Thermosetting polyurethane-based encapsulation of flexible perovskite solar cells: A step forward in devices stabilization in highly damp environment

Flexible perovskite solar cells (f-PSCs) are emerging as a promising technology for a wide range of applications, where flexibility, lightweight, and a high power-to-weight ratio are desirable features. To ensure f-PSCs long-term stability, a suitable encapsulation that provides protection from atmospheric agents (i.e., moisture, water, and oxygen) without compromising the overall flexibility of the device is required. Thermosetting polyurethanes (PUs) arise as promising candidates: they are inert toward the perovskite layer, and their curing reaction can easily be performed at room temperature, directly on the f-PSC. Moreover, minimal modifications on the precursors' skeleton can tune flexibility, barrier properties, and transparency. In this work, a low-cost thermosetting PU resin is successfully implemented as the primary encapsulant for 1 cm2 f-PSCs. Two encapsulation strategies are proposed: the PU is applied only on the back of the device (i.e., in contact with the metal electrode) or on both the back and the front, onto the PET substrate. Besides preserving the device flexibility (500 bending cycles without any PCE decrement), the double encapsulation strategy ensures for long term stability under a highly damp atmosphere (RH > 75 %), reaching T80 of over 550 h (i.e., 23 days), clearly outperforming the unencapsulated control devices, whose T80 is 6 h.