Precursor states for charge carrier generation in conjugated polymers probed by ultrafast spectroscopy

Photocurrent experiments using two femtosecond laser pulses are performed on a photodiode using a ladder-type conjugated polymer as the active layer. With a photon energy of 3.1 eV the first pulse excites singlet excitons. A time-delayed second pulse with a photon energy of 2.49 eV leads to a decrease of the photocurrent by exciton depletion due to stimulated emission. S1 excitons being dissociated during their entire lifetime are identified as the only relevant channel for charge carrier generation. Intrachain polaron pairs are also formed on an ultrafast time scale with a yield of approximately 10%. They can be efficiently dissociated by reexcitation with photons of an energy of 1.9 eV.     

Stark spectroscopy of excited-state transitions in a conjugated polymer

Stark spectroscopy, which is well established for probing transitions between the ground and excited states of many material classes, is extended to transitions between transient excited states. To this end, it is combined with femtosecond pump-probe spectroscopy on a conjugated polymer with appropriately introduced traps which harvest excitation energy and build up a sufficient excited state population. The results indicate a significant difference in the effective dipole moments between two short lived excited states.     

Direct visualization of the charge transfer in conjugated polymers

Ion-induced charge-transfer states in conjugated polyelectrolytes were experimentally investigated by Justin M.Hodgkiss and his co-workers [J Am Chem Soc,2009,131(25):8913].In this work,charged and neutral conjugated polyelectrolytes were further studied with quantum chemistry methods.The calculation result shows that the absorption spectra are roughly in visible and ultraviolet light regions,and the two absorption peaks are located in the wavelength span 300-400 nm for charged polyelectrolytes.However,in n...     

Role of mesoscopic morphology in charge transport of doped polyaniline

In doped polyaniline (PANI), the charge transport properties are determined by mesoscopic morphology, which in turn is controlled by the molecular recognition interactions among polymer chain, dopant and solvent, Molecular recognition plays a significant role in chain conformation and charge delocalization. The resistivity of PANI doped by camphor sulfonic acid (CSA)/2-acrylo-amido-1-propane sulfonic acid (AMPSA)/dodecyl benzene sulfonic acid (DBSA) is around 0.02 Ω cm. PANI-CSA and PANI-AMPSA show a metallic positive temperature coefficient of resistivity above 150 K. with a finite value of conductivity at 1.4 K; whereas, PANI-DBSA shows hopping transport at low temperatures. The magnetoresistance is positive (negative) for PANI-CSA (PANI-AMPSA); and PANI-DBSA has a large positive MR. The behavior of MR suggests subtle variations in mesoscopic morphology between PANI-CSA and PANI-AMPSA.     

Nonequilibrium mesoscopic transport: a genealogy

Models of nonequilibrium quantum transport underpin all modern electronic devices, from the largest scales to the smallest. Past simplifications such as coarse graining and bulk self-averaging served well to understand electronic materials. Such particular notions become inapplicable at mesoscopic dimensions, edging towards the truly quantum regime. Nevertheless a unifying thread continues to run through transport physics, animating the design of small-scale electronic technology: microscopic conservation and nonequilibrium dissipation. These fundamentals are inherent in quantum transport and gain even greater and more explicit experimental meaning in the passage to atomic-sized devices. We review their genesis, their theoretical context, and their governing role in the electronic response of meso- and nanoscopic systems.     

Spectroscopic studies on conjugated polymers in mesoporous channels: influence of polymer side-chain length

The influence of mesoporous environment on the conjugated polymers was studied by UV-Vis absorption and Photoluminescence spectroscopy. The applied polymers were three novel poly(p-phenylenevinylene) derivatives (DDMA-PPV). These polymers have dibenzothiophen-5,5-dioxide units in their backbones, but are different from each other in the length of alkoxy side-chains. The polymers were incorporated into the mesoporous channels of SBA-15 by sorption from their dilute solutions. The confined polymers exhibited different trends in the shifts of the absorption onsets and the emission peaks depending on the length of the side-chains. The polymer with shorter side-chain showed red-shifts in both the absorption and emission spectra, whereas the polymer with longer side-chain showed blue-shifts. These phenomena were caused by the combined influences from the electronic confinement and the conformation distortion. Moreover, these trends were enhanced when the polymers were loaded in amine-modified SBA-15.     

Semiconductor nanocrystals in carrier-transporting polymers: Charge generation and charge transport

By incorporating semiconductor nanocrystals in carrier-transporting polymers, an interesting class of photoconductive nanocomposites is created. The presence of semiconductor nanocrystals enhances the photoinduced charge generation efficiency and extends the sensitivity range, while the polymer matrix is responsible for charge transport. A wide variety of semiconductors and polymers have been used. In this paper, we review material synthesis and discuss the effects of semiconductor nanocrystals on the charge transport and charge generation properties.     

Time-resolved charge carrier generation from higher lying excited states in conjugated polymers

Sub-ps three-pulse transient differential transmission spectroscopy using two excitation pulses is used to directly investigate the generation of charge carriers in ladder-type poly(para)phenyl in bulk film. The role of higher excited singlet states of both even and odd symmetry is examined and the dynamics of the major processes involved is described quantitatively. The charge generation efficiency is found to depend strongly on the delay between the two excitation pulses. This is explained by the interplay between internal conversion, excitation energy migration, and on-site vibronic relaxation.     

