Charge polarons

Kigurc 7-35. Comparison of the energy levels and optical transitions of oppositely-charged polarons and polaron pairs. 

Fig. 4. Energy level diagrams showing possible electronic configurations for positively-charged polaron (a) and bipolaron (b) defects and (c) a schematic bipolaron band model. The negatively-charged polaron would carry three electrons and the bipolaron four. Also shown is the neutral polaron-exciton (d) which would decay to restore the chain structure.

Some authors claim that in polymers, recombination occurs between free charged polarons, so that the maximum efficiency corresponds to the internal quantum efficiency, ri t the number of photons emitted for every electron injected), as given by... 

Each of the above is in good agreement with mobile charge carriers (charged polarons) rather than neutral excitons as the primary photoexcitations in the PPVs. The stark contrast with the PDAs is evident. 

Thus, in some cases either model can provide a satisfactory explanation of the data. Consequently, the issue of excitons versus mobile charged polarons as the primary photoexcitations must be analyzed in greater detail. For a detailed review of these issues as of approximately 1996, the reader is referred to the collection of chapters on these issues in the recent volume edited by Sariciftci [19]. 

Thus, the relevant question is whether the photoconductive response in PPV results principally from secondary processes following the photogeneration of neutral geminate pairs, in agreement with the exciton model, or from separated, mobile, positive and negative charged polarons. 

Moreover, the remarkable sensitization of the photoconductivity of MEH-PPV by C o implies that the initial photo-excitations are mobile charged polarons (see... 

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