These are not the final page numbers! Ü Ü uniform. The low absorption/emission intensity of PNC supernatant is attributed to the low concentration of PNCs. This indicates that the as-formed PNCs can be easily collected by centrifugation (Supporting Information, Figure S5 b), which is desirable for cleaning and postprocessing in various applications.To explain the different capping effects of OA and APTES, we propose the following mechanism based on a dissolution-precipitation model (Figure 4 c). The dissolved precursors precipitate as PNCs at the DMF-toluene interface when the DMF precursor solution is injected into toluene. With OA as the capping ligand, the OA molecules adsorbed on the surface of the formed PNCs and PNSs diffuse from DMF to toluene, along with the products. However, the chain configuration of OA molecules cannot effectively prevent the products from dissolving back into DMF across the DMFtoluene interface and some OA ligands remain in the toluene phase because of their non-polar nature. The loss of OA ligands will result in a lack of ligands in the DMF phase, leading to the formation of large particles in the next round of precipitation. This effect becomes more pronounced when the concentrations of the precursors is high. On the contrary, the strong steric hindrance of APTES and the formation of silica can prohibit the dissolution of the as-formed PNCs back into DMF, which helps to maintain the original structural and optical properties of PNCs. Nevertheless, the PNC APTES-20 sample began to flocculate after standing for a few minutes (Supporting Information, Figure S7) because of hydrolysis of Si-O-C 2 H 5 groups attached to the PNC APTES surface, which generate hydroxy (-OH) groups (as indicated by FTIR spectra), resulting in a change in polarity and hydrogen bonding of the ligands.Water-induced degradation is a major problem for organic metal halide perovskites because protons are captured by methylammonium.[8] Similarly, they are also unstable towards other protic solvents such as alcohols.We hypothesized that PNC APTES may exhibit better stability because of the strong steric hindrance and hydrolysis properties of APTES, which reduces the access of protic solvent molecules to the PNCs surface. To test the stability of PNC APTES in protic solvents, 0.5 mg mL À1 of PNC precipitate capped by different ligands was dispersed in ethanol (Supporting Information, Figure S9 a and b). No emission was observed by the naked eye for PNC OA and PNC OABr dispersed in ethanol, and all XRD peaks (Supporting Information, Figure S9 c) of the decomposed products belong to rhombic PbBr 2 (JCPDS#31-0679). However, the PNC APTES precipitate showed high fluorescence intensity after sonication in ethanol, indicating better stability of PNC APTES in protic solvents.Long term stability tests were also conducted in different protic and polar solvents. As shown in Figure 5 a, the relative PL intensity of the PNC APTES-16 precipitate in isopropanol remained almost 70 % after 2.5 h. However, PNC precipitate showed p...

Organometal halide perovskites are highly promising materials for photovoltaic applications, yet their rapid degradation remains a significant challenge. Here, the light-induced structural degradation mechanism of methylammonium lead iodide (MAPbI3) perovskite films and devices is studied in low humidity environment using X-Ray Diffraction, Ultraviolet-Visible (UV-Vis) absorption spectroscopy, Extended X-ray Absorption Fine Structure spectroscopy, Fourier Transform Infrared spectroscopy, and device measurements. Under dry conditions, the perovskite film degrades only in the presence of both light and oxygen, which together induce the formation of halide anions through donation of electrons to the surrounding oxygen. The halide anions generate free radicals that deprotonate the methylammonium cation and form the highly volatile CH3NH2 molecules that escape and leave pure PbI2 behind. The device findings show that changes in the local structure at the TiO2 mesoporous layer occur with light, even in the absence of oxygen, and yet such changes can be prevented by the application of UV blocking layer on the cells. Our results indicate that the stability of mp-TiO2-MAPbI3 photovoltaics can be dramatically improved with effective encapsulation that protects the device from UV light, oxygen, and moisture.

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