Roshan Ali,^† Guo-Jiao Hou,^† Zhen-Gang Zhu,^{*,†,‡,⊥} Qing-Bo Yan,^§,‡ Qing-Rong Zheng,^‡ and Gang Su^{*,‡,∥,⊥}

^† School of Electronic, Electrical and Communication Engineering, University of Chinese Academy of Sciences, Beijing 100049, China

^‡ Theoretical Condensed Matter Physics and Computational Materials Physics Laboratory, College of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

^§ College of Materials Science and Optoelectronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China

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

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

Organic–inorganic halide perovskites are quite promising in applications of large scale photovoltaic technology. However, toxicity is one of the crucial issues in these materials, and searching for environmentally friendly perovskite materials for green energy applications is in high demand. Here we present a systematic ab initio study on the replacement of toxic Pb in the perovskite CH₃NH₃PbI₃ (MAPbI₃) with possible mono- and a few binary replacements. In the mono-replacements study, Ge and Sn are the best alternatives to Pb. In the binary replacements, we replace Pb by mixing Ca/Si and Zn/Si. In case of Ca/Si, a monotonic decrease in band gaps with a monotonic increase in the optical absorption was observed with increasing the Ca concentration. It is observed for the first time that the substitution of Ca/Si (or Zn/Si) at the B-site with various ratios would lead to remarkably high device absorption efficiencies. The band gaps of the studied mixed replacements are in the ideal ranges for single-junction solar cell and one cell in tandem architecture. As a result of the smaller effective masses, the mixed replacements could have better carrier mobility. An ab initio molecular dynamic simulation demonstrates the stability of the mixed replacements. More importantly, the mixed substituting elements are highly abundant in the earth. This work is helpful to gain further insights into developing green solar cells with low cost and high performance and would lead to wide applications in the future.