Optical studies of ordered and disordered α-sexithiophene films

We have studied (alpha) -sexithiophene ((alpha) -6T) films by photoinduced absorption (PA) and optically-detected magnetic resonance (ODMR). The PA of the disordered film is composed of two polaron PA bands and one bipolaron PA band, each with its own vibronic side bands. Measurements of an ordered film indicate that charge conjugation symmetry is broken and the polaron energy bands are shifted. The spin-1/2 ODMR of this film has a Gaussian lineshape with an 8 Gauss width. The PA of the ordered film shows no evidence for polarons. The bipolaron band is clearly split, indicating violation of charge conjugation symmetry. The ODMR of this film is significantly broader than that of the disordered film, which may be evidence for spin-spin interaction between paired charged excitations.