  C60 and C60 based molecules are efficient acceptor and electron transport
layers for planar perovskite solar cells. While properties of these molecules
are well studied by ab initiomethods, those of solid C60, specifically its
optical absorption properties, are not. We present a combined Density
Functional Theory - Density Functional Tight Binding study of the effect of
solid state packing on bandstructure and optical absorption of C60. The valence
and conduction band edge energies of solid C60 differ on the order of 0.1 eV
from single molecule frontier orbital energies. We show that calculations of
optical properties using linear response TD-DFT(B) or the imaginary part of the
dielectric constant (dipole approximation) can result in unrealistically large
redshift in the presence of intermolecular interactions compared to available
experimental data. We show that optical spectra computed from the
frequency-dependent real polarizability better reproduce the effect of C60
aggregation on optical absorption and may be more suited to study effects of
molecular aggregation.
