Elucidating the Mechanism of Self-Healing in Hydrogel-Lead Halide Perovskite Composites for Use in Photovoltaic Devices

Since the emergence of organometal halide perovskite(OMP) solarcells, there has been growing interest in the benefits of incorporatingpolymer additives into the perovskite precursor, in terms of bothphotovoltaic device performance and perovskite stability. In addition,there is interest in the self-healing properties of polymer-incorporatedOMPs, but the mechanisms behind these enhanced characteristics arestill not fully understood. Here, we study the role of poly-(2-hydroxyethylmethacrylate) (pHEMA) in improving the stability of methylammoniumlead iodide (MAPI, CH3NH3PbI3) anddetermine a mechanism for the self-healing of the perovskite-polymercomposite following exposure to atmospheres of differing relativehumidity, using photoelectron spectroscopy. Varying concentrationsof pHEMA (0-10 wt %) are incorporated into a PbI2 precursor solution during the conventional two-step fabricationmethod for producing MAPI. It is shown that the introduction of pHEMAresults in high-quality MAPI films with increased grain size and reducedPbI(2) concentration compared with pure MAPI films. Devicesbased on pHEMA-MAPI composites exhibit an improved photoelectric conversionefficiency of 17.8%, compared with 16.5% for a pure MAPI device. pHEMA-incorporateddevices are found to retain 95.4% of the best efficiency after ageingfor 1500 h in 35% RH, compared with 68.5% achieved from the pure MAPIdevice. The thermal and moisture tolerance of the resulting filmsis investigated using X-ray diffraction, in situ X-ray photoelectronspectroscopy (XPS), and hard XPS (HAXPES). It is found that exposingthe pHEMA films to cycles of 70 and 20% relative humidity leads toa reversible degradation, via a self-healing process. Angle-resolvedHAXPES depth-profiling using a non-destructive Ga K alpha sourceshows that pHEMA is predominantly present at the surface with an effectivethickness of ca. 3 nm. It is shown using XPS that this effective thicknessreduces with increasing temperature. It is found that N is trappedin this surface layer of pHEMA, suggesting that N-containing moieties,produced during reaction with water at high humidity, are trappedin the pHEMA film and can be reincorporated into the perovskite whenthe humidity is reduced. XPS results also show that the inclusionof pHEMA enhances the thermal stability of MAPI under both UHV and9 mbar water vapor pressure.