Deposition and patterning techniques for Organic Semiconductors

Transcription

1 Deposition and patterning techniques for Organic Semiconductors Maddalena Binda Organic Electronics: principles, devices and applications Milano, November 26-29th, 2013

2 Overview Organic materials DEPOSITION Drop casting Spin coating Doctor Blade Dip coating Layer-by-layer, Langmuir-Blodgett Spray coating PATTERNING Screen printing Soft Lithography NIL/Embossing Physical Delamination Photopatterning Ink-jet printing M. Caironi Vacuum Thermal Evaporation Organic Vapor Phase Deposition (OVPD) Organic Molecular Beam Deposition (OMBD) Shadow masking Vapor Jet Printing

3 Choice of the deposition technique Uniformity Waste of material Thickness Es. solar cells organic BHJ ~ hundreds nm dye-sensitized ~ μm Nanomorphology (molecules relative arrangement in the solid state) sol solvent evaporation non-sol post-processing deposition rate Material solubility polarity Substrate dimension shape surface features: ruoghness, wettability polymer or small molecule single or mix

4 Solution processable materials: deposition techniques Drop casting Spin coating Dip coating Layer-by-layer: Langmuir-Blodgett Spray coating

5 Drop Casting Dropping of solution and spontaneous solvent evaporation Evaporation Dropping Substrate Substrate Substrate Film thickness: Very simple No waste of material solution concentration X Limitations in large area coverage X Thickness hard to control X Poor uniformity Tricks... Combination of solvents Solvents evaporation time: heating of the substrate to speed up the evaporation process and improve film morphology

6 Spin Coating I Dropping on spinning substrate substrate Film thickness: dependent on many controllable parameters dω/dt, ω, t, solution viscosity, Good uniformity Reproducibility Good control on thickness down of 10nm or less Chang et Al. Chem. Mater.,16, 23, (2004) X Waste of material X No large area X Film dries fast less time for molecular ordering Tricks... Solvent evaporation time P3HT-based TFTs McCulloch et Al., Nat. Mater., 5, 328, (2006) 180ºC larger crystallinity POST-PROCESSING Thermal annealing Vapor annealing

7 Spin Coating II Film thickness: flow dominated evaporation dominated viscous flow rate = evaporation rate h FIN k c ( 1 c0) ( 0 0) most commonly reported experimental relationship between thickness and rotational speed 1 3 c 0 solids concentration (by volume) µ 0 viscosity ρ 0 liquid density S. L. Hellstrom, Basic Models of spin-coating", D. E. Bornside, C. W. Macosko and L. E. Scriven, "Spin Coating of a PMMA/Chlorobenzene Solution", J. Electrochem. Soc., 138, 317 (1991)

8 How to handle multilayer deposition? Post-deposition film insolubilization Polymer cross-linking Cross-linked polymer chains Thermally UV-light activated Host cross-linkable polymer Stable, high degree of control X Polymer intrinsic properties are affected Appliable to any kind of polymer (small molecule?) X Film properties are affected Doesn t affect polymer intrinsic properties X Less deterministic P. Keivanidis et al., Appl. Phys. Lett. 94, , Binda et al., Appl. Phys. Lett. 98, , 2011.

9 Doctor Blading I Spreading through a moving stationary blade onto a stationary substrate moving Film thickness: h0 film thickness: c U h0 c=concentration U=deposition speed h0=height of the gate Theoretical height of the wet layer thickness: surface tension, wetting, viscosity, coating speed,... J. Am. Ceram. Soc., 7010, Journal of Materials Engineering and Performance, 8, 4, 1999.

10 Doctor Blading II Spreading through a moving stationary blade onto a stationary substrate moving Large area No waste of material Good uniformity X Micrometric precision of blade regulation X Not suitable for very thin films: film thickness > 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)

11 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: H / 6 2 / 3 v 1/ 2 g Solution Quite good uniformity Very thin layers Large area coverage = fluid viscosity v = withdrawal speed = fluid density g = gravitational acceleration = surface tension (liquid-air) X Waste of material X Time consuming X Double side coverage Example: TFT based on P3HT in xylene single monolayer 2 nm thick Sandberg et Al., Langmuir, 18, 26, 2002.

