FTIR studies of charged photoexcitations in regio-regular and regio-random poly(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 which including regio-regular and regio-random stereo orders, different length of alkyl side group, and various film preparation techniques. In addition to the traditional one-dimensional (1D) polaron PA bands, we observed quasi two-dimensional excitations (DP) in regio-regular P3AT films, where the polymer chains self-organize to form 2D lamella structures. Due to increased interlayer and interchain coupling, the two plaron energy levels split by 2Δ (≈2t⊥), resulting in two allowed optical transitions where the lower energy band (DP1) dominates and red-shifts into 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.