Abstract
We investigated the linearly polarized emission of uniformly aligned poly(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thia-diazol-4,8-diyl) (F8BT) with a liquid crystalline phase on a rubbed alignment layer. The polarization ratio, defined by the ratio of luminous intensities polarized parallel and perpendicular to the rubbed direction, gradually decreased with increasing thickness of the F8BT film. In the photoluminescence (PL) process, the polarized light is emitted throughout the whole F8BT film, while in the electroluminescence (EL) process, the polarized light is emitted at a certain region within the F8BT film. The thickness-dependent polarization ratios in both PL and EL processes were successfully described based on a simple model wherein the mean optical birefringence was expressed as a function of the thickness of the F8BT film.
Highlights
The control of orientational ordering in conjugated, semiconducting polymer has been of interest as a means for improving the performance of many optoelectronic applications such as photovoltaic cells, light-emitting diodes, and field effect transistor [1,2,3]
We investigated the linearly polarized emission of uniformly aligned poly(9,9-din-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thia-diazol-4,8-diyl) (F8BT) with a liquid crystalline phase on a rubbed alignment layer
The governing mechanism of the PL and EL polarization ratio is theoretically investigated based on the nematic ordering
Summary
The control of orientational ordering in conjugated, semiconducting polymer has been of interest as a means for improving the performance of many optoelectronic applications such as photovoltaic cells, light-emitting diodes, and field effect transistor [1,2,3]. These LC conjugated polymers on a rubbed alignment layer sometimes acts as a hole injection/transport layer, and a high polarization ratio (I||/I⊥) was observed [7,8,9,10] This ratio is defined as the ratio of luminous intensities of parallel (I||) and perpendicularly (I⊥) polarized components relative to the rubbed direction. Based on such reports, the polarization ratio was strongly governed by thermal annealing condition, the anchoring strength of the alignment layer, or the molecular weight [8,9,10,13]. The EL and PL polarization ratios associated with the order parameter of the EL conjugated polymers have not been theoretically discussed yet
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