Session X49: Focus Session: Organic Electronics and Photonics - Polymer Dielectrics and Charge Transport

Inhomogeneous deformation and instability in soft dielectric transducers

Dielectric elastomer (DE) is assembled by sandwich an elastomeric membrane with compliant electrodes on both sides. They are capable of converting mechanical into electrical energy (generator)or electrical into mechanical energy (actuator). The large actuation strain of DE has inspired intense development of dielectric elastomers as applications as actuators and generators. DE transducers are lightweight, compliant, rust-free, and can convert higher energy than those of conventional transducers. DE transducers often undergo inhomogeneous deformation and instability during operation. Inhomogeneous deformation can cause the DE membranes in have inhomogeneous fields distribution and fail locally. Instability during actuation highly affects the performance and safety of the DE transducer. We present an analyitial model of a dielectric elastomer transducers undergoing inhomogeneous deformation and snap-through instability during operation.   

Dielectric Properties of Carbon, Silicon and Germanium Based Polymers: A First Principles Study

The field of high energy density capacitors would benefit from the discovery of new insulating polymers with high dielectric constant, low loss, large band gap and high breakdown strength. The current standards for capacitive energy storage applications are polypropylene and polyethylene which have large band gap and high breakdown strength, but a small dielectric constant. As an initial step aimed at the discovery of new polymers with better dielectric properties, we consider a class of chemically-modified polymers based on polyethylene. These polymers are composed of --XY$_{2}$-- building blocks, with X = C, Si or Ge, and Y = H, F or Cl. We use density functional perturbation theory and exchange-correlation functionals that include van der Waals and/or nonlocal exchange interactions to accurately predict the structure, dielectric constant (electronic and ionic) and band gap of this class of polymers. The computed properties have been correlated to the underlying electronic structure and phonon modes, and tradeoffs between the band gap and dielectric constant are established. The time-consuming dielectric computations have been optimized using a new ``single-chain'' method to allow for future extensive explorations of the polymer chemical space via automated high-throughput computations.   

Paraelectric Crystalline Polymers for High Energy Density and Low Loss Dielectrics

It is known that the high temperature phase above the Curie temperature (Tc) in poly(vinylidene fluoride-\textit{co}-trifluoroethylene) [P(VDF-TrFE)] is a typical paraelectric phase. Frequency-dependent electric displacement-electric field (D-E) loop tests are used to study the dielectric/ferroelectric properties of this paraelectric phase. It is observed that normal ferroelectric hysteresis loops are observed at a poling frequency of 1 Hz, while narrow paraelectric loops are observed at a high poling frequency of 1000 Hz. The poling mechanism is revealed by an in-situ electric field-dependent Fourier transform infrared study. From this study, we consider that paraelectric crystalline polymers are good candidates for high energy density and low loss dielectrics.   

Influence of Aqueous Electrolytes on Electrical Insulating Properties of Polyethylene

Polyethylene is commonly used as electrical insulation in high voltage (3-35 kV), underground electrical distribution cables. During service conditions the insulation ``ages'' and may fail. One method of ageing is a consequence of long-time exposure of the polyethylene to humidity and groundwater. Chemical analyses by other researchers indicated iron was frequently detected in degraded areas of aged cable. In the current research we examine the effect of a ferric chloride electrolyte on the electrical insulating character of polyethylene. In earlier research we discovered that in the presence of high DC voltages (approximately 3kV-6kV) ferric chloride electrolytes markedly enhance electron injection into and subsequent electron transport through polyethylene. The present research shows that ferric chloride complexes in solution are likely responsible for electron injection. The effect of exposure to ferric chloride solution was permanent, causing an increase in current density when the polyethylene was subsequently exposed to other electrolytes. The effect of FeCl$_{3}$ exposure was observed in additive free polyethylene as well as commercially processed polyethylene.   

Theoretically guided design of efficient polymer dielectrics

We have used theory and molecular dynamics simulation as an aid to the development of polymeric materials with favorable properties for energy storage with low dielectric losses. We build on the principle that the stored energy in a capacitor is a sum of the intrinsic energy of the electric field and the energy of distortion of the molecular bonds within the dielectric. We have attempted to maximize the energy of bond distortion by increasing the polarization without introducing large losses. In this initial study we simulate the behavior of a system consisting of parallel chains of the antiferroelectric $\alpha$ phase of polyvinylidene fluoride that has been modified to increase the separation between chains through crosslinks that prevent chain rotation. By varying the crosslink density, we identify the optimum length of unlinked chain such that the polar entities that rotate when subject to a strong electric field will be neither so long that they rotate collectively with little stored energy, nor so short that the large distortion of bond angles necessary for dipole rotation reduces the polarizability. We have adopted a combined-atom model in order to make feasible the study of systems comprising up to 100 chains, each consisting of up to 500 monomers.   

