Investigations on some selected conducting polymers and polymer ...

I ntroduction
Here holes are injected into the valence band of PPV from ITO
and electrons into its conduction band from aluminium. Their
recombination takes place in the PPV layer. The radiative decay of
singlet exciton results in light emission. One of the electrodes is
semitransparent. There are barriers to the injection of holes from ITO
and electrons from aluminium. Considering the ionization potential and
electron affinity of PPV, it is inferred that barrier to injection of
electrons is greater than barrier to injection of holes. The internal
quantum efficiency ofthe devicem) is defined as the ratio of number of
photons produced within the device to the number of electrons flowing
in the external circuit. It is given by,
ll=yrq
where y is the ratio of exciton formation events in the device to the
number ofelectrons flowing in the external circuit,
r the fraction of singlet excitons and q the efficiency of radiative decay
ofsinglet excitons
To get good internal quantum efficiency, the following conditions
must be satisfied.
a) Electron and hole currents must be balanced
b) There must be sufficient capture of electrons and holes within
the emissive layer
c) Efficient radiative decay ofsinglet excitons should take place.