Organic Field-Effect Transistors - Introduction -

Organic Field-Effect Transistors - Introduction -

1. Introduction to OFETs Field-Effect Transistor? Transistor A device composed of semiconductor material that amplifies a signal or opens or closes a circuit. The key ingredient of all digital circuits, including computers. Today's microprocessors contains tens of millions of microscopic c transistors. Field-Effect Transistor (FET) A voltage applied between the gate and source controls the current flowing between the source and drain.

1. Introduction to OFETs Field-Effect Transistor? - FET works like a dam! Gate Source Source Drain Base Gate Drain

1. Introduction to OFETs Organic Field-Effect Transistor (OFET) Organic transistors are transistors that use organic molecules rather than silicon for their active material. This active material can be composed of a wide variety of molecules. OFETs Advantages compatibility with plastic substances lower temperature manufacturing (60-120 C) lower-cost deposition processes such as spincoating, printing, evaporation less need to worry about dangling bonds makes for simpler processing [ I-V Characteristics] [ Pentacene] Disadvantages lower mobility and switching speeds compared to silicon wafers usually do not operate under inversion mode (more on this later) [ Device Configuration ]

1. Introduction to OFETs Applications of OFETs

1. Introduction to OFETs Applications of OFETs [ Lab on a Chip ] [ Portable Screen ] [ Smart Textile ] Organic Transistor [ RFID tag ] [ Smart Card ] [ e-paper ] [ Flexible Circuits ]

2. Operation Mechanism of OFETs Metal-Oxide-Semiconductor FET (MOSFET) - OFET is based on Si-based MOSFET technology in its fundamental principle, application, and operating mechanism. Principle of MOSFET qφ M =4.1eV qχ ox =0.95eV Ec qχ Si =4.05eV Vacuum level qφ S =4.9eV 0.4eV 3.15eV 3.1eV E g 9eV Al E V Silicon 4.6eV SiO 2 V s = 0 V V g >0 V V d >0 V I d Ideal switch I on n + Channel n + Depletion p-type MOSFET I off V th V g

2. Operation Mechanism of OFETs Device Configuration of OFETs Each of these devices has particular advantages and disadvantages in the fabrication process, which will be discussed.

2. Operation Mechanism of OFETs Basic Operation Mechanism of OFETs V S = 0 V Organic Semiconductor Source Drain Insulator V D Gate V G

2. Operation Mechanism of OFETs Basic Operation Mechanism of OFETs

2. Operation Mechanism of OFETs Current-Voltage (I-V) Characteristics Output (I d -V d ) Curve W C = μ L V 2 i d I d, lin ( Vg VTh ) -100 V d Linear region Saturation region I W C = μ 2L i 2 d, Sat ( Vg VTh) I d [μα] -80-60 -40-20 V g = -50 V V g = -40 V V g = -30 V V g > V Th 0 0-10 -20-30 -40-50 V d [V] V g < V Th

2. Operation Mechanism of OFETs Current-Voltage (I-V) Characteristics Transfer (I d -V g ) Curve Performance Parameters At saturation region -I d 1/2 [A 1/2 ] I on 1.2x10-2 V d = -30 V 1.0x10-2 8.0x10-3 6.0x10-3 4.0x10-3 Slope SS 2.0x10-3 0.0-60 -50-40 -30-20 -10 0 10 V g [V] 10-4 10-5 10-6 10-7 10-8 10-9 -I d [A] I off I W C = μ 2L i 2 d, Sat ( Vg VTh) 1. Field-Effect Mobility (μ) [cm 2 /Vs] μ = SS 2L WC i [ V = [log ( Slope) 2. Sub-threshold Swing (SS) [V/dec.] g ] I 10 d 3. On/Off Ratio (I On/ /I off ) ] 2 4. Threshold Voltage (V Th ) [V] V Th V on

3. Important Factors to Get High Performance OFETs Important Factors Requirements for high performance OFETs High Mobility High On/off Ratio (I on /I off ) Low Threshold Voltage (V Th ) Steep Sub-threshold Slope

