as a useful tool for further investigations of polymer photophysics, as well as for device applications.     

Mechanical properties of emulsion polymer blends

Polymer blend technology has been one of the most investigated areas in polymer science in the past 3 decades. The one area of polymer blends that has been virtually ignored involves simple emulsion blends, although several articles have recently appeared that address film formation and mechanical characteristics. In this study, we investigated the mechanical property behavior of emulsion blends composed of low/high‐glass‐transition‐temperature polymers (where low and high mean below and above the test temperature, respectively). The emulsions chosen for this study had similar particle sizes, and the mixtures were rheologically stable. Two conditions were chosen, a binary combination of polymers that were thermodynamically immiscible and another system that was thermodynamically miscible. The mechanical property results over the entire composition range were compared with the predictions of the equivalent box model (EBM) with the universal parameters predicted by percolation theory. An array of randomly mixed and equal‐size particles of differing moduli was expected to show excellent agreement with theory, and the emulsion blends provided an excellent experimental basis for testing the theory. For the immiscible blend, the EBM prediction for the modulus showed excellent agreement with experimental results. With tensile strength, the agreement between the modulus and theory was good if the yield strength for the higher glass‐transition‐temperature polymer was employed in comparison with the actual tensile strength. The phase inversion point (where both phases were equally continuous) was at a 0.50 volume fraction of each component (based on an analysis employing Kerner's equation), just as expected for a random mixture of equal‐size particles. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1093–1106, 2001     

Nanoorganization and Phase Behavior of Mixtures of Nicotinic Acid with Dodecylbenzenesulfonic Acid

Abstract

Mesomorphically ordered structures and phase behavior of the mixtures of nicotinic acid (NICA) and dodecylbenzenesulfonic acid (DBSA) were investigated by X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and polarized optical microscopy (POM). The POM observations revealed that the NICA–DBSA mixtures spontaneously formed liquid crystalline phases, although both NICA and DBSA were not liquid crystalline molecules. The NICA–DBSA mixtures formed ordered lamellar structures in DBSA‐rich mixtures and hexagonal cylinder structure in NICA‐rich mixtures. The mesomorphically ordered structures and optical anisotropy were caused by hierarchical interactions in the NICA–DBSA mixtures. The phase diagram divided into five regions—optically isotropic disordered phase, optically isotropic lamellar phase, optically anisotropic lamellar phase, optically anisotropic cylinder phase, and crystalline solid phase—is drawn by summarizing the XRD and POM results.     

Characterization of chemical heterogeneity in polymer systems using hydrolysis and tapping‐mode atomic force microscopy

Characterization of polymer coatings microstructure is critical to the fundamental understanding of the corrosion of coated metals. An approach for mapping the chemical heterogeneity of a polymer system using chemical modification and tapping‐mode atomic force microscopy (TMAFM) is demonstrated. This approach is based on the selective degradation of one of the phases in a multiphase polymer blend system and the ability of TMAFM to provide nanoscale lateral information about the different phases in the polymer system. Films made of a 70:30 polyethyl acrylate/polystyrene (PEA/PS) blend were exposed to a hydrolytic acidic environment and analyzed using TMAFM. Pits were observed to form in the PEA/PS blend films, and this degradation behavior was similar to that of the PEA material. Using these results, the domains in the 70:30 blend were identified as the PS‐rich regions and the matrix as the PEA‐rich region. This conclusion was confirmed by Fourier transform infrared‐attenuated total reflection analyses that revealed the hydrolysis of the PEA material. TMAFM phase imaging was also used to follow pit growth of the blend as a function of exposure time. The usefulness of the chemical modification/AFM imaging approach in understanding the degradation process of a coating film is discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci B Part B: Polym Phys 39: 1460–1470, 2001     

Multilayer films composed of conductive poly(3‐hydroxybutyrate)/carbon nanotubes bionanocomposites and a photoresponsive conducting polymer

In order to develop new electronic devices, it is necessary to find innovative solutions to the eco‐sustainability problem of materials as substrates for circuits. We realized a photoresponsive device consisting of a semiconducting polymer film deposited onto optically semitransparent and conductive biodegradable poly(3‐hydroxybutyrate) (PHB)/carbon nanotube (CNT) substrates. The experiments indicated that the PHB‐CNT bionanocomposite substrate behaves as an optical window trapping electric charges produced by the photoexcitation of the semiconducting polymer. Such PHB‐CNT functional substrates are expected to be attractive for eco‐friendly electronics. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 596–602     

Competition between the charge transfer state and the singlet states of donor or acceptor limiting the efficiency in polymer:fullerene solar cells

We study the appearance and energy of the charge transfer (CT) state using measurements of electroluminescence (EL) and photoluminescence (PL) in blend films of high-performance polymers with fullerene acceptors. EL spectroscopy provides a direct probe of the energy of the interfacial states without the need to rely on the LUMO and HOMO energies as estimated in pristine materials. For each polymer, we use different fullerenes with varying LUMO levels as electron acceptors, in order to vary the energy of the CT state relative to the blend with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). As the energy of the CT state emission approaches the absorption onset of the blend component with the smaller optical bandgap, E(opt,min) ≡ min{E(opt,donor); E(opt,acceptor)}, we observe a transition in the EL spectrum from CT emission to singlet emission from the component with the smaller bandgap. The appearance of component singlet emission coincides with reduced photocurrent and fill factor. We conclude that the open circuit voltage V(OC) is limited by the smaller bandgap of the two blend components. From the losses of the studied materials, we derive an empirical limit for the open circuit voltage: V(OC) ? E(opt,min)/e - (0.66 ± 0.08)eV.     

