Deposition and patterningtechniques for Organic Semiconductors Maddalena Binda Organic Electronics: principles, devices and applications Milano, November 15-18th, 2011 Overview DEPOSITION PATTERNING • Drop casting • Screen printing • Spin coating • Soft Lithography • Doctor Blade • NIL/Embossing • Dip coating • Physical Delamination • Layer-by-layer, • Photopatterning Organic Langmuir-Blodgett • Ink-jet printing materials • Spray coating M. Caironi • Vacuum Thermal Evaporation • Shadow masking • Organic Vapor Phase • Vapor Jet Printing Deposition (OVPD) • Organic Molecular Beam Deposition (OMBD) 1 Solution processable materials: deposition techniques • Drop casting • Spin coating • Dip coating • Layer-by-layer: Langmuir-Blodgett • Spray coating Drop Casting Dropping of solution and spontaneous solvent evaporation Evaporation Dropping Substrate Substrate Substrate Film thickness: solution concentration X Limitations in large area coverage Very simple X Thickness hard to control No waste of material X Poor uniformity Tricks... • Combination of solvents • Solvents evaporation time: heating of the substrate to speed up the evaporation process and/or improve film morphology 2 Spin Coating I Dropping on spinning substrate Film thickness: substrate dependent on many controllable parameters dω/dt, ω, t, solution viscosity,… Good uniformity/reproducibility X Waste of material Good control on thickness X No large area X Film dries fast Easy to obtain film of 10nm or less less time for molecular ordering Tricks... McCulloch et Al., 180ºC •Thermal annealing Nat. Mater., 5, 328, (2006) (post-deposition) •Solvent evaporation time Chang et Al. Chem. Mater.,16, 23, (2004) P3HT-based TFTs Spin Coating II Film thickness: 1/ 2 2v 1 4 = fluid viscosity ω = rotation speed 2 r h(t ) t z 2 3 = fluid density v = radial velocity It does not take into account solvent evaporation… 3 Spin Coating III Film thickness: evaporation flow dominated dominated viscous flow rate = evaporation rate 1 1 1 hFIN c0 (1 c0 ) 3 2 ( 0 ) 3 c(t)= solids concentration μ µ(t) most common reported experimental relationship between thickness and rotation speed D. E. Bornside, C. W. Macosko and L. E. Scriven, "Spin Coating of a PMMA/Chlorobenzene Solution", J. Electrochem. Soc., 138, 317 (1991) How to handle multilayer deposition? Post-deposition film insolubilization Thermally Polymer cross-linking activated UV-light Cross-linked polymer chains Host cross-linkable polymer Stable, high degree of control Appliable to any kind of polymer X Polymer intrinsic properties are (small molecule?) affected Doesn’t affect polymer intrinsic properties X Less “deterministic” X Film properties are affected P. Keivanidis et al., Binda et al., Appl. Phys. Lett. 94, 173303, 2009. Appl. Phys. Lett. 98, 073303, 2011. 4 Doctor Blading I Spreading through a moving blade onto a stationary substrate stationary moving Large area X Micrometric precision of blade regulation No waste of material Good uniformity film thickness > 150÷200nm Example. bladed organic solar cells (P3HT/PCBM) W.-B. Byun et al. / Current Applied Physics 11 (2011) Doctor bladed active material in comparison with spin-coated (~200nm) Doctor Blading II Film thickness: Theoretical height of the wet layer thickness: surface tension, wetting, viscosity, coating speed,... Y. Chou, Y. Ko, M. Yan, J. Am. Ceram. Soc., 7010, 1987. A.I.Y Tok, Journal of Materials Engineering and Performance, 8, 4, 1999. sol h02 P U=blade speed FilmThick ness h0 1 film 6 UL µ=fluid viscosity L=channel length =density ho=blade height P=slurry pressure head 5 Doctor Blading refinement I: zone casting Controlled orientation of deposited layers Controlled Deposition of Crystalline Organic Semiconductors Z. Bao, Adv. Mater. 21, 1217–1232, 2009. by suitably controlling solvent evaporation rate and solution supply Optical TEM microscope directional crystallization substituted hexabenzocoronenes (HBCs) Doctor Blading refinement II: solution-sheared deposition Z. Bao, Adv. Mater. 