The formation of a two-dimensional (2D) three-dimensional
(3D)
perovskite heterostructure has lately proved to be a promising way
to improve the interface between the perovskite and electron/hole
transport layers in perovskite solar cells, which is crucial for better
device efficiency and stability. Herein, a spacer cation, 4-fluorophenethylammonium
iodide, in isopropyl alcohol was used to form a thin 2D perovskite
layer on top of a 3D triple-cation perovskite by a spin-coating deposition
process. Therefore, a significant improvement in the device open-circuit
voltage is obtained, leading to an enhanced power conversion efficiency.
The formation mechanism of the 2D perovskite layer was studied by
analyzing the structural, chemical, and optoelectronic properties
of the layer, while varying several synthesis parameters. We reveal
the presence of bromide inside the 2D phase and conclude with the
existence of a concomitant formation mechanism, besides the most commonly
described one involving the lead iodide (PbI2) excess contained
in the 3D bulk. Therefore, we demonstrate how the stoichiometry of
the 2D perovskite is affected by the chemical composition of the 3D
layer underneath. This work provides new insights into the synthesis
mechanisms of 2D/3D perovskite heterostructures, which could help
to optimize their fabrication processes and develop new efficient
and functional 2D/3D structures.