  Electronic and stability properties of quasi-2D alkylammonium perovskites are
investigated using density functional theory (DFT) calculations and validated
experimentally on selected classes of compounds. Our analysis is focused on
perovskite structures of formula (A)$_2$(A$'$)$_{n-1}$Pb$_n$X$_{3n+1}$, with
large cations A = butyl-, pentyl-, hexylammonium (BA, PA, HXA), small cations
A$'$ = methylammonium, formamidinium, ethylammonium, guanidinium (MA,FA,EA,GA)
and halogens X = I, Br, Cl. The role of the halogen ions is outlined for the
band structure, stability and defect formation energies. Two opposing trends
are found for the absorption efficiency versus stability, the latter being
assessed with respect to possible degradation mechanisms. Experimental
validation is performed on quasi-2D perovskites based on pentylammonium
cations, namely: (PA)$_2$PbX$_4$ and (PA)$_2$(MA)Pb$_2$X$_7$, synthesized by
antisolvent-assisted vapor crystallization. Structural and optical analysis are
inline with the DFT based calculations. In addition, the thermogravimetric
analysis shows an enhanced stability of bromide and chloride based compounds,
in agreement with the theoretical predictions.
