First-principles calculations to investigate the structural, electronic and optical properties of lead-free double perovskites Rb2SeI6 and K2SeI6.

• The band gap values of Rb 2 SeI 6 and K 2 SeI 6 are 1.51 eV and 1.45 eV, respectively, which are very suitable for use as photosensitive materials of solar cells. • The optical parameters indicate that Rb 2 SeI 6 and K 2 SeI 6 have better light absorption characteristics than most lead-free perovskite materials, especially in the visible light range. • The effective mass of electrons and holes of Rb 2 SeI 6 and K 2 SeI 6 are much small than that of many perovskite materials. In this work, we have studied in detail the structural, electronic and optical properties of lead-free double perovskites Rb 2 SeI 6 and K 2 SeI 6 based on first-principles calculations. Their negative binding energy (E b) and formation energy (E f) ensure the stability of the studied materials. Our calculations give band gap values of Rb 2 SeI 6 and K 2 SeI 6 to be 1.51 eV and 1.45 eV, respectively. This illustrates that the two perovskite compounds belong to indirect band gap semiconductors, and are suitable for photosensitive materials of solar cells. For both Rb 2 SeI 6 and K 2 SeI 6 , the valence band maximum (VBM) are dominated by the I-p orbitals, and their conduction band minimum (CBM) are mainly contributed from the I-p orbitals and the Se-p orbitals. The results indicate that the effective mass of electrons and holes in Rb 2 SeI 6 and K 2 SeI 6 is very small, and therefore they are favorable for carrier transport. Moreover, the optical parameters display that Rb 2 SeI 6 and K 2 SeI 6 have significant light absorption properties, especially in the visible light range. Due to the suitable band gap values and excellent light absorption, Rb 2 SeI 6 and K 2 SeI 6 have great application potential in the absorption layer of solar cells. [ABSTRACT FROM AUTHOR]