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Electronic and Optoelectronic Polymers
Wen-Chang Chen
Department of Chemical Engineering
Institute of Polymer Science and Engineering
National Taiwan University
Outlines

History of Conjugated Polymers

Electronic Structures of Conjugated Polymers

Polymer Light-emitting Diodes

Polymer-based Thin Film Transistors

Polymer-based Photovoltaics
What’s Transistor?
Transistor

A device composed of semiconductor materials that amplifiers a signal or
opens or close circuit.
The key ingredient of all digital circuits, including computers.
Today’s microprocessors contains tens of millions of microscopic transistors.


Field-Effect Transistor

A voltage applied between the gate and source controls the current flowing
between the source and drain
What’s Transistor?
Field effect transistor works like a drain
Organic Thin Film Transistors (OTFTs)
Organic transistors are transistor that use
organic molecules rather than silicon for
their active material. This active materials
can be composed of a wide variety of
molecules.
Advantages



Compatibility with plastic substances
Lower-cost deposition process such as
spin coating, printing, evaporation
Lower temperature manufacturing
(60-120oC)
Disadvantages

Lower mobility and switching speeds
compared to silicon wafers
Subjects of the Polymer Optoelectronic Device
Ga
t
e
Polymer Solar Cells
Polymer Light-emitting Diodes
Organic Semiconductor
Source
Dielectric
Substrate
Drain
Polymer Thin Film Transistors
Integrated Optoelectronic Devices Based on
Conjugated Polymers
Sirringhaus H., Tessler N., Friend RH, Science 1998
All Organic Thin Film Transistors (OTFT)
source
drain
active organic layer
gate dielectric
gate
substrate
bottom contact
source
drain
Key Materials for OTFT:
(1)Active Organic Layer: Organic Semiconductor
(2)Source/drain electrodes: Electrical Conducting
Materials (PEDOT:PSS for organic case)
(3)Gate Dielectrics: Organic polymers
(4) Substrate: Highly thermal stable and
transparent polymer, e.g., PET, PSF, etc.
active organic layer
gate dielectric
gate
substrate
top contact
Optoelectronic Polymer Lab, NTU
Progress on Flexible Organic Display Devices
Reference：Science, 290, 2123 (2000))
Reference：Synthetic Metals 145, 83-85(2004)
In an active Matrix each pixel contains a light-emitting diodes (LED) driven by a
Field-effect transistor (FET). The FET performs signal processing while the LED
converts the electrical signal processing into optical output.
Applications of OTFTs
Applications of OTFTs
Flexible TFT arrays enabling technologies for a whole range of applications
Device Configuration of OTFTs
Working Principle of OTFTs
VTh Threshold Voltage
Vd Drain Voltage
Vg Gate Voltage
Id Drain Current
L Channel length
W Channel width
Linear regime
Start of saturation
regime at pinch-off
Saturation regime
Current-Voltage (I-V) Characteristics
X=0 to L, V(x)= O to Vds
Linear region Vds << Vg
Saturation region Vds ~ Vg - VTh
2
 ( I ds , sat )1 / 2  2 L
sat  


V
g

 WC i
Current-Voltage (I-V) Characteristics
Output (Id-Vd) Curve
Current-Voltage (I-V) Characteristics
Transfer (Id-Vg) Curve
at saturation region
Performance Parameters
Field Effect Mobility (μ) [cm2/VS]
 WCi 
slope  

 2L 
1/ 2
Threshold Voltage (VTh)
On/Off Current Ratio (Ion/Ioff)
Sub-threshold Slope (SS)
Important Performance Parameters
What’s important?