Interface conductivity contribution in Co-Phthalocyanines capacitive type devices

Metal Phthalocyanines are flat organic semiconductors which exhibit interesting magnetotransport properties. Recent experimental studies in a particular system together with theoretical calculations have shown that the temperature and thickness dependence of the ohmic conductivity can be universally described by two independent contributions: the organic film and the electrode-film interface [1]. In order to explore the implications of this model we performed transport measurements in sandwich devices with different bottom electrodes materials. These devices are grown in-situ by Organic Molecular Beam Epitaxy to assure ultra clean electrode-film interfaces. A combination of structural and transport studies are used to investigate the reason for the drastic change in ohmic conductance at metallo-organic film thickness around 100nm. \\[4pt] [1] C. N. Colesniuc, R. R. Biswas, S. A. Hevia, A. V. Balatsky, and I. K. Schuller, Phys. Rev. B 83, 085414 (2011).   

Nanoscale Correlation of Heterogeneous Morphology and Electrical Transport in Nanostructured Organic and Hybrid Solar Cells

The knowledge of correlation between morphology and electrical properties is essential both to understand fundamental physics and to facilitate device optimization. We report a systematic study of the organic blend of PCBM particles and P3HT fibers formed via the thermal annealing method. We use conductive atomic force microscopy (c-AFM) to simultaneously map the surface morphology and collect the electrical current. Cross-section AFM is attempted to get the internal 3D morphology so as to establish its correlation with the electrical properties. The obtained heterogeneity in the current map with a resolution up to 20 nm is attributed to the formation of cross-linked three dimensional fiber network, which is further supported by a three dimensional device model incorporating the geometry of nanowire and the c-AFM tip. Results on the hybrid cell of ZnO nanowires infiltrated with P3HT will also be discussed.   

Gigahertz Probing of Poly(3-hexylthiophene) With A Kilohertz Detection Scheme

Organic semiconducting polymers have been well studied at DC and low-frequencies, giving important information about charge transport and metal-polymer interaction. However, comparatively little is known about the operation of these polymers at radio frequencies (~1 GHz). RF frequencies can be a useful tool to investigate high-frequency mobility and charge carrier dynamics relevant for the possibility of using these polymers in RF applications. We present a novel technique that probes poly(3-hexylthiophene) (P3HT) response in the RF regime but allows for detection in the kHz regime. We show transport data of P3HT and discuss a theoretical framework for inferring behavior at GHz frequencies.   

Thickness dependence of hole mobility in regioregular poly(3-hexylthiophene) thin films

Methods of time of flight (TOF) and charge extraction by linearly increasing voltage (CELIV) are used to investigate the hole mobility in regioregular poly(3-hexylthiophene) (RR-P3HT) thin films. The film thickness was varied by changing the RR-P3HT solution concentration and spin-coating speed. The hole mobility is found to monotonically increase from 10$^{-4}$ cm$^{2}$/Vs to 10$^{-3}$ cm$^{2}$/Vs along with film thickness from 100 nm to 700 nm and saturate at 10$^{-3 }$cm$^{2}$/Vs beyond 700 nm. X-ray diffraction and ellipsometry data showed that the film morphology changes with the thickness. The structural change supports the weak dependence of energetic and positional disorder on thickness analyzed by the Gaussian Disorder Model (GDM).   

Contribution of Increased Extraction Efficiency to Increased Photo-Luminescence in Strained Polymer Films

Potential applications of Luminescent Conjugated Polymers in thin film diodes, solar cells and flat panel displays have been limited by low efficiency. Craze formation in MEH-PPV/polystyrene thin film leads to a factor of 2 or 3 increase in collected photo-luminescence (PL) due to a combination of factors such as MEH-PPV chain conformation and increased extraction efficiency of PL. In order to determine the contribution of the latter effect, we used Monte Carlo based Ray Tracing to analyze the trajectory of photons generated in the thin film under different strain conditions. Our results indicate that increased PL extraction due to the existence of crazes contributes $\sim$15\% of the observed increase in PL, the majority of this being due to light emitted near the craze edges.   

