Hole-transporting layer (HTL)-free CsPbI3 carbon-based
perovskite solar cells (C-PSCs) are regarded as a promising photovoltaic
candidate due to their low cost and enhanced device stability. However,
the imperfect perovskite/carbon interface, including surface defects
of CsPbI3 films, unmatched energy level alignment, etc.,
leads to a low power conversion efficiency (PCE) and thus hampers
its further development for commercialization. Herein, a multifunctional
interface modifier octylammonium iodide (OAI) is introduced into the
CsPbI3/carbon interface, which can not only reduce the
amount of residual PbI2 at grain boundaries by converting
PbI2 to the (OA)2PbI4 two-dimensional
(2D) phase but also passivate defects located at the surface and grain
boundaries of CsPbI3 films. Consequently, greatly reduced
defect density of CsPbI3 films as well as matched energy
level alignment of the CsPbI3/carbon interface are achieved,
which significantly boost the PCE of CsPbI3 C-PSCs from
12.97 to 14.64%. Moreover, due to the reduced amount of PbI2 at grain boundaries and the hydrophobic property of long-chain alkyl
in OAI, the unencapsulated CsPbI3 C-PSCs demonstrate excellent
long-term ambient stability, which can retain 91% of its initial PCE
after 30 days of storage in air.