electron

Transcrição

electron

Printed Electronics
Hans Martin Sauer
Institute of Printing Science and Technology
Materials for Printed Electronics SS2012
Materials
for Field Effect Transistors
Printed Electronics SS 2012 | Institut für Druckmaschinen und Druckverfahren | Hans Martin Sauer
Objective and Scope
Introduction to important classes of
organic and inorganic materials
for printed electronics
• Fundamentals on electrons and chemical bonds in
solids and organic compounds
• Metals, semiconductors, insulators
• What is an organic semiconductor ?
• Examples for organic semiconductors
| 2
Set-up of Field Effect Transistor
Semiconductor
Conductor
• Organic (polymers,
oligomers,
„small“ molecules)
• Ceramic (oxides, other
chalcogenides)
• Metals
• Conductive Pastes and
inks (composites)
• Polymers
• Ceramic (TCOs)
Insulator
• Ceramic (oxides, nitrides,
silicates)
• Polymers
Compatibility
Coating procedures
Semiconductor
Source
Drain
Dielectric
• Gas phase techniques
• Spin- oder Dipcoating
• Printing
Gate
Substrate
| 3
The aim: Fully printed TFTs
A. Hübler et al.
Applied Physics Letters (2005)
M. Halik et al.
Advanced Materials (2003)
• Metal electrodes (source and drain) created by vacuum depositon or by
printing (from conducting polymer PEDOT-PSS formulation)
• Organic semiconductors by gas phase deposition or coating
• Gate dielectrics (polymer: PMMA) by printing
• Gate electrode (silver or gold) by vacuum deposition
| 4
A crash course in Q. M.: how organic semiconductors work
| 5
Atomic electron states -- 1 elecron, 1 atom
•
•
•
•
•
•
Electrons are described by a wave function
Y(r,t) i.e. a stationary solution of the
Schrödinger equation. Also -Y(r,t) is a solution.
The wave function corresponds to a spatial
probability distribution |Y(r,t)|2 to find the
electron at a specific point in space.
Electron states: In an isolated atom, the
stationary states are a discrete set of highly
symmetric wave functions Ynlms(r,t), specified
by quantum numbers:
- electron shell n =1,2,3,…
- sub-shell l = 0,1,2,…,n (s, p, d, f,…)
- orbital within sub-shell m = -l,…,+l
- electron spin S = -1/2 („down“), +1/2 („up“)
Each wave function corresponds to a specific
binding energy Enlms of the electron to the
atom, Hydrogen atom: E100 = -13.6 eV
All 2(2L+1) states on the same sub-shell L are
(almost) degenerate (= equal energy)
Fermi rule: States can be „occupied“ by max. 1
electron, or be „empty“
Source: wikipedia
| 6
Atomic electron states – more than 1 electron in an atom
•
Energy minimization: If there are k electrons in an atom, the k states with lowest energies
are occupied by them, starting with the lowest shells
Hund‘s rule: when subsequently adding 1… 2(2l+1) electrons to a degenerate sub-shell l:
- the first 2l+1 electrons occupy distinct orbitals m = -l,…,+l,
and they all have the same spin s (say +1/2)
- the next 2l+1 complete the half-occupied orbitals, and their spin s is inverted (i.e. -1/2)
Charge neutrality: # electrons = # protons in the atom core, otherwise: ions (+) or (-)
charge -> periodic table of the elements
•
•
0
= „vaccum level“
energy
hydrogen helium
lithium
berylium
borium
carbon
1s1
1s2
1s2 2s1
1s2 2s2
1s2 2s2 2p1
1s2 2s2 2p2
| 7
Interacting atoms – band structure in solids
Electron energy
Wave function symmetries under atom exchange
 


Y2atoms (r1, r2 )  Y1 (r1 ) Y2 (r2 )
N=1
degenarate
Anti-symmetric: repulsion
 




 
Ya (r1, r2 )  Y1 (r1 ) Y2 (r2 )  Y1 (r2 ) Y2 (r1 )   Ya (r2 , r1 )
N=2
symmetric: attraction
 




 
Ys (r1, r2 )  Y1 (r1 ) Y2 (r2 )  Y1 (r2 ) Y2 (r1 )   Ys (r2 , r1 )
Totally anti-symmetric
N=4
Mixed symmetries
Electron energy
Totally symmetric

N=
Fermi energy
Formation of bands by step-wise alignment of n orbitals
| 8
Band structure and charge transport in molecular solids
• HOMO = Highest occupied molecular level, LUMO = Lowest unoccupied molecular level
• Energy gap = EHOMO - ELUMO
• Charge transport:
- shifting an electron into an unoccupied state
- Spatial mobility of electron and „hole“ between the atoms
| 9
Band structure in solids
Metal
Semiconductor
Insulator
„Work Function“
Conduction Band
Band Gap
Binding Energy
Vacuum Level
Fermi Level
Valence Band
Electron states exist at EF,
No Energy gap between
occupied and non-occupied
states
No electron states at EF,
Energy gap exists, but is
small (< 3 eV or so)
No electron states at EF,
Energy gap is large
(< 10 eV or so)
http://iwenzo.de/wiki/Halbleiter | 10
Band structure in solids: n- and p-doping in semiconductors
Semiconductor
(intrinsic)
Semiconductor
(n-Type)
Semiconductor
(p-Type)
Vacuum Level
Binding Energy
Conduction band
Donor
Fermi Level
Acceptor
Valence band
Doping of semiconductors = adding small quanties of atoms with more (donor)
or fewer (acceptor) electrons to the solid
Example: aluminium (3p1) acts as acceptor for silicon (3p2) , phosphor (3p3) as donor
| 11
Electron states and bonds in carbon atoms (1)
•
Electron scheme of a carbon atom: 1s2, 2s2, 2p2;
0
Energy per electron
Energy per electron
0
According to Hund‘s rule…
•
•
… and in reality: sp3 hybridization
The non-staturated electron state can form chemical
bonds with non-saturated states of other atoms.
CH4 (methane), C2H6 (ethane),…
Source: wiki
| 12
Electron states and bonds in carbon atoms (2): C=C vs. C-C
• The double bond consists of σ-bond + π-bond
• The σ-bond is formed by overlapping two hybrid orbitals;
The π-Bindung is formed by combination of two pzorbitals.
| 13
Electron states and bonds in carbon atoms (3)
*
LUMO
*