Modeling charge and energy transports in $\pi $-conjugated systems

A generic 3D model Hamiltonian is developed to simulate the charge and energy transports in $\pi $-conjugated composite materials. The intrachain interactions are described by our recently developed adapted Su-Schrieffer-Heeger Hamiltonian and the interchain $\pi \pi $ interactions are modeled using distance-dependent hopping integrals. Excellent agreements in their binding energetics and geometries with post-Hartree-Fock ab initio methods and experiments are found in the cases of a benzene dimer, graphene bi-layer, and poly-($p$-phenylene vinylene) (PPV) crystal. The computed photoinduced charge separated states and associated adsorption spectra of the PPV-C$_{60}$ and PPV-C$_{60}$-graphene agree perfectly with experimental measurements.   

NMR, magnetic susceptibility, and electrical conductivity investigation of doped poly-3-methylthiophene

We report $^{1}$H and $^{19}$F NMR spin-lattice relaxation rate (1/$T_{1})$ measurements over a wide range of temperature (3 K$ < T < $300 K) and magnetic field (0.9 T$ < B \quad < $23.4 T for $^{1}$H and 9.0 T for $^{19}$F) in the organic conductor poly-3-methylthiophene (P3MT) doped with hexafluorophosphate (PF$_{6})$. Also included are measurements of the electrical conductivity ($\sigma )$ at $B$ = 0 and 77 K $< \quad T \quad <$ 300 K and the magnetic susceptibility ($\chi )$ at $B$ = 0.1 T and 2 K $< \quad T \quad <$ 350 K. The doping level has been varied to tune the conductivity value at 300 K in the fully doped sample to $\sigma \quad \sim $ 120 S/cm and in the dedoped one to $\sigma \quad \sim $ 5 S/cm. This range enables investigation of the roles of carrier density and electron-electron interactions in the mechanisms for 1/$T_{1}$. A correlation between $\chi $ and the relaxation mechanisms is observed in these samples. The results are analyzed using the modified Korringa relation. Also, the proton and fluorine spin relaxation data give insight into the role of both inter-chain and intra-chain conduction mechanisms.   

Improving Order and Mobility in MEH-PPV Films by Reducing Polydispersity

The effect of polydispersity on morphology and charge transport in drop cast films of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) was investigated using grazing incidence X-ray diffraction and time-of-flight respectively. Morphologically, reducing polydispersity by removing short chain segments promoted the capability of crystallization. This resulted in higher hole mobility and non-dispersive transport down to lower temperatures for the lower polydispersity sample. The slope for the Poole-Frenkel relationship at 298 K was increased, and its change with temperature decreased, indicating reduced spatial inhomogeneity. Analysis using Bassler's Gaussian disorder model (GDM), found that the value for energy disorder ($\sigma \sim $53meV for both films) and infinite temperature zero field mobility ($\mu $o$\sim $3 x10$^{-6}$ cm$^{2}$/Vs) were similar for both films. However, a good fit for hopping site separation and spatial disorder was only possible for the lower polydispersity device, suggesting that the lower polydispersity films have less mesoscopic inhomogeneity.   

Charge Transport in Amorphous Polythiophene-Fullerene Blends

Energy-filtered transmission electron microscopy studies revealed that amorphous mixed phases are ubiquitous within mesostructured polythiophene/fullerene mixtures. Nevertheless, the role of mixing within nanophases on charge transport of organic semiconductor mixtures is not fully understood. We have examined the electron mobility in amorphous blends of poly(3-hexylthiophene) and phenyl-C$_{61}$-butyric acid methyl ester. Our studies reveal that the miscibility of the components strongly affects electron transport within amorphous blends. Immiscibility promotes efficient electron transport by promoting percolating pathways within organic semiconductor mixtures. As a consequence, partial miscibility may be important for efficient charge transport in polythiophene/fullerene mixtures and organic solar cell performance.   

Modeling energy transport in $\pi $-conjugated dendrimers containing triple bonds

An accurate, transferrable, and computational efficient adapted Su-Schrieffer-Heeger model Hamiltonian is developed to describe triple bonds in linear and fractal-dimensional $\pi $-conjugated systems. Chemical accuracy in the computed optical gaps is found for the cases of poly-(thiophene-ethynylene) and ploy-phenylacetylene of arbitrary lengths, with all errors less than 3{\%} as compared to existing UV-visible adsorption spectra. The computed exciton migration processes in the phenylacetylene dendrimers indicates that such conjugated Bethe tree structures are efficient energy transduction funnels.