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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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andrej golubkov
Andrej GolubkovCharge transport in organic semiconductors andorganic field effect transistors

IF – Seminar, Graz, 5.11.2007

co workers
Egbert Zojer
  • Peter Pacher
  • Harald Etschmaier
Co-Workers
outline
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
part i
Part I:Charge carrier transport models.

overview based on reviews from Gilles Horowitz

comparison

(Molecular) Crystal

Comparison

Covalent

Metallic

Gilles Horowitz

conjugated organic materials
sp2 hybridization of carbon
  • 3 σ bonds from (2s, 2px, 2py)‏
  • 1 π bond from 2pz
Conjugated organic materials
slide8

Simple band transport: Drude model

  • Temperature dependence of vth
  • Temperature dependence of scattering process:

mean free path between

phonons, charged impurities

W. Warta

polaron transport
Polarization by one single charge
  • dressed (by π- electros) charge
Polaron transport

Gilles Horowitz

residence time

bandwidth

el. polarization time

bandgap

polaron transport1
Polaron transport

Gilles Horowitz

Silinish E.A, Capek V.

polaron transport2

Charge modulated spectroscopy

Polaron transport

Silinish E.A, Capek V.

Peter J. Brown

Eg

hopping polaron transport

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

hopping polaron transport1

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
hopping polaron transport2

300 K

180 K

117 K

Hopping (polaron) Transport

Pentacene

A. R. Brown

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

Gilles Horowitz

multiple trapping thermal release1
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
slide18

Multiple trapping & thermal release

  • Alternative approach
    • effective mobility vs. effective charge density
    • Hall-effect measurements
slide19

Multiple trapping & thermal release

  • Hall-effect results on rubrene single crystal

Menard et al

Podzorov, Menard, Rogers, Gershenson

summary
Drude
  • Bässler
  • Vissenberg Matters
  • MTR
Summary
slide32

some differences to conventional OFETs

    • can potentially operate in electron and hole accumulation mode
OFET
slide33

basic OFET operation

    • related to a capacitor
OFET
slide34
OFET

output curve, logarithmic

output curve, linear

slide35

green: Vd = 10V

cyan: Vd = 45V

Symobols:

different devices

300 K

150 K

transition between transport modes
transition between transport modes

Trap dominated

Intrinsic