HOMO

LUMO Lowest Unoccupied Molecular Orbital
HOMO Highest Occupied Molecular Orbital
| 14
Electron states and bonds in carbon atoms (4): Conjugated bonds and electron
delocalization
*
LUMO

HOMO
NOT alternating double and single bonds,
but a delocalised Pi-system!
| 15
Electron states and bonds in carbon atoms (5): organic semiconductor backbone
LUMO
HOMO
Abbildung nach: A.G. Mac Diarmid, Rev. Mod. Phys. (2001)
Bonding and antibonding molecular orbitals
form valence and conduction band in the solid
| 16
Organic semiconductors as conductors: p- and n-type
•
Electric current = removing & extraction electric charge from the molecule
electron state : injection electron to a LUMO state
hole state: extraction electron from a HOMO state
•
Adding electrons by doping (silicon: 3p2): p- and n-type achieved by adding traces of
an element with less or more electrons in the valence shell (Al: 3p1, P: 3p3)
Organic semiconductors: doping is difficult, may break up the molecule or create
„traps“
However: mobilities of electrons injected to the LUMO and hole injected to the HOMO
level is usually different for most organic semiconductors
•
•
•
P-type semiconductor: electric current is transported by a positively charged hole
state, i.e. shifting an unoccupied electron state at HOMO-level along the backbone
•
N-Type semiconductor: electric current is transported by a negatively charged
electron state, i.e. shifting an occupied electron state at LUMO-level along the
backbone
| 17
Generation of p- or n-type organic semiconductors by doping
n-type ->
LUMO
LUMO
electron donating
electron withdrawing
substituent
substituent
Injection of electrons
at LUMO level
LUMO
HOMO
p-type ->
HOMO
HOMO
injection of holes
at HOMO level
H
H
C6F13
C6H13
C6H13
C6F13
Manufacturing of organic n-type semiconductors
• Essential for production of devices with complementary technology
• Employment of strongly electron withdrawing substituents (Flourinated or oxygenated side groups) at the
molecule backbone leads to lower binding energies of frontier orbitals and facilitates electron injection
• Modification and appropriate choice of dielectric for avoidance of charge traps
• Significant n-type conduction (besides p-type, i.e. ambipolar transport) in many organic semiconuctors
| 18
Semiconductors in TFTs : Polythiophene, Polythienylvinylene
Stereoregular Polythiophenes
Poly(thienylthiophenes)
C6H13
C14H29
*
*
*
*
µ~0.01-0.10cm2/Vs
P3HT;
Sensitive towards atmosphere and light
C14H29
2
µ~0.1-1.0cm /Vs
PBTTT
Processing under ambient conditions
Poly(quarterthiophenes)
C6H13
C12H15
C6H13
*
*
*
*
C6H13
C6H13µ~0.15-0.25 cm2/Vs
C12H15
PQT-12; µ~0.05-0.15cm2/Vs
Processing under ambient conditions
No thermal post-processing required
High long term stability
Scherf et al. Angewandte Chemie (2009) 4138-4167.
McCulloch et al. Advanced Materials (2009) 1091-1109.
| 19
Molecular Weight and mobility: Poly(alkyl-thiophenes)
• Typical chain lengths: 20…20000 monomers
• P3HT chains can form crystalline as well as amorphous molecular morphologies
• Longer polymer chains lead to a more effective connection of the crystalline domains
• Current transport between molecular chains by quantum mechaniical tunnel process (very slow)
| 20
Semiconductors in TFTs: Polyfluorenes (+Copolymeres), Polyindocarbazoles
Polyindocarbazoles
Polyfluorenes
C8H15
*
*
*
C8H15
C8H15
PFO; µ<0.001cm2/Vs
*
C8H15
Fluorenes: Blue emitting materials in OLEDs
Poly(octylindocarbazole); µ~0.01cm2/Vs
Polythienylvinylenes
*
*
*
C8H15 C8H15
F8T2; µ~0.008-0.02cm2/Vs
(Employed in „All printed“ OFET
by Sirringhaus et al. 2001)
*
Polythienylvinylene; µ~0.0015cm2/Vs
Sensitve towards atmosphere and light
Good stability against atmosphere and light
| 21
Semiconductors in TFTs: Polytriarylamines
Interesting alternative to thiophen systems:
R
• Low charge carrier mobility
• Not crystalline, but completely amorphous; i.e. good
reproducibility, small influence of processing parameters
• Good stability against air and moisture
*
*
• Good solubility in many organic solvents
Employment in
Hübner et al. 2005 in
„all-printed“ TFT
| 22
Semiconductors in TFTs: n-type conducting Polymers
• Polymer with Naphthalene-bis(carboxidiimide) (NDI) backbone
• Good solubility in organic solvents. Stable under ambient conditions.
• High regioregularity; no thermal alteration till 300°C (DSC)
• Mainly amorphous; also after annealing at 200°C
6.6.12
• µ~0.45-0.85cm2/Vs
Facchetti et al. Nature (2009). | 23
Thanks for
your attention!
| 24

electron

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