Session B48: Electrically and Optically Active Polymers

Micellar Electrolytes in Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) are promising for applications in sensing and bioelectronics. OECTs consist of a conducting polymer film (transistor channel) in contact with an electrolyte. A gate electrode immersed in the electrolyte controls the doping/dedoping level of the conducting polymer. OECTs can be operated in aqueous electrolytes, making possible the implementation of organic electronic materials at the interface with biology. The inherent signal amplification of OECTs has the potential to yield sensors with low detection limits and high sensitivity. In this talk we will present recent studies on OECTs using ionic surfactants (such as hexadecyl-trimethyl-ammonium bromide) as electrolytes. As the conducting polymer we used PEDOT:PSS, i.e. (Poly,3-4 ethylenedioxythiopene) doped with Poly(styrene sulphonate). Interestingly, ionic surfactant electrolytes result in large transistor current modulation, especially beyond the critical micellar concentration (CMC). Since micelles play a primary role in biological processes and drug-delivery systems, the use for micellar electrolytes opens new exciting opportunities for the use of OECTs in bioelectronics.   

Quadratic Electro-Optic Effect in the Nonconjugated Conductive Co-polymer Iodine-doped Styrene-Butadiene-Rubber Measured at 633 nm and 1550 nm

The quadratic electro-optic effect in the nonconjugated conductive \textit{co-polymer} film of styrene-butadiene-rubber (SBR) has been measured using field-induced birefringence method. Thin films of styrene-butadiene-rubber have been prepared on various substrates from a chloroform solution and characterized using optical absorption spectroscopy, FTIR and DSC before and after doping with iodine. The optical absorption spectrum at low doping shows two peaks: one at 4.27 eV and the other at 3.2 eV corresponding to the radical cation and charge-transfer transition. FTIR data indicate =C-H vibration bands (964 cm$^{-1}$ and 910 cm$^{-1})$ of polybutadiene decrease upon doping due to transformation of the double bonds into radical cations. The Kerr coefficients as measured at 633 nm and at 1550 nm are 3.1x10$^{-10}$ m/V$^{2}$ and 1.3x10$^{-10}$ m/V$^{2}$ respectively. These exceptionally large values have been attributed to the subnanometer metallic domains formed upon doping and charge-transfer involving isolated double-bonds.   

Photoluminescence of P3HT nanoparticles

Polythiophenes are semiconducting polymers that have been designed to crystallize. The photophysics of semicrystalline polythiophene and polythiophene-blends are the focus of intense research efforts across different disciplines. In these systems there is a competition between charge separation and recombination. Exciton diffusion length in organic-semiconductors is a major road-block for efficient solar energy harvesting devices since, for direct bandgap organic materials, this distance is about 10 nanometers. Thus, efficient extraction of photogenerated electrons and holes requires engineering polymer domain dimensions in this size range. In our initial investigations of the photophysics of isolated P3HT nanoparticles (15 - 130 nm), we have observed several intriguing size-dependent features in the single-particle photoluminescence (PL) connected with exciton diffusion and dissociation dynamics. In addition to the short-time behavior, we also observe size-dependent differences in PL decay at long times. In the 10 - 100 ns time regime, the PL originates not from radiative transitions of bound excitons, but rather from charge-separation followed by bi-polaron recombination--and thus provides an interesting measure of exciton fission probability within the nanoparticle.   

Crystallizing Conjugated Polymer Chains of P3HT Stretched for Dramatic Enhancements in Optoelectronic Efficiencies

Previously the amorphous conjugated polymer MEH-PPV was shown to illustrate huge enhancement in photoluminescence (PL) efficiencies upon stretching to large molecular strains. In this work the crystallizing polymer of poly(3-hexylthiophene) (P3HT), dispersed in the polystyrene (PS) matrix, was stretched in the PS local deformation zones to study the effect of molecular deformation. A huge enhancement of the PL intensity was observed that when normalized to the fraction of the strained polymer corresponded to an increase of 15 folds, significantly larger than that of the stretched MEH-PPV. Moreover, when examined under a con-focal micro-PL (spot size $\sim$5$\mu$m), the emission from the local deformation zones subject to a high stress ($\sim$40MPa) manifested marked increase of the intrachain emission relative to the effect on the interchain. These emission peak positions, however, were unaffected by the stretching. The PL enhancements were attributed to the depression of electron-phonon interactions of the stretched P3HT chains. Constraining conjugated polymers to yield high efficiencies thus may provide a feasible way for improving the performance of polymer-based devices.   

