Organic Light Emitting Diodes

This break leads to a free radical defect, a so-called neutral soliton which is relatively stable
(Figure 9). Addition of an acceptor removes an electron and creates a positive soliton (or a neutral one
if the electron removed is not the free electron). The resulting carbocation is stabilised by having the
charge spread over several monomer units and the charged solitons are responsible for making
polyacetylene a conductor
Figure 9: Charge defects in polyacetylene and oxidative doping.
A further type of charge storage occurs when the generated charge and the radical are coupled to
each other via local resonance of the charge and the radical. This type of charge transport is present in
polymers like PPV. This combination of a charge site and a radical dependent of each other is called a
polaron. A new localised electronic state is created in the band gap, with the lower energy states being
occupied by a single unpaired electron. Unlike the soliton, the polaron cannot move without first
overcoming an energy barrier so movement is by a hopping motion.
FIGURE 10: Radical cation (”polaron”) formed by removal of one electron on the 5th carbon
atom of a undecahexaene chain (a ―> b). The polaron migration shown in c ―> e.
If a second electron is removed from an already oxidised section of the polymer, either a second
independent polaron may be created or, if it is the unpaired electron of the first polaron that is
removed, a bipolaron is formed (with lower energy than 2 polarons) (Figure 11). The two positive
charges of the bipolaron are not independent, but move as a pair, like the Cooper pair in the theory of
superconductivity. While a polaron, being a radical cation, has a spin of 1/2, the spins of the
bipolarons sum to S = 0.
8