Encapsulation for improving the lifetime of flexible perovskite solar cells

The effect of encapsulation on improving the operational lifetime of flexible perovskite-based solar cells prepared on polymer substrates is presented. The devices were fabricated on polyethylene terephthalate films coated with indium-doped zinc oxide. Mesoporous TiO2 nanoparticles were used as the electron-transport layer and 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene as the hole-transport layer. The stability of non-encapsulated devices and devices encapsulated using two different architectures, referred to in the present work as 'partial' and 'complete' encapsulation, were evaluated on exposure to ambient conditions. The lifetime of the encapsulated flexible perovskite solar cell devices was extended significantly compared with that of the non-encapsulated devices. Permeation testing revealed that the post-encapsulation ingress of moisture through the adhesive layers and around electrical contacts constitutes a significant lifetime-limiting factor. Impedance spectroscopy indicates a gradual increase in the charge-transfer resistance at one of the device interfaces during degradation. These findings highlight the importance of continued development of encapsulation architectures to further prolong device lifetime.