Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J4: Conducting Polymers |
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Sponsoring Units: DPOLY Chair: Rachel Segalman, UC-Berkeley Room: LACC 515A |
Tuesday, March 22, 2005 11:15AM - 11:51AM |
J4.00001: Fundamental electronic processes in organic photovoltaic cells Invited Speaker: In most organic photovoltaic (PV) cells, excitons must diffuse to a donor-acceptor interface where they dissociate by electron transfer. It is therefore critically important to understand exciton diffusion in detail in order to optimize device architectures. We have measured the exciton diffusion length of various organic materials using steady-state photoluminescence quenching in well-defined polymer/titania bilayer heterostructures. We address how processing conditions can affect the exciton diffusion length in the investigated systems and suggest ways to engineer new materials with larger exciton diffusion lengths. To study charge transport of polymers confined in nanometer channels, we have formed films of anodic alumina with arrays of straight nanopores on indium tin oxide electrodes. We filled the pores with conjugated polymers by spin casting them over the pores and then melting the film. Measurements of the transmission and reflectance of polarized light as a function of angle show that the polymer in the nanopores is partially aligned in the direction perpendicular to the substrate as compared to a neat film. Fitting the current-voltage curves of the diodes to a space charge limited current model shows that the mobility can be increased by a factor of 20. By replacing the insulating alumina with a semiconductor, such as titania, we should be able to make efficient ordered bulk heterojunction solar cells. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:27PM |
J4.00002: Semiconducting block copolymers and their devices: the relationship between electronic properties, morphology and interfaces Invited Speaker: For the optimal performance of organic opto-electronic applications, such as light emitting diodes (LEDs) and photovoltaic devices (PVDs), the morphology of the active layer is of crucial importance. One way to control the morphology of organic materials is by making use of the self- assembling properties of block copolymers. Their well-known microphase separation into highly ordered lattices occurs on the length scale of the radius of gyration of the two blocks, which is comparable to the exciton diffusion length. The morphology depends mainly on the block copolymer composition and can thus easily be adapted in order to optimize device performances. This control over morphology therefore allows one to study the relationship between active layer morphology, interfaces and device performance. In order to fully exploit this block copolymer concept, several rod-coil semiconducting diblock copolymers consisting of a conjugated block and a second coil block functionalized with electron transporting and/or accepting materials (such as oxadiazole or C$_{60})$ were synthesized. The conjugated block acting as light absorbing, electron donating and hole transporting material. The donor/acceptor photovoltaic devices, with active layer the above mentioned semiconducting block copolymers, were used to investigate the relation between the photovoltaic cell performance and the thin- film morphology involving the spatial distribution of donor and acceptor phases within the active layer. [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 1:03PM |
J4.00003: Structural Influences on Conjugated Polymer Optoelectronic Properties Invited Speaker: This paper will focus on understanding the influence that structure has on the optoelectronic properties of device relevant conjugated polymer thin films. In particular, information will be provided on the development and utilization of spectroscopic probes to monitor changes in electronic properties for polyfluorene films and devices. Poly(9,9-dioctylfluorene) [PFO] and its copolymer derivatives provide a suitable test bed for such studies: PFO is especially instructive as it possesses a wide variety of morphologies amongst which a thin film can be readily transformed. Optical measurements using absorption, ellipsometry, fluorescence and Raman spectroscopies will be reported that probe phase transition temperatures, optical constants and their anisotropies (birefringence), and the influence of degradation and microscopic environment on the emission characteristics of thin films. In addition, device-related studies concerning charge transport and optical gain will also be reported. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:39PM |
J4.00004: Water-Soluble Conjugated Polymers: Self-Assembly and Biosensor Applications Invited Speaker: Homogeneous assays can be designed which take advantage of the optical amplification of conjugated polymers and the self-assembly characteristic of aqueous polyelectrolytes. For example, a ssDNA sequence sensor comprises an aqueous solution containing a cationic water soluble conjugated polymer such as poly(9,9-bis(trimethylammonium)-hexyl)-fluorene phenylene) with a peptide nucleic acid (PNA) labeled with a dye (PNA-C*). Signal transduction is controlled by hybridization of the neutral PNA-C* probe and the negative ssDNA target, resulting in favorable electrostatic interactions between the hybrid complex and the cationic polymer. Distance requirements for F\"{o}rster energy transfer are thus met \textit{only} when ssDNA of complementary sequence to the PNA-C* probe is present. Signal amplification by the conjugated polymer provides fluorescein emission $>$25 times higher than that of the directly excited dye. Transduction by electrostatic interactions followed by energy transfer is a general strategy. Examples involving other biomolecular recognition events, such as DNA/DNA, RNA/protein and RNA/RNA, will also be provided. The mechanism of biosensing will be discussed, with special attention to the varying contributions of hydrophobic and electrostatic forces, polymer conformation, charge density, local concentration of C*s and tailored defect sites for aggregation-induced optical changes. Finally, the water solubility of these conjugated polymers opens possibilities for spin casting onto organic materials, without dissolving the underlying layers. This property is useful for fabricating multilayer organic optoelectronic devices by simple solution techniques. [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 2:15PM |
J4.00005: Understanding the Intra- and Interchain Electronic Structure of Conjugated Polymers by Encapsulation in Mesoporous Silica Invited Speaker: Benjamin Schwartz It is becoming increasingly clear that the morphology of a conjugated polymer sample -- that is, the conformation of the individual polymer chains and the way they pack together -- plays a direct role in the electronic properties conjugated polymer films. In this talk, we describe the results of work that provides a new method for controlling conjugated polymer morphology and hence electronic properties: encapsulation of the polymer chains into the channels of aligned mesoporous silicas. We find, for example, that in small-pore silica samples where only a single polymer chain can fit into each pore, essentially no polarons are formed upon excitation and phenomena such as exciton-exciton annihilation that are prevalent in bulk films do not occur. We also find that energy transfer (exciton migration) occurs much more slowly along the polymer backbone than between polymer chains, suggesting that the predominant mechanism for exciton diffusion in bulk films is interchain Forster energy transfer. In medium-sized pores that contain just a few polymer chains, we find that the chains are still aligned by the encapsulation, and that polarons can be formed upon directly upon excitation. We also see that exciton-exciton annihilation is now possible, but that it occurs predominantly at kinks or defect sites. In still larger pores, the polymer chains exhibit more bulk film-like properties, including a high degree of photogeneration of polarons and exciton-exciton annihilation. We also show that in the intermediate pore size regime, the alignment of the polymer chromophores leads to significantly lower lasing thresholds than bulk films of comparable optical density, even though the physical density of the chromophores encapsulated into the porous silica is much lower than that in bulk films. Taken together, the results allow a new picture of how interactions between polymer chains control the electronic properties of conjugated polymer films. [Preview Abstract] |
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