FTIR studies of charged photoexcitations in regio-regular and regio-randompoly(3-alkylthiophene) films

We have studied the low energy photo-induced absorption (PA) band of photo- generated polarons in a variety of poly(3-alkylthiophene) (P3AT) films whic h including regio-regular and regio-random stereo orders, different length of alkyl side group, and various film preparation techniques. In addition t o the traditional one-dimensional (1D) polaron PA bands, we observed quasi two-dimensional excitations (DP) in regio-regular P3AT films, where the pol ymer chains self-organize to form 2D lamella structures. Due to increased i nterlayer and interchain coupling, the two plaron energy levels split by 2 Delta (approximate to 2t(perpendicular to)), resulting in two allowed optic al transitions where the lower energy band (DP1) dominates and red-shifts i nto the mid infrared range. The DP1 band blue-shifts when the length of the alkyl side group increases, or when using lower boiling point solvent, or when preparing films at higher temperature. (C) 2001 Elsevier Science B.V. All rights reserved.