Conduction at the semiconductor
dielectric interface
Contacts- injection of charges
Electronic and ambient stability
Fabrication technology
Requirements
for high performance OTFTs




High Mobility
High On/Off Ration
Low Threshold Voltage
Steep Sub-threshold Slope
Materials for OTFTs




Semiconductor Layer
Organic S.C.
Small molecules
(ex: pentacene, oligothiphene)
Conjugated polymers
(ex: P3HT, F8T2)
Inorganic S.C. (ex: a-Si, Zinc
oxide)
Insulator Layer
Organic Dielectric
(ex: Polyimide, PMMA, PVP)
Inorganic S.C.
(ex: SiO2, TiO2, Al2O3)
Electrode


Metal (ex: Au, Ca)
Conjugated Polymer (ex: PEDOT:PSS)
Materials Requirements of Organic Semiconductors for
OTFT
2
6

Target: > 1 cm /Vs on/off ratio >10 for n type or p/n type Organic
Semiconductors

Conjugated π-Electron System High Electron Affinity ( for n type) or
Ambipolar Characteristics (for p/n type)


Good Intermolecular Electronic Overlap
chemical bonding between molecules, molecular symmetry, the symmetry of the
crystal packing….
Good Film Forming Properties
polycrystalline film be highly oriented so that fast transports direction in the grains
lie parallel to the dielectric surface

Chemical Purity
charge trapping sites, dopants (increase the conductivity in off state)

Stability
device operation (Threshold Voltage Shift), air stability(O2, H2O)
Requirements of Materials for OTFTs
Factors Influencing OTFTs Performance
Evolution of OTFT mobility
for P type or N type Semiconductor
P type mobility
1-5 ~ 10-3 cm2/VS
N type mobility
1~ 10-5 cm2/VS
mobility (a Si-H μ~1cm2/VS)
Adv Mater 2002, 14, 4436
Characteristics of Organic Semiconductors

P type or N type
Charge transport by hole (Low IP) or electron (High EA)
Vacuum Level
EA
LUMO
Energy
IP
Bandgap
HOMO

Applications
Light emitting diode, photoconductor, thin film transistor,
sensor (PH or gas), solar cell, photovoltaic device…
Structures of P-Channel Semiconductors with
TFT Characteristics


Heterocyclic Oligomers
Linear Fused Rings


Two dimensional Fused Rings
Polymeric Semiconductors
Acc Chem Res 2001, 34, 359
Structures of P-Channel Semiconductors with Known TFT
Characteristics( Dimitrakopoulos and Malenfant, Adv. Mater.2002)
Mobility in the range of
10-3 ~ 1-5 cm2V-1S-1
mobility (a Si-H μ~1 cm2/Vs)
Single Crystal of High Mobility Organic Semiconductors
Materials Requirements for n-Channel
Organic Semiconductors

Conjugated π-Electron System with High Electron Affinity
(EA > 3.0 eV)

Good Intermolecular Electronic Overlap
chemical bonding between molecules, molecular symmetry, the symmetry of
the crystal packing….

Good Film Forming Properties
polycrystalline film be highly oriented so that fast transports direction in the
grains lie parallel to the dielectric surface

Chemical Purity
charge trapping sites, dopants

Stability
device operation (Threshold Voltage Shift), air stability(O2, H2O)
Chem Mater 2004, 16, 4436
Enhancement on the OTFT Characteristics

Materials issues

Materials Design and Preparation(HT%, regioregular, repeating
conjugated unit, substituent, synthesis method, refinement)

Key materials Optimization (gate, source, drain, substrate, dielectric)

TFT Structures

Chemical Treatment on dielectric film surface ( silane layer
pretreatment, SAMs thiol-based chemical modified contact)

Modifying the TFT structure (bottom contact or top contact)

Processing Optimization

Organic layer deposition (i) vacuum evaporation (ii) spin coating,
solution casting, printing

Controlling the deposition parameters (aging, deposition rate, anneal
process, solvent quality, channel length, channel dimension, deposition
thickness, solvent evaporation temperature)
Structures of n-Channel Semiconductors with known TFT
Characteristics ( C. D. Frisbie and coworkers, Chem. Mater. 2004)