3. Important Factors to Get High Performance OFETs Materials for OFETs Semiconductor layer V G C A : semiconductor B: metal electrode (Au, Ag) C: gate Key Characteristics High Mobility High I on/off Low Threshold Voltage Organic S.C.: : Small molecules (ex. pentacene, oligothiophene) : Conjugated polymers (ex. P3HT, F8T2) Inorganic S.C. (ex. a-si:h, zinc oxide) Insulator layer Organic Dielectrics (ex. PVP, PMMA, BCB, Polyimide) Inorganic Dielectrics (ex. SiO 2, Al 2 O 3, TiO 2 ) Electrodes Metal (ex. Au, Ca) Conducting Polymer (ex. PEDOT:PSS)

3. Important Factors to Get High Performance OFETs Requirements of Materials for OFETs

3. Important Factors to Get High Performance OFETs Factors influencing OFET Performance Performance Parameter On-Current Off-Current Mobility Sub-threshold slope Dominant Factor W/L Mobility Interface states Contact resistance Gap states W/L Fermi level Interface states Contact resistance Band gap Gate insulator Grain boundary Interface states Gap states Interface states

3-1. Organic Semiconductor Charge Transport in Organic Semiconductors Molecular Orbitals of the Ethene Molecule -10-5 0 5 10 r/a antibonding molecular π atomic p z atomic p z -10-5 0 5 10 r/a molecular π bonding

3-1. Organic Semiconductor Charge Transport in Organic Semiconductors

3-1. Organic Semiconductor Charge Transport in Organic Semiconductors Organic polymers Organic and molecular crystals Inorganic materials TMTSF 2 PF 6 (superconducting) Cu Conductivity (S cm 1 ) 10 5 10 0 10 5 Graphite Pyropolymers Polyacetylene Poly(p-phenylene) Poly(phenylene sulphide) Polypyrrole Polythiophene Polyphthalocyanine Polyacetylene (undoped) doped polymers TTF-TCNQ Cu-TCNQ Ag-TCNQ Hg Bi Si ZnO Ge (doped) Si Boron H 2 O Iodine metals semiconductors 10 10 10 15 10 20 Polydiacetylene Polythiophene Polypyrrole (undoped) Nylon Poly(p-phenylene) (undoped) Poly (phenylene sulphide) (undoped) PVC Polystyrene Polyimide (Kapton) PTFE (Teflon) Cu-phthalocyanine Anthracene ZnO Diamond SiO 2 insulators

3-1. Organic Semiconductor Organic Semiconductors for OFETs P-type N-type

3-1. Organic Semiconductor Organic Semiconductors for OFETs

3-1. Organic Semiconductor Organic Semiconductors for OFETs

3-1. Organic Semiconductor Conduction Mechanism in OFET Channel Field-Effect Mobility of OFET is determined by intrinsic or extrinsic charge transport mechanism of the organic semiconductors

3-1. Organic Semiconductor Conduction Mechanism in OFET Channel Charge Transport in Organic Single Crystal Upper limit of mobility in organic single crystal at room temperature is 1~10 cm 2 V -1 s -1 due to the weak intermolecular interaction forces (van der waals interaction) of 10kcal/mole (cf. 76kcal/mole for Si (convalent bond). F i : intermolecular interaction force F V : thermal vibration force Low Temperature High Temperature - Strong π-orbital overlap - Band transport - Negative temp. coefficient - Weak π-orbital overlap - Hopping transport - Positive temp. coefficient F i F V

3-1. Organic Semiconductor Conduction Mechanism in OFET Channel Charge Transport in Polymer Intra-molecular - Soliton propagation : μ= 1,000cm 2 /V.sec Inter-molecular - Hopping transport : μ= 10-3 cm 2 /V.sec» It is important to increase molecular ordering to obtain high field-effect mobility in OFET devices

