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 oTFTsgolubkov@sbox.tugraz.at

Co-Workers

Egbert Zojer

Peter Pacher

Harald Etschmaier

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.20053

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

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

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.20054

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Charge carrier transport models.

Part I:

overview based on reviews from

Gilles Horowitz

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.20055

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Comparison

(Molecular) Crystal

Covalent Metallic

Gilles Horowitz

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.20056

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Conjugated organic materials

sp2 hybridization of carbon

3 σ bonds from (2s, 2px, 2p

y)

1 π bond from 2pz

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.20057

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

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

scattering at phonons, impurieties

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.20058

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Temperature dependence of vth

Temperature dependence of scattering process:

mean free path between

phonons, charged impurities

Simple band transport: Drude model

W. Warta

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.20059

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Polaron transport

Polarization by one single charge

dressed (by π- electros) charge

residence time

el. polarization time

bandwidth

bandgap

Gilles Horowitz

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200510

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Polaron transport

Gilles Horowitz

Silinish E.A, Capek V.

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200511

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Polaron transport

Silinish E.A, Capek V.

Eg

Charge modulated spectroscopy

Peter J. Brown

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200512

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

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

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200513

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Hopping (polaron) Transport

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

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200514

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Hopping (polaron) Transport

Pentacene

300 K 180 K 117 K

A. R. Brown

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200515

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Multiple trapping & thermal release

Assumptions– carriers arriving at trap -> capture

– release is thermaly activated

– 2 sorts of carriers

Gilles Horowitz

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200516

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Multiple trapping & thermal release

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

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200517

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Multiple trapping & thermal release

Gate Voltage (charge carrier density) dependence

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200518

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Multiple trapping & thermal release

Alternative approach– effective mobility vs. effective charge density

– Hall-effect measurements

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200519

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Multiple trapping & thermal release

Hall-effect results on rubrene single crystal

Menard et al

Podzorov, Menard, Rogers, Gershenson

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200520

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Summary Drude

Bässler

Vissenberg Matters

MTR

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200521

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Device fabrication

and

measurement setups

Part II:

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200522

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

OTFT, Fabrication and measurements

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200523

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200524

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200525

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200526

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200527

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200528

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200529

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Cryostat

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200530

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200531

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Parameter extraction

and

first results

Part III:

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200532

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

OFET

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

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200533

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

OFET

basic OFET operation– related to a capacitor

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200534

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

OFET

output curve, logarithmic output curve, linear

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200535

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

green: Vd = 10V

cyan: Vd = 45V

Symobols:

different devices

300 K 150 K

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200536

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

Thank You for Your Attention!

University of Technology Graz - Institute of Solid State Physics

Professor Horst Cerjak, 19.12.200537

Andrej Golubkov Charge transport in organic semiconductors and oTFTsgolubkov@sbox.tugraz.at

transition between transport modes

Trap dominated Intrinsic