PFO Tight Binding / Beta Phase

PFO Tight Binding / Beta Phase

Based on part of a talk as delivered to MRS Boston December 2012

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Jarvist Moore Frost

December 05, 2012
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Transcript

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    Computational Design of Materials for Organic Photovoltaics Jarvist Moore Frost,

    Sam Foster, James Kirkpatrick, Thomas Kirchartz, Jenny Nelson Imperial College London jarvist.frost@imperial.ac.uk Originally: MRS Boston Fall 2012 – O8.08 Wednesday 28th November
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    Tight Binding - The Hückel Method [[ S J 0.

    0. ] [ J S J 0. ] [ 0. J S J ] [ 0. 0. J S ]] H = Hamiltonian very easy to solve Parameters semi-empirical (can derive from DFT) Can treat extremely large systems
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    PF8 Proposed Structures Grell et al. (Macromolecules 1999, 32, 5810-5817)

    Chen et al. (Macromolecules 2004, 37, 6833-6838) Planar Zig-Zag [2,1 Helix] Complex Unit Cell [Longer Helix]
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    Predicted Mobility in PF8 vs. PF5:8 cm2 / Vs Simulated

    Hole Mobilities Simulated Tetramer Densities
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    Processing magic → di-octyl Polyfluorene can be persuaded to have

    high mobilities Samples are glassy Induction of the Beta-phase strongly reduces mobility
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    β-Phase α-Phase HOMO Spin Density B3LYP/6-31g* "Formation of the β-phase

    effectively corresponds to crystallization in one dimension, a remarkably uncommon phenomenon in nature." http://pubs.acs.org/doi/pdfplus/10.1021/nl071207u
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    0 1 2 3 4 5 6 7 eV In

    Vacuum, Beta Phase structures are strained But at accessible energies
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    Need larger systems for representative DoS → Python TightBinding http://www.physics.rutgers.edu/pythtb/

    Hybrid DFT on Octamers (B3LYP/6-31g* & tuned BNL/6-31g*) - get some trap formation (up to 140 meV)
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    Beta Phase Energetic Trap (10 sites) For Wide Enough trap

    ~10 sites (no confinement), Trap depth = site Energy change
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    P3HT–like polymer (normally flat) A pproxim ate torsional distribution as

    norm al distribution C onsider beta phase to be sm all pockets of flat regions – (i.e. J is m axim ised is these regions to J0)
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    Polyfluorene–like polymer (Helix / twisted rod) • Approximate torsional distribution

    as normal distribution (around 45 degree minima) • Consider beta phase to be small pockets of flat regions of polymer – (i.e. J is maximised is these regions to J0)
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    Conclusions • In polymers, Torsion is important – In P3HT,

    torsion suffcient to describe DoS – In PFO, natural background twist makes extended regions (Beta phase) deep traps – Traps are not always what one imagines (and be careful what you wish for) Even (or especially?) 1930s electronic-structure techniques can offer useful insights