Exact analytical results on electronic transport of conjugated polymer junctions: Renormalization method

We study the electrical coherent transport in a typical conjugated polymer connected to molecular electrodes. We provide an exact analytical method to reduce such a system to a uniform wire by employing Green’s function theory and the tight-binding Hamiltonian. The formalism is useful to investigate transport properties of periodic quasi-one dimensional molecular nanowires and polymers in the presence or absence of defects. Some illustrative examples including polyacetylene and poly(p-phenylene) polymers are given to show the ability of our approach. For ideal infinite polyacetylene and poly(p-phenylene) polymers due to dimerization respectively, one and two gaps will appear in the density of states spectra.     

Singlet-triplet transition in a quantum dot: effect on mesoscopic transport

The T=0 transport properties of a wire interacting with a lateral two-level quantum dot are studied by using an exact numerical calculation. The wire conductance, the spin–spin correlation and the Kondo temperature are obtained as a function of the dot level energy spacing. When the dot has two electrons and spin S_D1, the wire current is totally quenched by the S=1 Kondo effect. The Kondo temperature is maximum at the singlet–triplet transition and its dependence upon the dot energy spacing follows a non-universal scaling law.     

Charge transport in low-concentration MEH-PPV conjugated polymer/fullerene composites

Charge transport was studied in composites of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV) conjugate polymer and low-concentration fullerenes (C₆₀) below the percolation threshold. The electron mobility showed a linear increase with the fullerene concentration, to which the hole mobility was insensitive. Our results indicate that fullerene–polymer networks provide a conduction path to the electrons, whereas the holes are transported through the polymer-only paths. The microscopic environments of the two distinct conduction paths in the composites as revealed by the electric field dependence of the mobilities are also discussed.     

Photoinduced charge currents in mesoscopic rings

The temporal and spatial controllability of charge distribution in submicron structures opens new avenues for potential applications and for the understanding of nonequilibrium processes. Here we suggest a novel way to trigger and control within picoseconds charge currents and magnetic moments in nanoscopic and mesoscopic ring structures by applying two shaped, time-delayed light pulses. Our quantum dynamic calculations show that the magnitude and direction of the induced currents are tunable by varying the time delay and strengths of the pulses. Furthermore, in an array of rings desirable magnetic orders are generated depending on the ring sizes and particle number.     

Simultaneous Raman and fluorescence spectroscopy of single conjugated polymer chains

Simultaneous surface enhanced Raman scattering (SERS) and fluorescence is demonstrated from single conjugated polymer chains. As resonance enhancement of SERS depends on the spectral overlap of the polymer's absorption and the incident laser, resonance Raman and fluorescence effectively probe the absorbing and emitting part of the polymer, respectively. The optical phonon energies change along the polymer chain, providing a window to spatially track excited state relaxation. Whereas a mean spatial redistribution of the excitation is witnessed by a change in vibronic fingerprint following interchromophoric energy transfer, intrachromophoric exciton self-trapping leaves the vibrations unchanged.     

Glass transition in charge carrier transport phenomena of amorphous photoconductive polymers

The critical temperature T_o in Gill's empirical equation for the charge carrier transport was found to have aclose connection with the glass transition temperature T_g for poly-N-vinylcarbazole containing model compound systems. Interpretations of T₀ and μ₀, i.e., the drift mobility at T₀, were presented.     

Four-photon polarization spectroscopy of water in a millimeter-wave radiation field

Resonance (frequency 1.4 cm⁽⁻¹) changes induced in the four-photon optical spectrum of water by a millimeter-wave electromagnetic field are observed experimentally. Comparison with the spectrum of ice in the range 0‒2 cm⁽¹ shows that the action of such a field is of a structure-forming character. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 266–268 (25 August 1998)     

Polymeric quasicrystal: mesoscopic quasicrystalline tiling in ABC star polymers

A mesoscopic tiling pattern with 12-fold symmetry has been observed in a three-component polymer system composed of polyisoprene, polystyrene, and poly(2-vinylpyridine) which forms a star-shaped terpolymer, and a polystyrene homopolymer blend. Transmission electron microscopy images reveal a nonperiodic tiling pattern covered with equilateral triangles and squares, their triangle/square number ratio of 2.3 (approximately equal to 4/sqrt[3]), and a microbeam x-ray diffraction pattern shows dodecagonal symmetry. The same kind of quasicrystalline structures have been found for metal alloys (approximately 0.5 nm), chalcogenides (approximately 2 nm), and liquid crystals (approximately 10 nm). The present result (approximately 50 nm) confirms the universal nature of dodecagonal quasicrystals over several hierarchical length scales.     

Two-step dissociation of a polaron in conjugated polymers

We have studied the electric-field-driven motion of a polaron by solving the time-dependent Schr\"{o}dinger equation nonadiabatically and the lattice equation of motion simultaneously. It is found that the polaron may experience two sequent transitions under high fields; one is the transition from the subsonic to the supersonic state, and the other from the supersonic to dissociated state. The acoustic mode is decoupled from the charge when the polaron moves at a speed faster than the sound speed, and then the optical mode is decoupled at the second transition to make the polaron dissociate completely.