Stability of Perovskite Films Encapsulated in Single- and Multi-Layer Graphene Barriers

This paper describes the efficacy of barrier films coated with single- and multi-layer graphene in preventing degradation of perovskite films in air. Despite the impermeability of graphene to small species such as water and oxygen, the presence of numerous grain boundaries and defects in chemical vapor deposition (CVD)-grown graphene monolayer films can present pathways for permeation. However, the availability of these pathways can in principle be reduced by stacking multiple layers of graphene on top of each other. The barrier material considered here consists of the semi-permeable polymer parylene laminated with either 0, 1, 2, or 3 monolayers of graphene. These composite films are used to encapsulate triple cation perovskite films, which are then subjected to a degradation test under damp heat. We find that a monolayer of graphene confers a 15-fold reduction in degradation compared to the parylene films with no graphene and that three-layer graphene can yield a further 2× reduction in degradation. Although all of our films encapsulated with graphene/parylene exhibited substantial degradation compared to films encapsulated in glass with polyisobutylene edge seals, our results nonetheless reinforce the utility of graphene barriers for less demanding applications, including lightweight or flexible perovskite solar cells with shorter anticipated lifetimes.