Dünne organische Filme: Plastik-Elektronik · OLED TV announced for 2nd half of 2012 55 Zoll (140...
Transcript of Dünne organische Filme: Plastik-Elektronik · OLED TV announced for 2nd half of 2012 55 Zoll (140...
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• Introduction: Organic electronics
• Fabrication and characterization of organic thin films
• Devices: solar cells, OLEDs, OFETs
blackboard part: OFET
Organic Electronics
MatWi II (summer term 2012)
Priv. Doz. Bert Nickel ([email protected])
Organische Elektronik
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Philips
1990 2000 2010
300
Pap
ers
/ yea
r
year
organic light emitting diode OR OLED
organic field effect transistor OR OFET
organic solar cell
organic field effect transistor sensor
Trends in organic electronics
40 inch Samsung OLED display OLED
FETs
Solar
Georgia Institute of Technology (Pentacene+C60 solar cell, 2.7 % efficiency)
AMOLED displays
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OLEDs for lightening
AP photo
Anil Duggal, who heads up GE Global
Research's Organic Electronics Project, says
sheets of organic light-emitting diodes, such as
the one above, might be the future of lighting.
OLED TV announced for 2nd half of 2012
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O-Solar cells
Alan Heeger / University of California - Santa
Barbara
Science 2007
"The result is six and a half
percent efficiency," said
Heeger. "This is the highest
level achieved for solar cells
made from organic materials. I
am confident that we can make
additional improvements that
will yield efficiencies sufficiently
high for commercial products."
He expects this technology to
be on the market in about three
years.
How is this possible,
or what do we need for organic
electronics ?
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Silicon: microstructering and doping
Intel 80486DX2
Strukturierter Si-wafer
Organic materials (hydrocarbons) in
everydays life …
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… are good insulators
all electrons covalently bound
Polyethylen (PE)
electron density
MatWi I
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electron mobility
(Drude Model)
sp2 hybridisation (C2H4):
p-orbital
s
s
s
s
s
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conjugated and aromatic molecules: p-electrons
Molekulare Orbitale (MO) von
aromatischen Molekülen: Bsp. Benzol
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Bezeichnung der Orbitale: HOMO und LUMO
• Nobel Preis Chemie 2000: Alan J. Heeger
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poly(styrenesulfonate),
Doped polymers are conducting: PDOT : PSS
Nobelprices for aromatic and conjugated materials
Graphene 2010 physics
Polyacetylene 2000 chemistry
Fullerenes 1996 chemistry
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Linear acenes:
HOMO – LUMO band gap
Bsp. Pentacene: Absorption
Translation
valence band, conduction band, electron-hole pair, doping, traps for charge carriers, phonons, energy bands, Drude model, electroluminescence, surface states
LUMO, HOMO, excitonic states, chemical impurities, vibrations, hopping, fluorescence and phosphorescence, singulets, triplets, anhilation, oxidation, reduction
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The vision of organic electronics: mass printing
Status Quo
• ... 90% of OLEDs, by far the most
important application of organic electronics
at the moment, are produced by vapor
deposition of small molecules
[Source: ICB 10 July 2008 ]
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Organic materials deposition and
growth
1422HC
Properties:
•hole mobility larger than electron
mobility
•forms well-ordered layers when
evaporated in vacuum at RT
•gold contact are reasonably well
matched
Record material: Pentacene
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Pentacene-deposition by vacuum
sublimation, accuracy ca. 0.1 nm (QMC)
0
10 mm
0
10 mm
H H H H H H
Si
OH OH OH OH OH OH
Si
SiO2
Pentacene film growth
Phys. Rev. B 67 125406 (2003)
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diffusion nucleation
island formation
= Diffusionsenergie
= Diffusionskonstante
• temperature
• surface modification
• deposition rate
Diffusion limited Aggregation (DLA)
R = D / F
Molecular structure: Bragg-scattering
APL (2004)
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Pentacene growth (thick films)
Comparison: Coronene films
M. Huth (LMU), diploma thesis (2006)
1.4µm
1.4
µm
3µm
1.5
µm
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surface energy determines growth mode (Wulf Konstruktion)
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Summary: Deposition and structure
• Growth mode of small molecules largely determined by molecular shape and surface energy
• molecular arrangement can be determined by x-ray experiments
Carrier mobility
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Güte des Kristalls
little overlap - bands are flat – mass of the carriers is high – mobility is low
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Measurement of mobility: Time-Of-Flight:
1. Generation of Elektron-Hole pairs by
hard light pulse (pulse 0.76ns)
Charge carriers are generated at
the surface due to adsorption
2. measurement of displacement
current
injection free method
electron and hole current
separately
Experiment
TOF-Geometry Kepler & LeBlanc 1960 for
Anthracene crystals
typical
mobility
for organics
m = 1 cm2/Vs
same as
amorphous
Si
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traps reduce mobility
mobility, in the presence of
shallow traps:
where m0(T) ~ T-n
n 1.5 – 3 depending on material,
scattering mechanisms, etc.
