First-principles study of thermodynamic miscibility, structures, and optical properties of Cs2Sn(X1−xYx)6 (X,Y = I, Br, Cl) lead-free perovskite solar cells

The toxicity of lead (Pb) and the volatility of organic cations in typical Pb-based organic-inorganic hybrid perovskite materials are the two key challenges in emerging perovskite solar cells. Thus, the development of lead-free and inorganic perovskite materials for solar cells is of great interest. Here, the structural and optical properties of all-inorganic lead-free mixed Cs2Sn(X1−xYx)6 (X, Y = I, Br, Cl) perovskites are explored via first-principles calculations. The calculated Helmholtz free energies of mixing indicate that, at room temperature, Cs2Sn(I1−xBrx)6 and Cs2Sn(Br1−xClx)6 have good thermodynamic stability and miscibility in the whole range of Br/Cl contents. On the other hand, Cs2Sn(I1−xClx)6 has thermodynamic instability and immiscibility when the ratio of the Cl and I contents approaches 1. The bandgap of Cs2Sn(X1−xYx)6 increases as the concentration of doping (Y = Br, Cl) increases, while the lattice constant and volume generally decrease. The calculated optical absorption spectra of the perovskites show a significant blue shift as the doping concentration increases. These results demonstrate that the bandgap and optical absorption of the perovskites can be systematically tuned across a wide range upon dopant incorporation. This work may be helpful for designing all-inorganic lead-free perovskite solar cells and optoelectronic materials.