Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session A18: Photophysics of Electronic Polymers |
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Sponsoring Units: DPOLY DMP Chair: Rachel Segalman, University of California, Berkeley Room: Colorado Convention Center 103 |
Monday, March 5, 2007 8:00AM - 8:12AM |
A18.00001: Identification of the Possible Defect States in Poly(3-hexylthiophene) Thin Films Danqin Feng, Anthony Caruso, Yaroslav Losovyj, Douglas Schulz, Peter Dowben We find evidence for a gradual change in the electronic properties of regioregular poly(3-hexylthiophene) thin films with temperature. The conduction properties appears to be mediated by hopping conduction dominated by a low density of defects states within the highest occupied molecular orbital to lowest unoccupied molecular orbital gap, not by a change in band gap. The possible origins of a low density of defects states within the highest occupied molecular orbital to lowest unoccupied molecular orbital gap are suggested. A number of ``chemical'' defects, impurities and structural defects can contribute to features in photoemission for regioregular poly(3-hexylthiophene). A density of states within the highest occupied molecular orbital to lowest unoccupied molecular orbital gap may affect the transport properties of regioregular poly(3-hexylthiophene) and like polymers. These gap electronic states are not expected in the perfectly ordered polymer. [Preview Abstract] |
Monday, March 5, 2007 8:12AM - 8:24AM |
A18.00002: Ab intio study of ladder-type metallic polymers Simon Pesant, Guillaume Dumont, Sebastien Langevin, Michel Cote The electronic structure of recently synthesized ladder-type polythiophene polymer is studied with density-functional theory based calculations. It is found, in the local density approximation (LDA) that upon a simple substitution of the sulfur atoms by nitrogen and boron atoms, the band structure of the resulting polymer (called LPPyB) exhibits bands overlap between the occupied and the unoccupied states that is characteristic of metallic systems. The band structure is further validated by GW calculations confirming the assessment of the LDA results. Calculations using the B3LYP functional, which contains exact exchange, show a different electronic behavior, a small HOMO-LUMO band gap of 0.67 eV is obtained. In order to better assess the energy gap of the polymer, a TDDFT (Time-dependent density-functional theory) study of the LPPyB was performed on olygomers and verifies the metallic structure of this polymer. Other calculations were done using TDDFT on different polymers to validate the last result. In parallel, similar electronic properties were computed on an isoelectronic polymer of LPPyB, having the atomic structure of the ladder- type polythiophene, with only one sulfur atom replaced by a boron atom. [Preview Abstract] |
Monday, March 5, 2007 8:24AM - 8:36AM |
A18.00003: Illumination induced metastable polaron-supporting phase in poly p-phenylene- vinylene films E. Ehrenfreund, E. Gershman, Y. Eichen, T. Drori, C.X. Sheng, Z.V. Vardeny We found a new illumination induced metastable polaron-supporting phase in pristine films of a soluble derivative of poly-p-phenylene vinylene (MEH-PPV). In the pristine, un-illuminated MEH-PPV phase $A$, the polymer films do not show any long-lived photogenerated polarons. Prolonged UV illumination, however, was found to induce a reversible, metastable phase $B$, characterized by its ability to support the existence of abundant long-lived photogenerated polarons. In the dark, films of phase $B$ revert back to the original phase A within about thirty minutes at room temperature. Relying on the well-established ubiquitous reversible photoinduced cyclization of diarylethenes into dihyrophenanthrene derivatives, we propose a reversible mechanism in which UV illumination creates metastable deep defects that substantially increase the photogenerated polaron lifetime. [Preview Abstract] |
Monday, March 5, 2007 8:36AM - 9:12AM |
A18.00004: Ultrafast and Nonlinear Optical Spectroscopies of Excited States in Pristine and Doped $\pi $-Conjugated polymers Invited Speaker: A variety of ultrafast and optical nonlinear spectroscopies were applied to pristine and doped $\pi $-conjugated polymers, for elucidating the excited states energy levels and primary photoexcitation species in these materials. These spectroscopies include fs pump-probe photomodulation (PM), two-photon absorption (TPA), photoluminescence up-conversion (PL(t)), and electroabsorption (EA); as well as THz time domain spectroscopy (THz-TDS). The $\pi $-conjugated polymers include derivatives of PFO, PPV and PT, as well as t-(CH)$_{x}$; doping includes fullerene molecules, as well as heavily doping with strong acceptors. The results have been analyzed in terms of the exciton picture advanced by Mazumdar et al. The primary photoexcitations are singlet excitons of which PM spectrum is composed of two