Metal Halide Perovskite Alloying: Expanding Understanding of the A-site, and Unlocking New Applications of the B-site

Metal halide perovskite (MHP) has received an immense amount of attention in the past decade for its rapid advancement in high efficiency, solution-processed optoelectronic devices such as solar cells, light emitting diodes, detectors, and scintillators. The high performance of MHP is predominantly due to the low rates of charge carrier recombination and tolerance of defects, the sources of which are currently under intense debate. The wide variety of MHP materials possible with the ABX3 stoichiometry introduces a large parameter space; one which this dissertation aims to understand and utilize through the use of compositional alloys. In the first part of this dissertation, the impact of the A-site monovalent cation on charge carrier recombination and defect tolerance will be investigated. The second part of this dissertation will explore metal doping at the B-site with the application of producing a new class of scintillation materials.
The role of the A-site monovalent cation has long been disputed, particularly in the distinction between the inorganic cesium (Cs+) cation, and the organic methylammonium (MA+) and formamidinium (FA+) cations. This dissertation approaches the comparison through the use of highly controlled mixed-cation alloys. By reducing variability between samples from morphology, structure, and density of defects, the connection between the monovalent cations can more easily be distinguished. In the FAxMA1-xPbI3 system, the dipole of the cation was found to not impact band-to-band recombination, however, the presence of multiple cations in the alloyed samples showed reduced band-to-band recombination. This is in support of the theories regarding the Rashba effect and polaron delocalization through lattice deformation. Defect-mediated recombination was found to have a strong dependence on the nature of the monovalent cation, with the defect-mediated recombination rate decreasing by over two orders of magnitude with increasing content of the MA+ cation. This is in agreement with related work on “defect healing” as a result of interactions between the dipolar MA+ cation and deep defects. The CsxMA1-xPbI3 system further supports these results, and similarly shows a dependence of decreasing defect-mediated recombination rate constant with increasing MA+ content. These results present evidence that the monovalent cation impacts charge carrier recombination through two separate mechanisms, and highlights the potential to tune band-to-band and defect-mediated recombination independently through selection of the cation.
Doping at the B-site has proven to be an effective method of increasing the photoluminescence of MHP. Ytterbium (Yb3+) doping in particular has been shown to boost the photoluminescence quantum yield of CsPbX3 above 100% through a quantum cutting mechanism. This dissertation presents the first fabrication of Yb3+ doped CsPbX3 through low-temperature water-based processing, and the first use of this material as a scintillator. Yb3+ doped CsPbCl3 powder was produced with high purity and crystallinity with a large Stokes shift preventing re-absorption of emitted photons. The champion 5% Yb3+ composition exhibits radioluminescence at 1,000 nm with a light yield of 102,000 photons/MeV. This light yield is higher than one of the brightest commercial scintillators, CsI(Tl), which has a light yield of 65,000 photons/MeV. This work is the first evidence of radioluminescence in Yb3+ doped MHP, and introduces the material as an exciting new class of scintillators for X-ray detection.