Theoretical Study of the Organic Photovoltaic Electron Acceptor PCBM: Morphology, Electronic Structure, and Charge Localization

The electronic structure of the lowest unoccupied orbitals of [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) is computed using a combination of classical molecular dynamics simulations (used to determine the morphology) and approximate quantum chemical calculations (used to determine the energy spectrum and localization length). The time-dependent coupling between localized states and the electron-vibration coupling is also computed. The results show that PCBM possesses an unusual distribution of localized and delocalized states, both thermally accessible at room temperature, which cannot be mapped into standard models of transport in disordered media. The coupling between these states is found to be too strong for simple perturbative treatments. At the same time, the local electron-vibration coupling, dominated by high frequency modes, is too weak to allow the formation of localized small polarons, as the zero point energy is above the barrier for electron hopping.