3-1. Organic Semiconductor Organic & Inorganic Semiconductors Organic Semiconductor Inorganic Semiconductor Weak Van der Waals interaction forces : π-bond overlapping - Slight change of electronic structure - Retaining molecule s identity - Between molecular gas and covalent crystal (Molecular property + crystal property) - Hopping-type charge carrier transport dominant - Low charge carrier mobility and small mean free path Strong covalent bonds :ρ bond - Complete loss of individual properties (Only crystal properties) - Band-type charge carrier transport dominant - High carrier mobility and large mean free path

3-1. Organic Semiconductor Bipolar OFETs OSC in Interfacial Properties Idealized energy level diagram of OFETs N-channel operation V G = 0 LUMO HOMO V D = 0 P-channel operation Ambipolar behavior of OFETs source drain V G > 0 ---- V D = 0 V G < 0 ++++ V D = 0 Electron accumulation Hole accumulation V G > 0 V D > 0 V G < 0 + + + + V D < 0 Protecting OH functional with BCB Electron transport Hole transport Friend et. al. Nature, 2005

3-1. Organic Semiconductor Extrinsic Effects on Charge Transport Charge Transport in Polycrystalline Semiconductors 100 10 Mobilty (cm 2 V -1 s -1 ) 1 0.1 0.01 1E-3 1E-4 Band Grain boundary polaron hopping Far range hopping Transport mechanism Disorder

3-1. Organic Semiconductor Extrinsic Effects on Charge Transport Scattering Mechanism in Organic Thin-Film

3-1. Organic Semiconductor Extrinsic Effects on Charge Transport Free and Trapped Carriers Field-Effect Induced Carrier Density - - - - - - - - - - - - Gate + + + + + + + E F + + Organic Semiconductor Traps Accumulation Layer E C E V p C i V g /e p = p f + p t Q = p f /(p f +p t ) m FET Q m o < m o Sample Dependence of m FET (Growth Conditions, Preparation ) Gate Voltage Dependence Degradation / Instabilities Insulator

3-1. Organic Semiconductor Extrinsic Effects on Charge Transport Traps in OFETs Insulator Interface Bulk + + + + E v Grain Boundary Source of trap: impurity, grain boundary, misfit etc. If the interface trap density (C it ) is high, the subthreshold slope (S) is increased kt Cdm + C S = 2.3 1+ q COx it

3-1. Organic Semiconductor Disorder and OFET Characteristics

3-2. Gate Insulators Requirements for OFET Dielectrics Requirements for OFET Dielectrics 1) High dielectric constant for low-voltage operating 2) Good heat and chemical resistance 3) Pinhole free thin film formability with high breakdown voltage and long term stability 4) Comparable with organic semiconductor in interfacial properties Low k CYMM Commonly used

3-2. Gate Insulators Potential Dielectric Effects in OFETs The conduction mechanism in organic semiconductors is different from that of inorganics. 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 n in inorganic materials. Dielectric Effects in OFETs 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 effectse The polarity of the dielectric interface may also play a role, as it can affect local morphology or the distribution of electronic states in OSC

3-2. Gate Insulators Inorganic Insulators for OFETs Surface states on inorganic oxides are particular problem leading g to interface trapping and hysteresis,, also impacts the semiconductor morphology. Large number of surface treatment studies

3-2. Gate Insulators Surface Treatments of Inorganic Insulators Self-Assembly Monolayer (SAM) SAM studies Hexamethyldisilazene (HMDS) Octadecyltrichlorosilane (OTS) Other silanes Alkanephosphonic acid Cinnamic agents - Increasing the molecular ordering - Obtained improved OFET characteristics Mobility increases owing to increased grain size of OSC : increased grain size of OSC : assisted by high molecular surface mobility and reduced interaction with the surface of the hydrophobic substrates

3-2. Gate Insulators Surface Treatments of Inorganic Insulators Mobility enhancement controlled by molecular ordering at the interface

3-3. Electrode and Contact Organic Insulators for OFETs Organic dielectrics offer the freedom to build both top and bottom om gate devices more easily by the use of solution coating techniques and printing ng (cf. The surface treatments employed on inorganics would be practically impossible to use in top gate devices)

