Two-dimensional
(2D) materials are promising components for defect
passivation of metal halide perovskites. Unfortunately, commonly used
polydisperse liquid-exfoliated 2D materials generally suffer from
heterogeneous structures and properties while incorporated into perovskite
films. We introduce monodisperse multifunctional 2D crystalline carbon
nitride, poly(triazine imide) (PTI), as an effective defect passivation
agent in perovskite films via typical solution processing. Incorporation
of PTI into perovskite film can be readily attained by simple solution
mixing of PTI dispersions with perovskite precursor solutions, resulting
in the highly selective distribution of PTI localized at the defective
crystal grain boundaries and layer interfaces in the functional perovskite
layer. Several chemical, optical, and electronic characterizations,
in conjunction with density functional theory calculations, reveal
multiple beneficial roles from PTI: passivation of undercoordinated
organic cations at the surface of perovskite crystal, suppression
of ion migration by blocking diffusion channels, and prevention of
hole quenching at perovskite/SnO2 interfaces. Consequently,
a noticeably improved power conversion efficiency is achieved in perovskite
solar cells, accompanied with promoted stability under humid air and
thermal stress. Our strategy highlights the potential of judiciously
designed 2D materials as a simple-to-implement material for various
optoelectronic devices, including solar cells, based on hybrid perovskites.