(Invited) Graphene Ion Blocking Layers for Robust Metal-Halide Perovskite Heterostructures

Abstract

Despite being only one atomic layer thick, a continuous monolayer of graphene should be completely impermeable to atoms and ions. As such, graphene monolayers could potentially be employed as ion blocking layers in numerous technologies where charge and/or energy transport across a key interface is desirable, but ionic or atomic transport is not. As a key example, the development of metal-halide perovskite/perovskite heterostructures is hindered by rapid interfacial halide diffusion leading to mixed alloys rather than sharp interfaces. This halide diffusion hinders the creation of tailored interfaces that funnel energy or charge in specified directions within energy technologies such as photovoltaic solar cells or light-emitting diodes. To circumvent this issue, we developed an ion-blocking layer consisting of single layer graphene (SLG) deposited between the metal-halide perovskite layers and demonstrate that it effectively blocks anion diffusion in perovskite/perovskite heterostructures. We study these interfaces with a suite of spectroscopy and elemental analysis tools to demonstrate that halides do not diffuse across the graphene-modified interface, whereas control samples without the SLG show rapid homogenization of the halides and loss of the sharp interface. The SLG has little electronic impact on the band alignment and optical properties of the individual semiconductors, as supported by density functional theory calculations, facilitating photoinduced energy transfer across the abrupt heterointerface and light-emitting diodes with no evidence of ion-diffusion during operation. Our results demonstrate that monolayer graphene can be used as an ion blocking layer in a variety of different heterostructures for advanced opto-electronic and electrochemical applications. Further, it establishes a route towards robust perovskite/perovskite heterostructures that are difficult to achieve by other means.