Investigation of the Roles of Metal Halide Additives Used for the Preparation of Metal Halide Semiconductors

Metal halide (MHs) including halide perovskites emerge as a class of promising materials for solar energy harvesting and ionizing radiation detection applications. Preparation of some high performance MHPs can involves the use of impurity additives. In this work, the role of metal halide additives, HgI 2 , ZnI 2 , SrI 2 , MnI 2 , and CdI 2 , in preparation of MA 4 Bi 6 I 22 (where MA = methylammonium) is investigated using the ultraviolet–visible optical absorption spectroscopy, small‐angle X‐ray scattering, and electrospray ionization mass spectrometry measurement techniques. These experimental studies are supported by density functional theory calculations to reveal the impact of the specific metal halide additive used in the synthesis of MA 4 Bi 6 I 22 . It is shown that CdI 2 and HgI 2 additives act to reduce the concentration of free iodide anions by forming metal halide polyhedral units in the solution phase (e.g., [HgI 3 ] − ), and thus tune the molar ratio of reactants (MAI:BiI 3 ) and lead to the preferential formation of MA 4 Bi 6 I 22 . In all other cases, the high stability phase MA 3 Bi 2 I 9 is formed. This work provides insights for understanding how the metal halide additives impact the solution chemistry, thereby assisting the synthesis and discovery of new metal halide compositions for optoelectronic applications.