Crystalline structure, molecular motion and photocarrier formation in thin films of monodisperse poly(3-hexylthiophene) with various molecular weights

A better understanding of the carrier formation process for semiconducting polymers, especially in thin films, is essential for designing and constructing highly functionalized polymeric optoelectronic devices. Here, the effects of aggregation states and thermal molecular motion on photocarrier formation in melt-crystallized thin films of monodispersed poly(3-hexylthiophene) (P3HT) are discussed. Grazing incidence X-ray diffraction measurements revealed that the crystalline ordering in the films was greatly influenced by the molecular weight (MW) of P3HT. In contrast, dynamic mechanical analysis (DMA) revealed that the MW had no significant effects on the α₁ relaxation process, which corresponded to the twisting motion of thiophene rings in the crystalline phase unless the MW was quite small. Femtosecond transient absorption (TAS) spectroscopy showed that better crystalline ordering led to the direct formation of polarons (P) from hot excitons at room temperature. Once the temperature went beyond T_α1, at approximately 310 K, the P formation process from polaron pairs (PP) was activated. Thus, it can be claimed that the P formation for P3HT could be regulated by the thermal molecular motion in addition to the crystalline structure. The knowledge obtained here should be useful for a better molecular design of semiconducting polymers that can be applied to optoelectronic devices.