Metal-Phthalocyanines
~ 0.6 cm2V-1S-1

Addition of Electron Withdrawing Groups (cyano,
perfluoroalkyl) to p Type Cores
10-4 ~ 0.1 cm2V-1S-1

Perylene or Naphthalene Derivatives
10-4 ~ 0.6 cm2V-1S-1

C60
~ 0.3 cm2V-1S-1
10-1 ~ 10-5 cm2/VS
Need to develop polymer semiconductors with
high electronic mobility(>1 cm2/Vs)!
Introduction to PTCDA and PTCDI-R
Year
Compound
Mobility (cm2V-1S-1)
Ion/Ioff
1997
PTCDA
10-4~10-5
-
1996
PTCDI
1×10-4
-
2000
PTCDI-C18H37
0.11
-
2002
PTCDI-C8H17
0.6
>105
2004
PTCDI-C5H11
0.05
-
O
O
R= C8H17
R N
O
N R
R= CH2C6H4CF3
O
Optoelectronic Polymer Lab, NTU
Air stable PTCDI-R or NTCDI-R
NTCDI-C6H4CF3
NTCDI-C8H17

Less negative reduction potential of fluorinated
chains may be stabilized during operation in air

Denser packing of fluorinated chains could be more
permeable to oxygen and water
NTCDI-CH2C7F15
H.E. Katz et al., Nature 2000, 404, 479
H.E. Katz et al., JACS 2000, 122, 7787
Optoelectronic Polymer Lab, NTU
Introduction to PTCDI-R
Single-crystal-like packing
π stacking occurs parallel to
the substrate surface
Optoelectronic Polymer Lab, NTU
Why Using PTCDI-R as N Type OTFTs

Single-step synthesis

Impart additional electron withdrawing character to the
conjugated backbones to stabilized electron injection.

Provide screening against penetration of environmental
contaminants (H2O, O2..)into the channel region.

The side group could induce a more favorable packing
geometry that increases intermolecular overlaps or reduces
phonon scattering.
Optoelectronic Polymer Lab, NTU
Mobility for Semiconducting Polymers
HOMO / LUMO (eV)
Hole / Electron mobility (cm2V-1S-1)
5.7 / 2.4
3X10-4 / 5X10-3
PFO
5.0 / 2.8
5X10-4 / 8X10-5
OCC10-PPV
5.9 / 3.3
NA / 4X10-3
F8BT
5.0 / 2.8
5X10-5 / 3X10-5
MEH-PPV
5.5 / 3.1
5X10-3 / 6X10-3
F8T2
PPV
5.4 / 3.2
NA / 4X10-5
CN-PPV
5.2 / 2.7
NA / 1X10-4
4.9 / 2.7
2X10-4 / 6X10-4
P3HT
Ca s-d electrode
RH Friend et al, Nature 2005, 434, 194
Comparable Electron & Hole Mobility for OTFT:
Donor-Acceptor Systems
Compound
Hole/Electron Mobility
(cm2V-1S-1)
Ref.
0.004
0.005
Science 1995, 269,1560
1.1×10-5
4.3×10-5
J Mater Chem 2004,14,
2840
2.5×10-3
NA
3.4×10-4
5.4×10-3
O
C7H15
O
N
N
N
N
N
n
S
C7H15
N
n
10-4
10-5
Chem Mater 2004,16, 4616
Macromol Rapid Commun.
2005, 26, 1214
Chen and Jenekhe (to be
submitted to
Macromolecules)
Conduction Mechanism in OTFT Channel
Charge carrier mobility is dependent
on molecular order within the
semiconducting thin film
Current modulation is
achieved by electric
field-induced charge
build-up at the
interface between the
organic semiconductor
and the insulator
IBM J. Res. and Devel. 2001, 45, 11
Charge Transport in Organic Crystal
Limit of mobility in organic single crystal at room temperature is due to the
weak intermolecular interaction forces (van der waals interaction) of 10
kcal/mole (cf 76 kcal/mole for Si convanlent bond)
Fi >> Fv
Fi ~ Fv
Band transport



