Conjugated Polymers & Applications

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

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

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.

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

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

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)

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.

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

Tuning the bandgap of polythiophene derivatives which one has the largest bandgap? Mats Andersson et al. J. Mater. Chem., 9 (1999) p. 1933.

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. 5017.

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

Stille Coupling Pd(II)L2Cl2 + 2R’SnBu3 R-R’ R’-R’ + 2Bu3SnCl Pd(0)L2 RX RPdL2X RPdL2R’ R’SnBu3 Bu3SnX 3. Reductive Elimination 1. Oxidative addition 2. Transmetalation

Suzuki Coupling Ullmann Reaction Heck Reaction Sonogashira Coupling http://www.organic-chemistry.org

Negishi Coupling Kumada Coupling

Different approaches to one polymer

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

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.

The Nobel Prize in Chemistry 2000

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

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

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.

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.

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. 1249.

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.

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.)

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.

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)