1. Conjugated Polymers & Applications

2. Outline p Conjugation Bandgap of conjugated polymers Synthesis
Processing
Applications: OLED, OFET, OPV

3. …… p Orbitals + _ atom diatomic molecule extended molecule p* band p*

4. 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.

5. 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

6. Benzene The p electrons The s electrons Top down view
of the p orbitals
Atkins, Physical Chemistry

7. 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)

8. Band diagram for poly(para-phenylene)
Benzene
levels
PPP Eg
-p/a p/a k
E.K. Miller et al., Phys. Rev. B, 60 (1999) p

9. Tuning the bandgap of conjugated polymers
~ 3 eV
~ 2 eV

10. Tuning the bandgap of polythiophene derivatives
which one has the largest bandgap?
Mats Andersson et al. J. Mater. Chem., 9 (1999) p

11. How to make conjugated polymers: 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

12. How to make conjugated polymers: 2. polycondensationX X Y Y [ ] n
Commonly Used Coupling Reactions
Stille Coupling
Suzuki Coupling
Heck Reaction
Ullmann Reaction
Sonogashira Coupling
Kumada and Negishi Coupling

13. Stille Coupling Pd(II)L2Cl2 + 2R’SnBu3 R-R’ R’-R’ + 2Bu3SnCl Pd(0)L2RX
RPdL2X
RPdL2R’
R’SnBu3
Bu3SnX
3. Reductive Elimination
1. Oxidative addition
2. Transmetalation

14. Suzuki Coupling
Ullmann Reaction
Heck Reaction
Sonogashira Coupling

15. Negishi Coupling
Kumada Coupling

16. Different approaches to one polymer

17. 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 -> [(Na+)y(p-polymer)-y]n + y(Naphth)0
Oxidized
molecule
Reduced
polymer
Counter-ion

18. 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.

19. The Nobel Prize in Chemistry 2000

20. 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 >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.
anthracene
tetracene
pentacene

21. Thermal evaporation of small molecules
Substrate
Host
Dopant
Heater
Pump 
Shutter
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

22. 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.

23. 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.

24. 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 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

25. Advantages and disadvantages of screen printing
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.

26. 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.)

27. 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.

28. 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)