Stretchable Hole Extraction Layer for Improved Stability in Perovskite Solar Cells

Flexibility and stretchability of solar cells are crucial factors for enhancing their real-life application for wearable devices. Although poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been conventionally employed as a hole extraction layer (HEL) in flexible organic or perovskite solar cells, the inherent stretchability of PEDOT:PSS has yet to be convinced. Here, we report a highly stretchable and mechanically stable PEDOT:PSS-based thin film and its application on flexible perovskite solar cells. We synthesized a chemically linked copolymer, P(SS-co-TFPMA), consisting of PSS and tetrafluoropropyl methacrylate (TFPMA) followed by graft copolymerization with poly(ethylene glycol) methyl ether methacrylate (PEGMA) to form a P(SS-co-TFPMA)-g-PEGMA dopant for the PEDOT HEL. The PEDOT:P(SS-co-TFPMA)-g-PEGMA (PEDOT:PTP) copolymer solution has excellent homogeneity and high phase stability, and its developed HEL film exhibits outstanding stretching capability. After stretching of 300%, PEDOT:PTP films sustain conductivity of over 80% of their original conductivity, whereas the conventional PEDOT:PSS films completely lose their conductivity after a strain of 300%. In addition, the PEDOT:PTP-incorporated flexible perovskite solar cells exhibited improved mechanical stability compared with the unassisted cells, retaining 92% of the initial power conversion efficiency after 1500 bending cycles at a 7 mm radius.