| Abstract [eng] |
A majority of non-organic semiconductor devises have found organic substitution: organic light emitting devices (OLED), thin film transistors (OTFT), photovoltaics (OPV) and etc. However, an electrically pumped organic laser is still on demand. Bifluorene compounds have been investigated in previous researches and the promising properties such as low spontaneous emission threshold, low reabsorption and controllable intermolecular coupling have been found. Without properties mentioned above organic laser operation depends on excitonic processes such as exciton diffusion and annihilation. This reason encouraged to investigate these properties in bifluorene single crystals in more detail. Exciton diffusion coefficient and diffusion length in current research were determined by measuring efficiency of exciton-exciton annihilation. This was realized by measuring and further theoretical modeling of transient absorption decay curves measured by pump-probe experiment. Results were compared with diffusion coefficient determined by light induced transient grating (LITG) method. Both experimental approaches provided similar results: determined values of exciton diffusion coefficient were in range of 0,15-0,2 cm2s-1 and corresponding values of exciton diffusion length were in range of 115-140 nm. At high exciton densities (4.5×1017) increase of diffusion coefficient up to 0.5 cm2s-1 was observed. This can be an outcome of local heating which occurs at high excitation intensities. To approve this assumption diffusion coefficient was measured at different sample temperatures. This research revealed that diffusion coefficient is temperature dependent and increases at high temperatures. |
