2D/3D
perovskite heterostructures have emerged as a promising material
composition to reduce nonradiative recombination in perovskite-based
LEDs and solar cells. Such heterostructures can be created by a surface
treatment with large organic cations, for example, n-butylammonium bromide (BABr). To understand the impact of the BABr
surface treatment on the double-cation (Cs0.17FA0.83Pb(I0.6Br0.4)3) (FA = formamidinium)
perovskite thin film and further optimize the corresponding structures,
an in-depth understanding of the chemical and electronic properties
of the involved surfaces, interfaces, and bulk is required. Hence,
we study the impact of the BABr treatment with a combination of surface-sensitive
X-ray photoelectron spectroscopy and bulk-sensitive resonant inelastic
soft X-ray scattering (RIXS). A quantitative analysis of the BABr-treated
perovskite thin film shows a modified chemical perovskite surface
environment of carbon, nitrogen, bromine, iodine, and lead, indicating
that the treatment leads to a perovskite surface with a modified composition
and bonding structure. With K-edge RIXS, the local environment at
the nitrogen and carbon atoms is probed, allowing us to identify the
presence of BABr in the perovskite bulk albeit with a modified bonding
environment. This, in turn, identifies a “hidden parameter”
for the optimization of the BABr treatment and overall performance
of 2D/3D perovskite solar cell absorbers.