  The effect of 14 organic cations on the optical properties of the lead iodine
perovskite is analyzed. The electronic and optical properties are obtained
using first principles. The absorption coefficients are split into inter-atomic
species components in order to quantify all of the contributions. For energies
close to the bandgaps, the main contribution is from the Pb-Pb intra-species
transitions. For higher energy this contribution is still important in addition
to I-I and Pb-I contributions, and to the 3 and 4 species term. Almost all
absorption properties are qualitatively similar. Furthermore, this absorption
coefficient splitting also allows the optical characteristics that the
substitution of Pb by another element should satisfy to be identified in order
to reduce the toxicity because of Pb while maintaining a high absorption
capacity.