3-2. Gate Insulators Issues on Organic Insulators Cap. = ε 0 k/d (ε 0 : vacuum permittivity, k: dielectric constant, d: insulator thickness => thin insulator layer (less than 100 nm), and high k High Capacitance Low voltage operation In thin film, k increasing, but leakage current also increasing High K inorganic dielectrics + polymer layer TiO 2 (7 nm) + PAMS (10 nm) Reduced leakage current by PAMS (poly(α-methylstyrene)) OTFT operation below 1 Voltage M. Crell et. Al. Adv. Mater. 2005

3-3. Electrode and Contact Charge transport in OFETs Contact Effects in OFETs also very important! I I d d 1 1 1 + R Injection : contact effects of charge I d ( injection) Id ( drift) injection at source electrode and depletion region resistances ( RInjection + 2RDepletion) + R R Channel Depletion :depletion region resistances at source and drain elecctrodes = ( injection & drift) 1 ( injection & drift)

3-3. Electrode and Contact Contact Problems Most organic semiconductors look a lot like insulators contacts can be problematic Details of film growth and morphology and device structure often amplify contact problems

3-3. Electrode and Contact Contact Problems Contact Barrier Source Drain Gate

3-3. Electrode and Contact Reducing the Contact Resistance Requirement for S/D Electrodes : No interface barrier with the active layer - High carrier injection, low contact resistance : Low diffusivity for top contact Au Voltage drop (V c = I D R c ) between electrodes and semiconductor : Mainly used as source/drain electrodes because of its high work function (5.1 ev) and low injection barrier. : However, still remain injection barrier due to interface effects such as the dipole barrier. Controlling charge injection in OFET using metal-oxide layer or SAM treatments Source (Au) Charge injection layer Pentacene Charge Injection Transport

3-3. Electrode and Contact Electrode Organic Semiconductor Barrier

3-4. Electronic and Environmental Stability Bias-Stress Effects Bias-Stress Effects» Threshold voltage shift during operation : A slow decrease in the ON-current when is turned on for an extended time F8T2 : Large and stable PT RR : Smaller and rapidly reversed» Reversible, but it takes from second to days of longer» Bias stress is due to gate potential shielding by trapped charge (=C 0 ΔV T )

3-4. Electronic and Environmental Stability Bias-Stress Effects Where is the charge trapped? 1. In states within the gate dielectric 2. At the semiconductor/dielectric interface 3. In the semiconductor itself 4. Slow ion migration or electrochemical degradation 5. Structure transformation in polymer

3-4. Electronic and Environmental Stability Bias-Stress Effects Trapping depends on effective stress not on dielectric or dielectric and semiconductor interface Recovery rate scales with absorption of band-gap radiation in the polymer A. Salleo and R. A. Street, J. Appl. Phys.

3-4. Electronic and Environmental Stability Bipolaron Model for Bias-Stress Effects» Charge trapping occurs within Charge trapping occurs within the polymer» Slow trapping of holes into a deep electronic state Bipolaron Model h + + h + B.P dn dt h kn 2 h A. Salleo and R. A. Street, Phys. Rev. B 68, 085316

3-4. Electronic and Environmental Stability Environmental Stability Off- current increase by oxygen doping process. Sub-threshold swing increased

4. Fabrication of OFETs How to Get High Mobility? Organic materials tend to pack into either a herringbone or a π-π stacking structure, governed by intermolecular interactions : Better overlap between the π molecular orbitals generally results in a higher mobility To achieve high field effect mobility, the semiconducting molecules should have an orientation in which the π-π stacking direction between molecules is arranged in the same direction as that of the current flow Three Ways of Mobility Improvement To strengthen the intermolecular interaction force-works of chemists To use a single macromolecule bridging between source and drain electrode;1,000cm 2 /Vsec for the conjugated conducting polymer -difficult Fabrication To control the assembling conditions : large π-conjugation length; : close packing along the short molecular axes-feasible process

4. Fabrication of OFETs Important Factors to Get High Mobility 1. Shortest intermolecular distance b Triclinic P-1 a(å)= 5.7676 (11) b (Å)= 8.5369 (16) c (Å)= 22.949 (4) α = 100.272 (4) β = 94.846 (3) γ = 92.070 (4) a Monoclinic P2(1)/a a(å)= 8.0077 (8) b (Å)= 5.6402 (5) c (Å)= 23.749 (2) α = 90 β = 99.215 (2) γ = 90