12 Extreme thickness control: Langmuir-Blodgett film I based on hydrophobicity/hydrophilicity Hydrophilic head Hydrophobic tail Amphiphilic molecules Molecules move as in a bi-dimensional ideal gas, with a well defined surface pressure P, area A, and density water 3D Gas P [N/m 2 ] V [m 3 ] N [m -3 ] T [k] 2D Gas P [N/m] A [m 2 ] n [m -2 ] T [k]

13 Extreme thickness control: Langmuir-Blodgett film II P [Nm -1 ] P c solid l/s Isothermal curve liquid g/l gas Reducing the available area, pressure increases and eventually a phase-change occurs: gas liquid solid Once PC is reached, a compact molecular mono-layer is formed ( solid state) and floats on the water surface. At this stage the area cannot be further reduced without destroying the mono-layer. A [m 2 ] KSV Instruments Application Note:

14 Extreme thickness control: Langmuir-Blodgett film III Movable barrier hydrophilic substrate Wilhelmy plate: measures P Feedback Head-to-tail Tail-to-tail Head-to-head

15 Extreme thickness control: Langmuir-Blodgett film IV Excellent control of thickness. An ideal monolayer can be grown Homogeneity over large areas Multilayer structures with varying layer composition Example. C60 dendrimer n-type TFT Polar X Only amphyphilic molecules can be deposited X Non trivial setup LB film: 5 layers 15nm Apolar Higher mobility than spin-coated film higher morphological order Kawasaki et al., Appl. Phys. Lett. 91, , 2007.

16 Spray Coating Substrate is hit by a vaporized solution flux Nozzle trajectory substrate single pass technique: droplets merge on the substrate into a full wet film before drying The film morphology can be controlled by: o air pressure o solution viscosity o solvent properties (evaporation rate, ) o gun tip geometry o distance between nozzle and substrate multiple pass technique: droplets dry independently smooth and uniform films analogous to spin-coating rougher films, but topology and wettability issues can be overcome and thickness can be adjusted Large area coverage? Waste of material

17 Current [A] Spray Coating Example 1. Girotto et al. Adv. Funct. Mater. 2011, 21, Spray-coated organic solar cells Example 2. Adv.Mater.2013,25, Organic light sensor directly deposited onto a Plastic Optical Fiber (POF) 100n 1mW Spray coated bottom electrode and active material 10n LIGHT 10 W 1 W 1n 100p DARK Time [s]

18 Compression molding Solid state processing from powders Baklar et al. Adv. Mater. 2010, 22, 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 No waste of material Good uniformity Highly ordered films X Thickness 1-200µm

19 Compression molding Radial molecular flow during compression molding Free-standing flexible film Material anisotropy X-ray diffraction Baklar et al. Adv. Mater. 2010, 22,

20 Solution processable materials: patterning techniques Screen printing through a mask Shadow masking Photopatterning Soft Lithography NIL/Embossing Physical Delamination Atomic force nanolithography beyond mask limitation

21 Substrate Screen Printing + Shadow masking The solution of the active material is squeezed by a moving blade through a screen mask onto the substrate surface Mask Z. Bao et al., A. J. Chem. Mater. 9, , Simple X Limited resolution: X Waste of material m Masking applied to spray coating: shadow masking Mask

22 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, , 2011 Hydrophobic SAM (Self Assembled Monolayer) UV-light damages the ODTS film PEDOT/PSS: conductive polymer from aqueous suspension

23 Shadow masking+selective wettability Journal of Polymer Science B: Polymer Physics, 49, , 2011

24 Photopatterning Same principles of standard photo-lithography resist is the active material! UV Mask Active material UV-Crosslinkable Etching (solvent) Example: patterning of pixels in OLED display: Patterning of the hole transport layer Feature size 5 m Nuyken et Al., Macromol. Rapid Commun., 25, , 2004.

25 Soft Lithography Earliest motivation: overcome cost of photolithography for sub μm features Basic idea: replicate patterns generated by photolithography through an elastomeric mold. Master Photolithography X-Ray Litho EB Litho FIB writing Elastomer Mold Critical aspect ratios Whitesides et al., Angew. Chem. Int. Ed. 1998, 37,

26 Soft Lithography Printed material has to adhere to the substrate while the interaction with the mold has to be minimal Micro Contact Printing ( CP) Micro Transfer Molding ( TM) Micromolding in Capillaries (MIMIC)

27 Soft Lithography Printed material has to adhere to the substrate while the interaction with the mold has to be minimal Micro Contact Printing ( CP) Micro Transfer Molding ( TM) Micromolding in Capillaries (MIMIC) Appl. Phys. Lett. 88, , Org. Elec. 8,94 113, (film grown by LBL onto the mold) Angew. Chem. Int. Ed. 1998, 37, Adv. Mater. 14, 1565, (2002) Organic Electronics 8, , (2007) Organic Electronics 10, , (2009)

28 Micro-Contact Printing (µcp) III 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 Dilute solution Very dilute solution Solution pinned to the edges b) AFM image of the stamp; c) printed AlQ 3 film using dilute solution; d) very dilute solution; e) line profile of stamp and films Cavallini, Nano Lett., Vol. 3, No. 9, 2003.

