Unusual hexagonal NaCl structures at lead halide perovskite surfaces: New insights from first-principles calculations.

Sodium chloride NaCl is a ionic compound that is used in heterogeneous junctions for tuning the electronic properties of the interface. In photovoltaics, NaCl is often used in perovskite solar cells as interlayer between the photoactive material and the charge transport layers. Despite its simplicity, unexpected complex NaCl interfacial structures can be found at high pressure or low dimensions. The most stable surface facet of cubic NaCl is the (100), nevertheless recent experiments found different surface termination depending on the chemical nature and the structure of the substrate, for example a hexagonal surface of NaCl has been found on the diamond (110) surface. In this framework, this contribution explores the interface between NaCl and the prototypical lead halide perovskite (MAPI) with first-principles calculations at the DFT level of theory. Our results show different possible NaCl surface reconstructions depending on the MAPI terminations and the nature of the interactions at play. Effects on MAPI electronic structure (work function, band edge potentials) are also discussed. These findings will help the design of new and better performing perovskite solar cells.