Since the emergence of organometal halide perovskite
(OMP) solar
cells, there has been growing interest in the benefits of incorporating
polymer additives into the perovskite precursor, in terms of both
photovoltaic device performance and perovskite stability. In addition,
there is interest in the self-healing properties of polymer-incorporated
OMPs, but the mechanisms behind these enhanced characteristics are
still not fully understood. Here, we study the role of poly(2-hydroxyethyl
methacrylate) (pHEMA) in improving the stability of methylammonium
lead iodide (MAPI, CH3NH3PbI3) and
determine a mechanism for the self-healing of the perovskite–polymer
composite following exposure to atmospheres of differing relative
humidity, using photoelectron spectroscopy. Varying concentrations
of pHEMA (0–10 wt %) are incorporated into a PbI2precursor solution during the conventional two-step fabrication
method for producing MAPI. It is shown that the introduction of pHEMA
results in high-quality MAPI films with increased grain size and reduced
PbI2concentration compared with pure MAPI films. Devices
based on pHEMA-MAPI composites exhibit an improved photoelectric conversion
efficiency of 17.8%, compared with 16.5% for a pure MAPI device. pHEMA-incorporated
devices are found to retain 95.4% of the best efficiency after ageing
for 1500 h in 35% RH, compared with 68.5% achieved from the pure MAPI
device. The thermal and moisture tolerance of the resulting films
is investigated using X-ray diffraction, in situ X-ray photoelectron
spectroscopy (XPS), and hard XPS (HAXPES). It is found that exposing
the pHEMA films to cycles of 70 and 20% relative humidity leads to
a reversible degradation, via a self-healing process. Angle-resolved
HAXPES depth-profiling using a non-destructive Ga Kα source
shows that pHEMA is predominantly present at the surface with an effective
thickness of ca. 3 nm. It is shown using XPS that this effective thickness
reduces with increasing temperature. It is found that N is trapped
in this surface layer of pHEMA, suggesting that N-containing moieties,
produced during reaction with water at high humidity, are trapped
in the pHEMA film and can be reincorporated into the perovskite when
the humidity is reduced. XPS results also show that the inclusion
of pHEMA enhances the thermal stability of MAPI under both UHV and
9 mbar water vapor pressure.