Stong π-orbital overlap
Band transport
Negative temp coefficient

Hopping transport


Weak π-orbital overlap
Hopping transport
Positive temp coefficient
Fi intermolecular interaction force ; Fv thermal vibration force
Charge Transport in Polymer
Intra-Molecular
Soliton Propagation :μ~1000 cm2/VS
Inter-Molecular
Hopping transport :μ~10-2cm2/VS
It is important to increase molecular ordering to obtain high
mobility in OTFT devices
Organic & Inorganic Semiconductors
Organic Semiconductor





Weak Van der Waals interaction forces
π-bond overlapping
Molecular gas property (molecule’s
identity)
Hopping type charge transport
dominant
Low mobility and small mean free path
Inorganic Semiconductor





Strong covalent bonds
ρ-bond
Only crystal property
Band type charge transport
dominant
High mobility and large mean
free path
Bipolar OTFTOrganic Semiconductors in Interfacial Properties
Idealized energy level diagram of OTFTs
P- & N- Channel OTFT Operation
Scattering Mechanism in Thin Film
For high mobility



Flat & clean surface
Large grain
No doping
Operation Energy Diagram and Important Parameters
Field Effect Mobility (μ)
How strongly the motion of an electron or hole is
influenced by an electric field
 WC i 


2
L


1/ 2
The Slope of ID1/2-VG @ saturation region
On/Off Current Ratio (Ion/Ioff)
(a) Off ：the state of a transistor is then on
voltage is applied between the gate and
source electrode
(b) On：drain and source current increases due to
the increased number of charge carriers
Mobility (a Si-H electron μ ~1cm2/VS)
N type
P type
Electron transport
Hole transport
Ion/Ioff current ratio (diving circuits in LCD Ion/Ioff >106)
Enhancement on Performance of OTFTs
 Chemical surface treatment on dielectric film surface or electrode
(SAMs silane layer pretreatment, plasma treatment)
 Modify the TFT structure
(bottom contact or top contact)
 Control the processing parameters
(deposition rate, anneal process, solvent power, channel dimension,
deposition thickness, heat treatment, film forming method)
 Choose materials
(gate, source, drain, substrate, dielectric)
 Organic P3HT selection
(HT% regioregularity, molecular weight, substituent, synthesis method,
refinement)
Surface treatment of Inorganic Dielectric
Self-Assembly Monolayer (SAM)








Hexamtehyldisilazene (HMDS)
Octadecyltrichlorosilane (HMDS)
Other silanes
Alkanephosphonic acid
Increased grain boundary of OSC
Hydrophilic to hydrophobic
attachment (smooth surface)
Increasing molecular ordering
Obtain improved OTFT characteristics
Surface Treatment of Inorganic Dielectric
Self-Assembly Monolayer (SAM)
Adv Funct Mater, 2005, 15, 77
Chemical Treatment on Dielectric Surface
Plasma pretreatment
Plasma treatment
Un-treatment
Plasma treatment
RMS roughness:
RMS roughness:
0.8 ~ 1.3 nm
0.3 ~ 0.5 nm
untreatment
Higher mobility after
plasma treatment
Synth Met, 2003, 139, 377
Dielectric
Requirements for OTFT Dielectrics




High dielectric constant for low-voltage operating
Good heat and chemical resistance
Pinhole free thin film formability with high breakdown voltage and
long term stability
Comparable with organic semiconductor in interfacial properties
Polymeric Dielectrics
Adv Mater, 2005, 17, 1705
Dielectric
The conduction mechanism in organic semiconductor is different from
that of inorganic.
Due to the weak intermolecular forces in OSC, the number of effects
through which the dielectric can influence carrier transport and mobility
is much broader than in inorganic materials.
Dielectric Effect in OTFTs




