Anion effect on absorption and photovoltaic efficiency of some complex iodobismuthates

The photovoltaic use of iodoplumbates perovskites is limited because of lead toxicity and its instability. An alternative is the iodobismuthate compounds. In this work, the microscopically optical properties of two iodobismuthate compounds, (C28H40N4)(Bi4I18) and (C6H8N)(BiI5), are obtained using first principles. To relate these properties to the structure and to analyze the effect of the iodine linker atoms in these compounds, we split the absorption coefficients into an exact many-specie expansion. The results indicate that the optical gap and the optical properties are determined by the anionic fragment of these compounds. The presence of iodine linker atoms results in a reduction in the energy gap and a larger absorption near the gap.