Vertical phase separation of conjugated polymer and fullerene bulk heterojunction films induced by high pressure carbon dioxide treatment at ambient temperature

The morphology of bulk-heterojunctions (BHJ) is critically important for conjugated polymer and fullerene blend solar cells. To alter the morphology, high pressure (gas phase) carbon dioxide (CO(2)) treatment is applied to poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blend films under ambient temperature. This process can achieve vertically phase separated morphology such that PCBM distributes toward the film surface, which is suggested by secondary ion mass spectroscopy (SIMS), contact angle, X-ray photoelectron spectroscopy (XPS) and cross-sectional scanning electron microscope (SEM) studies. While pristine P3HT films do not show a significant change upon CO(2) treatment, pristine PCBM films are plasticized in high pressure CO(2). Thus, PCBM is selectively plasticized by CO(2) in the blend film and is drawn towards the surface due to depressed surface energy, although P3HT tends to distribute around the surface without CO(2). This stratification process can enhance solar cell performance. 55% improvement is achieved in the power conversion efficiency of the CO(2) treated device compared to the untreated one, indicating that CO(2) treatment can be a good candidate for optimizing the morphology and enhancing the performance of BHJ polymer solar cells.     

In situ thermo-optical studies of polymer:fullerene blend films

Optical properties of a blend thin film (1:1 wt) of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) exposed to a stepwise heating and cooling, have been reported and compared with the properties of pure PCBM and P3HT films. The UV–Vis(T) absorption measurements were performed in situ, during annealing and cooling runs, at the precisely defined temperatures, in a range of 20–210 °C. It was demonstrated that this new method allows to observe the changes of absorption coefficient spectra and absorption edge parameters: the energy gap (E_G) and the Urbach energy (E_U), connected with the length of conjugation and structural disorder of thin film, respectively. Several stages, during annealing/cooling runs, were distinguished for the P3HT:PCBM blend film and related to the following processes, as an increase of P3HT crystallinity in the blend, the orderly stacking of polymer chains, thermally induced structural defects and the phase separation, caused by an aggregation of PCBM in the polymer matrix. These changes were also observed on the P3HT:PCBM film surface, by means to the microscopic studies.     

Design of bicontinuous donor/acceptor morphologies for use as organic solar cell active layers

In recent works, we demonstrated the achievement of bicontinuous donor/acceptor morphologies by the addition of conjugated block copolymers to a blend of conjugated homopolymer donors and fullerene acceptors. However, the domain sizes resulting in experiments were much larger than those of interest for high‐performance organic solar cells. Moreover, a significant concentration of fullerene acceptors was present in the donor domains. Here, we utilize simulations to study the bicontinuous donor/acceptor morphologies that result for different parametric conditions. Using such results, we provide guidelines for how to blend polymer materials to give rise to bicontinuous phases with the smaller and more compositionally pure domains that are desirable for organic photovoltaic applications. Our results can be generalized to treat a large range of donor and acceptor monomers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 884–895     

Optical absorption of the β‐phase of poly(9,9‐dioctylfluorene)

We outline a theory for the optical absorption of the β‐phase of poly(9,9‐dioctylfluorene) (PFO) that is based on the Frenkel exciton model. The absorption peak at 435 nm is attributed to polymer segments having torsion angles equal to π that are weakly perturbed by the presence of random monomer junctions with torsion angles equal to 0. The broad band below 435 nm is associated with disordered segments having a broad distribution of random torsion angles. The effects of small random deviations from π in the torsion angles are discussed. The calculations support the interpretation that the β‐phase is characterized by alternating segments of highly ordered and strongly disordered regions. PFO is a widely studied fluorene‐based polymer with interesting and potentially useful photophysical properties. In this work, Frenkel exciton states in the β‐phase of PFO are studied, and a two‐region model—weak torsional disorder and strong torsional disorder—is presented. The peak in the optical absorption at 435 nm is associated with the weakly disordered regions. The broad background in the absorption is attributed to the strongly disordered regions. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1109–1111     

Preparation of a unique microporous structure via two step phase separation in the course of drying a ternary polymer solution

A unique porous polymeric film was prepared by drying a ternary polymer solution: a polystyrene (PS), polyethylene glycol (PEG), and toluene solution. Highly ordered micropores, ranging from 5 to 12 mum in diameter, were formed on the film surface, and the rim of each micropore was surrounded by a ring of PEG. The effects of the weight ratio of the polymer blend and molecular weight of the polymer (PEG) on the porous structure were investigated. Based on in situ visual observation and light scattering measurements, the formation mechanism of the porous structure was speculated to be a two step phase separation: the phase separation into PEG-rich and PEG-poor (i.e., PS-rich) phases occurred first at the surface area of the ternary solutions, where polymers were condensed due to solvent evaporation. The PEG-rich phase became droplets and had an ordered structure on the surface. The PEG-poor phase became a matrix where PS and solvent coexisted as a single phase solution. Secondary phase separation then followed in the PEG droplets, which was induced by further solvent evaporation, and formed into solvent-rich and PEG-rich domains within the droplets. Solvent evaporation and secondary phase separation created a cavity structure in each PEG droplet structured on the film surface.