First-principles study of thermodynamic miscibility, structures, and optical properties of Cs2Sn(X1−<i>x</i>Y<i>x</i>)6 (X,Y = I, Br, Cl) lead-free perovskite solar cells
Shiyu Liu, Shiyu Liu, Meng Sun, Shuoxin Zhang, Shiyang Liu, Shiyang Liu, Dejun Li, Zhiqiang Niu, Yaping Li, Sanwu Wang
Abstract
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.