Polymer Synthesis CHEM 421

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Polymer Synthesis
CHEM 421
Conjugated Polymers &
Applications
Outline
Polymer Synthesis
CHEM 421
• p Conjugation
• Bandgap of conjugated polymers
• Synthesis
• Processing
• Applications: OLED, OFET, OPV
p Orbitals
+
_
p*
p
p
atom
diatomic molecule
Polymer Synthesis
CHEM 421
……
p* band
p band
extended molecule
p bonds
Polymer Synthesis
CHEM 421
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.
Larger molecules
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.
Polymer Synthesis
CHEM 421
butadiene
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
Benzene
Polymer Synthesis
CHEM 421
The p electrons
The s electrons
Top down view
of the p orbitals
Atkins, Physical Chemistry
p Conjugated polymers
Polymer Synthesis
CHEM 421
PA: polyacetylene
(1st conducting polymer)
PPV: poly(phenylene-vinylene)
(used in 1st polymer LED)
n
S
PT: polythiophene
(widely used in transistors)
S
n
PPP: poly(para-phenylene)
(large bandgap)
n
Synthesis
Band diagram for poly(para-phenylene) Polymer
CHEM 421
Benzene
levels
n
PPP
Eg
-p/a
0
p/a
k
E.K. Miller et al., Phys. Rev. B, 60 (1999) p. 8028.
Tuning the bandgap of conjugated
polymers
~ 3 eV
Polymer Synthesis
CHEM 421
~ 2 eV
Tuning the bandgap of polythiophene
Polymer Synthesis
derivatives
CHEM 421
which one has the largest
bandgap?
Mats Andersson et al. J. Mater. Chem., 9 (1999) p. 1933.
How to make conjugated polymers:Polymer Synthesis
CHEM 421
1. 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.
How to make conjugated polymers:Polymer Synthesis
CHEM 421
2. polycondensation
X
X
Y
Commonly Used Coupling
Reactions
• Stille Coupling
• Suzuki Coupling
• Heck Reaction
• Ullmann Reaction
• Sonogashira Coupling
• Kumada and Negishi Coupling
Y
[
]
n
Stille Coupling
Polymer Synthesis
CHEM 421
Pd(II)L2Cl2 + 2R’SnBu3
R-R’
3. Reductive
Elimination
R’-R’ + 2Bu3SnCl
Pd(0)L2
1. Oxidative
addition
RX
RPdL2R’
2. Transmetalation
Bu3SnX
RPdL2X
R’SnBu3
Polymer Synthesis
CHEM 421
Suzuki Coupling
Ullmann Reaction
Heck Reaction
Sonogashira Coupling
http://www.organic-chemistry.org
Polymer Synthesis
CHEM 421
Negishi Coupling
Kumada Coupling
Different approaches to one
polymer
Polymer Synthesis
CHEM 421
n-type doping
Polymer Synthesis
CHEM 421
Energy
e-
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 -> [(Na+)y(p-polymer)-y]n + y(Naphth)0
Counter-ion
Reduced
polymer
Oxidized
molecule
p-type doping
Polymer Synthesis
CHEM 421
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.
The Nobel Prize in Chemistry 2000
Polymer Synthesis
CHEM 421
Small conjugated molecules
anthracene
tetracene
Polymer Synthesis
CHEM 421
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 >1 cm2/V-s can be
achieved by thermally evaporating
thin films. The mobility is limited by
grain boundaries.
Mobilities > 3 cm2/Vs can be
achieved by growing single crystals.
pentacene
Thermal evaporation of small molecules
Polymer Synthesis
CHEM 421
The pressure is low enough for the mean
free path of a molecule to be larger than the
size of the chamber.
Substrate
Shutter
Impurities with a higher vapor pressure than
the desired molecule are deposited on the
shutter.

Host
Heater
Pump
Dopant
Peter Peumans
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.
Polymer Synthesis
CHEM 421
Features of spin coating
Polymer Synthesis
CHEM 421
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.
Screen printing
Polymer Synthesis
CHEM 421
• 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 doesn’t have perfectly uniform emission, but it isn’t bad
for the first demonstration of this deposition method.
Ghassan Jabbour et al. Adv. Mater. 12 (2000) p. 1249.
Advantages and disadvantages ofPolymer Synthesis
CHEM 421
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.
Drop casting
Polymer Synthesis
CHEM 421
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.)
Summary of drop casting
Polymer Synthesis
CHEM 421
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.
Ink Jet Printing (IJP)
Polymer Synthesis
CHEM 421
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)
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