Electronic and Optoelectronic Polymers

1
Electronic and Optoelectronic Polymers
Wen-Chang Chen Department of Chemical
Engineering Institute of Polymer Science and
Engineering National Taiwan University
2
Outlines

• History of Conjugated Polymers
• Electronic Structures of Conjugated Polymers
• Polymer Light-emitting Diodes
• Polymer-based Thin Film Transistors
• Polymer-based Photovoltaics

3
Whats 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.
• Todays 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

4
Whats Transistor?
Field effect transistor works like a drain
5
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

6
Subjects of the Polymer Optoelectronic Device
Polymer Solar Cells
Polymer Light-emitting Diodes
Polymer Thin Film Transistors
7
Integrated Optoelectronic Devices Based on
Conjugated Polymers
Sirringhaus H., Tessler N., Friend RH, Science
1998
8
All Organic Thin Film Transistors (OTFT)
Key Materials for OTFT (1)Active Organic Layer
Organic Semiconductor (2)Source/drain electrodes
Electrical Conducting Materials (PEDOTPSS for
organic case) (3)Gate Dielectrics Organic
polymers (4) Substrate Highly thermal stable and
transparent polymer, e.g., PET, PSF, etc.
9
Progress on Flexible Organic Display Devices
ReferenceScience, 290, 2123 (2000))
ReferenceSynthetic 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.
10
Applications of OTFTs
11
Applications of OTFTs
Flexible TFT arrays enabling technologies for a
whole range of applications
12
Device Configuration of OTFTs
13
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
14
Current-Voltage (I-V) Characteristics
X0 to L, V(x) O to Vds
Linear region Vds ltlt Vg
Saturation region Vds Vg - VTh
15
Current-Voltage (I-V) Characteristics
Output (Id-Vd) Curve
16
Current-Voltage (I-V) Characteristics
Transfer (Id-Vg) Curve
Performance Parameters
at saturation region
Field Effect Mobility (µ) cm2/VS
Threshold Voltage (VTh)
On/Off Current Ratio (Ion/Ioff)
Sub-threshold Slope (SS)
17
Important Performance Parameters
Whats 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

18
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 PEDOTPSS)

19
Materials Requirements of Organic Semiconductors
for OTFT

• Target gt 1 cm2/Vs on/off ratio gt106 for n type
  or p/n type Organic Semiconductors
• Conjugated p-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)

20
Requirements of Materials for OTFTs
21
Factors Influencing OTFTs Performance
22
Evolution of OTFT mobility for P type or N type
Semiconductor
P type mobility
N type mobility
1-5 10-3 cm2/VS
1 10-5 cm2/VS
mobility (a Si-H µ1cm2/VS)
Adv Mater 2002, 14, 4436
23
Characteristics of Organic Semiconductors

• P type or N type
• Charge transport by hole (Low IP) or electron
  (High EA)

• Applications
• Light emitting diode, photoconductor, thin film
  transistor, sensor (PH or gas), solar cell,
  photovoltaic device

24
Structures of P-Channel Semiconductors with TFT
Characteristics

• Heterocyclic Oligomers

• Two dimensional Fused Rings

• Linear Fused Rings

• Polymeric Semiconductors

Acc Chem Res 2001, 34, 359
25
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 µ1 cm2/Vs)
26
Single Crystal of High Mobility Organic
Semiconductors
27
Materials Requirements for n-Channel Organic
Semiconductors

• Conjugated p-Electron System with High Electron
  Affinity
• (EA gt 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
28
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)

29
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(gt1 cm2/Vs)!
30
Introduction to PTCDA and PTCDI-R
R C8H17
R CH2C6H4CF3
31
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
32
Introduction to PTCDI-R
Single-crystal-like packing
p stacking occurs parallel to the substrate
surface
33
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.

34
Mobility for Semiconducting Polymers
HOMO / LUMO (eV)
Hole / Electron mobility (cm2V-1S-1)
Ca s-d electrode
RH Friend et al, Nature 2005, 434, 194
35
Comparable Electron Hole Mobility for OTFT
Donor-Acceptor Systems
36
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
37
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 gtgt Fv
Fi Fv
Band transport

• Stong p-orbital overlap
• Band transport
• Negative temp coefficient

• Weak p-orbital overlap
• Hopping transport
• Positive temp coefficient

Hopping transport
Fi intermolecular interaction force Fv thermal
vibration force
38
Charge Transport in Polymer
Intra-Molecular Soliton Propagation µ1000 cm2/VS
Inter-Molecular Hopping transport µ10-2cm2/VS
It is important to increase molecular ordering to
obtain high mobility in OTFT devices
39
Organic Inorganic Semiconductors
Organic Semiconductor
Inorganic Semiconductor

