posted on 2023-12-06, 12:05authored byJoong Il Jake Choi, Luis K. Ono, Hunyoung Cho, Ki-Jeong Kim, Hyung-Been Kang, Yabing Qi, Jeong Young Park
While organic–inorganic hybrid perovskites are
emerging
as promising materials for next-generation photovoltaic applications,
the origins and pathways of perovskite instability remain speculative.
In particular, the degradation of perovskite surfaces by ambient water
is a crucial subject for determining the long-term viability of perovskite-based
solar cells. Here, we conducted surface characterization and atomic-scale
analysis of the reaction mechanisms for methylammonium lead bromide
(MA(CH3NH3)PbBr3) single crystals
using ambient-pressure atomic force microscopy (AP-AFM) and near-ambient-pressure
X-ray photoelectron spectroscopy (NAP-XPS) in environments ranging
from ultrahigh vacuum to 0.01 mbar of water vapor. MAPbBr3 single crystals, grown by a solution process, were mechanically
cleaved under UHV conditions to obtain an atomically clean surface.
Consecutive topography and friction force measurements in low-pressure
water (pwater ≈ 10–5 mbar) revealed the formation of degraded patches, one atomic layer
deep, gradually increasing their coverage until the surface was entirely
covered at a water exposure of 4.7 × 104 langmuir
(L). At the perimeters of these degraded patches, a higher friction
coefficient was observed, along with an interstitial step height,
which we attribute to a structure equivalent to that of the MA–Br
terminated surface. Combined with NAP-XPS analysis, our results demonstrate
that water vapor induces the dissociation of surface methylammonium
ligands, eventually resulting in the depletion of the surface MA and
the full coverage of hydrocarbon species after exposure to 0.01 mbar
of water vapor.