Morphology of organic semiconductor and orientation of molecular
segments via their interaction with the dielectric (especially in bottom
gate devices)
Interface roughness and sharpness may be influenced the dielectric
itself, the deposition conditions, and the solvent used
Gate voltage dependent mobility, which together with the variation of
the threshold voltages, can be a signature of dielectric interface effects
The polarity of dielectric interface may also play a role, as it can affect
local morphology or the distribution of electronic states in OSC.
Dielectric
Inorganic Insulator for OTFTs
Surface states on inorganic oxides are particular problem leading to interface
trapping and hysteresis, also impacts the semiconductor morphology
Large number of surface treatment studies!
Dielectric
Organic Insulator for OTFTs
Organic dielectrics offers the freedom to build both top and bottom gate
devices more easily by the use of solution coating technique and printing
Why high K insulators have better OTFT performances?
For parallel plate capacitor filled with dielectrics
C
k o A
d
The mobility depends on the concentration of carriers accumulated in
the channel in the OTFTs, the insulators should be thinner and its dielectric
constant should be higher to induce a larger number of carriers at a lower
voltage.
High K gate dielectric is the expansion of design space due to the possibility
of using thinker gate length.
d 
k
d SiO2
3 .9
Optoelectronic Polymer Lab, NTU
K value
Ta2O5：25-40
TiO2：40-80
Si3N4 ：7.5
Al2O3 ：10
Optoelectronic Polymer Lab, NTU
Use High k Materials as Gate Dielectrics
k A
C o
d
k
d 
d SiO2
3 .9
Threshold Voltage (Vt)
Vt  V 't  MS  
P type
1
V 't  2 F 
COX
F
d
kT

q
k
The x-axis intercept of ID1/2-VG
Q
COX
N type
4qN A S F
V 't  2 F 
 NA 
ln

 ni 
F 
c
1
COX
4qNA S  F 
ND 
kT 
 ln

q 
ni 
Vt
but high leakage current (high off current) !!
Smooth interface between the polymer-semiconductor and dielectric to
reduced scattering at the smooth interface
IEEE Trans. Electron Devices. 2001, 14, 281
Optoelectronic Polymer Lab, NTU
Why choosing Organic materials as insulators?
The drawbacks of using inorganic materials as insulators：Difficulty on
building electronic devices on plastic substrate; High processing
temperature、adhesion to substrate、processing method、Cost、
large area?
Organic Polymers
Year
Organic
active layer
Dielectrics
Substrate
Mobility
(cm2/VS)
Fabrication
1994
DH6T
Polyester(3)
PET
0.06
spin coating
1997
P3HT
Polyimide
Polyester
0.03
spin coating
1998
P3HT
Polyimide
PET
0.05
Screen printing
1999
PTV
PVP
polyimide
3×10-4
2000
F8T2
PVP
2001
Pentacene…
Organosilsesquioxane
2002
Pentacene
P3HT
2002
0.02
spin coating
PET
0.1
spin coating
P4VP(4.2)
Glass
0.05
spin coating
Pentacene…
Organosilsesquioxane
ITO/Mylar
0.1
spin coating
2002
Pentacene
PVP
glass
PEN
0.3
0.05
spin coating
2003
Pentacene
PVP
PEN
0.7
spin coating
2003
pentacene
PVA(3)
glass
0.01
spin coating
2003
pentacene
JSS-362
PET
0.12
spin coating
2003
pentacene
Al2O3 /JSS-362
(2.2-1.7)
PEN
1.4×10-2
Sputtering
/Spin coating
Al2O3 /JSS-362 as dielectric double layers
Low dielectric constant of organic materials :
reducing leakage current
Inorganic materials：supply the adhesion
force between the dielectric layer and S and
D electrode
Synth. Met. 2003, 139, 445
Optoelectronic Polymer Lab, NTU
Contact Electrode
Requirement for S/D Electrodes



No interface barrier with the active layer
No metal diffusivity
High carrier injection, low contact resistance
Au


