Band gap tuning of metal halide perovskites for multi-junction all perovskite solar cells
ABX3 perovskite solar cells, where A is methyl ammonium, formamidinium or Cs, B is Pb or Sn and X is I, Br or Cl, have already exceeded 21% efficiency in single junction devices, and when combined with crystalline silicon in a tandem architecture, are set to surpass 25% efficiency in the near future. The ideal semiconductor for multijunction optoelectronic applications must have a broadly tuneable band gap and operate at peak performance for any desired band gap. We have recently demonstrated a completely tunable band gap between 1.5 to 2.2eV with very good operation for a CsyFA1-y Pb(IxBr1-x)3 perovskite, an ideal range for the “top cell” in a tandem solar cell. However, perovskites composed mixed Pb-Sn perovskites could push this band gap much lower towards 1.1eV, opening the possibility of all perovskite tandems. We have recently made considerable progress with these low band gap perovskites, it therefore should now feasible to construct an all perovskite tandem solar cell with ultimate optimised efficiencies eventually surpassing 30%.