Investigations on some selected conducting polymers and polymer ...

Cliapter-l
formation ofextended excited states which would be desribed as charge
transfer excitons within the frame work of molecular semiconductors.
Excimers are excitons extending over identical molecular units.
Exciplexes are excitons extending over two or more different molecular
units.
Organic thin film electroluminescence started to flourish since the
early 1970s.Electroluminescence from conjugated polymers was first
reported using poly (p-phenylenevinylene) (PPY) as the single
semiconductor layer between organic electrodes [65]. The lI-Ilenergy
gap of PPY is about 2.5eY.It produces luminescence in a band below
this energy. The major drawback of PPY is that it is an intractable
material with a rigid rod microcrystalline structure. So it is infusible
and insoluble in common solvents. But PPY can be synthesized
conveniently by in situ chemical conversion of films of a suitable
precursor polymer that itself is processed from solution by spin
coating[9].Most frequently used derivative of PPY is poly(2-methoxy­
5-(2'ethyl) hexyloxy-p-phenylenevinylene) (MEH -PPY).
In a polymer LED, positive and negative charge earners are
injected from opposite electrodes. These oppositely charged carriers
combine within the polymer layer, resulting in the formation of a
singlet exciton. The singlet exciton thus formed decays radiatively to
the ground state resulting in light emission [9].
22