• Strong covalent bonds
• ?-bond
• Only crystal property
• Band type charge transport dominant
• High mobility and large mean free path

• Weak Van der Waals interaction forces
• p-bond overlapping
• Molecular gas property (molecules identity)
• Hopping type charge transport dominant
• Low mobility and small mean free path

40
Bipolar OTFT-
Organic
Semiconductors in Interfacial Properties
Idealized energy level diagram of OTFTs
P- N- Channel OTFT Operation
41
Scattering Mechanism in Thin Film
For high mobility

• Flat clean surface
• Large grain
• No doping

42
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
The Slope of ID1/2-VG _at_ 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)
Ondrain and source current increases due to the
increased number of charge carriers
Mobility (a Si-H electron µ 1cm2/VS) Ion/Ioff
current ratio (diving circuits in LCD Ion/Ioff
gt106)
P type
N type
Electron transport
Hole transport
43
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)

44
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

45
Surface Treatment of Inorganic Dielectric
Self-Assembly Monolayer (SAM)
Adv Funct Mater, 2005, 15, 77
46
Chemical Treatment on Dielectric Surface
Plasma pretreatment
Plasma treatment
Un-treatment
Plasma treatment
RMS roughness 0.8 1.3 nm
RMS roughness 0.3 0.5 nm
untreatment
Higher mobility after plasma treatment
Synth Met, 2003, 139, 377
47
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
48
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.

49
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!
50
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
51
Why high K insulators have better OTFT
performances?
For parallel plate capacitor filled with
dielectrics
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.
52
K value Ta2O525-40 TiO240-80 Si3N4 7.5 Al2O3
10
53
Use High k Materials as Gate Dielectrics
Threshold Voltage (Vt)
The x-axis intercept of ID1/2-VG
N type
P type
d
k
c
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
54
Why choosing Organic materials as insulators?
The drawbacks of using inorganic materials as
insulatorsDifficulty on building electronic
devices on plastic substrate High processing
temperature?adhesion to substrate?processing
method?Cost?large area?
55
Organic Polymers
56
(No Transcript)
57
Al2O3 /JSS-362 as dielectric double layers
Low dielectric constant of organic materials
reducing leakage current Inorganic
materialssupply the adhesion force between the
dielectric layer and S and D electrode
Synth. Met. 2003, 139, 445
58
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

59
Contact Electrode
60
Environment Stability
Off current increase by oxygen doping process
61
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)

62
Control the Processing Parameters
Solvent Power
Appl Phys Lett, 1996, 69, 4108
63
Control the Processing Parameters
Solvent Power
P3HT in chloroform
Lamellar layer structure
p - p interchain stacking
Less crystalline
P3HT in TCB
Mobility increase with higher bp of solvent
Chem Mater 2004, 23, 4775
Nanoribbons µm
64
Control the Processing Parameters
Annealing
Alignment
65
Organic P3HT Selection
Molecular weight
Adv Mater, 2003, 15, 1519
Adv Funct Mater, 2004, 14, 757
66
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
67
Organic P3HT Selection
HT regioregularity
Nature, 1999, 401, 685
Synth Met, 2000, 111-112, 129
68
Organic Compound Selection
Alkyl chain length
Synth Met, 2005, 148, 169
69
Chemical Treatment on Dielectric Surface
Plasma pretreatment
Plasma treatment
Un-treatment
Plasma treatment
RMS roughness 0.8 1.3 nm
RMS roughness 0.3 0.5 nm
untreatment
Higher mobility after plasma treatment
Synth Met, 2003, 139, 377
70
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
71
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
  p-pstacking
• The crsytalline domains of the semiconductor must
  cover the area between the S and D contacts
  uniformly.

72
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
• Newman, C. R. Frisbie, C. D. da silva Filho, D.
  A. Bredas, J. L. P. C. Ewbank, Mann K. R. Chem.
  Mater. 2004, 16, 4436
• Veres, J. Ogier, S. Lloyd, G. Chem. Mater.
  2004, 16, 4543
• Ling, M. M. Bao, Z. Chem. Mater. 2004, 16, 4824
• Chua, L. L. Zaumsell, J. Chang, J. F. Ou, E.
  C. W. Ho, P. K. H. Sirringhaus, H. 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
• Facchetti, A. Materials Today 2007, 10, 28
• Zaumseil, J. Sirringhaus, H. Chem. Rev. 2007,
  107, 1296