Bending fatigue damage reduction in indium tin oxide (ITO) by polyimide and ethylene vinyl acetate encapsulation for flexible solar cells

Abstract

Hybrid organic-inorganic perovskites hold tremendous promise for next-generation portable power source applications. However, the device reliability under severe mechanical deformations is limited due to the use of brittle indium tin oxide (ITO). In the current study, we have attempted to enhance the bendability of the device by employing an encapsulation on the ITO coated polyethylene terephthalate (PET) substrate. The aim of this encapsulant buffer layer is to shift the location of the maximum bending stresses away from the ITO as well as provide barrier properties against the environment. Ethylene-vinyl acetate (EVA), as well as Polyimide (PI) films, have been used as encapsulating materials with the aim to shift the location of the neutral axis to the ITO. Finite element analysis has been done to evaluate the principal stresses generated in ITO during monotonic bending under different modes of bending, the thickness of encapsulant layers as well as radii of bending. The effect of the number of bending cycles, mode of bending as well as the bending radii on the conductivity of the ITO have been studied experimentally. A large change in resistance of ITO film has been used as a metric to determine the failure of the ITO film. Samples encapsulated with an optimum thickness of PI shows a significant reduction in percentage change in resistance as compared to raw samples for both cases of bending, while EVA encapsulated samples hardly show any improvement. Moreover, both the modulus and thickness of an encapsulant play an important role in enhancing the bendability.