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Charge transport in organic semiconductors and organic field effect transistors

Andrej Golubkov. Charge transport in organic semiconductors and organic field effect transistors. IF – Seminar, Graz, 5.11.2007. Egbert Zojer Peter Pacher Harald Etschmaier. Co-Workers. Part I: Charge transport models Comparison to inorganic semiconductors Drude model

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Charge transport in organic semiconductors and organic field effect transistors

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  1. Andrej Golubkov Charge transport in organic semiconductors andorganic field effect transistors IF – Seminar, Graz, 5.11.2007

  2. Egbert Zojer • Peter Pacher • Harald Etschmaier Co-Workers

  3. Part I: Charge transport models • Comparison to inorganic semiconductors • Drude model • Hopping transport models • Part II: Building and analyzing FETs • Building process • temperature dependend Measurements • Part III: Parameter extraction and first results • Parameter extraction • Mobility vs. temperature Outline

  4. Part I: Charge carrier transport models. overview based on reviews from Gilles Horowitz

  5. (Molecular) Crystal Comparison Covalent Metallic Gilles Horowitz

  6. sp2 hybridization of carbon • 3 σ bonds from (2s, 2px, 2py)‏ • 1 π bond from 2pz Conjugated organic materials

  7. free moving carriers, acceleration by external field • scattering at phonons, impurieties Simple band transport: Drude model

  8. Simple band transport: Drude model • Temperature dependence of vth • Temperature dependence of scattering process: mean free path between phonons, charged impurities W. Warta

  9. Polarization by one single charge • dressed (by π- electros) charge Polaron transport Gilles Horowitz residence time bandwidth el. polarization time bandgap

  10. Polaron transport Gilles Horowitz Silinish E.A, Capek V.

  11. Charge modulated spectroscopy Polaron transport Silinish E.A, Capek V. Peter J. Brown Eg

  12. useful for disordered materials (polymers)‏ • Bässler's model • transport by hopping between localized states • (Polarisation) Energy of the states fluctuates • DOS is described by gausian distribution of variance σ • charge transport: random walk Hopping (polaron) Transport energy difference intersite distance

  13. Perculation theory by Vissenberg & Matters • Variable range hopping among exponential DOS • Conduction through 'resistor network': infinite cluster with the highest conductivity is relevant • Gate voltage (charge density) dependence Hopping (polaron) Transport

  14. 300 K 180 K 117 K Hopping (polaron) Transport Pentacene A. R. Brown

  15. Assumptions • carriers arriving at trap -> capture • release is thermaly activated • 2 sorts of carriers Multiple trapping & thermal release Gilles Horowitz

  16. Gate Voltage (charge carrier density) dependence • Upon aplied Vg a potential Vs develops at insulator-semiconductor interface • shift of EF towards EC • trapped carrier release becomes easier Multiple trapping & thermal release

  17. Gate Voltage (charge carrier density) dependence Multiple trapping & thermal release

  18. Multiple trapping & thermal release • Alternative approach • effective mobility vs. effective charge density • Hall-effect measurements

  19. Multiple trapping & thermal release • Hall-effect results on rubrene single crystal Menard et al Podzorov, Menard, Rogers, Gershenson

  20. Drude • Bässler • Vissenberg Matters • MTR Summary

  21. Part II: Device fabricationandmeasurement setups

  22. OTFT, Fabrication and measurements

  23. Cryostat

  24. Part III: Parameter extractionand first results

  25. some differences to conventional OFETs • can potentially operate in electron and hole accumulation mode OFET

  26. basic OFET operation • related to a capacitor OFET

  27. OFET output curve, logarithmic output curve, linear

  28. green: Vd = 10V cyan: Vd = 45V Symobols: different devices 300 K 150 K

  29. Thank You for Your Attention!

  30. transition between transport modes Trap dominated Intrinsic

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