Manipulating meso-structure and electrical conductivity in polymer-acid doped polyaniline by exploiting redox chemistry

Template synthesis of polyaniline on poly(2-acrylamido-2-methyl-1-propane sulfonic acid) yields electrostatically stabilized particles that can be aqueously dispersed and cast into thin films; electrical conductivity in these films scales with inter-particle connectivity. Solvent annealing with dichloroacetic acid induces structural relaxation of the polymer acid, thereby eliminating the particulate nature of thin films and consequently increasing their conductivity by up to two orders of magnitude (from 0.4 to 40 S/cm). Alternatively, the electrostatic interactions between polyaniline and its template can be neutralized through chemical reduction with hydrazine monohydrate, after which the polymer acid can be plasticized by water vapor to encourage structural relaxation. Exposure to nitric oxide leads to oxidation of polyaniline and concurrent reassociation with its polymer acid dopant. Enhanced conductivity is observed following this redox process, and is attributed to extensive polymer chain relaxation and the simultaneous elimination of the particulate nature of template-synthesized polyaniline.   

An Integrated Electrochromic Nanoplasmonic Optical Switch

We describe an electrochemically-driven optical switch based on absorption modulation of surface plasmon polaritons (SPPs) propagating in a metallic nanoslit array waveguides containing the electrochemical polymer polyaniline (PANi). Optical transmission modulation of near 100{\%} is achieved by electrochemically switching PANi between oxidized and reduced states using voltages below 1 V. High spatial overlap and long interaction length between the SPP and the active material are achieved by preferential growth of PANi on the nanoslit sidewalls. The resulting orthogonalization between the directions of light propagation, and that of charge transport from the electrolyte to ultra-thin active material inside the nanoslit waveguide offers significant promise for the realization of electrochromic devices with record switching speeds.   

Resistive Switching in Ag Nanowire/Polymer Composite Materials

Bulk composites of electrically conductive nanoparticles within an insulating polymer matrix are insulating when the conductive particle concentration is below the electrical percolation threshold and conductive above it. However, we have observed reversible resistive switching with increasing voltage at room temperature in Ag nanowire/polystyrene composites with nanowire concentrations close to the percolation threshold. We have found the reversibility of the observed switching behavior to be temperature dependent which implies a diffusive process is involved. We propose the basis for resistive switching in these materials is the formation of field-induced filaments between adjacent nanowires that extend the percolated electrical network and increase the overall conductivity of the system. Here, we will compare our observations of resistive switching in Ag nanowire/polystyrene and Ag nanowire/poly(methylmethacrylate) bulk nanocomposites, explore the breadth of metal nanowire and polymer systems that exhibit resistive switching, and explore the underlying mechanism for filament formation.   

Far-infrared through visible optical characterization of polymer-based electrochromic devices on single-walled carbon nanotube electrodes

Electrochromic polymers (ECPs) exhibit reversible optical modulation in a wide spectral range as a function of an externally applied voltage. In this work, ECPs have been used in absorptive/transmissive electrochromic devices as candidates for smart window applications. The electrochromic devices were fabricated on flexible polyethylene substrates and used ECPs sandwiched between thin films of single-walled carbon nanotubes serving as conductive and flexible electrodes. Unlike ITO, the nanotube films are highly transmissive in the visible and infrared region of the spectrum. The transmission and reflection of the individual device components as well as assembled devices were measured over a wide spectral range (FIR to UV). The devices were switched in situ in the spectrometers. The optical constants of the constituent layers were calculated using the Drude-Lorentz model. The devices demonstrated high transmission contrasts between their colored and bleached states in the VIS, NIR, and MIR spectra, enabling electrically tunable control over the transmission or reflection of both light and heat. This control could lead to reduced heating or cooling costs in real world applications and the flexible nature of the device components allows many applications.   

Consequence of the miscibility and mesostructure of the photoactive layer on organic solar cell performance

Recent work has found that mixed phases exist in polythiophene/fullerene solar cells. Nevertheless, the consequence of miscibility between the electron donor and acceptor is not fully understood. Through model polythiophene/fullerene mixtures, we have characterized charge transport in amorphous mixed phases. These results suggest that partial miscibility may be important for device performance, due to the interplay between minimizing large scale phase separation and maximizing charge transport in the photoactive layer. However, in some systems crystallization of either the electron donor or acceptor complicates the role of miscibility on device performance by modifying the composition of amorphous phases. As a result, we utilize grazing-incidence small angle X-ray scattering results to quantitatively describe solar cell device performance from the structure of the photoactive layer.   

Magnetic field alignment of supramolecular perylene/block copolymer complexes for electro-optic thin films

The realization of nanostructured electro-optic materials by self-assembly is complicated by the persistence of structural defects which render the system properties isotropic on macroscopic length scales. Here we demonstrate the use of magnetic fields to facilitate large area alignment of a supramolecular system consisting of a poly(styrene-b-acrylic acid) (PS-b-PAA) diblock copolymer host and a semiconducting perylene ligand. Hydrogen bonding between the carboxylic acid groups of PAA and imidazole head group of the perylene species results in hierarchically ordered materials with smectic perylene layers in a matrix of hexagonally packed PS cylinders at appropriate stoichiometries. The smectic layers and the PS domains are strongly aligned by the application of large ($>$ 2T) magnetic fields in a manner reflective of the positive diamagnetic anisotropy and the planar anchoring of perylene units at the PS interface. We use a combination of SAXS studies in-situ with applied magnetic fields, GISAXS and polarized optical transmission measurements to characterize the system. Magnetic fields thus offer a viable route for directing the self-assembly of functional materials based on rigid chromophores and further, that supramolecular approaches can be complementary to such efforts.   