21, 1217–1232, 2009. Diluited solution Pre-heated substrates Top wafer lyophobic Fast evaporation of the front produces a seeding film with crystal grains that act as nucleation sites TMS-4T dbo-P2TP 6 Dip Coating The substrate is dipped into the solution and then withdrawn at a controlled speed. Film thickness: Thickness (H) determined by the balance of forces at the liquid-substrate interface Landau and Levich equation: 0.94 v 2 / 3 H 1 / 6 g 1 / 2 = fluid viscosity v = withdrawal speed Solution = fluid density g = gravitational acceleration = surface tension (liquid-air) Quite good uniformity X Waste of material Very thin layers X Time consuming Large area coverage X Double side coverage Example: TFT based on P3HT in xylene single monolayer ≈ 2 nm thick Sandberg et Al., Langmuir, 18, 26, 2002. Extreme thickness control: Langmuir-Blodgett film I based on hydrophobicity/hydrophilicity Hydrophilic head Hydrophobic tail Molecules move as in a bi-dimensional ideal gas, with a well defined surface pressure P, area A, and density Amphiphilic molecules 3D Gas 2D Gas water P [N/m2] P [N/m] V [m3] A [m2] N [m-3] n [m-2] T [k] T [k] 7 Extreme thickness control: Langmuir-Blodgett film II Isothermal curve P [Nm-1] solid Reducing the available area, pressure increases and eventually a phase-change occurs: “gas” “liquid” “solid” Pc l/s Once PC is reached, a compact molecular mono-layer is formed (”solid” state) and liquid floats on the water surface. At this stage the area cannot be further reduced g/l without destroying the mono-layer. gas A [m2] KSV Instruments Application Note: http://www.ksvltd.fi/Literature/Application%20notes/LB.pdf Extreme thickness control: Langmuir-Blodgett film III hydrophilic substrate Movable barrier Wilhelmy plate: measures P Feedback Head-to-tail Tail-to-tail Head-to-head 8 Extreme thickness control: Langmuir-Blodgett film IV Excellent control of thickness. X Only amphyphilic molecules can An ideal monolayer can be grown be deposited Homogeneity over large areas X Non trivial setup Multilayer structures with varying layer composition Example. C60 dendrimer – n-type TFT LB film: Polar 5 layers 15nm Higher mobility than spin-coated film higher morphological order Apolar Kawasaki et al., Appl. Phys. Lett. 91, 243515, 2007. Spray Coating Substrate is hit by a vaporized solution flux Nozzle trajectory The film morphology can be controlled by: o air pressure o solution viscosity o solvent properties (evaporation rate,…) o gun tip geometry substrate o distance between nozzle and substrate single pass technique: multiple pass technique: droplets merge on the substrate droplets dry independently into a full wet film before drying smooth films rough film, but wettability issues can be overcome Adjustable layer thickness X Homogeneity of the film Large area coverage Independence on substrate topology 9 Spray Coating Example 1. Girotto et al. Adv. Funct. Mater. 2011, 21, 64–72 Spray-coated organic solar cells Example 2. Organic light sensor directly deposited onto a Plastic Optical Fiber (POF) 100n 1mW 10W Current [A] Spray coated 10n LIGHT bottom electrode 1W 1n DARK 100p 0 10 20 30 40 50 Time [s] Compression molding Solid state processing from powders Baklar et al. Adv. Mater. 2010, 22, 3942–3947 • Applicable to NON-soluble materials • Solid, powdered material placed in a hot press and compressed well below the melting temperatures of the species Medium area X Thickness 1-200µm No waste of material Good uniformity Highly ordered films 10 Compression molding Radial molecular flow during compression molding Free-standing flexible film Material anisotropy X-ray diffraction Baklar et al. Adv. Mater. 2010, 22, 3942–3947 Solution processable materials: patterning techniques • Screen printing • Soft Lithography • NIL/Embossing • Physical Delamination • Photopatterning 11 Screen Printing The solution of the active material is squeezed through a screen mask onto the substrate surface It can be applied to spray coating and doctor blading Substrate Mask Mask Z. Bao et al., A. J. Chem. Mater. 