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Deposition and patterning techniques for Organic Semiconductors

Maddalena Binda Organic Electronics: principles, devices and applications Milano, November 15-18th, 2011

Overview

Organic materials

DEPOSITION

PATTERNING

• • • • •

Drop casting Spin coating Doctor Blade Dip coating Layer-by-layer, Langmuir-Blodgett • Spray coating

• Screen printing • Soft Lithography • NIL/Embossing • Physical Delamination • Photopatterning • Ink-jet printing

• Vacuum Thermal Evaporation • Organic Vapor Phase Deposition (OVPD) • Organic Molecular Beam Deposition (OMBD)

• Shadow masking • Vapor Jet Printing

M. Caironi

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

Film thickness:

Substrate

Substrate

solution concentration

 Very simple  No waste of material

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/or improve film morphology

2

Spin Coating I Dropping on spinning substrate Film thickness: dependent on many controllable parameters dω/dt, ω, t, solution viscosity,…

substrate

 Good uniformity/reproducibility  Good control on thickness

X Waste of material X No large area X Film dries fast  less time for molecular ordering

Easy to obtain film of 10nm or less

Tricks...

180ºC

McCulloch et Al.,

•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:



 2v    2 r z 2

h(t ) 

1  4   t   3 

1/ 2

 = fluid viscosity  = fluid density

ω = rotation speed v = radial velocity

It does not take into account solvent evaporation…

3

Spin Coating III Film thickness: flow dominated

evaporation dominated

viscous flow rate = evaporation rate

hFIN  c0 (1  c0 )

1

3



1

2

( 0  )

1

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 Polymer cross-linking Cross-linked polymer chains

 Stable, high degree of control X Polymer intrinsic properties are affected

Thermally UV-light

activated

Host cross-linkable polymer

 Appliable to any kind of polymer (small molecule?)  Doesn’t affect polymer intrinsic properties X Less “deterministic”

X Film properties are affected P. Keivanidis et al., Appl. Phys. Lett. 94, 173303, 2009.

Binda et al., Appl. Phys. Lett. 98, 073303, 2011.

4

Doctor Blading I Spreading through a moving blade onto a stationary substrate stationary moving

 Large area  No waste of material

X Micrometric precision of blade regulation

 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   FilmThick ness   h0 1   film  6 UL 

U=blade speed 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 microscope

TEM

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:

H

0.94 v 2 / 3

 1 / 6 g 1 / 2

= fluid viscosity v = withdrawal speed = fluid density g = gravitational acceleration = surface tension (liquid-air)

Solution  Quite good uniformity  Very thin layers

X Waste of material 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

Amphiphilic molecules

water

Molecules move as in a bi-dimensional ideal gas, with a well defined surface pressure P, area A, and density 3D Gas

2D Gas

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]

Pc

solid

Reducing the available area, pressure increases and eventually a phase-change occurs: “gas”  “liquid”  “solid”

l/s

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.

liquid g/l 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. An ideal monolayer can be grown

X Only amphyphilic molecules can be deposited

 Homogeneity over large areas

X Non trivial setup

 Multilayer structures with varying layer composition

Example. C60 dendrimer – n-type TFT LB film: 5 layers

Polar

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

substrate

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

single pass technique: droplets merge on the substrate into a full wet film before drying smooth films  Adjustable layer thickness

multiple pass technique: droplets dry independently rough film, but wettability issues can be overcome 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) 1mW

100n

Spray coated bottom electrode

Current [A]

10W 10n

LIGHT 1W 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  No waste of material

X Thickness 1-200µm

 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

Substrate

It can be applied to spray coating and doctor blading

Mask

 Simple

Mask

Z. Bao et al., A. J. Chem. Mater. 9, 1299-1301, 1997.

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)

Mold

• Micro Transfer Molding (TM) • 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 ~few nm OTS:

modifies substrate chemical proprieties and the wettability Cl Cl Si Cl

(CH2)17

CH 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=140m/2m

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. Film grown by LBL technique onto the PDMS mold J.R. Tischler et al., Org. Elec. 8,94–113, 2007.

Patterned LBL deposited Organic Active Material

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

Dilute solution

Very dilute solution Solution pinned to the edges

b) AFM image of the stamp; c) printed AlQ3 film using dilute solution; d) very dilute solution; e) line profile of stamp and films 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! Patterning mechanism?

Test with oxygen plasma

• Lift-off due to adhesion forces between polymer and stamp

glass

PDMS stamp

before • 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

PDMS 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, B,C,D: epoxy on glass

Microstructures of 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:

2R

L Z

Z: length of the liquid column into the capillary

dZ R lv cos   dt 4Z

: liquid viscosity 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 300nm

Ag nanodispersion (AFM image) A. Blumel et al., Organic Electronics 8, 389–395, (2007)

• Examples: 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

Polymer adhere to the substrate where OTS is not present

Optical and AFM images of patterned PBTTT 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

“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).

Vacuum Thermal Evaporation I Growth rate and substrate temperature affect film morphological order PC on thermal SiO2

Pentacene on SiO2 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

PC on sputtered palladium

X Waste of material X Expensive equipments X Very low throughput  high production costs 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 the evaporation rate

Organic evaporation limited by the flow of the carrier gas

Deposition regimes Equilibrium limited: Substrate kept at a temperature high enough to establish an adsorption/desorption equilibrium

Transport limited: Substrate at room temperature Better morphology

22

OVPD: examples TFT based on Pentacene

TPD-Alq3 OLED heterostructure SiO2-OTS

SiO2

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 Substrate

Quartz microbalance

Extremely slow deposition rate  epitaxial growth Single crystal

pC ,TC

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 subpixels 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 source material

Fluorescence from Alq3 stripes

N2 flow rate x=feature size substrate-nozzle distance

25