Roshan Ali,^ab  Guo-Jiao Hou,^a  Zhen-Gang Zhu,*^abe  Qing-Bo Yan,^bc  Qing-Rong Zheng^b  and  Gang Su*^bde 

^a School of Electronic, Electrical and Communication Engineering, University of Chinese Academy of Sciences, Beijing 100049, China. E-mail: zgzhu@ucas.ac.cn

^b Theoretical Condensed Matter Physics and Computational Materials Physics Laboratory, College of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China. E-mail: gsu@ucas.ac.cn

^c College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China

^d Kavli Institute for Theoretical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China

^e CAS Center for Excellence in Topological Quantum Computation, University of Chinese Academy of Sciences, Beijing 100190, China

In just a few years the efficiency of organic–inorganic perovskites has reached 22.1%. Long term stability and lead toxicity are their two main hurdles to overcome for commercialization as solar cells. Keeping these significant challenges in mind, we use ab initio density functional theory (DFT) and incorporate mixed Ca/(Ge, Sn) and Sr/(Ge, Sn) at the B-site of CH₃NH₃PbI₃ (MAPbI₃) to get Pb-free perovskites with excellent properties. Tunability of the band gaps is observed while decreasing the ratios of Sn and Ge. The predicted band gaps (<2.01 eV) determined using the HSE06 functional reveal that all of the studied mixed perovskites are in a highly favorable range for solar cells. Out of the 12 studied, five materials are in the ideal range for the top cell in tandem solar cells (1.70–1.90 eV); meanwhile 2 perovskites fall in the optimum band gap range of single junction solar cells. The enthalpy of formation study confirms that all of the mixed Pb-free perovskites are more stable than MAPbI₃. We also calculated the effective masses, uncovering the lower effective mass of these materials. We expect excellent carrier mobility as a consequence of this. This work is of high importance for the solar industry due to the low cost (Ca and Sr are highly earth abundant), non-toxic nature and higher absorption of the perovskites investigated as compared to the toxic MAPbI₃.