Electronic correlations in oligo-thiophene molecular crystals

The Coulomb interaction between two holes on oligo-thiophene molecules is studied systematically as a function of the oligomer length using first principles density function calculations. The effect of molecular geometry relaxation upon this interaction is found to be small. In contrast, electronic polarization of the molecules that surround the charged oligomer in the crystal lattice reduces the bare Coulomb repulsion between the holes by approximately a factor of 2. In all cases, the effective hole-hole repulsion is much larger than the valence band width, which means that at high doping levels, strong correlation effects should become important.