Stable mixed group II (Ca, Sr) and XIV (Ge, Sn) lead-free perovskite solar cells

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 (MAPbI(3)) 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 MAPbI(3). 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 MAPbI(3).