Organic Materials and Electronic Transport

Organic Electronic, Fall 2005, Dr. Dietmar Knipp

Structural properties and electronic transport

Information:http://www.faculty.iu-bremen.de/course/c300331a/

Source: Apple

Ref.: Apple

Information:http://www.faculty.iu-bremen.de/course/c420411/

Dielectric

e e e e e e e e

Gate

VGNeutral substrate

Source Drain

VD

h h h h h h h h h h

101110-110-210-310-410-510-610-8

Critical dimension (m)10-710-9

Introduction to Organic Electronics(Nanomolecular Science Seminar I)(Course Number 420411 ) Fall 2005

Organic materials and electronic Transport

Instructor: Dr. Dietmar Knipp



Introduction to Organic Electronics

2 Structural properties and Electronic Transport2.1 General properties of organic and polymeric semiconducting material2.2 Organic and polymeric semiconducting materials2.3 Organic molecules2.4 From a single molecule to a solid2.5 Bandgap in organic solids2.6 Structural order of materials2.7 The unit cell2.8 Structural order in molecular solids2.9 Electronic Transport

2.9.1 Thermal movement of carriers2.9.2 Band-like transport2.9.3 Grain boundaries in polycrystalline material2.9.4 Trap-controlled transport2.9.5 Hopping transport

References



2.1 General properties of organic and polymeric semiconducting materials

Advantages:

Low cost processing

Large area compatible

Low temperature processing

Tailoring of electronic and optical properties

Certain properties not easily attainable with conventional materials

Disadvantages:

Low carrier mobility

Stability

Patterning of films

Novel fabrication technology required

Tetracene



2.2 Organic and polymeric semiconducting materialsTwo general classes of materials exist:

Antracene

Tetracene

Pentacene

Phthalocyanine

N

N

N

N

N

N

N

N

M

Organic molecules

Perylene

SS

C8 C8

poly(9,9-dioctylfluorene-co-bithiophene) (F8T2)

Polymers

Poly(3-hexyl thiophene) (P3HT)

XPT: regio-regular poly(thiophene)



2.3 Organic Molecules

Hydrocarbons, the simplest organic molecules, contain only carbon and hydrogen atoms. They can be subdivided in Alkanes, Alkenes, Alkynes, Arenes.

Arenes and AromaticityArenes are hydrocarbons based on benzene units. The simplest, yet the most important compound in this class of organic compounds is benzene.

"Aromatic" was originally used to describe these compounds since many have pleasent smells. To the chemist, the word aromatic also carries with it stabilityand reactivity implications. The unusual stability of benzene compared to closely related alkenes is what makes it important and gives benzene its own set of characteristic reactions.



2.3 Organic Molecules

Polyaromatic Hydrocarbons

Larger systems of benzene rings fused together are known.

These are the polyaromatic hydrocarbons. A collection of images of some common systems are shown.

Chemical stability of these molecules decreases as the size of the molecule increases. (e.x. pentacene and hexacene oxidize readily in air, while benzene, naphtalene, and anthraceneare stable in absence of light).



-electron overlap between adjacent carbon atoms: leads to delocalization Within a single molecule there is very good electronic overlap

pi molecular orbitals for benzene. With 6 C atoms contributing to the p system, we need to create 6 molecular orbitals.

2.3 Organic MoleculesEnergy distribution of Benzene

Ref.: I. Hunt, University of Calgary



2.4 From a single molecule to a solid

Orbital overlap The extent of p-orbital overlap between adjacent molecules Depends on the direction (in 3-D) Extent of orbital overlap determines bandgap

Bandgap The gap or distance between the min. and max points of a band. Typical bandgaps are in the range of 1.5 to 5 eV

Structural order in the material The structural order of the material is closely related to the electronic

properties of the material. (This even applies to polymers.)



2.5 Bandgap in organic solids

Electronic states given rise to valence (HOMO level) and conduction bands (LUMO level). The bands are shown for a series of materials from benzene to pentacene. The dashed line corresponds to the Fermi level. The electronic states are given for the gas phase and a solid. Ref.: N. Karl, University Stuttgart



Amorphous materials

Poly crystallinematerials

(Mono)Crystallinematerials

No long-range order

Completely ordered in segments

Entirely ordered solid

Ref.: R.F. Pierret, Semiconductor Fundamentals

The structural properties of organic solid depends on the molecule itself, its electrical structure, the substrate and the growth conditions (temperature, deposition rate, flow of material) or preparation conditions.

