Conjugated Polymers

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Conjugated Polymers Applications
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Outline

• p Conjugation
• Bandgap of conjugated polymers
• Synthesis
• Processing
• Applications OLED, OFET, OPV

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p Orbitals

_
p band
p
p
p
p band
atom diatomic molecule extended
molecule
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p bonds
Bonding combination of p orbitals (p)
electron density builds up between the nuclei
Antibonding combination of p orbitals (p)
The wavefunctions cancel between the nuclei.
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Larger molecules
butadiene
To generate the molecular orbitals of larger
molecules, we take linear combinations of the
atomic orbitals from each atom. If there are n
atoms, there will be n different combinations.
With a little bit of math, the energy levels and
wavefunctions to the right can be found.
fully antibonding
3 nodes
partially antibonding
2 nodes
partially bonding
1 node
fully bonding
0 nodes
Top down view of the p orbitals
Atkins, Physical Chemistry
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Benzene
The p electrons
The s electrons
Top down view of the p orbitals
Atkins, Physical Chemistry
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p Conjugated polymers
PA polyacetylene (1st conducting polymer)
PPV poly(phenylene-vinylene) (used in 1st
polymer LED)
PT polythiophene (widely used in transistors)
PPP poly(para-phenylene) (large bandgap)
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Band diagram for poly(para-phenylene)
Benzene levels
PPP
Eg
-p/a 0 p/a
k
E.K. Miller et al., Phys. Rev. B, 60 (1999) p.
8028.
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Tuning the bandgap of conjugated polymers
3 eV
2 eV
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Tuning the bandgap of polythiophene derivatives
which one has the largest bandgap?
Mats Andersson et al. J. Mater. Chem., 9 (1999)
p. 1933.
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How to make conjugated polymers1. precursor
methods
The Durham precursor route to polyacetylene.
The Wessling-Zimmermann route to PPV.
water-soluble polyelectrolyte
Feast et al. Polymer 37 (1996) p. 5017.
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How to make conjugated polymers2.
polycondensation
X
Y
X

Y

n

• Commonly Used Coupling Reactions
• Stille Coupling
• Suzuki Coupling
• Heck Reaction
• Ullmann Reaction
• Sonogashira Coupling
• Kumada and Negishi Coupling

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Stille Coupling
3. Reductive Elimination
1. Oxidative addition
2. Transmetalation
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Suzuki Coupling
Ullmann Reaction
Heck Reaction
Sonogashira Coupling
http//www.organic-chemistry.org
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Negishi Coupling
Kumada Coupling
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Different approaches to one polymer
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n-type doping
e-
Energy
Lithium
Polymer
Reducing agents donate electrons to the
conduction band. Solids like calcium, lithium
and sodium tend to dope the polymer only near the
surface since they cannot diffuse into the film.
Electrolytes (see below) can be used to dope an
entire film. (p-polymer)n (Na(Naphthalide)-y
-gt (Na)y(p-polymer)-yn y(Naphth)0
Oxidized molecule
Reduced polymer
Counter-ion
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p-type doping
Polyethylene dioxythiophene polystyrene
sulphonate (PEDOT/PSS) can be bought from Bayer
as an aqueous solution under the trade name
Baytron.
PEDOT
PSS
The sulphonic acid group on the PSS dopes the
PEDOT to make it conductive.
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The Nobel Prize in Chemistry 2000
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Small conjugated molecules
It is not impossible to solution deposit small
molecules, but it is usually hard to make high
quality films because the solution viscosity is
too low. Small molecules are usually deposited
from the vapor phase. Mobilities of gt1 cm2/V-s
can be achieved by thermally evaporating thin
films. The mobility is limited by grain
boundaries. Mobilities gt 3 cm2/Vs can be
achieved by growing single crystals.
anthracene
tetracene
pentacene
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Thermal evaporation of small molecules
The pressure is low enough for the mean free path
of a molecule to be larger than the size of the
chamber. Impurities with a higher vapor pressure
than the desired molecule are deposited on
the shutter.
Peter Peumans
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Depositing polymers Spin coating
Procedure 1. Dissolve the material. 2. Cast the
solution onto the substrate. 3. Spin the
substrate at 1000 to 6000 revolutions per minute.
Most ( 99 ) of the solution is flung off of
the substrate, but a high-quality thin film is
left behind. The thickness of the films goes
up with increasing solution concentration and
down with increasing spin speed. Polymers with
larger molecular weights tend to result in more
viscous solutions, which yield thicker films.
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Features of spin coating

• Advantages
• Spin coating can be done at atmospheric pressure
  and is very cheap.
• Film thickness of up to several hundred
  nanometers can be obtained.
• The thickness can be controlled (but not as well
  as with evaporation).
• The thickness is fairly uniform across the
  substrate (except at the edge).
• Disadvantages
• The whole substrate is coated. Patterning must
  be done separately.
• In most cases, part of the material must be
  nonconjugated so that the molecules are soluble.
• In can be difficult to make multilayer
  structures because the deposition of
• one layer can dissolve the layer underneath.

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Screen printing

• This technique is used to put patterns on
  T-shirts.
• The squeegee is used to press the dye through
  the screen.
• Recently screen printing of polymers has been
  used to make LEDs and photovoltaic cells.

This LED doesnt have perfectly uniform emission,
but it isnt bad for the first demonstration of
this deposition method.
Ghassan Jabbour et al. Adv. Mater. 12 (2000) p.
1249.
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Advantages and disadvantages of screen printing

• Advantages
• Large areas can be covered at low cost.
• Atmospheric pressure.
• Patterning is possible.
• Disadvantages
• Controlling film thickness might be difficult.
• The material must be soluble and have a
  viscosity within a certain range.

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Drop casting
Drying solution that leaves a film behind.
Petri Dish
Substrate
Hotplate
Covering the sample with a Petri dish slows down
the evaporation rate, which results in more
uniform films. Keeping the substrate level
results in much more uniform films. Since the
solvent evaporates slowly, the material can
crystallize or aggregate into fairly well ordered
structures. (This can be good or bad.)
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Summary of drop casting

• Advantages
• Film thickness can be far greater than 1 mm.
• This method is very inexpensive.
• In many cases the film can be removed from the
  substrate by peeling or by dissolving the
  substrate (which would be NaCl or KBr).
• Disadvantages
• The film thickness is difficult to control and
  is not very uniform.
• Very thin films are difficult to make.
• The material must be soluble.

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Ink Jet Printing (IJP)

• Polymers can be deposited from a printer onto a
  substrate.
• Advantages
• Patterning with resolution approaching 5-10 mm
  is possible.
• No material is wasted. ( 99 is wasted with
  spin casting)
• Cost can be extremely low
• Disadvantages
• Controlling film thickness is difficult.
• Fabrication of multilayer structure is difficult
  (compared to evaporation)