4. Fabrication of OFETs Important Factors to Get High Mobility 2. Large grain size T s =30 o C, 3 μm x 3μm T s =150 o C, 10 μm x 10μm Mobilty = 0.01 cm 2 V -1 s -1 Mobilty = 0.1 cm 2 V -1 s -1

4. Fabrication of OFETs Important Factors to Get High Mobility 3. Lowest surface roughness and surface energy

4. Fabrication of OFETs Important Factors to Get High Mobility 4. Higher molecular ordering

4. Fabrication of OFETs Important Factors to Get High Mobility 5. Orientation and direction In-plane mobility -0.5 S (Au) D (Au) PtOEP RT wet transfer -50 Drain current (μa) -0.4-0.3-0.2-0.1 0.0 SiO 2 Silicon substrate 0-10 -20-30 -40-50 Source-Drain voltage (V) Higher mobility -40-30 -20 0 Gate Voltage (V) > Drain current (na) -3.5-3.0-2.5-2.0-1.5-1.0-0.5 0.0 S (Au) SiO 2 D (Au) Silicon (Gate) 0-10 -20-30 -40-50 Drain voltage (V) PtOEP, 50 o C, wet transfer -50-40 -30-20 -10 0 Gate Voltage (V)

4. Fabrication of OFETs Important Factors to Get High Mobility 6. Reorganization energy (small electron-phonon coupling) Mobilty = 1~ 2 cm 2 V -1 s -1 Mobilty = 0. 1 cm 2 V -1 s -1 Even though they have same packing manner (herringbone) and intermolecular distance, they show different mobility due to difference of Reorganization energy. Generally, fused structure show low reorganization energy.

4. Fabrication of OFETs Important Factors to Get High Mobility 7. Carefully designed polymers form ordered films Regiorandom PHT Regioregular PHT Low mobility High mobility Slow evaporation Carefully choice of solvent

4. Fabrication of OFETs Important Factors to Get High Mobility 8. Optimized treatment and process Annealing Alignment

4. Fabrication of OFETs Semiconductor Deposition Methods In general, organic semiconductors are deposited either from vapor or solution phase depending on their vapor pressure and solubility Device performance of OFETs is greatly influenced by various deposition conditions due to the different resulting molecular structure and thin film morphology Vacuum Deposition Organic small molecules and oligomers that are solution-insoluble High film uniformity Good run-to-run reproducibility Multi-layer deposition and codeposition of several organic semiconductors are possible Relative high materials consumption High initial cost for equipment setup Thermal evaporation deposition Molecular beam deposition Organic vapor phase deposition Materials Advantages Disadvantages Examples Solution Deposition Solution processable materials Not limited by size of vacuum chamber No pumping down time is required Compatible with large-area thin film fabrication Lower production cost per device Low film uniformity Delamination or dissolution of the pervious layers Spin coating Ink-jet printing Screen printing

4. Fabrication of OFETs Vacuum Deposition 1. Organic Thermal Deposition High vacuum machine is needed Exact and easy control of crystallinity

4. Fabrication of OFETs Vacuum Deposition 2. Organic Vapor Phase Deposition (OPVD) Growth occurs by infusing a hot boat containing the organic source material with an inert carrier gas. The gas becomes saturated with the organic vapors and carries them downstream to a cooled substrate where they physisorb onto its surface. Shown immediately in front of the substrate is a hydrodynamic boundary layer of thickness d. Depending on the reactor pressure and rate of molecular pickup by the carrier gas, the boundary layer thickness can be varied from very thin to thick

4. Fabrication of OFETs Vacuum Deposition 3. Organic Molecular Beam Deposition (OMBD) Monolayer control over the growth of organic thin films with extremely high chemical purity and structural precision is possible Schematic of processes relevant in thin film growth, such as adsorption (as a result of a certain impingement rate), (re-)desorption, intra-layer diffusion (on a terrace), interlayer diffusion (across steps), nucleation and growth of islands.

