The performance of the best-certified organic–inorganic halide perovskite solar cells has improved by more than 150% relative and 8% absolute (Figure 3.1) over the last few years making it the fastest advancing photovoltaic technology. 1 There has been a move from the widely researched CH3NH3PbI3 (MAPbI3) 1–2 toward mixed perovskites by incorporating HC(NH2)2 + (FA+) to take advantage of lower bandgap and therefore higher current but is often compromised by the addition of Br ions for better perovskite phase stability. 3 The addition of Cs 4 and most recently Rb 5,6 also produce cells with excellent efficiencies (although not certified) and stability.