Abstract
Singlet exciton migration has been studied in films of the conjugated polymer polyfluorene (PF) by doping the samples with a fluorescent probe molecule, tetraphenyl porphyrin (TPP). Energy transfer in such systems has often been described in terms of F\"orster transfer, a dipole-dipole mechanism. TPP emission from the films was measured in steady-state, as a function of temperature and dopant concentration. The intensity of the TPP emission was found to be constant up to $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and then to increase with temperature. Therefore, the energy transfer cannot be occurring solely by F\"orster transfer as that process is temperature-independent. Instead, energy transfer between PF and TPP is considered to take place via thermally-activated exciton diffusion through the polymer followed by F\"orster transfer between the polymer and dopant. Moreover, TPP emission as a function of dopant concentration could not be described by F\"orster transfer alone, but could be well fitted at low temperature $(15\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and room temperature by the Yokota-Tanimoto model, which combines diffusion and F\"orster transfer. Diffusion lengths of $11\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ $(15\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and $20\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ (room temperature) were found. The nonzero exciton diffusion at low temperature is believed to be due to downhill migration to low energy polymer segments.
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