Low-Energy Inverse Photoemission Study on the Electron Affinities of Fullerene Derivatives for Organic Photovoltaic Cells

The lowest unoccupied molecular orbital (LUMO) levels of six fullerene derivatives frequently used for organic photovoltaic cells are examined in the solid state. We employed a new experimental technique, low-energy inverse photoemission spectroscopy (LEIPS), which allows us to determine the electron affinities of solid samples within the uncertainties of 0.1 eV. Using the precisely determined electron affinities, the energy difference between the ionization energy of donor and the electron affinity of acceptor is compared with the open circuit voltage for various polymer/fullerene combinations taken from the literature. The result suggests that the well-known relation between the energy difference and the open circuit voltage should be reconsidered.