A computational view of the change in the geometric and electronic properties of perovskites caused by the partial substitution of Pb by Sn.

Recently, solar cells with hybrid organic-inorganic lead halide perovskites have achieved a great success and their power conversion efficiency reaches about 17.9%. For practical applications, one has to avoid the toxicology issue of lead, to develop lead-free perovskite solar cells by using metal substitution. It has been shown that tin is one of possible candidates as a replacement for lead. Herein, a step-by-step protocol based on the first-principles calculations is performed to investigate the geometrical and electronic properties of mixed Sn and Pb perovskite MAPbxSn1-xI3 with different crystal symmetries. At first, a GGA functional with the inclusion of the van der Waals interaction, vdW-DF3, is used to optimize the geometries and it reproduces closely the unit cell volume. Then, a more accurate hybrid functional PBE0 combined with the spin-orbit coupling effect is used to perform electronic-structure calculations. The calculated results reveal that the band gaps of MAPbxSn1-xI3 are sensitive to the ratio of Sn/Pb, and are proportional to the x component, consistent with the previous reports. Further investigations show that the crystal symmetry can also modify the band gap in an order of Pnma > I4cm > P4mm at x = 0.5. The random rotation of organic cations, which makes the band alignments in the compounds, facilitates the separation and transfer of holes and electrons. Interestingly, the computed binding energies of the unrelaxed exciton have the same trend as band gaps, which decreases with decreasing x, the binding energies of MAPb0.5Sn0.5I3 also decrease as the crystal symmetry decreases, implying a faster exciton dissociation with lower x and lower symmetry at an ambient temperature.