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  • 1

    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

  • 2

    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

    Very simple

    No waste of material

    Combination of solvents

    Solvents evaporation time: heating of the substrate to speed up the

    evaporation process and/or improve film morphology

    Dropping

    Evaporation

    Substrate Substrate Substrate

    X Limitations in large area coverage

    X Thickness hard to control

    X Poor uniformity

    Film thickness: solution concentration

    Tricks...

  • 3

    Spin Coating I Dropping on spinning substrate

    Good uniformity/reproducibility

    Good control on thickness

    Easy to obtain film of 10nm or less

    X Waste of material

    X No large area

    X Film dries fast

    less time for molecular ordering

    Film thickness:

    dependent on many controllable parameters

    d/dt, , t, solution viscosity,

    Thermal annealing

    (post-deposition)

    Solvent evaporation time

    McCulloch et Al.,

    Nat. Mater., 5, 328, (2006)

    substrate

    Tricks... 180C

    Chang et Al.

    Chem. Mater.,16, 23, (2004) P3HT-based TFTs

    Film thickness:

    rz

    v 22

    2

    2/1

    3

    41)(

    tth

    It does not take into account

    solvent evaporation

    = fluid viscosity = rotation speed

    = fluid density v = radial velocity

    Spin Coating II

  • 4

    Film thickness:

    Spin Coating III

    viscous flow rate = evaporation rate

    c(t)= solids concentration

    (t)

    31

    02

    13

    1

    00 )()1(

    cchFIN

    D. E. Bornside, C. W. Macosko and L. E. Scriven, "Spin Coating of a

    PMMA/Chlorobenzene Solution", J. Electrochem. Soc., 138, 317 (1991)

    most common reported experimental relationship

    between thickness and rotation speed

    flow dominated

    evaporation

    dominated

    How to handle multilayer deposition?

    Post-deposition film insolubilization

    Thermally

    UV-light activated Polymer cross-linking

    Host cross-linkable polymer Cross-linked polymer chains

    Stable, high degree of control Appliable to any kind of polymer

    (small molecule?)

    Doesnt affect polymer intrinsic properties

    X Polymer intrinsic properties are

    affected

    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.

  • 5

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

    stationary moving

    Large area

    No waste of material

    Good uniformity

    X Micrometric precision of blade regulation

    film thickness > 150200nm

    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

    Theoretical height of the wet layer thickness:

    surface tension, wetting, viscosity, coating speed,...

    Film thickness:

    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.

    UL

    PhhessFilmThickn

    film

    sol

    61

    2

    00

    U=blade speed

    =fluid viscosity

    L=channel length

    =density

    ho=blade height

    P=slurry pressure head

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    Doctor Blading refinement I:

    zone casting Controlled orientation of deposited layers

    Z. Bao, Adv. Mater. 21, 12171232, 2009.

    Controlled Deposition of Crystalline

    Organic Semiconductors

    directional crystallization

    by suitably controlling solvent

    evaporation rate and solution supply

    Optical

    microscope TEM

    substituted hexabenzocoronenes (HBCs)

    Doctor Blading refinement II:

    solution-sheared deposition

    TMS-4T dbo-P2TP

    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

    Z. Bao, Adv. Mater. 21, 12171232, 2009.

  • 7

    Dip Coating The substrate is dipped into the solution and then withdrawn at a controlled speed.

    Thickness (H) determined by the balance

    of forces at the liquid-substrate interface

    Solution

    Film thickness:

    Landau and Levich equation:

    2161

    32940

    //

    /

    g

    v.H

    = fluid viscosity v = withdrawal speed

    = fluid density g = gravitational acceleration

    = surface tension (liquid-air)

    Quite good uniformity

    Very thin layers

    Large area coverage

    X Waste of material

    X Time consuming

    X Double side coverage

    Sandberg et Al., Langmuir, 18, 26, 2002.

    Example: TFT based on P3HT in xylene single monolayer 2 nm thick

    water

    Molecules move as in a bi-dimensional

    ideal gas, with a well defined surface

    pressure P, area A, and density

    3D Gas

    P [N/m2]

    V [m3]

    N [m-3]

    T [k]

    2D Gas

    P [N/m]

    A [m2]

    n [m-2]

    T [k]

    based on hydrophobicity/hydrophilicity

    Extreme thickness control:

    Langmuir-Blodgett film I

    Amphiphilic molecules

    Hydrophilic head Hydrophobic tail

  • 8

    Extreme thickness control:

    Langmuir-Blodgett film II

    Reducing the available area, pressure

    increases and eventually a phase-change

    occurs: gas liquid solid

    A [m2]

    P [Nm-1]

    gas

    liquid

    solid

    g/l

    l/s Pc 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.

    KSV Instruments Application Note: http://www.ksvltd.fi/Literature/Application%20notes/LB.pdf

    Isothermal curve

    hydrophilic substrate

    Movable barrier Wilhelmy plate:

    measures P

    Feedback

    Extreme thickness control:

    Langmuir-Blodgett film III

    Head-to-tail Head-to-head Tail-to-tail

  • 9

    Example. C60 dendrimer n-type TFT

    LB film:

    5 layers 15nm

    Apolar

    Polar

    Kawasaki et al., Appl. Phys. Lett. 91, 243515, 2007.

    Higher mobility than

    spin-coated film

    higher morphological

    order

    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

    X Only amphyphilic molecules can

    be deposited

    X Non trivial setup

    Spray Coating

    single pass technique:

    droplets merge on the substrate

    into a full wet film before drying

    smooth films

    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

    Substrate is hit by a vaporized solution flux

    multiple pass technique:

    droplets dry independently

    rough film, but wettability issues

    can be overcome

    substrate

    Nozzle trajectory

    Adjustable layer thickness

    Large area coverage

    Independence on substrate topology

    X Homogeneity of the film

  • 10

    Spray Coating Example 1. Girotto et al. Adv. Funct. Mater. 2011, 21, 6472

    Spray-coated organic solar cells

    Example 2. Organic light sensor directly deposited onto a Plastic Optical Fiber (POF)

    Spray coated

    bottom electrode

    0 10 20 30 40 50100p

    1n

    10n

    100n

    Cu

    rre

    nt [A

    ]

    Time [s]

    DARK

    LIGHT

    1mW

    10W

    1W

    Baklar et al. Adv. Mater. 2010, 22, 39423947

    Compression molding Solid state processing from powders

    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 1-200m

  • 11

    Compression molding

    Free-standing flexible film

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