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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 (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 (60-120oC) 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 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 Vd > 0 L W Source Drain - - - - - - - - - - - - Why Polymer? Printability Polymer Semiconductor Flexibility Low cost & weight Dielectric Layer Substrate/Gate Electrode Easy fabrication Architecture of transistor device ++++++++++++ Vg > 0 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 cm2/VS) High On/Off current Ratio (>104) 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 WCi 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) 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 Transfer (Id-Vg) Curve at saturation region Performance Parameters Field Effect Mobility (μ) [cm2/VS] WCi slope 2 L 1/ 2 Threshold Voltage (VTh) On/Off Current Ratio (Ion/Ioff) Sub-threshold Slope (SS) 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 WCi Current-Voltage (I-V) Characteristics Output (Id-Vd) 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: SiO2, TiO2, Al2O3) Electrode Metal (ex: Au, Ca) Conjugated Polymer (ex: PEDOT:PSS) Materials Requirements of Organic Semiconductors for OTFT Target: > 1 cm2/Vs on/off ratio >106 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 The OTFTs Performance Evolution of The OTFT mobility for P type or N type Semiconductors 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) Donor-Acceptor Conjugated Polymer Semiconductors with High FET Mobility (Literature~2009) Hole/Electron mobility C16H33 C16H33 O C12H25 S N S N S n S C12H25 O N S N 0.1 cm2/VS 0.1 cm2/VS 1.4 cm2/VS S S n Adv. Mater. 2009, 21, 209 Adv. Mater. 2008, 20, 2217 NC N CN C12H25 S S S C12H25 NC N 0.1 n cm2/VS S S CN C8H17 O N C12H25 0.2 cm2/VS J. Mater. Chem. 2009, 19, 591 C10H21 J. Am. Chem. Soc. 2008, 130, 8580 C12H25 C12H25 C12H25 n O C14H29 C14H29 S S N S S N S S S S n C14H29 C14H29 J. Am. Chem. Soc. 2009, 131, 2521 0.2 cm2/VS O N O C8H17 n 0.85 cm2/VS Nature. 2009, 457, 679 C10H21 Q: Could we develop new solution-processable semiconducting polymers with mobility 38 2 > 1 cm /Vs 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 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 WCi 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 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 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 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! Dielectrics 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 thicker 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 C OX The x-axis intercept of ID1/2-VG Q C OX N type 4qN A S F V 't 2 F kT N A F ln q ni d k F c 1 COX 4qN A 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? 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 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