Exploring the properties of lead-free hybrid double perovskites using a   combined computational-experimental approach
  Density functional theory screening of the hybrid double perovskites (MA)2BIBiX6 (BI=K,Cu,Ag,Tl; X=Cl,Br,I) shows that systems with band gaps similar to those of the MAPbX3 lead compounds can be expected for BI=Cu,Ag,Tl. Motivated by these findings, (MA)2TlBiBr6, isoelectronic with MAPbBr3, was synthesised and found to have a band gap of ~2.0eV. 
Zeyu Deng, Fengxia Wei, Shijing Sun, Gregor Kieslich, Anthony K.   Cheetham and Paul D. Bristowe
cond-mat.mtrl-sci