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