Conductive Polymers

The Nobel Prize in Chemistry was awarded in 2000 to Heeger MacDiarmid Shirakawa for the discovery of electrically conductive polymers.

Steps leading to the 2000 Nobel Prize in Chemistry

The original discovery focused on the role of conductivity in a simple conjugated polymer called polyacetylene
Made in 1974 by Shirakawa and co-workers using the Zieglar-Natta catalyst. The material was initially non-conducting but appeared metallic.
In 1977 Shirakawa, Heeger and MacDiarmid discovered that oxidation with chlorine, bromine or iodine vapor made polyacetylen 9-11 orders of magnitude more conductive
The "doped" polyacetylene had a conductivity of <math>10^5 S/m</math>. Teflon is <math>10^{-16} S/m</math>, and silver is <math>10^8S/m</math>.

Conductivity of polyacetylene increases with temperature, while conductivity of silver decreases.

Application of a simple Huckel theory gives a pretty good approximation to the electronic eigenfunctions of the molecule.
- Ground state properties are adequately described by Huckel like models
- Excited states involve lattice interactions
There are two types of bonds in the system characterized by an <math>sp^2</math> hybridization
Angular momentum 0 orbitals called <math>\sigma</math> bonds in the plane of the molecule and <math>\pi</math> bonds comprising of superposition of atomic <math>p_z</math> orbitals out of the x-y planes.
Electronic transport properties determined by the <math>\pi</math> bonds.
- Given the delocalized nature of the <math>\pi</math> molecular orbitals, shouldn't polyacetylene be a metal?
- In fact pure (undoped) polyacetylene is a semiconductor with a bandgap around <math>1.7 eV</math>.
- Electronic distribution is predicted to be across the entire chain implying equal bond lengths
Focus on the HOMO(<math>\pi</math>) and LUMO(<math>\pi^*</math> ) which correspond to the valence and conduction bands discussed previously in inorganic SC.

Shirakawa discovered how to produce all trans polyacetylene using the Ziegler Natta organo-metallic <math>Ti(OBu)_4</math> catalyst
trans polyacetylen first isolated in early 70's demonstrated a conductivity <math>10^{-2}S/m</math> in all <math>cis</math> polyacetylene

Like a classical semiconductor the sample is transparent to light with a photon energy smaller than the band gap (<math>1.7 eV</math>).
There is an emergence of a midgap state in the doped material around <math>0.7 eV</math>

Pierls showed that a hypothetical chain of sodium atoms would undergo a metal to insulator transition because the equi-spaced lattice is unstable with respect to an alternating bond configuration (1930s).
This small conformation change leads to the emergence of a small bandgap

Conductivity increases with increased doping
The role of the dopant (halogen <math>I_2/Br_2</math>) is to remove an electron from the polymer
- The resulting cation is not completely delocalized.
Radical cation ("polaron" formed by the removal of one electron. The polaron migrates
The radical cation is localized partly because of the interaction with a counter ion.
A high concentration of dopants is needed in order to facilitate polaron mobility because the anion is typically of very low mobility.

===SSH model of soliton formation in polyacetylene

===Properties of PPV(poly (p-phenylene))

Electroluminescence from conjugated polymers was first reported in 1990 by R. Friend and coworkers
In PPV the polymer backbone is held together by <math>\sigma</math> bonds.
- The remaining electron in each carbon is in a <math>p_z</math> or <math>p_y</math> atomic orbital which overlaps to produce a <math>\pi</math> MO.
The HOMO LUMO separation in PPV is aboout 2.5 eV which corresponds to a green-yellow emission.
The magnitude of the gap depends on the conjugation length
- Longer lengths correspond to smaller gaps with an asymptote reached at about ten repeat units.
In crystalline semiconductors the electron and hole pairs are delocalized in three-dimensions which leads to a small overlap. In organic semiconductors the electron and holes are confined to the same chain.
- This leads to bound neutral excited states called excitons.
- Excitons play an important role in determining the PL properties of PPV and other conjugated polymers.

semiconducting polymer is sandwiched between a hole injecting material (ITO) and a low work function metal such as <math>Ca</math>, <math>Al</math>, or <math>Mg</math>
Upon external bias, electrons are injected from the <math>Ca</math> electrodes. Holes come from the ITO and recombine in the semiconductor layer and emit light corresponding to the HOMO-LUMO separation of the polymer
Devices that are <math>80%</math> efficient have been produced.

Doped polyaniline is used as a conductor and for electromagnetic shielding of electronic circuits. Polyaniline is also manufactured as a corrosion inhibitor.
Poly(ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonic acid is manufactured as an antistatic coating material to prevent electrical discharge exposure on photographic emulsions and also serves as a hole injecting electrode material in polymer light-emitting devices.
Poly(phenylene vinyliden) derivatives have been major candidates for the active layer in pilot production of electroluminescent displays (mobile telephone displays).
Poly(dialkylfluorene) derivatives are used as the emissive layer in full-color video matrix displays
Poly(thiophene) derivatives are promising in use in field-effect transistors: they may possibly find use in a supermarket checkouts
Poly(pyrrole) has been tested as microwave-absorbing "stealth" (radar-invisible) screen coatings and also as the active thin layer of various sensing devices.