Roll-transferred graphene encapsulant for robust perovskite solar cells

Perovskite solar cells (PSCs) have demonstrated enormous potential for use in next-generation photovoltaics because of their promising properties. However, the poor stability of PSCs as a result of severe degradation under outdoor conditions precludes their commercialization. Therefore, ideal encapsulation is a pivotal technique for achieving good long-term stability of PSCs, which is highly demanded. Here, we first introduce a highly flexible and stable graphene encapsulant by adopting the dry transfer method based on the roll-based process. Under well-controlled moisture conditions, the optical and structural properties of encapsulated perovskite films with different numbers of layers were systematically investigated through optical spectroscopy and grazing-incidence wide-angle X-ray scattering studies. As a result, the graphene-encapsulated perovskite films exhibited outstanding stability compared with a pristine film, and the graphene device retained 95% of its initial efficiency after 100 h of exposure to 85-90% relative humidity and retained 82.4% of its initial efficiency after 3700 h in ambient air. In addition, graphene-encapsulated flexible PSCs exhibited enhanced stability even when subjected to bending stress while submerged in water. On the basis of the results, we expect the proposed graphene encapsulation technique to be compatible with diverse graphene applications and to contribute to the development of not only stable perovskites but also next-generation flexible solar cells such as organic and quantum-dot solar cells.