Very thin photoalignment films for liquid crystalline conjugated polymers: Application to polarized light-emitting diodes

Photoaligned polyimide films with different film thicknesses were prepared on quartz substrates, and uniaxially aligned glassy poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) layers were formed on top. The photoluminescence polarization ratio of the PFO layer rapidly increased with increasing polyimide film thickness, and beyond a thickness of 1.6 nm, it was saturated at similar to 11. This result shows that the 1.6-nm-thick photoaligned polyimide film works as a good alignment layer for PFO. We succeeded in fabricating a polarized light-emitting diode with a polarization ratio of 29 at 459 nm and a brightness of 700 cd/m(2) by using a 2.8-nm-thick polyimide photoalignment layer. (C) 2007 American Institute of Physics.