Elucidating the mechanism of self healing in hydro gel lead halide perovskite composites for use in photovoltaic devices

Since the emergence of organometal halide perovskite (OMP) solar cells, there has been growing interest in the ben-efits of incorporating polymer additives into the perovskite precursor, both in terms of photovoltaic device performance and perov-skite 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 spectros-copy. Varying concentrations of pHEMA (0 – 10 wt. %) are incorporated into a PbI2 precursor 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 PbI2 concentration compared with pure MAPI films. Devices based on pHEMA-MAPI composites exhibit an improved photoelectric conversion efficiency (PCE) 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 hours 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 dif-fraction (XRD), in situ X-ray photoelectron spectroscopy (XPS) and hard X-ray 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 hard XPS (HAXPES) depth-profiling using a non-destructive Ga K 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.