m(T) = m0(T)
1 + [exp( ) -1] NT N0
ET kBT
µ0(T): intrinsic mobility
ET : trap energy
NT/N0: trapping vs. conduction states
grain boundaries reduce mobility
Horowitz Adv. Mat. (2000)
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Comparison of organic and inorganic Semiconductors
Evac
Ecb
Evb
Egap
0
E [eV]
2.2
5.8
EF
x
narrow bands
( ca. 100meV at 300K )
high mass
bandgap Egap 3.6eV
(diamond Egap = 5.5eV)
kBT = 26meV at 300K
no free carriers in thermal equilibrium
Compared to tetracene:
strong influence of traps
Summary – electronic properties • conjugated and aromatic molecules have delocalized
electrons (p electrons)
• small organic molecules form highly ordered crystals, while polymer films are only partially ordered
• Details der Anordnung der MO und Symmetrie bestimmen die elektronischen Eigenschaften
• organic molecules have large band gaps (typically 2 eV), few or no intrinsic carriers at RT
• bands are flat, dispersion typically 100 meV
• Quality matters for mobility (grain boundarys, traps)
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Thin-film devices
• transistors
• p-n contacts (diodes, solar cells, oLEDs)
Thin film transistors
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Thin film transistors - design
Contacts
(60 nm Au + 3nm Ti)
Thermal SiO2 (200nm) )/108.1( 28 cmFCox
-
~ 48 nm Pentacene
Gold-Structure(bottom-contact)
250 µ m
200 µ m
750 µ m 250 µ m
m
750 µ m 250 µ m
m
750 µ m
Channel
w = 10 µm
l = 20 µm
2.5 µm
5 µm 25 µm
12.5 µm
Transistor channel Pentacene on SiO2
Fieldeffect-Transistors from pentacene molecules
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0 -5 -10 -15 -20 -25 -300.0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
VG = 0 V
VG = -10 V
VG = -20 V
VG = -30 V
I SD *
10
-5 [A
]
VSD
[V]
Precise saturation behaviour
Ohmic contacts
Linear regime
p-type pentacene bottom-contact OTFTs
Organic field effect transistors
(see blackboard for characteristic curve)
Pentacene TFTs : Trap density
Hysteresis V shallow traps
• Energy level near HOMO level
• Can be released thermally
ID 0099:
Threshold shift VT deep traps
• Energy level far from HOMO level
• Fixed interface charges
ID 0099:
1st measurement
last measurement
M. Fiebig (LMU, LS Kotthaus), Diploma thesis 2005
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Diodes, solar cells
Si – the p/n- junction enegy band deformation by doping
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Singulet excitons diffuse within the organic crystal until they decay fluorescent or
radiationless . Lifetime is very short (fs-ps) , therefore diffusion length is only 10-20 nm.
Optical excitations in organic crystals
Exciton splitting at hetero-junctions
Appl. Phys. Lett. 48, 183 (1986)
Two‐layer organic photovoltaic cell
C. W. Tang
Modellsystem: Pentacene/C60
(C60 erst ab 1985 bekannt)
p n
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Organic solar cells
http://www.adsdyes.com/solarcells.html
e
h
Wikipedia: Solar cell
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Summary
• all important devices can be made by organic electronics: oLEDs Field-Effect-Transistors (FETs) Solar cells
• Materials science aspects have huge influence on performance: structural defects, chemical impurities, lifetime and stability