Electronic States at Low-Angle Grain Boundaries in Polycrystalline Naphthalene

We investigated the influence of grain boundaries on electronic properties of polycrystalline organic semiconductor naphthalene. The atomic structure of grain boundaries was found using a Monte Carlo method, whereas electronic structure calculations were performed using the charge patching method. We found that grain boundaries introduce trap states within the band gap of the material. Our results show that spatial positions and energies of trap states can be predicted solely from geometrical arrangement of molecules near the boundary. Wave functions of these states are localized on closely spaced pairs of molecules from opposite sides of the boundary. The energies of trap states are strongly correlated with the distances between the molecules in the pair. These findings were used to calculate the electronic density of trap states, which was found to exhibit a qualitatively different behavior for grain boundaries perpendicular to the a and b directions of the crystal unit cell.