Entropy of Charge Separation in Organic Photovoltaic Cells: The Benefit of Higher Dimensionality

The role of entropy in charge separation processes is discussed with respect to the dimensionality of the organic semiconductor. In 1-D materials, the change in entropy, Delta S, plays no role, but at higher dimensions, it leads to a substantial decrease in the Coulomb barrier for charge separation. The effects of Delta S are highest in equilibrium systems but decrease and become time-dependent in illuminated organic photovoltaic (OPV) cells. Higher-dimensional semiconductors have inherent advantages for charge separation, and this may be one reason that C(60) and its derivatives, the only truly three-dimensional organic semiconductors yet known, play such an important role in OPV cells.