Download Introduction to Organic Electronics

Organic Electronic, Fall 2005, Dr. Dietmar Knipp
Introduction to Organic Electronics
(Nanomolecular Science Seminar I)
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http://www.faculty.iuhttp://www.faculty.iubremen.de/course/c30
bremen.de/course/c42
0331a/
0411/
(Course Number 420411 ) Fall 2005
Organic materials and electronic Transport
Instructor: Dr. Dietmar Knipp
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Critical dimension (m)
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
Introduction to Organic Electronics
2
Structural properties and Electronic Transport
2.1 General properties of organic and polymeric semiconducting material
2.2 Organic and polymeric semiconducting materials
2.3 Organic molecules
2.4 From a single molecule to a solid
2.5 Bandgap in organic solids
2.6 Structural order of materials
2.7 The unit cell
2.8 Structural order in molecular solids
2.9 Electronic Transport
2.9.1 Thermal movement of carriers
2.9.2 Band-like transport
2.9.3 Grain boundaries in polycrystalline material
2.9.4 Trap-controlled transport
2.9.5 Hopping transport
References
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.1 General properties of organic and polymeric
semiconducting materials
Advantages:
Tetracene
•Low cost processing
•Large area compatible
•Low temperature processing
•Tailoring of electronic and optical
properties
•Certain properties not easily
attainable with conventional materials
Structural properties and electronic transport
Disadvantages:
•Low carrier mobility
•Stability
•Patterning of films
•Novel fabrication
technology required
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.2 Organic and polymeric semiconducting materials
Two general classes of materials exist:
Organic molecules
Polymers
Poly(3-hexyl
thiophene) (P3HT)
Antracene
Tetracene
S
Pentacene
S
C8
N
N
N
N
M
N
C8
poly(9,9-dioctylfluorene-cobithiophene) (F8T2)
N
N
N
Phthalocyanine
Perylene
XPT: regio-regular poly(thiophene)
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
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 Aromaticity
Arenes 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 stability
and 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.
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
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 anthracene
are stable in absence of
light).
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.3 Organic Molecules
Energy distribution of Benzene
pi molecular orbitals for
benzene. With 6 C atoms
contributing to the p system,
we need to create 6 molecular
orbitals.
Ref.: I. Hunt, University of Calgary
π-electron overlap between adjacent carbon atoms: leads to delocalization
Within a single molecule there is very good electronic overlap
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
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.)
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
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
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.5 Structural order of materials
Amorphous
materials
No long-range
order
Poly crystalline
materials
Completely ordered
in segments
(Mono)Crystalline
materials
Entirely ordered
solid
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.
Ref.: R.F. Pierret, Semiconductor Fundamentals
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
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.
Primitive unit cell.
R=ma+nb+pc
The vectors a, b, and c are called the
lattice constants.
Ref.: M.S. Sze, Semiconductor Devices
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.7 The unit cell
Different unit cells based on cubic unit cells
Simple
cubic unit
cell
Body centered
cubic unit cell
Face centered
cubic unit cell
(bcc)
(fcc)
Ref.: M.S. Sze, Semiconductor Devices
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.7 The unit cells
Ref.: Joseph R. Smyth, Geology 3010: Introduction to Mineralogy
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.8 Structural order in molecular solids
Organic materials can form very highly
ordered crystals
Van der Waals forces keep these crystals
together.
These crystals can have a band
structure just like any other
semiconductor if the crystals are
highly order and the concentration
of impurities is very low.
Structural properties and electronic transport
Antracene single crystal (Ref.:
University Stuttgart, N. Karl).
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.8 Structural order in molecular solids
γ
c
Pentacene, C22H14 :
a
α
Aromatic hydrocarbons based on linear arranged
benzene rings
b
β
Crystal structure: Triclinic: a ≠ b ≠ c, α ≠ β ≠ γ ≠ 90°
Electronic transport due to an overlap of π−orbitals
Thermal Evaporation: Source temperature: 275300°C
Substrate view
Structural properties and electronic transport
Material: 0, 1 or 2 times sublimation purified
Substrate temperature: rt-110°C
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.8 Structural order in molecular solids
Substrate temperature
Pentacene on thermal oxide
2.5µm
Substrate at rt
2.5µm
Substrate at 70°C
2.5µm
Substrate at 90°C
Crystal size
Atomic force micrographs of thermally evaporated pentacene films (200nm).
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.8 Structural order in molecular solids
X-ray diffraction pattern of pentacene on thermal oxide
Diffraction intensity [a.u.]
90
75
60
(002)
11.46°
16°
(001)
5.743°
(003)
17.22°