4. Fabrication of OFETs Vacuum Deposition Mechanism of Thin-Film Growth

4. Fabrication of OFETs Solution Deposition Polymeric organic semiconductors offer process advantages

5. Printing Technology in OFETs Alternatives to Photolithography

5. Printing Technology in OFETs Screen Printing

5. Printing Technology in OFETs Ink-Jet Printing

5. Printing Technology in OFETs Surface Energy Assisted Ink-Jet Printing

5. Printing Technology in OFETs Dip-Coating Ink-jet printing of wax is used to define the device structure

5. Printing Technology in OFETs Micro-contact Printing

5. Printing Technology in OFETs Micro-contact Printing

5. Printing Technology in OFETs Micro-contact Printing

5. Printing Technology in OFETs Roll-to-Roll Micro-contact Printing

5. Printing Technology in OFETs Micro-contact Printing OTFTs by Microcontact printing OLED by Microcontact printing E-ink by Microcontact printing

6. Future Prospects in OFETs Future Trend Molecular-Scale Electronic Devices Moore Moore s 1st st Law Moore s 2nd Law Chip functionality increases a factor of 4 every 3 years Manufacturing cost increases by a factor of 2 every 3 years Physics Today, 53, 38 (2000)

6. Future Prospects in OFETs CMOS Technology at 2010 Scaling of electron devices expects that only 8 electronsare required for on/off by 2010 Physics Today, 53, 38 (2000)

6. Future Prospects in OFETs Technology Roadmap SIA 2001

6. Future Prospects in OFETs How Can Go Beyond Roadmap? Nanostructures

6. Future Prospects in OFETs Top-Down and Bottom-Up Approach

6. Future Prospects in OFETs How Can Go Beyond Roadmap?

6. Future Prospects in OFETs Molecular Rectifier

6. Future Prospects in OFETs Molecular Rectifier

6. Future Prospects in OFETs Molecular Rectifier

6. Future Prospects in OFETs Single Molecular Transistor

6. Future Prospects in OFETs Nanotube Transistor Room Temperature CNTFET S.J. Tans et al., Nature, 393, 49 (1998)

6. Future Prospects in OFETs Fabrication Methods for Molecular Devices 1. E-Beam Lithography CNT

6. Future Prospects in OFETs Fabrication Methods for Molecular Devices 2. Nano Imprint Lithography

6. Future Prospects in OFETs Fabrication Methods for Molecular Devices 3. Dip-Pen Nanolithography Dip-PenNanolithography(DPN) is a scanning probe nano-patterning technique in which an AFM tip is used to deliver molecules to a surface via a solvent meniscus, which naturally forms in the ambient atmosphere. D. Piner, J. Zhu, F. Xu, and S. Hong, C. A. Mirkin, Science, 283, 661(1999)

6. Future Prospects in OFETs Fabrication Methods for Molecular Devices 4. Break Junction Single molecule transport studied in Mechanically Controllable Break Junctions M.A. Reed et al., Science 278, 252 (1997)

6. Future Prospects in OFETs Fabrication Methods for Molecular Devices 5. Self-Assembly Monolayer Self-AssembledMonolayer(SAM) A single layer of order molecules adsorbed on a substrate due to bonding b/w the surface and molecular head groups.-modification of adhesion-corrosion protection-chemical and biological functionalization-insulating layers in electronic devices

6. Future Prospects in OFETs Fabrication Methods for Molecular Devices 6. Langmuir-Blogett Deposition

6. Future Prospects in OFETs What Is Wrong with Si? Nothing! Smaller no Faster no Cheaper yes Polymers are NOT going to replace Si. Si processing is NOT cheap...low cost has been achieved through small size and high levels of integration. Packaging and placement are a large part of the cost of simple devices. In many applications the performance of Si is not required. There appears to be an opportunity that exists for a disruptive technology that has good enough performance and can achieve very low cost without regard to the size of the finished device

6. Future Prospects in OFETs Roadmap of Organic Electronics