Structure/property relationships in high hole mobility regioregular PT based copolymers

The synthesis of novel solution processable conjugated polymers with high hole mobilites is an active field of study due to the potential to fabricate low cost, high though-put, lightweight organic field effect transistors (OFET). Two regioregular copolymers, based on cyclopenta[2,1-$b$:3,4-$b'$]dithiophene (CDT) and pyridal[2,1,3]thiadiazole (PT) structural units, have been prepared by using polymerization reactions involving reactants specifically designed to avoid random orientation of the PT heterocycle. Compared to their regiorandom counterpart, the regioregular polymers exhibit a two orders of magnitude increase in hole mobility from 0.005 to 0.6 cm$^{2 }$V$^{-1}$ s$^{-1}$. Grazing incidence wide angle X-ray scattering (GIWAXS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and transmission electron microscopy were carried out to obtain further insight into possible differences of structural order within the bulk and interfaces of the thin films. It was found that the backbone regioregularity leads to significant differences in the structural arrangement of the chains and indicates the importance of regioregularity for achieving optimal electronic properties.   

Electric Field Effect on Adhesion of Poly(ethylene oxide) Physical Hydrogels

The objective of this study was to characterize the effect of electric field on adhesion of poly(ethylene oxide) physical hydrogels to an aluminum substrate. The normal load necessary to disbond two plain aluminum surfaces joined by a thin layer of a poly(ethylene oxide) physical hydrogel can be reduced by about at least one order of magnitude if a reduced normal load is applied to aluminum-hydrogel interfaces simultaneously with an electric potential difference. Two aluminum surfaces joined by the hydrogel serve as a cathode and an anode. The current densities of about ten amperes per square meter determine the tenths-of-watt power dissipated in a sample. The effect of electric field on the adhesion strength of poly(ethylene oxide) hydrogels to aluminum depends on polymer concentration.   

Optical behavior of the conjugated polymer MEH-PPV thin films stretched in bi-layer dwetting by an unstable layer

Molecular packing and chain conformation play important roles in the optoelectronic performance of conjugated polymer thin films. It has been shown that by virtue of stretching via dewetting, the photoluminescence (PL) efficiencies of rarefied MEH-PPV thin films may be dramatically enhanced. To result similar effects in the stable non-diluted pristine MEH-PPV thin films, bi-layer dewetting was attempted in samples of MEH-PPV thin films ($\sim$7nm) covered by one layer of polystyrene (PS) ($\sim$40nm) that dewetted in toluene vapor to form droplets (height $\sim$300 nm) and ultrathin residual layer ($\sim$3nm) on the substrate. The instability was initiated from the PS layer in which small pinholes first emerged upon the intake of the solvent vapor. The pinholes then expanded and deepened into the underlying MEH-PPV, forcing the conjugated film to dewet. As a result of the stretching induced by the dewetting, the PL peak blue-shifted 20 nm to 540 nm and the intensity was enhanced around 10 times. Revealed by the position-sensitive confocal PL data, the huge enhancement came from both the droplet and residual layer, caused by molecular separation and stretching. Electroluminescence devices are being made based on these stretched MEH-PPV films.   

High pressure optical studies of donor-acceptor polymer heterojunctions

Bulk heterojunction polymer solar cells are based on a composite blend of two materials with electron donating and electron accepting properties. We present optical studies of a ladder-type poly(para-phenylene) and a regioregular poly(3-hexylthiophene) polymer blended with a fullerene derivative under hydrostatic pressure. The photoluminescence and absorption spectra reveal different pressure coefficients for the pristine polymer compared with the blended system. Using a phenomenological model to determine the volume change of the system under pressure, we attribute the difference in the pressure coefficient to a change in the band-edge offset at the heterojunction upon enhanced interaction. The band-edge offset is found to increase with increasing pressures for both the ladder-type and thiophene systems.   

Electrical transport properties of FeCl$_{3}$ doped poly(phenylenevinylene-co-3,4-ethylenedioxythiophene)

Poly(arylenevinylene) copolymers, in which 3,4-ethylenedioxythiophene (EDOT) and dialkoxy phenylenes are alternatively linked by vinylene unit, were synthesized by the Horner-Emmos reaction. The samples were doped with FeCl$_{3 }$and the temperature dependence of conductivity, magnetoresistance (MR), and thermoelectric power (TEP) were measured. The temperature dependence of conductivity follows exp[-(T$_{0}$/$T)^{1/2}$] and positive MR is observed up to $H $= 14 tesla. The TEP can be described by $S (T)$ = A + B/$T$ + C$T$. These behaviors are understood in the frame of charging energy limited tunneling conduction between metallic islands separated by insulating barriers.