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Page 8 of 34 Wang et al. Energy Mater. 2026, 6, 600064

Figure 3. Degradation pathways in perovskite precursors. Reprinted with permission . Copyright 2025, John Wiley and Sons.
[81]
anion vacancies. During crystallization and subsequent thermal annealing, the inevitable loss or volatilization
of halide ions at grain boundaries and film surfaces generates vacant anionic sites . The physical absence of
[83]
these halides simultaneously strips the adjacent lead ions of their coordinating bonds and exposes them as
unsaturated Pb centers. Consequently, these exposed metal sites function as detrimental deep-level electron
2+
traps within the semiconductor bandgap, severely exacerbating non-radiative recombination pathways and
thereby imposing a rigid limitation on the open-circuit voltage (V ) of the photovoltaic devices. One
OC
common approach is the use of Lewis-base molecules such as amines [84-86] , carbonyls [87-89] , thiols [90-92] , and
phosphates [93-95] , which form strong coordination bonds with undercoordinated Pb ions. For instance,
2+
Xie et al. demonstrated that phosphoric acid derivatives (e.g., 3-phosphonopropionic acid (H3pp))
[96]
passivate undercoordinated Pb via O-Pb interactions, stabilizing the film and suppressing ion migration.
2+
Another effective strategy involves multifunctional molecules such as urea, biuret, and thiourea derivatives.
Liu et al. demonstrated that amidinethiourea (ATU) effectively passivates undercoordinated Pb via a
[97]
2+
targeted coordination mechanism in which the sulfur atom of the thiourea group donates lone-pair electrons
to the empty orbitals of exposed metal centers to form robust coordination bonds. This strong chemical
interaction successfully neutralizes the detrimental deep-level trap states associated with undercoordinated
lead species, leading to significantly improved crystallinity, reduced defect density, and enhanced carrier
mobility across the perovskite film. A third approach employs polydentate chelating agents such as
2-deoxy-2,2-difluoro-D-erythro-pentafuranous-1-ulose-3,5-dibenzoate (DDPUD) to implement a molecular
locking strategy . Distinct from conventional multisite additives that offer flexible or transient surface
[74]
coordination, this locking paradigm leverages abundant electronegative functionalities including carbonyl
and carbon-fluorine bonds to establish a rigid chelating network with undercoordinated Pb . This firm
2+
chemical anchoring physically locks the perovskite surface lattice to relieve residual tensile strain, a critical
mechanical stabilization that standard additives often fail to achieve. By securing the crystal surface and grain
boundaries, the DDPUD molecule not only passivates interfacial defects but also establishes a robust barrier
against environmental moisture, directly improving charge transport and long-term device stability.

Cation vacancies

While mitigating undercoordinated lead sites addresses a primary source of deep traps, the overall structural
integrity of the perovskite lattice remains highly susceptible to the concurrent loss of A-site cations. This