CONDUCTING POLYMERS

DISCOVERY OF CONDUCTING POLYMERS -an accidental discovery- At the beginning of the 1970s Shirakawa was studying the polimerisation of acetylene. In his reaction vessel polyacetylene appeared in the form of an unremarkable black powder. On once occasion a visiting researcher accidentally added one thousand times more catalyst than usual. Imagine the researchers’ surprise when a beautiful silvery film formed on the surface of the liquıd in the vessel. The obvious question was:’If the plastic film shines like a metal, can it conduct electricity, too?’ - Official Nobel Website

NOBEL PRIZE IN CHEMISTRY 2000 “For the Discovery and Development of Conductive Polymers” Hideki Shirakawa, University of Tsukuba Alan Heeger, Alan MacDiarmid, University of California University of Pennsylvania

POLYACETYLENE ■Polyacetylene is the simpliest conjugated polymer. ■There are two forms of polyacetylene: ■Films prepared by catalytic polymerisation of gaseous acetylene. ■Network of randomly oriented fibrils. ■Room temperature conductivity of undoped PA.

FORMS OF TRANS-POLYACETYLENE ■The minimum energies occur for u =±u0 ' ±0.04 ˚A, corresponding to the A and B phases of the twofold degenerate ground state of polyacetylene.

■As synthesized conductive polymers exhibit very low conductivities.It is not until an electron is removed from the valence band (p-doping) or added to the conduction band (n-doping, which is far less common) does a conducting polymer become highly conductive. ■Doping (p or n) generates charge carriers which move in an electric field. Positive charges (holes) and negative charges (electrons) move to opposite electrodes. This movement of charge is what is actually responsible for electrical conductivity.

How the conjugated organic polymer is converted into a conducting polymer ? ■A full orbital can not conduct electrons, so to get a conjugated material to conduct, we must add or remove charges (doping process) ■Two options: ■Remove electrons from the HOMO (createholes) ■Add electrons to LUMO

A simple defect can occur when these two degenerate forms of the polymer are joined. ■The defect results in one unpaired p-electron, but the entire system is electrically neutral. ■The soliton will be mobile due to the translational symmetry of the chain.

Soliton structures in Polyacetylene

■ If the localized state contains one electron, the soliton is neutral. The unpaired electron will have a spin of ½ and can be detected by electron spin resonance, ESR.

■When the electron in the localized state is removed, for example by p-doping, the soliton is positively charged with spin = 0.

■Similarly, if n-doping occurs, a negative soliton is obtained with spin = 0.

CONDUCTING MECHANISM ■While the addition of a donor or an acceptor molecule to the polymer is called "doping ", the reaction that takes place is actually a redox reaction. ■The first step is the formation of a cation (or anion) radical, which is called a solitonor a polaron. Pn ⇔ [Pn + A - ] reduction oxidation

■This step may then be followed by a second electron transfer with the formation of a dication(or dianion) known as a bipolaron. [Pn + A - ] ⇔ [Pn 2+ 2A - ] reduction oxidation ■Alternatively after the first redox reaction, charge transfer complexes may form between charged and neutral segments of the polymer when possible. [Pn + A - ] + Pm → [(PnPm) + A – ] CONDUCTING MECHANISM

DOPING PROCESS ■The halogen doping transforms polyacetylene to a good conductor. ■Oxidation with iodine causes the electrons to be jerked out of the polymer, leaving "holes" in the form of positive charges that can move along the chain.

■The iodine molecule attracts an electron from the polyacetylene chain and becomes I 3 -. The polyacetylene molecule, now positively charged, is termed a radical cation, or polaron. ■The lonely electron of the double bond, from which an electron was removed, can move easily. As a consequence, the double bond successively moves along the molecule. ■The positive charge, on the other hand, is fixed by electrostatic attraction to the iodide ion, which does not move so readily.

IODINE DOPING OF MEH-PPV ■Iodine diffuses into MEH-PPV Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] ■electrons are removed from HOMO of MEH-PPV. ■iodine is reduced to I5-complex.

ORIGINS OF NEW TRANSITIONS ■Electrons are removed from HOMO of MEH-PPV. ■Structural relaxation occurs. ■Levels are “pulled into the band-gap”. ■Additional transitions are allowed.

EFFECT OF CONJUGATION LENGTH ■Polymer film is like a bowl of spaghetti. ■Disorder creates kinks in polymer chain → termination in conjugation.

EFFECT OF DOPING ON CONDUCTIVITY ■The conductivity of MEH-PPV increases > 5 orders of magnitude upon doping. ■Conductivity falls again after de-doping under vacuum.

SELECTIVE DOPING ■Iodine will dope MEH-PPV but not CN-PPV. ■More energy is needed to remove e– from CN-PPV HOMO. ■Insufficient potential to drive the reaction from the oxidation of the iodine.

■For metals; higher the temperature, Lower the conductivity. ■For semiconductors; higher the temperature, higher the conductivity. ■Even when doped to a highly conductive state most p-conjugated polymers behave as classic semiconductors.

FACTORS THAT AFFECT CONDUCTIVITY ■Concentration of charges (density) ■Speed of the charges (mobility) ■Direction of the charges ■Length of conjugation ■Temperature ■Density of dopants.

CONDUCTIVITIES

APPLICATIONS ■Conducting polymers have many uses. The most documented are as follows: ■Anti-static substances for photographic film ■Corrosion Inhibitors ■Compact Capacitors ■Anti Static Coating ■Electromagnetic shielding for computers

■A second generation of conducting polymers have been developed these have industrial uses like: ■Transistors ■Light Emitting Diodes (LEDs) ■Lasers used in flat televisions ■Solar cells ■Displays in mobile telephones and mini-format television screens APPLICATIONS

WHAT ARE THE CONSEQUENCES OF DOPING?

MOLECULAR VERSUS BAND MODELS