First-principles density-functional theory (DFT) method has been used to investigate the structural and electronic properties of regioregular head-to-tail poly(3-hexylthiophene-2,5-diyl) (rrP3HT) crystal. Among the different configurations investigated for the rrP3HT crystal, the most stable is a staggered structure which is 875 meV/monomer more stable than isolated rrP3HT chains. Electronic properties of the rrP3HT crystal are strongly influenced by the magnitude of $\pi \text{−}\pi$ interactions that can be described by structural organization parameters such as the interchain distance in the stacking direction. From a disordered to a tightly packed polymer crystal, the direct band gap significantly decreases by $\sim 0.8\text{eV}$, while the dispersion of highest occupied molecular-orbital and lowest unoccupied molecular-orbital bands increase by 0.60 and 1.33 eV, respectively.

• Received 1 December 2008

DOI:https://doi.org/10.1103/PhysRevB.79.115207