Polyaniline doped with orthophosphoric acid - A material with prospects for optoelectronic applications

Polyaniline (PANI) is one of the highly pursued conducting polymers in recent times owing to its excellent electrical and optical properties, better air stability and easy and cheap synthesis route. Doped PANI has been extensively investigated as a hole-injecting electrode in polymer light en-fitting diodes (LEDs). However, the work on doped PANI as emissive layers in polymer LEDs is quite scanty. Though there are reports on the photoluminescence characteristics of doped PANI, a detailed investigation on a comparative study of the photo luminescence in PANI doped with different dopants, which is the essence of our work, has not been carried out earlier. In the present work, we compare the photoluminescence (PL) emission intensity of PANI doped with hydrochloric acid (HCl), camphor sulphonic acid (CSA) and orthophosphoric acid, with a view to assessing the prospects of doped PANI as emissive layer in polymer LEDs. The PL emission peak is centered around 450 nm when excited at 365 nm. PANI doped with orthophosphoric acid exhibits the highest photoluminescent intensity followed by PANI doped with HCl and CSA. A comparison of the X.R.D spectra of the samples show that both HCl doped PANI and CSA doped PANI, do not show any significant crystalline nature whereas orthophosphoric acid doped PANI exhibits considerable crystallanity. The observed long-range order in orthophosphoric acid doped PANI could be a consequence of the more orderly arrangement of benzenoid and quinonoid units, resulting in enhanced PL emission intensity. (C) 2007 Elsevier B.V. All rights reserved.