Mainly used as S/D electrodes due to its high
work function (5.1 eV) and low injection barrier
Still remain dipole barrier
Contact Electrode
Environment Stability
Off current increase by oxygen doping process
Improvement of P3HT OTFTs
 Chemical surface treatment on dielectric film surface or electrode
(SAMs silane layer pretreatment, plasma treatment)
 Modify the TFT structure
(bottom contact or top contact)
 Control the processing parameters
(deposition rate, anneal process, solvent power, channel dimension,
deposition thickness, heat treatment, film forming method)
 Choose materials
(gate, source, drain, substrate, dielectric)
 Organic P3HT selection
(HT% regioregularity, molecular weight, substituent, synthesis method,
refinement)
Control the Processing Parameters
Solvent Power
Appl Phys Lett, 1996, 69, 4108
Control the Processing Parameters
Solvent Power
P3HT in chloroform
Less crystalline
P3HT in TCB
Lamellar
layer
structure
π-π
interchain
stacking
Mobility increase with higher bp of solvent
Nanoribbons ~μm
Chem Mater 2004, 23, 4775
Control the Processing Parameters
Annealing
Alignment
Organic P3HT Selection
Molecular weight
Adv Mater, 2003, 15, 1519
Adv Funct Mater, 2004, 14, 757
Organic P3HT Selection
Molecular Weight
Low Mw P3HT
Chare carriers trapped on the nanorod
High Mw P3HT
Mobility increase with higher MW
Interconnect ordered area and soften the boundary
Macromolecules 2005, 38, 3312
Organic P3HT Selection
HT% regioregularity
Nature, 1999, 401, 685
Synth Met, 2000, 111-112, 129
Organic Compound Selection
Alkyl chain length
Synth Met, 2005, 148, 169
Chemical Treatment on Dielectric Surface
Plasma pretreatment
Plasma treatment
Un-treatment
Plasma treatment
RMS roughness:
RMS roughness:
0.8 ~ 1.3 nm
0.3 ~ 0.5 nm
untreatment
Higher mobility after
plasma treatment
Synth Met, 2003, 139, 377
Semiconductor Deposition Methods
Organic semiconductors are deposited either from vapor or solution
phase depending on their vapor pressure and solubility
Device performance of OTFTs is greatly influenced by various
deposition conditions due to the different resulting molecular
structure and thin film morphology
How to Get High Mobility ?
Ways of Mobility Improvement

Homo/LUMO of the individual molecules must be at levels
where hole/electrons can be induced at accessible applied
electric fields.
 The solid should be extremely pure since impurities act as
charge carrier traps.
 The molecules should be preferentially oriented with the long
axes approximately parallel to the substrate since most efficient
charge transport occurs along the direction of intermolecular
π-πstacking
 The crsytalline domains of the semiconductor must cover the
area between the S and D contacts uniformly.
Reference
 G. Horowitz, Adv. Mater. 2000, 14, 365
 Katz, H. E.; Bao, Z., J. Phys. Chem. B., 2000, 104, 671
 Dimitrakopoulous, C. D.; Mascaro, D. J., IBM J. Res. & Dev. 2001, 45,11
 Katz, H. E.; Bao, Z.; Gilat, S. L., Acc. Chem. Res., 2001, 34, 359
 Dimitrakopoulous, C. D.; Malenfant, D. R. L. Adv. Mater. 2002, 14, 99
 Horowitz, G. J. Mater. Res. 2004, 19, 1946
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K. R. Chem. Mater. 2004, 16, 4436
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Friend, R. H. Nature, 2005, 434, 194
 Sun, Y.; Liu, Y.; Zhu, D. J. Mater. Chem. 2005, 15, 53
 Facchetti, A.; Yon, M. H.; Marks, T. J. Adv. Mater. 2005, 17, 1705
 Sirringhaus, H. Adv. Mater. 2005, 17, 2411
 Reichmanis, E.; Katz, H. E.; Kloc, C.; Maliakal, A. Bell Labs Technical J. 2005, 10, 87
 Dodabalapur, A. Materials Today 2006, 9 , 24
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