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

We report poly[(9,9‐bis(3’‐((N,N‐dimethyl)‐N‐ethylammonium)‐propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctylfluorene)]dibromide (PFN−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 emission polymer F8BT/Red F (Ref F=poly(triphenylamine‐co‐4,7‐di(thiophen‐2‐yl)benzo[c][1,2,5]thiadiazole‐co‐benzo[c]thiadiazole‐co‐9,9‐dioctyl‐9H‐fluorene)), demonstrated clear polarized absorption when PFN−Br was used as the orientation layer. We have studied the effects of rubbing times of a PFN−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 PFN−Br. Conjugated polyelectrolyte PFN−Br (poly[(9,9‐bis(3‐((N,N‐dimethyl)‐N‐ethylammonium)‐propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctylfluorene)]dibromide) was successfully used as rubbing alignment layer for chain orientation of liquid crystal conjugated polymer PFO, F8BT and F8BT/Red F blends. The effect of rubbing times of the PFN−Br layer, and molecular weight, film thickness and annealing temperature of conjugated polymers on the optical anisotropy was investigated. A high optical anisotropy ratio of 12.22 was achieved. The amplified spontaneous emission thresholds of PFO and F8BT decreased and showed anisotropy. (PFO=poly(9,9‐dioctylfluorene); F8BT=poly(9,9‐dioctylfluorene‐cobenzothiadiazole); Red F=poly(triphenylamine‐co‐4,7‐di(thiophen‐2‐yl)benzo[c][1,2,5]thiadiazole‐co‐benzo[c]thiadiazole‐co‐9,9‐dioctyl‐9H‐fluorene).