Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
The electron transport in conjugated polymers is hindered by a universal electron trap with a concentration of typically 1017-1018 cm-3. Remarkably, the exciton diffusion length amounts to about 5-7 nm for a whole range of conjugated polymers. It was observed that for polymers with less energetic disorder the resulting increase of the exciton diffusion coefficient is compensated by a decrease of the PL decay time (exciton lifetime). A possible explanation for a disorder independent diffusion length could be that the PL decay time in pristine polymer films is determined not by the intrinsic exciton lifetime, but by the diffusion-limited exciton quenching at non-radiative recombination centers in the polymers. In that case the PL decay time reflects the time that excitons need to diffuse to these recombination centers. This will lead to shorter PL decay times in materials with faster diffusion and consequently to a constant exciton diffusion length if the different polymers would have an equal amount of non-radiative recombination centers. From time-resolved photoluminescence measurements, using randomly distributed fullerene quenchers, we demonstrate that the background concentration of exciton quenching defects in pristine polymer films of derivatives of poly[p-phenylene vinylene] (PPV) is nearly the same as the electron trap concentration. This observation strongly suggests that the exciton quenching defects and the electron traps share the same origin. Since the electron traps are universal this also explains why a similar exciton diffusion length of 5-8 nm is observed for a whole range of conjugated polymers.
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