9, 1299-1301, 1997. Simple X Limited resolution: 50-100 m X Waste of material Shadow masking+selective wettability Exploiting the difference in wettability between hydrophobic surfaces and hydrophilic surfaces to make the patterns Journal of Polymer Science B: Polymer Physics, 49, 1590–1596, 2011 Hydrophobic SAM (Self Assembled Monolayer) UV-light damages the ODTS film PEDOT/PSS: conductive polymer from aqueous suspension 12 Shadow masking+selective wettability Journal of Polymer Science B: Polymer Physics, 49, 1590–1596, 2011 Soft Lithography Earliest motivation: overcome cost of photolithography for sub 100nm features Basic idea: replicate patterns generated by photolithography through an elastomeric mold. Master • Photolithography • X-Ray Litho • EB Litho • FIB writing… Elastomer • Micro Contact Printing (CP) • Micro Transfer Molding (TM) Mold • Micromolding in Capillaries (MIMIC) Whitesides et al., Angew. Chem. Int. Ed. 1998, 37, 550 -575. 13 Micro-Contact Printing (µCP) I Stamping of an “ink” through the elastomeric mold Printed material has to adhere to the substrate while the interaction with the mold has to be minimal Critical aspect ratio PDMS distortion Micro-Contact Printing (µCP) II Indirect... ... mediated by SAMs Monomolecular layer modifies substrate chemical proprieties ~few nm and the wettability Cl OTS: Cl Si (CH2)17 CH Cl 3 Hydrophobic SAM Organic material Hydrophobic region C. R. Kagan, Appl. Phys. Lett. 79, 3536, 2001 14 Micro-Contact Printing (µCP) III Direct... & additive Substrate pre-treated with O2 plasma to imrpove idrophilicity activating OH- groups Pentacene TFTs: PEDOT S/D contacts, W/L=140m/2m Baking step at 80ºC to ensure chemical bonding (an adhesive agent is added to PEDOT:PSS solution) D. Li and J. J. Guo, Appl. Phys. Lett. 88, 063513, 2006. Example 2. Patterned LBL deposited Film grown by LBL technique onto the PDMS mold Organic Active Material J.R. Tischler et al., Org. Elec. 8,94–113, 2007. Micro-Contact Printing (µCP) IV Direct... & subtractive As the stamp is placed in contact with a liquid thin film spread on a substrate, capillary forces drive the solution to form menisci under the stamp protrusions b) AFM image of the stamp; c) printed Dilute solution Very dilute solution AlQ3 film using dilute solution; d) very dilute solution; e) line profile of stamp and Solution pinned to films the edges Cavallini, Nano Lett., Vol. 3, No. 9, 2003. 15 Micro-Contact Printing (µCP) V Direct... & subtractive C. Packard et al., Langmuir 2011, 27, 9073–9076 Applicable also to evaporated/solid films! Test with oxygen plasma Patterning mechanism? • Lift-off due to adhesion forces PDMS between polymer and stamp glass stamp before • Organic film removal due to organic molecules diffusion into the PDMS stamp during contact (suggested by time dependence of material removal) glass PDMS Molecules stimulated fluorescence stamp after Micro-Contact Printing (µCP) VI Direct... & subtractive C. Packard et al., Langmuir 2011, 27, 9073–9076 Applicable also to evaporated/solid films! Controllable thickness, depending on contact time Nanometer-scale accuracy of thickness control! 16 Micro Transfer Molding A liquid polymer precursor is poured in the cavities of the elastomeric mold, put in contact with the substrate and finally cured. Polymer must not shrink too much after curing Typical materials: polyurethane, polyacrylates and epoxies 3D structures A: polyurethane on Ag, Microstructures of B,C,D: epoxy on glass glassy carbon Whitesides et al., Angew. Chem. Int. Ed. 1998, 37, 550 -575 Micro Molding in Capillaries I Based on the flow of a liquid in capillaries Flow dynamics: Z: length of the liquid column into the capillary dZ R lv cos 2R L dt 4Z : liquid viscosity Z lv: liquid-vapor surface energy θ: liquid-capillary surface contact angle Whitesides et al., Angew. Chem. Int. Ed. 1998, 37, 550 -575 Pisignano et Al., Adv. Mater. 