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Page 10 of 34 Wang et al. Energy Mater. 2026, 6, 600064

toward the band edge and extends photocarrier lifetimes from 452 to 791 ns. Advancing beyond these
localized coordination effects, the multifunctional small molecule 4,7-bromo-5,6-fluoro-2,1,3-phenylpropyl
thiadiazole (M4) incorporates a diverse array of Br, F, and S donor groups . This broad functionalization
[104]
enables M4 to address multiple defect types by concurrently suppressing V along with both Pb and I Pb
I
I
defects to restore proper lattice coordination. Ultimately, while advanced multifunctional molecules
successfully address complex arrays of chemical defects, this chemical coordination paradigm is further
elevated by macroscopic molecular locking strategies that provide comprehensive structural and mechanical
stabilization across the crystal surface. As previously discussed in the context of undercoordinated Pb 2+
passivation, the polydentate DDPUD molecule exemplifies this advanced paradigm by utilizing its C=O and
-F groups to simultaneously bind Pb centers . To fully appreciate the significance of this structural
[74]
I
evolution, it is instructive to distinguish the DDPUD molecular locking strategy from the tridentate PAA
coordination mentioned above. Both approaches transcend single-site passivation through multiple
functional groups to establish robust networks with metallic centers and antisite defects. However, while the
PAA strategy primarily exerts a thermodynamic influence by elevating specific defect formation energies, the
DDPUD approach executes a macroscopic mechanical locking effect. Beyond chemical passivation, this rigid
anchoring physically locks the top interface and grain boundaries to relieve residual tensile strain, an effect
that is uniquely coupled with the formation of a robust hydrophobic barrier to significantly enhance
long-term environmental stability.

Surface, grain boundary, and interface defects
While atomic-scale defects are critical, their spatial distribution and aggregation at boundaries give rise to
mesoscale defects that dominate non-radiative losses in polycrystalline films. These imperfections can be
broadly categorized into surface defects, grain boundary defects, and interface defects located between the
perovskite layer and the charge transport layers (CTL). Among these diverse regions, the external boundaries
are particularly problematic. Specifically, in mixed halide perovskite films, imperfections are primarily
concentrated at the surface and interface regions, where their density is approximately 100 times higher than
that of the bulk defects [105,106] .

Surface defects

Because the physical termination of the perovskite lattice naturally exposes a high density of
undercoordinated lead centers and atomic vacancies, the diverse coordination strategies discussed previously
fundamentally serve as effective surface passivation mechanisms. Illustrating this functional overlap, targeted
organic molecules such as triphenylphosphine oxide (TPPO) readily coordinate with these exposed surface
species to yield a marked increase in PCE [107] . Similarly, phenethylammonium bromide (PEABr) effectively
modulates the surface chemistry to reduce non-radiative recombination and improve overall film stability .
[108]
Although these tailored molecular interventions successfully fortify the external top interface, the
polycrystalline nature of perovskite thin films introduces a vast network of internal interfaces. These internal
domains, commonly known as grain boundaries, are highly susceptible to defect accumulation and moisture
infiltration.

Grain boundary defects

Grain boundary defects fundamentally arise from lattice misalignment and the incomplete physical fusion
between adjacent crystalline domains during the film drying process. Because these internal interfaces act as
primary channels for ion migration and non-radiative recombination, researchers frequently deploy tailored
organic molecules and targeted inorganic salts to regulate local crystallization. A highly advanced
methodology to eliminate these boundary defects involves the synergistic incorporation of the organic spacer