Charge transport in organic semiconductors and organic field effect transistors

Charge transport in organic

semiconductors and

organic field effect transistors

Andrej Golubkov

IF – Seminar, Graz, 5.11.2007

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.20052

Andrej Golubkov Charge transport in organic semiconductors and [email protected]

Co-Workers

Egbert Zojer

Peter Pacher

Harald Etschmaier




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




Charge carrier transport models.

Part I:

overview based on reviews from

Gilles Horowitz




Comparison

(Molecular) Crystal

Covalent Metallic

Gilles Horowitz




Conjugated organic materials

sp2 hybridization of carbon

3 σ bonds from (2s, 2px, 2p

y)

1 π bond from 2pz




Simple band transport: Drude model free moving carriers, acceleration by external field

scattering at phonons, impurieties




Temperature dependence of vth

Temperature dependence of scattering process:

mean free path between

phonons, charged impurities

Simple band transport: Drude model

W. Warta




Polaron transport

Polarization by one single charge

dressed (by π- electros) charge

residence time

el. polarization time

bandwidth

bandgap

Gilles Horowitz




Polaron transport

Gilles Horowitz

Silinish E.A, Capek V.




Polaron transport

Silinish E.A, Capek V.

Eg

Charge modulated spectroscopy

Peter J. Brown




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

energy difference

intersite distance





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





Pentacene

300 K 180 K 117 K

A. R. Brown




Multiple trapping & thermal release

Assumptions– carriers arriving at trap -> capture

– release is thermaly activated

– 2 sorts of carriers

Gilles Horowitz





Gate Voltage (charge carrier density) dependence– Upon aplied Vg a potential Vs develops at insulator-semiconductor

interface

– shift of EF towards E

C

– trapped carrier release becomes easier





Gate Voltage (charge carrier density) dependence





Alternative approach– effective mobility vs. effective charge density

– Hall-effect measurements





Hall-effect results on rubrene single crystal

Menard et al

Podzorov, Menard, Rogers, Gershenson




Summary Drude

Bässler

Vissenberg Matters

MTR




Device fabrication

and

measurement setups

Part II:




OTFT, Fabrication and measurements






















Cryostat







Parameter extraction

and

first results

Part III:




OFET

some differences to conventional OFETs– can potentially operate in electron and hole accumulation mode




OFET

basic OFET operation– related to a capacitor




OFET

output curve, logarithmic output curve, linear




green: Vd = 10V

cyan: Vd = 45V

Symobols:

different devices

300 K 150 K




Thank You for Your Attention!




transition between transport modes

Trap dominated Intrinsic

Charge transport in organic semiconductors and organic field effect transistors

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