2.5 Structural order of materials



2.7 The unit cell

The periodic arrangement of atoms is called lattice!

A unit cell of a material represents the entire lattice. By repeating the unit cell throughout the crystal, one can generate the entire lattice.

A unit cell can be characterized by a vector R, where a, b and c are vectors and m, n and p are integers, so that each point of a lattice can be found.

R=ma+nb+pc

The vectors a, b, and c are called the lattice constants.

Primitive unit cell.

Ref.: M.S. Sze, Semiconductor Devices



2.7 The unit cell

Different unit cells based on cubic unit cells


Simple cubic unit

cell

Body centered cubic unit cell

(bcc)

Face centered cubic unit cell

(fcc)



2.7 The unit cells

Ref.: Joseph R. Smyth, Geology 3010: Introduction to Mineralogy



2.8 Structural order in molecular solids

Organic materials can form very highlyordered crystals

Van der Waals forces keep these crystalstogether.

These crystals can have a bandstructure just like any othersemiconductor if the crystals are highly order and the concentration of impurities is very low.

Antracene single crystal (Ref.: University Stuttgart, N. Karl).



Electronic transport due to an overlap of orbitalsThermal Evaporation: Source temperature: 275-300C

Material: 0, 1 or 2 times sublimation purified

Substrate temperature: rt-110C

a

c

b

Pentacene, C22H14 :Aromatic hydrocarbons based on linear arranged benzene rings

Crystal structure: Triclinic: a b c, 90

Substrate view




Substrate temperature

2.5m 2.5m2.5m

Pentacene on thermal oxide

Substrate at rt Substrate at 70C Substrate at 90C

Crystal size

Atomic force micrographs of thermally evaporated pentacene films (200nm).




X-ray diffraction pattern of pentacene on thermal oxide

0

15

30

45

60

75

90

10 20 30

(003)17.22

(002)11.46

(001)5.743

2 scan

16

Substrate view

D

i

f

f

r

a

c

t

i

o

n

i

n

t

e

n

s

i

t

y

[

a

.

u

.

]

The pentacene film was prepared at room temperature.




0.01

0.1

1

0.1 1 10

room temperature70C90C

R

a

t

i

o

o

f

x

-

r

a

y

d

i

f

f

r

a

c

t

i

o

n

(

0

0

1

)

/

(

0

0

1

)

average crystal size [m]

X-ray diffraction pattern of pentacene on thermal oxide

Relation between the average crystal size and the ratio of the diffraction peaks.




2.9 Electronic Transport

The structural order of the material is closely related to the electronic properties of the material.

Structural Order10-6

10-4

10-2

100

M

o

b

i

l

i

t

y

[

c

m

2

/

V

s

]

Band-like transport

Hopping transport

Disorder

Grain boundaries or trap-controlled

transport

- Transistors High mobility materials and its appications

Low mobility materials and its applications- Photoconductors- Organic LEDs



2.9 Electronic TransportElectrons in the conduction band and holes in the valence band are able to move upon thermal activation, a gradient or an applied electric field. In the following the concepts of electronic transport in crystalline materials will be described.

2.9.1 Thermal movement of carriersElectrons in the conduction or holes in the valence band can essentially be treated as free carriers or free particles. Even in the absence of an electric field the carriers follow a thermally activated random motion. In thermal equilibrium the average thermal energy of a particle (electron or hole) can be obtained from the theorem for equipartition of

The thermal energy of the particle is equal to the kinetic energy of the electron, so that the velocity of the particle can be calculated. The mass of the electron is equal to the effective mass of the electron.

kTEthermalaverage 23= Average thermal energy of an electron / hole



2.9.1 Thermal movement of carriers

Furthermore, the velocity of the electron corresponds to the thermal velocity of the electron, so that the thermal velocity can be determined by:

At room temperature the average thermal velocity of an electron is about 105m/s in silicon and GaAs.