29 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

30 Nano Imprint Lithography/Embossing II Example. Nanometer-sized electrodes for OTFTs - Nanoimprint of photoresist (a,b,c) - Dry etching in O 2 plasma (d) - Metallization Au/Ti (e) - Lift-off in acetone (f) d Silicon stamp O nm Kam et al., Microelectronic Engineering, 73, , 2004.

31 Physical Delamination Based on a photolithographic process previous to semiconductor deposition Polymer adhere to the substrate where OTS is not present Optical and AFM images of patterned PBTTT Sirringhaus, Adv. Mater., 21, 1 6, 2009.

32 Atomic force nanolithography Resolution < 100 nm... but on 100x100 μm area! J. Phys.: Condens. Matter 21 (2009) Nanoshaving, Nanografting Dip-pen...

33 Atomic force nanolithography: thermochemical patterning NATURE NANOTECHNOLOGY, 4, (2009)

34 Non-soluble materials: deposition techniques Vacuum Thermal Evaporation Organic Vapor Phase Deposition (OVPD) Organic Molecular Beam Deposition (OMBD)

35 Vacuum Thermal Evaporation I Sublimation of small molecules due to high-vacuum and high temperature Substrate holder Pressure torr Source boat : contains the material and it is heated at hundreds of C Molecules mean free path: tens of cm - m Evaporated molecules travel in straight lines inside the chamber and they condense on the substrate Growth rate controlled by tuning 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).

36 Vacuum Thermal Evaporation I Growth rate and substrate temperature affect film morphological order Pentacene on SiO 2 Dimitrakopoulos, Adv. Mater., 14, 99, (2002) High quality, ordered thin films Good control and reproducibility of film thickness Multilayer deposition and codeposition of several organic materials X Waste of material X Expensive equipments X Very low throughput high production costs X No large area coverage

37 Organic Vapor Phase Deposition Based on low pressure carrier gas instead of high vacuum Hot inert carrier gas (Ar, N 2 ) transport source material to the cooled substrate where condensation occurs Heated walls vapor does not condense

38 OVPD: examples TFT based on Pentacene SiO 2 SiO 2 -OTS Shtein et al. Appl. Phys. Lett., 81, No. 2 (2002) Advantages over standard VTE: Higher deposition rates Less waste of material (no condensation on the internal walls) Better film thickness control and uniformity

39 Organic Molecular Beam Deposition Flow of focalized molecules in ultra high vacuum Substrate p C,T C Quartz microbalance Extremely slow deposition rate epitaxial growth Single crystal 10-9 mbar Pin hole p H,T H Effusion cell (Knudsen) Shutter

40 Non-soluble materials: patterning techniques Shadow masking Vapor Jet Printing

41 Patterning of vacuum deposited materials Shadow mask (same principle of screen printing). Resolution limited to tens of m. Patterning of RGB subpixels in OLED displays Fukuda et Al., Synthetic Metals, (2000)

42 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 (MEMS processing) Low gas flow rate 6 tubes with different source material G.J. McGraw, Appl. Phys. Lett. 98, , 2011

43 Examples

44 Micro-Contact Printing (µcp) II Additive Substrate pre-treated with O 2 plasma to improve idrophilicity activating OH- groups Pentacene TFTs: PEDOT S/D contacts, W/L=140 m/2 m Baking step at 80ºC to ensure chemical bonding (an adhesive agent is added to PEDOT:PSS solution) Example 2. Film grown by LBL technique onto the PDMS mold Patterned LBL deposited Organic Active Material

45 Micro-Contact Printing (µcp) IV Subtractive C. Packard et al., Langmuir 2011, 27, Applicable also to evaporated/solid films! Patterning mechanism? Test with oxygen plasma Lift-off due to adhesion forces between polymer and stamp Organic film removal due to organic molecules diffusion into the PDMS stamp during contact (suggested by time dependence of material removal) Molecules stimulated fluorescence glass glass PDMS stamp before PDMS stamp after

46 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, B,C,D: epoxy on glass Microstructures of glassy carbon Whitesides et al., Angew. Chem. Int. Ed. 1998, 37,

47 Micro Molding in Capillaries I Based on the flow of a liquid in capillaries Flow dynamics: 2R Z L Z: length of the liquid column into the capillary dz dt R lv cos 4 Z : liquid viscosity lv : liquid-vapor surface energy θ: liquid-capillary surface contact angle Whitesides et al., Angew. Chem. Int. Ed. 1998, 37, Pisignano et Al., Adv. Mater. 14, 1565, (2002)

48 Micro Molding in Capillaries II EXAMPLE 1 EXAMPLE 2 FEATURES 300nm Examples: Ag nanodispersion (AFM image) A. Blumel et al., Organic Electronics 8, , (2007) Polyurethane in PDMS mold EXAMPLE 3 PEDOT:PSS as S/D contacts in all organic TFTs H. Kang et al., Organic Electronics 10, , (2009)