Large Cation Engineering in Two-Dimensional Silver–Bismuth Bromide Double Perovskites

Double
perovskites are promising candidates for less toxic and
highly stable metal halide perovskites, but their optoelectronic performances
still lag behind those of the lead halide counterpart, due to the
indirect nature of the bandgap and the strong electron–phonon
coupling. Reducing the dimensionality of Cs2AgBiBr6 down to a 2D layered form is strategic in order to tune the
band gap from indirect to direct and provides new insights into the
structure–property relationships of double perovskites. Herein,
we report on a series of monolayer 2D hybrid double perovskites of
formula (RA)4AgBiBr8, where RA represents different
primary ammonium large cations with alkyl- and aryl-based functionalities.
An in-depth experimental characterization of structure, film morphology,
and optical properties of these perovskites is carried out. Interestingly,
the variation of the ammonium cation and the interplanar distance
between adjacent inorganic monolayers has peculiar effects on the
film-forming ability and light emission properties of the perovskites.
Experiments have been combined with DFT calculations in order to understand
the possible origin of the different emissive features. Our study
provides a toolbox for future rational developments of 2D double perovskites,
with the aim of narrowing the gap with lead halide perovskite optoelectronic
properties.