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.