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Dispositivi e sensori a semiconduttore organico
Annalisa Bonfiglio University of Cagliari, Italy
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1: Semiconduttori organici
2: Organic Thin Film Transistors (OTFT)
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Outline prima parte
Proprieta’ degli orbitali del Carbonio
Molecole coniugate
Semiconduttori organici: orbitali molecolari e bandgap
Portatori di carica e trasporto nei semiconduttori organici
Influenza della morfologia sul comportamento elettrico dei film sottili organici
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E
2s
2p
Fundamental state
hybrid sp3
sp3
hybrid sp
2p
sp
hybrid sp2
2p
sp2
c c c c c
1σ 1π
134 pm 1σ
154 pm
1σ 2π
120 pm
Hybridization of orbitals in Carbon
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C C
H
H
H
H
Π bonding
σ bonding σ bonding
C C H
H
H
H H H C C H
H
Carbon-Carbon bonding
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fig.1.1: un atomo di carbonio ibridizzato sp2 [3].
Hybrid orbital sp2 Top view Side view
Hybridization of orbitals in Carbon
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fig.1.2: la sovrapposizione orbitalica nel doppio legame carbonio-carbonio [3].
Molecular Orbital= linear combination of atomic orbitals (LCAO)
Carbon-Carbon bonding
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C C
H
C
C C
H
C H
H H
H
C C
H
C
C C
H
C H
H H
H
C C
H
C
C C
H
C H
H H
H C6H6
C C
H
C
C C
H
C H
H H
H
Real molecule
Limit formula
Coniugated molecules
Limit formula
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MEH-PPV LPPPT PTV
A B C
D E F fig.1.6: struttura molecolare di: A) trans-poliacetilene (PA); B) poli-para-fenilene (PPP); C) poli-fenile-vinilene (PPV); D) poli-pirrolo (PPy); E) poli-tiofene (PT); F) poli-furano (PF) [4].
Coniugated molecules
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fig.1.7: orbitali molecolari π leganti e antileganti [3].
Band Gap
Legame Energia di legame (kJ • mol-1)
C—C C—H C—O C—S C—F C—Cl C—Br C—I
350 410 350 260 440 330 280 240
Legame Energia di legame
(kJ • mol-1) Si—Si Si—H Si—O Si—S Si—F Si—Cl Si—Br Si—I
180 300 370 230 540 360 290 210
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fig.1.10: funzioni d’onda dell’elettrone π nella buca di potenziale profonda infinito [6].
L = coniugation length = length of the shortest molecular segment with a perfect alternation of single and double bonds; N= number of atoms (each with 2 electrons); d = atomic distance
2
22
8mLhnEn =
( )2
22
82)(Ndm
hN
HOMOE⎟⎠
⎞⎜⎝
⎛
=
( )2
22
8
12)(Ndm
hN
LUMOE⎟⎠
⎞⎜⎝
⎛ +=
( )( ) Nmd
hNdmhNHOMOELUMOEEG 2
2
2
22
881)()( ≈
+=−=
The highest N, the lowest the gap
Band Gap
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Charge transport in organic semiconductors
Å
nm
µm
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Intrachain Interchain Intergrain
+
3 coesistent mechanisms+ charge injection from contacts
Measurements difficult to explain
Charge transport in organic semiconductors
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Doping and charge transport
In inorganic semiconductor, doping is substitutional and causes an increase in free charge carrier concentration. These carriers move in a periodic potential with no interaction with the crystal (band model). In organic, doping is not substitutional: supplementary charge physically interact with molecules causing a perturbation in the coniugated chain. This perturbation (charge + deformation induced by the charge) is called polaron
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In analogy with inorganic semiconductors, the polaron can be roughly described as a charge free of moving along the chain strongly interacting with the semiconductor lattice
m eff (polaron) >> m eff (free charge carrier)
è mobility (polaron) << mobility (free charge carrier)
Doping and charge transport
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Intermolecular transport: hopping
hopping = tunneling between crossing chains
To move, the charge carrier needs energy that can be provided by phonons (increasing with temperature)
Tkde
BJD ⋅⋅
⋅=
τµ
2
2σ( T )= σo ( T )⋅exp[-( To /T )1/(1+d)]
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But, in organic materials there is a great variety of behaviour from material to material. A unifying theory has not yet been developed.