14, 1565, (2002) 17 Micro Molding in Capillaries II EXAMPLE 1 EXAMPLE 2 FEATURES Ag nano- 300nm dispersion (AFM image) A. Blumel et al., • Examples: Organic Electronics 8, 389–395, (2007) Polyurethane in PDMS mold EXAMPLE 3 PEDOT:PSS as S/D contacts in all organic TFTs H. Kang et al., Organic Electronics 10, 527–531, (2009) Nano Imprint Lithography/Embossing I Similar to SL but based on hard mold/stamp. I It allows obtaining smaller features ( 10 nm) T>Tg Tg: polymer glass phase transition Hot Embossing Room Temperature NIL 18 Nano Imprint Lithography/Embossing II Example. Nanometer-sized electrodes for OTFTs Silicon stamp - Nanoimprint of photoresist (a,b,c) - Dry etching in O2 plasma (d) - Metallization Au/Ti (e) - Lift-off in acetone (f) O2 d 100 nm Kam et al., Microelectronic Engineering, 73, 809–813, 2004. Physical Delamination Based on a photolithographic process previous to semiconductor deposition Optical and AFM images Polymer adhere to the substrate of patterned PBTTT where OTS is not present Sirringhaus, Adv. Mater., 21, 1–6, 2009. 19 Photopatterning Same principles of standard photo-lithography resist is the active material! UV Mask Active material Etching (solvent) UV-Crosslinkable Example: patterning of pixel in OLED display: Patterning of the hole transport layer Feature size 5 m Nuyken et Al., Macromol. Rapid Commun., 25, 1191–1196, 2004. Non-soluble materials: deposition techniques • Vacuum Thermal Evaporation • Organic Vapor Phase Deposition (OVPD) • Organic Molecular Beam Deposition (OMBD) 20 Vacuum Thermal Evaporation I Sublimation of small molecules due to high-vacuum and high temperature Substrate holder Pressure 10-5-10-7 torr Molecules mean free path: tens of cm - m •Evaporated molecules travel in straight lines inside the chamber and they condense on the “Source boat”: substrate contains the material and it is heated at •Growth rate controlled by tuning hundreds of °C the temperature of the source boat Thickness of the film is monitored with the crystal microbalance (change of the resonating frequency of a piezo resonator). Vacuum Thermal Evaporation I Growth rate and substrate temperature affect film morphological order PC on thermal Pentacene SiO2 on SiO2 PC on Dimitrakopoulos, sputtered Adv. Mater., 14, palladium 99, (2002) High quality, ordered thin films X Waste of material Good control and reproducibility of X Expensive equipments film thickness X Very low throughput Multilayer deposition and co- high production costs deposition of several organic materials X No large area coverage 21 Organic Vapor Phase Deposition Based on low pressure carrier gas instead of high vacuum Hot inert carrier gas (Ar, N2) transport source material to the cooled substrate where condensation occurs Heated walls vapor does not condense OVPD: working regimes Evaporation regimes Equilibrium evaporation: Kinetic evaporation: Organic evaporation limited by Organic evaporation limited by the evaporation rate the flow of the carrier gas Deposition regimes Equilibrium limited: Transport limited: Substrate kept at a temperature Substrate at room temperature high enough to establish an adsorption/desorption equilibrium Better morphology 22 OVPD: examples TFT based on Pentacene TPD-Alq3 OLED heterostructure SiO2 SiO2-OTS TPD Alq3 Shtein et al. Appl. Phys. Lett., 81, No. 2 (2002) Forrest et Al., Appl. Phys. Lett., 71 (21), (1997) • Advantages over standard VTE: Higher deposition rates Less waste of material (no condensation on the internal walls) Better film thickness control and uniformity in large area substrates Organic Molecular Beam Deposition Flow of focalized molecules in ultra high vacuum Quartz Substrate microbalance Extremely slow deposition rate epitaxial growth pC ,TC Single crystal 10-9 mbar “Pin hole” pH ,TH Effusion cell (Knudsen) Shutter 23 Non-soluble materials: patterning techniques • Shadow masking • Vapor Jet Printing Shadow masking Shadow mask (same principle of screen printing). Resolution limited to tens of m. Patterning of RGB sub- pixels in OLED displays Fukuda et Al., Synthetic Metals, 111–112 (2000) 24 Organic Vacuum Jet Printing (OVJP) Organic small molecule material carried by hot inert gas to a nozzle array that collimates the flow into jets Condensation onto a proximally located cold substrate, that can moove relatively to the print-head for patterning For high resolution (~20μm): •Narrow nozzles ~10μm •Low gas flow rate Fabricated by MEMS processing G.J. McGraw, Appl. Phys. Lett. 98, 013302, 2011. Organic Vacuum Jet Printing (OVJP) 6 tubes with different Fluorescence from Alq3 stripes source material N2 flow rate x=feature size substrate-nozzle distance 25