Water-resistance of macromolecules adsorbed on CH3NH3PbI3 surfaces: A first-principles study

Enhancing the stability of perovskites by constructing heterojunctions of graphene/MASnI3

In this paper, the geometric and electronic properties of heterojunctions constructed using a graphene sheet and an MASnI₃ surface were investigated by performing first-principles calculations based on the density functional theory. Our results show that the interaction between graphene and the MASnI₃ surface is in the scope of van der Waals interactions. In the heterojunctions, electrons transfer from graphene to the MASnI₃ surface, resulting in the formation of a built-in electric field in the interface, which is favorable for the separation of electrons and holes. The absorption spectra showed that the absorption intensity of the heterojunction in the visible region is slightly smaller than that of the pristine MASnI₃ surface. The energy barriers of water molecules diffusing through MASnI₃ surfaces are relatively low, but when a water molecule penetrates the graphene sheet into the interior of the MASnI₃ it has to overcome an energy barrier of as high as 9 eV. It is found that the water diffusions through the surfaces cause very severe damage to the structures of the graphene sheet and MASnI₃ surface. So, the graphene can block the penetration of water into the inside of the material and retard the degradation of the perovskites. Coating a graphene sheet onto the MASnI₃ surface to form a heterojunction is an effective strategy of enhancing the stability and performance of perovskite solar cells. This study could provide an in-depth understanding of the properties of graphene/MASnI₃ heterojunctions and contribute to the design strategy of perovskite-based solar cells.

Siloxane-Encapsulated Upconversion Nanoparticle Hybrid Composite with Highly Stable Photoluminescence against Heat and Moisture

Herein, we report a siloxane-encapsulated upconversion nanoparticle hybrid composite (SE-UCNP), which exhibits excellent photoluminescence (PL) stability for over 40 days even at an elevated temperature, in high humidity, and in harsh chemicals. The SE-UCNP is synthesized through UV-induced free-radical polymerization of a sol-gel-derived UCNP-containing oligosiloxane resin (UCNP-oligosiloxane). The siloxane matrix with a random network structure by Si-O-Si bonds successfully encapsulates the UCNPs with chemical linkages between the siloxane matrix and organic ligands on UCNPs. This encapsulation results in surface passivation retaining the intrinsic fluorescent properties of UCNPs under severe conditions (e.g., 85 °C/85% relative humidity) and a wide range of pH (from 1 to 14). As an application example, we fabricate a two-color binary microbarcode based on SE-UCNP via a low-cost transfer printing process. Under near-infrared irradiation, the binary sequences in our barcode are readable enough to identify objects using a mobile phone camera. The hybridization of UCNPs with a siloxane matrix provides the capacity for highly stable UCNP-based applications in real environments.