Toward Water-Resistant,\nTunable Perovskite Absorbers\nUsing Peptide Hydrogel Additives

In recent years, hydrogels have been demonstrated as\nsimple and\ncheap additives to improve the optical properties and material stability\nof organometal halide perovskites (OHPs), with most research centered\non the use of hydrophilic, petrochemical-derived polymers. Here, we\ninvestigate the role of a peptide hydrogel in passivating defect sites\nand improving the stability of methylammonium lead iodide (MAPI, CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) using closely controlled, <i>in situ</i> X-ray photoelectron spectroscopy (XPS) techniques\nunder realistic pressures. Optical measurements reveal that a reduction\nin the density of defect sites is achieved by incorporating peptide\ninto the precursor solution during the conventional one-step MAPI\nfabrication approach. Increasing the concentration of peptide is shown\nto reduce the MAPI crystallite size, attributed to a reduction in\nhydrogel pore size, and a concomitant increase in the optical bandgap\nis shown to be consistent with that expected due to quantum size effects.\nEncapsulation of MAPI crystallites is further evidenced by XPS quantification,\nwhich demonstrates that the surface stoichiometry differs little from\nthe expected nominal values for a homogeneously mixed system. <i>In situ</i> XPS demonstrates that thermally induced degradation\nin a vacuum is reduced by the inclusion of peptide, and near-ambient\npressure XPS (NAP-XPS) reveals that this enhancement is partially\nretained at 9 mbar water vapor pressure, with a reduced loss of methylammonium\n(MA<sup>+</sup>) from the surface following heating achieved using\n3 wt % peptide loading. A maximum power conversion efficiency (PCE)\nof 16.6% was achieved with a peptide loading of 3 wt %, compared with\n15.9% from a 0 wt % device, the former maintaining 81% of its best\nefficiency over 480 h storage at 35% relative humidity (RH), compared\nwith 48% maintained by a 0 wt % device.