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 (or Field Effect) Transistors



Polymer-based Photovoltaics



Polymers for Memory devices

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 drain 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 transistors that use organic molecules rather than silicon for their active materials. These 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 (60-120 o C)



Disadvantages Lower mobility and switching speeds compared to silicon wafers

Subjects of the Polymer Optoelectronic Device

Polymer Solar Cells Polymer Light-emitting Diodes G at e Polymer Thin Film Transistors

Integrated Optoelectronic Devices Based on Conjugated Polymers

Sirringhaus H., Tessler N., Friend RH, Science 1998

source

All Organic Thin Film Transistors (OTFT)

drain active organic layer gate source gate dielectric substrate bottom contact

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.

drain 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

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

Polymer Field Effect Transistors

L Source W Drain - - - - - - - - - - -

+ + + + + + + + + + + +

V g > 0 V d > 0 Polymer Semiconductor Dielectric Layer Substrate/Gate Electrode Why Polymer?

    Printability Flexibility Low cost & weight Easy fabrication

Architecture of transistor device What’s important?

 Conduction at the semiconductor dielectric interface    Contacts- injection of charges Electronic and ambient stability Fabrication technology

Requirements for high performance

  High mobility (>0.1 cm 2 /VS) High On/Off current Ratio (>10 4 )  Low Threshold Voltage (within ± 5 V) 13

Device Configuration of OTFTs

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 I D 1/2 -V G @ saturation region

On/Off Current Ratio (I on /I off )

(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

N type Electron transport P type Hole transport

Mobility (a Si-H electron μ ~1cm 2 /VS) I on /I off current ratio (diving circuits in LCD I on /I off >10 6 )

Working Principle of OTFTs

V Th Threshold Voltage V d Drain Voltage V g Gate Voltage I d Drain Current L Channel length W Channel width Linear regime Start of saturation regime at pinch-off Saturation regime

Current-Voltage (I-V) Characteristics

Transfer (I d -V g ) Curve at saturation region Performance Parameters Field Effect Mobility (μ) [cm 2 /VS]

slope



W



C i

 1 / 2 2

L

Threshold Voltage (V Th ) On/Off Current Ratio (I on /I off ) Sub-threshold Slope (SS)

Current-Voltage (I-V) Characteristics

X=0 to L, V(x)= O to V ds Linear region V ds << V g Saturation region V ds ~ V g - V Th



sat

     (

I ds

,

sat



V g

) 1 / 2    2 2

L WC i

Current-Voltage (I-V) Characteristics

Output (I d -V d ) Curve

Materials for OTFTs 

Semiconductor Layer Organic S.C.



Small molecules (ex: pentacene, oligothiophene) 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: SiO 2 , TiO 2 , Al 2 O 3 )

 

Electrode Metal (ex: Au, Ca) Conjugated Polymer (ex: PEDOT:PSS)

Materials Requirements of Organic Semiconductors for OTFT

   

Target: > 1 cm 2 /Vs on/off ratio >10 6 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(O 2 , H 2 O)

Requirements of Materials for OTFTs

Factors Influencing The OTFTs Performance

Evolution of The OTFT mobility for P type or N type Semiconductors

P type mobility N type mobility 1-5 ~ 10 -3 cm 2 /VS 1~ 10 mobility (a Si-H μ~1cm 2 /VS) -5 cm 2 /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 IP Energy 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



Two dimensional Fused Rings



Linear 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 cm 2 V -1 S -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(O 2 , H 2 O)

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 cm 2 V -1 S -1

Addition of Electron Withdrawing Groups (cyano, perfluoroalkyl) to p Type Cores

10 -4 ~ 0.1 cm 2 V -1 S -1

Perylene or Naphthalene Derivatives

10 -4 ~ 0.6 cm 2 V -1 S -1

C60

~ 0.3 cm 2 V -1 S -1

10 -1 ~ 10 -5 cm 2 /VS

Need to develop polymer semiconductors with high electronic mobility(>1 cm 2 /Vs)!

Introduction to PTCDA and PTCDI-R

Year 1997 1996 2000 2002 2004 Compound PTCDA PTCDI PTCDI-C 18 H 37 PTCDI-C 8 H 17 PTCDI-C 5 H 11 Mobility (cm 2 V -1 S -1 ) 10 -4 ~10 -5 1 × 10 -4 0.11

0.6

0.05

I on /I off >10 5 O R N O O N R R= C 8 H 17 R= CH 2 C 6 H 4 CF 3 O Optoelectronic Polymer Lab, NTU

Air stable PTCDI-R or NTCDI-R

NTCDI-C 6 H 4 CF 3 NTCDI-C 8 H 17 NTCDI-CH 2 C 7 F 15   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

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 (H 2 O, O 2 ..) 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

PFO F8BT

Mobility for Semiconducting Polymers HOMO / LUMO (eV) Hole / Electron mobility (cm 2 V -1 S -1 ) 5.7

/ 3X10 2.4

-4 / 5X10 5.9

/ 3.3

NA / 4X10 -3 -3

OCC 10 -PPV MEH-PPV

5.0

/ 2.8

5X10 -4 / 8X10 -5 5.0

/ 5X10 2.8

-5 / 3X10 -5

F8T2 PPV Ca s-d electrode

5.5

/ 3.1

5X10 -3 / 6X10 -3

CN-PPV

5.2

/ 2.7

NA / 1X10 -4

P3HT

5.4

NA 4.9

/ / / 2X10 3.2

4X10 2.7

-4 / -5 6X10 -4 RH Friend et al, Nature 200 5,

434

, 194

O N N

Comparable Electron & Hole Mobility for OTFT: Donor-Acceptor Systems

Compound Hole / Electron Mobility (cm 2 V -1 S -1 ) 0.004

0.005

1.1

× 10 -5 4.3

× 10 -5 2.5

× 10 -3 NA 3.4

× 10 -4 5.4

× 10 -3 Ref.

