Stabilization of hybrid perovskite CH₃NH₃PbI₃ thin films by graphene passivation

We report the long-term stability of water-sensitive hybrid perovskites CH₃NH₃PbI₃ that were protected with monolayer graphene. This successful passivation was enabled by our development of a new water-free and polymer-free graphene transfer method. Monolayer graphene samples grown by plasma-enhanced chemical vapor deposition and transferred onto different substrates with the water/polymer-free method were found to preserve their high-quality characteristics after the transfer, as manifested by the studies of Raman, X-ray and ultraviolet photoemission spectroscopy (XPS and UPS), optical absorption, and sheet resistance. Additionally, XPS, UPS and optical absorption studies of fully graphene-covered CH₃NH₃PbI₃ thin films showed spectral invariance even after 3 months, which was in sharp contrast to the drastic spectral changes after merely one week in control CH₃NH₃PbI₃ samples without graphene protection. This successful demonstration of the graphene-enabled passivation and long-term stability of CH₃NH₃PbI₃ thin films therefore opens up a new pathway towards realistic photovoltaic applications of hybrid perovskites.