strong photoinduced absorption bands in the mid and near ir spectral range, that are correlated with a stimulated emission band and PL(t). These bands are in agreements with transitions from the lowest exciton with odd symmetry into higher lying excitons with even symmetry, as revealed by TPA and EA spectroscopies. Polaron excitations are also formed and are characterized by two PA bands in the mid-ir range, and correlated ir-active vibrations. Surprisingly t-(CH)$_{x}$ is not different from many other $\pi $-conjugated polymers, except that the primary polarons recombine at a later time to form charged solitons. In fullerene-doped polymers the primary singlet excitons are trapped and undergo ultrafast nonradiative decay in doping-related defects, and this explains in part the weak cw PL in these compounds. In heavily doped polymers with strong acceptors the ground state no longer is neutral, but rather contains substantial amount of free charge carriers characterized by the Drude free carrier response in the THz to mid ir spectral ranges. [Preview Abstract] |
Monday, March 5, 2007 9:12AM - 9:24AM |
A18.00005: Temperature dependence and anisotropy of charge-carrier mobilities in crystalline durene Frank Ortmann, Karsten Hannewald, Friedhelm Bechstedt We report on the theoretical analysis of charge-carrier mobilities in durene crystals. The crystal is studied with DFT methods to examine structural, vibrational, and electronic properties. On that basis we employ a Holstein-Peierls model (see Hannewald et al. PRB \textbf{69}, 075211 (2004); PRB \textbf{69}, 075212 (2004)) to simulate the temperature dependence of the mobilities. The relation between the anisotropy of electron/hole mobilities and the band structure as well as lattice vibrations is discussed. [Preview Abstract] |
Monday, March 5, 2007 9:24AM - 9:36AM |
A18.00006: Ultrafast polarization memory dynamics of photoexcitations in $\pi $-conjugated polymers. Sanjeev Singh, Minghong Tong, Josh Holt, Zeev Vardeny For better understanding ultrafast photoexcitation dynamics in $\pi $-conjugated polymers, we study the polarization memory dynamics in the pump/probe photomodulation (PM) spectrum of these materials. The transient PM spectrum of polymers contain singlet excitons with prominent photoinduced absorption (PA) band, stimulated emission and photobleaching bands in the near ir/visible spectral range; polarons with PA bands in the mid- and near-ir; and polaron pairs in the visible range. Each of these spectral feature shows polarization memory, P(t) where the PM signal with parallel pump/probe polarizations is $\sim $ twice larger than with perpendicular polarizations. P(t) has a specific dynamics for each photoexcited species, and, in addition it also depends on the excitation pump photon energy. Results for MEH-PPV films and solutions will be thoroughly discussed, in comparison with the photoluminescence efficiency. [Preview Abstract] |
Monday, March 5, 2007 9:36AM - 9:48AM |
A18.00007: Experimental Determination of Charge/Neutral Branching Ratio in $\pi $-Conjugated Polymers by Broad-band Ultrafast Spectroscopy$^{1}$ Chuanxiang Sheng, Minghong Tong, Sanjeev Singh, Z. Valy Vardeny We demonstrate a reliable method of determining the branching ratio, $\eta $ of photogenerated charge (polarons) to neutral (excitons) photoexcitations in various $\pi $-conjugated polymer films and solutions using femtosecond ultrafast spectroscopy with broad spectral range from 0.14 to 2.7 eV. We found that both excitons and polarons are instantaneously photogenerated, but $\eta $ critically depends on the film nanomorphology, which, in turn controls the interchain coupling. In films, $\eta $ varies between 1{\%} for derivatives of poly(p-phenylene vinylene) casted from chloroform solution, to more than 30{\%} for regio-regular poly-3-hexyl thiophene. Our results show that charge photogeneration quantum efficiency in these materials is an interchain process; and this has ramifications for their use in solar cell applications. $^{1}$Supported in part by the DOE. [Preview Abstract] |
Monday, March 5, 2007 9:48AM - 10:00AM |
A18.00008: In-situ characterization of the mesophase of a high performance semiconducting polymer L.J. Richter, A.J. Moad, D.M. Delongchamp, R.J. Kline, D.J. Gundlach, D.A. Fischer Poly(2,5-bis(3-alkylthiophen-2yl)thieno[3,2-b]thiophene) is a semiconducting polymer with exceptional hole mobility in thin film transistors upon annealing into a mesophase. We have identified the structural motifs of both the mesophase and the high performance, thermally processed film with a variety of in-situ techniques: NEXAFS, spectroscopic ellipsometry (SE), and IR absorption. Upon cooling from the mesophase, the films exhibit pi-stacked lamella with molecular terraces (AFM) with a high degree of order of the conjugated backbone (NEXAFS). The side chains are well ordered (IR) and interdigitated, which may be a driving factor in the growth of large crystals. Upon re-heating, the side chains (IR) and conjugation length (SE) monotonically disorder until entry into the mesophase which is characterized by highly disordered side chains and moderate torsional disorder of the backbone but near ideal in-plane order of the polymer long axis. Side-chain order is reestablished upon re-cooling into the ordered phase. The hysteresis of the side chain order mimics the DSC of powders. As-cast films exhibit greater disorder in all degrees of freedom; entry into the mesophase is necessary to achieve high order. The spectroscopic data can be correlated with in-situ mobility measurements. [Preview Abstract] |