45
30
15
0
10
20
2 Θ scan
Substrate view
The pentacene film was prepared at room temperature.
Structural properties and electronic transport
30
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.8 Structural order in molecular solids
Ratio of x-ray diffraction (001)/(001‘)
X-ray diffraction pattern of pentacene on thermal oxide
1
room temperature
70°C
90°C
Relation between the
average crystal size and
the ratio of the diffraction
peaks.
0.1
0.01
0.1
1
average crystal size [µm]
Structural properties and electronic transport
10
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.9 Electronic Transport
0
Mobility [cm2/Vs]
10
Low mobility materials and its applications
- Photoconductors
- Organic LEDs
Band-like
-2
10
-4
10
Hopping
transport
Grain boundaries or
trap-controlled
transport
transport
Disorder
- Transistors
High mobility materials and its appications
-6
10
Structural Order
The structural order of the material is closely related to the electronic
properties of the material.
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.9 Electronic Transport
Electrons 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 carriers
Electrons 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
thermal
Eaverage
=
3
kT
2
Average thermal energy of an electron / hole
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.
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
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:
Ekin =
1
meff vth2
2
Kinetic energy of an electron / hole
At room temperature the average thermal velocity of an electron is about
105m/s in silicon and GaAs.
3kT
vth =
meff
Thermal velocity of an electron
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 path
we can introduce a mean free time τ. A typical mean free path is in the range
of 100nm and the mean free time is in the range of 1ps.
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.9.2 Band-like transport
When a small electric field is applied to the semiconductor material each
free carrier will experience an electro static force
Force = − qF
So that the carrier is accelerated along the field (in opposite direction of the
field).
F=0
F
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.
Ref.: M.S. Sze, Semiconductor Devices
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.9.2 Band-like transport
An 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.
P = mn vn
P = − q ⋅ F ⋅τ C
Based on momentum conservation the drift velocity can be calculated. The drift
velocity is proportional to the applied electric field F.
vn = −
qτ C
F
mn
Structural properties and electronic transport
Electron drift velocity
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.9.2 Band-like transport
q ⋅τ C
µn ≡
mn
q ⋅τV
µp ≡
mp
Electron and hole mobility
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
v
µn = n
F
µp =
vp
F
Electron and hole mobility
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.
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.9.2 Band-like transport
Carriers can move from one molecule to
the next molecule.
Quantified by mobility
Mean free path > intermolecular spacing
Temperature dependent behavior:
µ (T ) ∝ T
−
3
2
Ref.: N. Karl, University Stuttgart
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.9.2 Band-like transport
10
2
Influence of traps on the electronic
transport:
µ tf
N traps
N bulk
E 
exp traps 
 kT 
Exponential Decrease of µ
Et ~ 40 - 50 meV
Dependence on Trap Density Nt
Nt ~ 1016 - 1018 cm-3
Trap-Free Limit : Power Law T-n
n ~ 1.6 - 2.3, phonon scattering
10
1
10
0
2
1+
Mobility (cm /Vs)
µ 0 (T )
µ eff (T ) =
10
-1
10
-2
10
-3
20
Ref.: Dodabalapur, University Texas, lecture notes EE 396K
Structural properties and electronic transport
increasing
Nt
100
Temperature (K)
300
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.9.3 Grain boundaries in polycrystalline material
1
EB
Pentacene thin
film transistor
EC
Ei
EF
mobility [cm2/Vs ]
NT
EV
0.1
L
−1
µ −1 = µ 0−1 + µ GB
2-5µm
0.01
0.001
100
100-200nm
The mobility decreases with decreasing
characterization temperature.
Exp.
fit
Temperature dependent mobility can be
explained by a barrier model.
300 400
Smaller crystals leads to higher grain
boundary traps density.
200
temperature [K]
Structural properties and electronic transport
 EB 

kT


µ GB = µ GB 0 ⋅ exp −
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.9.4 Trap-controlled transport
Mobility edge (E = 0)
E =0
DOS, g(E)
Energy
(Activation) energy
Extended states
Band-tail states
Localized states
Deep traps
Trapping and release of charges
µeff =µc - carrier mobility in extended states
Density-of-states
distribution
Ref.: V. Arkhipov, IMEC
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
2.9.5 Hoping transport
Energy
Jumps over-barriers dominate at
higher temperatures
At lower temperatures tunneling of
carriers take over
Energy
Most hopping models assume:
-positions of hopping sites are
completely random
- positions and energies of hoping
sites are uncorrelated
Ref.: V. Arkhipov, IMEC
Structural properties and electronic transport
Organic Electronic, Fall 2005, Dr. Dietmar Knipp
References
Pope and Swenburg, Electronic Processes in organic crystals and polymers, 2 nd
Ed., Oxford
Organic molecular crystals, E.A. Sininsh EA and V. Capek.
http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/6973Organic-OptoelectronicsSpring2003/CourseHome/
(Organic optoelectronic lecture MIT)
Structural properties and electronic transport