Rubbed PFN−Br as an Alignment Layer for Liquid Crystalline Conjugated Polymer Chain Orientation**

Abstract

We report poly [(9,9-bis(3'-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]dibromide (PFNÀ Br), a previously reported charge transfer interlayer for organic optoelectronic devices, as an orientation/alignment layer and rubbing as a means to achieve chain orientation for liquid crystalline conjugated polymers (LCCP). Polyfluorene and its derivatives, namely blue emission polymer poly(9,9-dioctylfluorene) (PFO), green-yellow emission polymer poly(9,9-dioctylfluorene-cobenzothiadiazole) (F8BT), and binary blend red, demonstrated clear polarized absorption when PFNÀ Br was used as the orientation layer. We have studied the effects of rubbing times of a PFNÀ Br layer, and molecular weight, film thickness and annealing temperature of LCCP on the orientation degree. A high absorption anisotropy ratio of 12.22 was achieved for 106 nm thick F8BT film with a molecular weight of 55000 at the annealing temperature 265 °C, which is close to the theoretical limitation of the dichroic ratio of F8BT derived previously. Finally, we studied the amplified spontaneous emission (ASE) in chain oriented F8BT and PFO films. It showed that chain orientation made the ASE threshold anisotropic and lowers the threshold compared to the isotropic films if pumped under the favored configuration. This work provides a novel approach to reduce laser threshold and improve charge transfer simultaneously for electrically pumped devices when combined with the conductive property of PFNÀ Br.