Two-dimensional (2D) halide perovskites, HaPs, can provide
chemical
stability to three-dimensional (3D) HaP surfaces, protecting them
from exposure to ambient species and from reacting with contacting
layers. Both actions occur with 2D HaPs, with the general stoichiometry
R2PbI4(R: long or bulky organic amine) covering
the 3D ones. Adding such covering films can also boost power conversion
efficiencies of photovoltaic cells by passivating surface/interface
trap states. For maximum benefit, we need conformal ultrathin and
phase-pure (n= 1) 2D layers to enable efficient
tunneling of photogenerated charge carriers through the 2D film barrier.
Conformal coverage of ultrathin (<10 nm) R2PbI4layers on 3D perovskites is challenging with spin coating; even
more so is its upscaling for larger-area devices. We report on vapor-phase
cation exchange of the 3D surface with the R2PbI4molecules and real-timein situgrowth monitoring
by photoluminescence (PL) to determine limits for forming ultrathin
2D layers. We characterize the 2D growth stages, following the changing
PL intensity–time profiles, by combining structural, optical,
morphological, and compositional characterizations. Moreover, from
quantitative X-ray photoelectron spectroscopy (XPS) analysis on 2D/3D
bilayer films, we estimate the smallest width of a 2D cover that we
can grow to be <5 nm, roughly the limit for efficient tunneling
through a (semi)conjugated organic barrier. We also find that, besides
protecting the 3D against ambient humidity-induced degradation, the
ultrathin 2D-on-3D film also aids self-repair following photodamage.