Thermal motion of free carriers can be seen as random collision (scattering) of the free carriers with the crystal lattice. A random motion of an electron or hole leads to zero net displacement of the free carrier over a sufficient long distance / period of time. The average distance between two collisions within the crystal lattice is called mean free path. Associated to the mean free pathwe can introduce a mean free time . A typical mean free path is in the range of 100nm and the mean free time is in the range of 1ps.

effth m

kTv 3= Thermal velocity of an electron

2

21

theffkin vmE = Kinetic energy of an electron / hole



2.9.2 Band-like transportWhen a small electric field is applied to the semiconductor material each free carrier will experience an electro static force

So that the carrier is accelerated along the field (in opposite direction of the field).

Schematic path of an electron in a semiconductor (a) random thermal motion, (b) combined motion due to random thermal motion and an applied electric field.

F=0 F

qFForce =




2.9.2 Band-like transportAn additional velocity component will be superimposed upon the thermal motion of the electron. The additional velocity is caused by an applied electric field F. The additional component is called drift velocity. The drift of the electrons can be described by a steady state motion since the gained momentum is lost due to collisions of the electrons and the lattice.

Based on momentum conservation the drift velocity can be calculated. The drift velocity is proportional to the applied electric field F.

nnvmP =

Fmqvn

Cn

=

CFqP =

Electron drift velocity



2.9.2 Band-like transport

The mobility is an important electronic transport parameter. The mobility directly related a the material properties. Rewriting of the expression for the drift velocity leads to

The mobility is directly related to the mean free time between two collisions, which is determined by various scattering mechanisms. The most important scattering mechanisms are lattice scattering and impurity scattering. Lattice scattering is caused by thermal vibrations of the lattice atoms at any temperature above 0K. Due to the vibrations energy can be transferred from the carriers and the lattice.

n

Cn mq

p

Vp mq

Fvp

p =

Electron and hole mobility

Fvn

n = Electron and hole mobility




Carriers can move from one molecule to the next molecule.Quantified by mobility

Mean free path > intermolecular spacing

Temperature dependent behavior:

( ) 23TT

Ref.: N. Karl, University Stuttgart



20 100 30010-3

10-2

10-1

100

101

102

increasing Nt

tf

Temperature (K)

M

o

b

i

l

i

t

y

(

c

m

2

/

V

s

)

Exponential Decrease of Et ~ 40 - 50 meV

Dependence on Trap Density NtNt ~ 1016 - 1018 cm-3

Trap-Free Limit : Power Law T-nn ~ 1.6 - 2.3, phonon scattering

Ref.: Dodabalapur, University Texas, lecture notes EE 396K


Influence of traps on the electronic transport:

( ) ( )

+

=kTE

NN

TTtraps

bulk

trapseff

exp1

0



Pentacene thin film transistor

Exp.fit

2-5m

100-200nm

0.001

0.01

0.1

1

100 200 300 400

temperature [K]

m

o

b

i

l

i

t

y

[

c

m

2

/

V

s

]

2.9.3 Grain boundaries in polycrystalline material

The mobility decreases with decreasing characterization temperature.

Temperature dependent mobility can be explained by a barrier model.

Smaller crystals leads to higher grain boundary traps density.

L

EC

EV

EF

Ei

EB

NT

=+= kTEB

GBGBGB exp011

01



2.9.4 Trap-controlled transport

eff =c - carrier mobility in extended states

Deep traps

Band-tail states

E = 0

E

n

e

r

g

y

DOS, g(E)

Extended states

Mobility edge (E = 0)

Localized states

(Act

iva t

ion)

ene

rgy

Density-of-states distribution

Trapping and release of charges

Ref.: V. Arkhipov, IMEC



2.9.5 Hoping transport

Jumps over-barriers dominate at higher temperatures

At lower temperatures tunneling of carriers take over

Most hopping models assume:

-positions of hopping sites are completely random

- positions and energies of hoping sites are uncorrelated E

nerg

yE

n erg

y

Ref.: V. Arkhipov, IMEC



References

Pope and Swenburg, Electronic Processes in organic crystals and polymers, 2 ndEd., Oxford

Organic molecular crystals, E.A. Sininsh EA and V. Capek.

http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/6-973Organic-OptoelectronicsSpring2003/CourseHome/

(Organic optoelectronic lecture MIT)

Documents

Organic Materials and Electronic Transport