Science 1995, 269,1560 J Mater Chem 2004,14,

2840

Chem Mater 2004,16, 4616

Macromol Rapid Commun.

2005, 26, 1214 O N N n S C 7 H 15 N C 7 H 15 N n

10 -4 10 -5 Chen and Jenekhe ( to be submitted to Macromolecules)

Donor

-

Acceptor

Conjugated Polymer Semiconductors with High FET Mobility (Literature~2009)

C 12 H 25 S S O N N O S Hole / Electron mobility C 16 H 33 C 16 H 33 N S N S n 0.1 cm 2 /VS C 12 H 25 0.1 cm 2 /VS S S n

Adv. Mater. 2009 ,

21

, 209

1.4 cm 2 /VS

Adv. Mater. 2008 ,

20

, 2217

NC CN C 12 H 25 S S n C 12 H 25 NC CN

J. Am. Chem. Soc. 2008 ,

130

, 8580

C 14 H 29 C 14 H 29 S S N S N S S S C 14 H 29

J. Am. Chem. Soc. 2009 ,

131

, 2521

n C 14 H 29 0.1 cm 2 /VS 0.2 cm 2 /VS N N S S C 12 H 25 C 12 H 25 S 0.2 cm 2 /VS n C 12 H 25 C 12 H 25

J. Mater. Chem. 2009 ,

19

, 591

C 10 H 21 C 8 H 17 O O N O N O C 8 H 17 C 10 H 21 S S n 0.85 cm 2 /VS

Nature. 2009 ,

457

, 679

Q: Could we develop new > 1 cm 2 /Vs solution-processable semiconducting polymers with mobility and good environmental stability ?

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 Develop. 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 covalent bond) Fi >> Fv Fi ~ Fv Band transport

  

Strong 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 cm 2 /VS Inter-Molecular Hopping transport :μ~10 -2 cm It is important to increase molecular ordering to obtain high mobility in OTFT devices 2 /VS

Organic & Inorganic Semiconductors

Organic Semiconductor Inorganic 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

    

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

Bipolar OTFT Organic 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 I D 1/2 -V G @ saturation region

On/Off Current Ratio (I on /I off )

(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

N type Electron transport P type Hole transport

Mobility (a Si-H electron μ ~1cm 2 /VS) I on /I off current ratio (diving circuits in LCD I on /I off >10 6 )

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 on Inorganic Dielectric

Self-Assembly Monolayer (SAM)

   

Hexamethyldisilazene (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 on Inorganic Dielectric

Self-Assembly Monolayer (SAM) Adv. Funct. Mater. 2005, 15, 77

Chemical Treatment on Dielectric Surface

Plasma pretreatment Plasma treatment Un-treatment RMS roughness: 0.8 ~ 1.3 nm Plasma treatment RMS roughness: 0.3 ~ 0.5 nm untreatment Higher mobility after plasma treatment Synth Met. 2003, 139, 377

Dielectrics

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

Dielectrics

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.

Dielectrics

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!

Organic Insulator for OTFTs

Dielectrics

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 thicker gate length.

d

 

k

3 .

9

d SiO

2 Optoelectronic Polymer Lab, NTU

K value Ta 2 O 5

：

25-40 TiO 2

：

40-80 Si 3 N 4

：

7.5

Al 2 O 3

：

10

Optoelectronic Polymer Lab, NTU

Use High k Materials as Gate Dielectrics

C



k



o A d d

 Threshold Voltage (V t )

V t



V

'

t

 

MS

 

Q C OX

P type

V t

'  2 

F

 1

C OX



F



kT

  ln

q N A

 

n i

4

qN A



S



F



k

3 .

9

d SiO

2 The x-axis intercept of I D 1/2 -V G N type

V

'

t

 2 

F

 1

C OX

4

qN



F



kT q

   ln

N D n i

 

A



S

  

F

 d k c V t 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?

Year 1994 1997 1998 Organic active layer DH6T P3HT P3HT 1999 2000 2001 PTV F8T2 Pentacene… Dielectrics Polyester(3) Polyimide Polyimide PVP PVP Substrate PET Polyester PET polyimide Organosilsesquioxane PET 2002 2002 Pentacene P3HT Pentacene… P4VP(4.2) Organosilsesquioxane Glass ITO/Mylar 2002 Pentacene PVP 2003 2003 2003 Pentacene pentacene pentacene PVP PVA(3) JSS-362 glass PEN PEN glass PET 2003 pentacene Al 2 O 3 /JSS-362 (2.2-1.7) PEN Mobility (cm2/VS) 0.06

0.03

0.05

3 × 10 -4 0.02

0.1

0.05

0.1

0.3

0.05

0.7

0.01

0.12

1.4

× 10 -2 Fabrication spin coating spin coating Screen printing spin coating spin coating spin coating spin coating spin coating spin coating spin coating spin coating Sputtering /Spin coating

Al 2 O 3 /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 Nanoribbons ~μm Mobility increase with higher bp of solvent Chem Mater 2004, 23, 4775

Annealing

Control the Processing Parameters

Alignment

Molecular weight

Organic

P3HT Selection

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 Interconnect ordered area and soften the boundary Mobility increase with higher MW Macromolecules 2005, 38, 3312

HT% regioregularity

Organic P3HT Selection

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 RMS roughness: 0.8 ~ 1.3 nm Plasma treatment RMS roughness: 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 crystalline 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



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