Unraveling the real structures of solution-based and surface-bound poly(3-hexylthiophene) (P3HT) oligomers: a combined theoretical and experimental study

Abstract

While the crystalline structure for regio-regular poly(3-hexylthiophene) (P3HT) in thin films is well established, the conformation of P3HT chains in solution has received less attention. Nevertheless, the control of this in-solution structure can be used for managing the structure-processing relationship, which can be used to improve the optoelectronic behavior and thus the efficiency of devices exploiting electroactive polymers. In the current study, we report a combined theoretical and experimental study of P3HT and a series of oligomers, both in the solid state and in solution. (3HT)_n oligomers were simulated in a variety of planar and non-planar conformations by means of density functional theory (DFT) and time-dependent DFT (TDDFT), comparing results for various functionals with and without dispersion correction in order to evaluate the role of intermediate and long-range effects. Our calculations show that regio-regular P3HT chains adopt a twisted conformation in solution (dihedral angle of about 40°), which contrasts with the well-established planar (θ = 0°) conformation when deposited onto a substrate, due to inter-chain interactions. Determining the Raman spectra, electronic gaps, quasi-particle energies and optical spectra, a good agreement between experimental and simulated optical absorption spectra was obtained for the in-solution case. This study will help to promote the development of alternative strategies for controlling the optoelectronic features of conjugated polymers and polymer blends by exploiting the in-solution structure.