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8. Optical processes in conjugated materials

8. Optical processes in conjugated materials. Cambridge Display Technology. Full color display. - Active matrix. - 2 inch diagonal. - 200 x 150 Pixels. 8.1. Electron-Phonon Coupling. E. Excitations. Lowest excitation state. Relaxation effects. Emission. Absorption. Ground state.

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8. Optical processes in conjugated materials

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  1. 8. Optical processes in conjugated materials Cambridge Display Technology Full color display - Active matrix - 2 inch diagonal - 200 x 150 Pixels

  2. 8.1. Electron-Phonon Coupling E Excitations Lowest excitation state Relaxation effects Emission Absorption Ground state Q

  3. 8.1.1. Fluorescence Polyfluorene (F8) - Weak self-absorption - Vibronic structure Carlos Silva, University of Cambridge

  4. 3 2 3 2 6 2 5 4 3 4 6 4 1 6 4 4 8.1.2. Intrachain Exciton INDO/SCI Lowest excited state Exciton=electron-hole pair Binding energy  0.3 eV Exciton size Probability to find the e- and h+ at one site Site number

  5. 8.2. Conjugation length 8.2.1. Optical transition versus chain size emission absorption absorption emission 1/m (m=number of bonds) Cornil, J. et al. Chem. Phys. Lett. (1997), 278, 139 The “conjugation length” is the length of the oligomer emitting the same luminescence spectrum as the polymer. While the polymer may easily be 10-100 times longer than a conjugation length, the chain is effectively operating as a sequence of conjugation lengths along a common string. This description is valid for the behaviour of absorption processes; where emission is relevant, the excited state is often more localized.

  6. 8.2.1. Conformation changes • Switch between different structures by applying mechanical force while monitoring the Langmuir monolayer's optical spectra. The figure shows the chemical structures, conformations and spatial arrangements at the air–water interface of the polymer. • Compression causes a transition from the face-on to the zipper structure, which breaks the conjugation, i.e. decreases the π-conjugation length and generates a large blue shift (34-nm). Kim et al. Nature411, 1030 - 1034 (2001)

  7. PRIMÄR EFFEKT R Gult område: pz densitet R SEKUNDÄR EFFEKT ENERGI k PRIMÄR EFFEKT: Tillför (tar bort) laddning R R R ENERGI w < W R k SEKUNDÄR EFFEKT Vridning av ring minskar pz-pz överlapp Bandgap och Dispersion via Sidogrupper

  8. 8.3. Influence of Electroactive Substituents Al Need for small energy barriers to optimize hole/electron injection  Molecular engineeringto modulate the energy of the band edges E + 0.08 eV - 1.17 eV + 0.27 eV - 0.99 eV n n n

  9. Donor: • σ-donor (electronegativity): symmetric destabilization • π-donor: asymmetric destabilization • Acceptor: • σ-acceptor : symmetric stabilization • π-acceptor : asymmetric stabilization • Note that ”-O-CH3” acts as a globally as a donor. This is the results of a competition between its π-donor and σ-acceptor characters.

  10. 8.4. Modulation of the Optical Properties 8.4.1 Structure of the conjugated chain - Molecular backbone Red Yellow-Green Blue - Chain size K. Müllen and co

  11. 8.4.2 Optical properties and Doping E L H Polaron Bipolaron

  12. Electrochromism Doubly charged Neutral Singly charged

  13. Electrochromism in a substituted polythiophene, under electrochemical doping in contact with an electrolyte. The suppression of bandgap absorption in the polymer (with a maximum at 500 nm) due to doping is highly visible; formation of polarons is hardly visible, but the two optical transitions due to bipolarons are found, one peaking at 800 nm and another below 1200 nm. From Peter Åsberg, work in progress, Biorgel, IFM, LiU

  14. Electrochromic Displays on Papers Prof. M. Berggren, Norrköping

  15.  qK rK =   qK= 0 - 1Ag 1Bu + K K 8.5. Solid State Effect: Exciton Splitting 8.5.1. Transition Dipole Moment N = 20 1Ag 1Bu INDO/SCI Atomic transition densities

  16. - - + + + - - + 8.5.2. H-Aggregate 1 Cofacial dimer 2 E2 tot = 2  E E 2 tot = 0 E1  G G

  17. - - + + + - - + 8.5.3.J-Aggregate E2 tot = 0 E E 2 tot = 2  E1  G G

  18. 8.6. Charge and energy transfer in conjugated polymers Glass ITO e h LUMO LUMO HOMO HOMO Organic Solar Cells Energy transfer Charge transfer

  19. 8.6.1.Photoinduced Charge Transfer E LUMO LUMO Photoinduced ELECTRON transfer HOMO HOMO E LUMO LUMO Photoinduced HOLE transfer HOMO HOMO

  20. Chemical Sensors TNT Photoinduced Electron Transfer Land-mine detector L (Detection limit : 10-15 g) H Polymer TNT Tim Swager and co, MIT

  21. 8.6.2. Polymer / Polymer Interfaces DMOS-PPV CN-PPV MEH-PPV L L 0.63 eV 0.55 eV H H 0.17 eV 0.44 eV

  22. 8.6.3. Charge transfer MEH-PPV / CN-PPV Blend MEH-PPV CN-PPV L 0.63 eV H 0.44 eV J.J.M. Halls, J. Cornil, et al., Phys. Rev. B 60, 5721 (1999)

  23. 8.6.4. Energy transfer DMOS-PPV / CN-PPV Blend DMOS-PPV CN-PPV L 0.55 eV H 0.17 eV

  24. 8.6.5. Charge versus Energy Transfer MEH-PPV / CN-PPV Blend : Charge transfer Penalty to pay to dissociate an exciton on the order of 0.35 eV Excited states One-electron levels L 0.63 eV INTRA MEH-PPV 0.28 eV 0.19 eV INTRA CN-PPV INTRA INTER INTER INTRA H 0.44 eV GROUND STATE MEH-PPV CN-PPV Charge transfer at the polymer/polymer interface

  25. DMOS-PPV / CN-PPV Blend : Energy transfer Penalty to pay to dissociate an exciton on the order of 0.35 eV Excited states One-electron levels L 0.55 eV INTRA DMOS-PPV 0.20 eV INTER INTRA INTER 0.38 eV INTRA CN-PPV INTRA H 0.17 eV GROUND STATE DMOS-PPV CN-PPV Energy transfer towards the CN-PPV chains

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