Intermolecular transport: hopping
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Materials for “Plastic Electronics”
Intergrain transport: Pentacene (C22 H14)
• 5 benzene rings • Non soluble. Deposited by evaporation
• Molecules stand perpendicular to the substrate. • Molecules form separated domains (grains)
15Å
15Å
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Conduction: - hopping - thermically activated - limited by traps at grain boundaries
Correlation btw grain dimensions and mobility
trapsbulk µµµ
111+=
Intergrain transport: Pentacene (C22 H14)
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8 10 12 14 16 18 20
0.000
0.005
0.010
0.015
0.020
0.025
0.030
Freq
uenc
y
height (nm)
500 nm
Influence of surface morphology
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1 µm 1 µm
Pentacene on Mica Pentacene on SiO2
Influence of surface morphology
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1 µm
Pentacene on SiO2 Pentacene on Mylar
1 µm
Influence of surface morphology
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Outline seconda parte
Organic Thin Film Transistors (OTFTs)
Contatti metallo-semiconduttore organico
Comportamento elettrico degli OTFT
Modello elettrico del dispositivo
Tecnologia
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++++++++++++++++++++++++++
Source Drain semiconductor
Insulating layer
OTFT = ORGANIC THIN FILM TRANSISTOR
Field Effect Transistors
Interface metal - semiconductor
Gate
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Charge injection from metal contacts
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TFT model was first developed for poorly conductive semiconductors like amorphous Si
Tipically p-type!
Thin Film Transistor
++++++++++++++++++++++++++
Source Drain semiconductor
Insulating layer
0 -20 -40 -60 -80 -1000
-10µ
-20µ
-30µ
-40µ
-50µ a) Vg=100V Vg=80V Vg=60V Vg=40V Vg=20V Vg=0V Vg=-20V Vg=-40V Vg=-60V Vg=-80V Vg=-100V
I D(A
)
VD(V)
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How an OTFT works
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Typical values: µ ~ 10-2: 10-1
Ion/Ioff ~104 :105
VT ~ -10:+10 V
Linear region:
Saturation region:
( )
( )221
TGoxD
DTGoxD
VVLZCI
VVVLZCI
−=
−=
µ
µ
Toff
on VII ,,µ
Important parameters:
( )TGoxsatG
D VVLZC
VI
−=∂
∂µ
100,0 50,0 0,0 -50,0 -100,00,0
-500,0n
-1,0µ
-1,5µ
-2,0µ
-2,5µ Bottom Contact Au electrodesVt=-7.5Vµ=5.2x10-3cm2/VsZ/L=53
I D(A
)
VG(V)
Ioff
Ion
Equations and extraction of parameters
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Structures and fabrication techniques
Bottom gate
Top gate
Si/SiO2 substrate Substrate-free
Free-standing insulating
layer
OSC S D
G
I Contact scheme
channel
Top contact Bottom contact Bottom contact Bottom contact
OSC S D
G
I Contact scheme
channel
OSC
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Materials and fabrication techniques OSC
S D
G
I Contact scheme
channel
Materials: Electrodes: metals, conductive polymers Insulating layers: polyimide, PVA, PVP,…
Organic semiconductors
Solution processable Polymers
Small molecules
(deposited by evaporation)
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Low cost technique for liquid phase materials
Deposition
Spin up
Spin off
Spin down
Process parameters: 1. Spin up time 2. Spin speed 3. Spin off time 4. Spin down time 5. Dewetting time 6. Dewetting T
Thermal annealing (dewetting) Adv: simple and low cost
Fabrication techniques - Spin Coating
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1) 2)
3) 4)
5) 6)
Fabrication technique – Soft Lithography
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Inks instead of resist for patterning metals
Fabrication technique – Soft Lithography
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MicroContact Printing
Fabrication technique – Soft Lithography
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Printing & Roll to Roll Printing
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Possible applications for OTFTs