On the Inapplicability of Electron-Hopping Models for the Organic Semiconductor Phenyl-C61-butyric Acid Methyl Ester (PCBM)

Phenyl-C61-butyric acid methyl ester (PCBM) is one of the most popular semiconductors in organic photovoltaic cells, but the electron-transport mechanism in the microcrystalline domains of this material as well as its preferred packing structure remain unclear. Here we use density functional theory to calculate electronic-coupling matrix elements, reorganization energies, and activation energies for available experimental and model crystal structures. We find that the picture of an excess electron hopping from one fullerene to another does not apply for any of the crystalline phases, rendering traditional rate equations inappropriate. We also find that the cohesive energy increases in the order body-centered-cubic < hexagonal < simple cubic < monoclinic < triclinic, independently of the type of dispersion correction used. Our results indicate that the coupled electron-ion dynamics needs to be solved explicitly to obtain a realistic description of charge transfer in this material.