Surface
passivation by various organic molecules is a widely used
approach to compensate surface defects and to improve a stability
of hybrid halide perovskites. For commonly used cationic passivators,
the formation of 2D phases and related heterostructures is considered
as an essential part of the passivation process. However, there is
an intriguing fundamental question: is it possible to achieve effective
and stable passivation by the thinnest possible layer? In this article,
we applied an iodide salt of a new bulky bifunctional 11-carboxy-decylammonium
cation (further AUDA+) as a passivator which can not only
passivate both VI
• and VMA
′ defects but can also expectedly form on the 3D perovskite surface
a dense and stable “monolayer” assembled due to the
strong hydrogen bonds between the terminal carboxylic groups and,
hereby, suppressing the passivator migration in the bulk of the perovskite.
Applying a wide set of methods such as steady-state and time-resolved
photoluminescence spectroscopy, X-ray diffraction, and scanning electron
microscopy, we revealed that the passivation regime could be controlled
by adjustment of two treatment parametersthe concentration
of AUDA+I– and the post-annealing temperature
resulting in either the formation of 2D/3D heterostructures or surface
defect passivation by the “monolayer” without the formation
of additional phases. The “monolayer” regime was found
to provide a greater improvement of optical properties: photoluminescence
intensity and average charge carrier lifetime increase by 10 and 2.5
times, respectively, and demonstrate significantly better properties
after long-time light soaking. These results indicate that a fine
tuning of passivation conditions provides a significant increase in
photostability even without formation of a 2D capping layer, thus
revealing new possibilities to enhance perovskite solar cells’
lifetime.