Monday, March 5, 2007 10:00AM - 10:12AM |
A18.00009: Electro-optic Measurements in Single-Crystal Films of a Combination of Materials Involving DAST and IR-125 A. Narayanan, J. Titus, M. Thakur Single crystal films of a combination of materials involving DAST and a dye molecule IR-125 have been prepared using the modified shear method. X-ray diffraction results indicate a [001] orientation of the film similar to a DAST single-crystal film. The electro-optic measurements of the DAST-IR125 films have been performed using field induced birefringence in the cross polarized geometry at 633 nm and 1550 nm. A modulation of 14 percent has been observed in a single pass through the film for a field of 1 Volt/micron at 633 nm. The results indicate exceptionally high electro-optic coefficients at both of the wavelengths (633 and 1550 nm). [Preview Abstract] |
Monday, March 5, 2007 10:12AM - 10:24AM |
A18.00010: Nonlinear Refractive Index in a Novel Nano-optical Material Based on the Nonconjugated Conductive Polymer, Poly($\beta$-pinene) A. Narayanan, J. Titus, Mrinal Thakur Two-photon absorption in a novel nano-optical polymer based on the nonconjugated conductive polymer, iodine-doped poly($\beta $-pinene) has been recently reported. In the present report, we will discuss measurement of the nonlinear refractive index (n$_{2})$ of iodine-doped poly($\beta $-pinene). The measurement has been made using 150 fs pulses from a Ti:Sapphire laser. Time-resolved measurement has been made using pump-probe technique in which the phase change in the probe beam was measured from the intensity-induced birefringence while the pump pulse was overlapped. The measured value of the nonlinear refractive index is larger than 10$^{-5}$cm$^{2}$/MW at 800 nm. The results show that the measured n$_{2}$ is of electronic origin. This exceptionally large magnitude of n$_{2}$ has been attributed to the special electronic structure of doped poly($\beta $-pinene) confined in a sub-nanometer domain. [Preview Abstract] |
Monday, March 5, 2007 10:24AM - 10:36AM |
A18.00011: Quadrupolar dyes for NLO applications: solvent-induced symmetry breaking and huge TPA cross-sections in aggregates Anna Painelli, Gabriele D'Avino, Francesca Terenziani Quadrupolar dyes, where electron donor (D) and acceptor (A) groups are linked by $\pi$-conjugated bridges to yield symmetrical structures (D-$\pi$-A-$\pi$-D or A-$\pi$-D-$\pi$-A) are intensively studied for TPA applications. In an essential-state model for the solvated dyes, symmetry-broken dipolar solutions are found for either the ground or the one-photon excited state. Dyes are accordingly classified in three different classes, with distinctively different spectroscopic behavior. The model provides useful guidelines for the design of molecules for TPA applications and represents a general frame to understand energy transfer processes in multipolar molecular systems. [1] The same essential state model applies to aggregates of quadrupolar dyes. Relaxing the dipolar approximation for electrostatic intermolecular interactions, bound-biexcitons appear with important spectroscopic consequences. Specifically, the large TPA cross-section of quadrupolar dyes is amplified by orders of magnitude as a result of aggregation. [1] F. Terenziani, A. Painelli, C. Katan, M. Charlot, M. Blanchard-Desce, {\it J. Am. Chem. Soc.} {\bf 2006}. [Preview Abstract] |
Monday, March 5, 2007 10:36AM - 10:48AM |
A18.00012: Infrared probe of charge dynamics in single crystal rubrene organic field-effect transistors Zhiqiang Li, Vitaly Podzorov, Na Sai, Michael Martin, Michael Gershenson, Massimiliano Di Ventra, Dimitri Basov We report on infrared (IR) spectroscopy of charge dynamics in organic field-effect transistors based on single crystal rubrene. IR microscopy measurements show uniform charge injection over macroscopic length scales of several millimeters in these devices. IR measurements uncover anisotropic optical conductivities of these transistors in agreement with earlier transport studies. The field-induced electronic excitations in rubrene reveal optical constants with the Drude-like form and low effective masses. I will discuss several new aspects of the charge dynamics in organic molecular crystals uncovered by this work. [Preview Abstract] |
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