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. Introduction.Lead iodine perovskites are promising semiconductors for solar energy conversion. The most studied so far is the methylammonium because of its electronic properties, low non-radiative recombination, and its high device efficiencies15-20 % [1,2]. In addition to their efficiencies, the perovskites have attracted interest because of the novel device structures and new perovskite materials [3][4][5][6][7].

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Cu-Sb-S semiconductors are a large group of materials promising for a variety of applications, especially as thin-film solar cell absorbers. We present first-principles calculations of the optical and electronic properties of Cu3SbS4 in the famatinite structure. The effect of the microscopic properties on macroscopic optical and photovoltaic parameters is analyzed. By splitting the optical properties, we identified microscopically the origin of the high absorption coefficients. It is because an elevated contribution from S-S and Sb-Sb transitions. In addition, we estimate the macroscopic potential of these compounds for photovoltaic applications obtaining solar cell efficiencies using different spectra. The results indicate the great potential of these materials for photovoltaics.

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