Extending the Compositional Space of Mixed Lead Halide Perovskites by Cs, Rb, K, and Na Doping

A trend in high performing lead halide perovskite solar cell devices has been increasing compositional complexity by successively introducing more elements, dopants, and additives into the structure; and some of the latest top efficiencies have been achieved with a quadruple cation mixed halide perovskite Cs x FA y MA z Rb 1- x - y - z PbBr q I 3- q . This paper continues this trend by exploring doping of mixed lead halide perovskites, FA 0.83 MA 0.17 PbBr 0.51 I 2.49, with an extended set of alkali cations, i.e., Cs +, Rb +, K +, and Na +, as well as combinations of them. The doped perovskites were investigated with X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning electron microscopy, hard X-ray photoelectron spectroscopy, UV–vis, steady state fluorescence, and ultrafast transient absorption spectroscopy. Solar cell devices were made as well. Cs + can replace the organic cations in the perovskite structure, but Rb +, K +, and Na + do not appear to do that. Despite this, samples doped with K and Na have substantially longer fluorescence lifetimes, which potentially could be beneficial for device performance.