As a prospect of future research in the field of PSCs, it is pivotal to investigate the cation segregation from material and device. Despite the significant effectiveness of multifunctional molecules in regulating the cation uniformity of MCPs thin films, most efforts rely on trial-and-error strategy to identify suitable molecular materials for optimizing MCPs. There remains a lack of systematic strategies for designing and optimizing molecular materials specifically aimed at achieving uniformity in cation distribution. Further research on the universal optimization strategy to mitigate cation inhomogeneity and suppress ion migration is pursuit of highly efficient and stable PSCs toward ultimate commercialization necessitates.

In addition to the segregation initiated during crystallization, aging conditions can also contribute to segregation. Under operational conditions, illumination increases the Gibbs free energy of the system, destabilizing mixed perovskites. Illumination not only increases the system's energy but also reduces the activation energy for ion migration, ultimately promoting cation segregation^[9]. This drives the separation of mixed components into several single phases. Simultaneously, some components may convert into a photo-inactive yellow phase perovskite at room temperature. Heat accumulation during device operation further accelerates ion migration, exacerbating segregation^[10]. However, the driving force behind cationic phase segregation in mixed-cation perovskites remains an open question. As a whole, the cation inhomogeneity arises from suboptimal fabrication procedures, imbalanced crystallization, and cation migration under external stimuli, as shown in Fig. 1(c).

Since cation segregation is a collective consequence of cation migration, many researchers have aimed to suppress phase segregation by increasing the ion migration activation energy. Strategies such as lattice interstitial occupancy and polydentate ligand engineering have proven effective in raising the activation energy for ion migration^{[13, 14]}. For instance, Cao et al. demonstrated that introducing external alkali metal cations into perovskites formed ion migration barriers within the lattice due to their interstitial occupation^[15]. In addition, different Lewis base additives have been used based on the electronegativity of specific defects. These additives interact with charged defects via coordination, weakening ion migration dynamics and mitigating phase segregation. This strategy often involves forming intermediate phases through coordination with the B-site metal (BX₂) in the precursor. During this process, BX₂ acts as an electron acceptor, while exogenous modulation molecules serve as electron donors. This coordination inhibits nucleation and facilitates the regulation of the perovskite film layer's properties^[16]. In addition, the AX component has also been identified as an effective method for improving the quality and phase purity of photovoltaic devices. For example, based on the anion-π interactions, hexafluorobenzene (HFB) with its electron−accepting capability has been used as an AX retardant. The addition of HFB has been shown to inhibit nanoscale impurities during perovskite fabrication^[17]. Bai et al.^[18] fabricated perovskite films by incorporating selenophene, which interacted with Pb to disrupt the formation of large colloidal clusters in the precursor. This approach resulted in homogeneous cation distribution (Fig. 2(c)), effectively retarding cation aggregation during material processing and device operation.

Manipulation strategy of cation inhomogeneity. As the type and distribution of phase segregation depend on the composition and preparation process of the perovskite layer, various effective strategies have been employed (Fig. 2(a)). These include alleviating the external stress, optimizing film fabrication and regulating crystallization, which based on solute and solvent engineering, coordination engineering, and ion migration regulation strategy.

Introducing size-matched additive molecules has proven highly advantageous in coordinating chemical effects and physical interactions for crystal geometry design. It has been demonstrated that the crystallization rate of perovskites can be delayed by leveraging the size effect, achieved through the size matching between the functional groups of additive molecules and the lattice size of the perovskite. This approach effectively passivates defects, leading to solar cells with superior performance. Liu et al.^[19] found that BDAI₂, with its appropriately matched size, can synergistically passivate defect sites on perovskite surfaces, exhibiting an excellent defect passivation effect. The modified perovskite films retained the 3D perovskite phase with considerably reduced trap-state density and enhanced carrier extraction capabilities. Liang et al.^[20] developed 1-(phenylsulfonyl)pyrrole (PSP), whose molecular size (~6.21 Å) closely matched the lattice size of PbI₂ (~6.32 Å). The introduction of PSP improved the homogeneity of cation distribution in perovskite films and increased the defect formation energy of various types of defects, particularly Pb and I vacancies. This led to lower defect densities and longer carrier lifetimes, as confirmed by experimental results (Fig. 2(d)).

During spin-coating, partial solvent evaporation leads to the formation of a gel film. At this stage, processing temperature and solvent composition play critical roles. The duration of the gel stage, affected by the processing temperature, further influences the crystallization kinetics of the perovskite^[6]. In addition, solvent complexes not only delay the reaction between precursor compositions but also act as scaffolds to initiate subsequent crystal growth. This delays perovskite formation, yielding films with more uniform morphology and larger grain sizes. As depicted in the time evolution of crystallization of pristine MCPs precursor, a significant phase segregation occurs in the as-cast films (Fig. 1(a))^[7]. The variation in cation solubility are a key factor in segregation, a phenomenon that becomes more pronounced when alkali metal cations are involved. Besides, inappropriate fabrication procedures also result in cation inhomogeneity of MCPs. Zhu et al.^[8] demonstrated that the temperature gradient during annealing is a main reason for the composition evolution and gradient residual strain in mixed-cation perovskite film (Fig. 1(b)).

Studies have demonstrated that perovskite precursor solutions are colloidal dispersions rather than real solutions, and the CH₃NH₃I : PbI₂ ratio determines the coordination of the octahedral framework, further determining the colloidal size^[4]. During spin-coating, colloids act as nucleation sites to initiate crystallization. Higher colloidal concentrations in the precursor result in rapid perovskite crystallization due to the abundance of nucleation sites. In addition to the crystal growth period, variations in the size and coordination properties of lead polyiodides alter the crystallization pathway, leading to different assembly behaviors^[5]. As a result, the final film composition may not be uniform.

Mechanisms of element and phase segregation. Element and phase segregation occurs both during material fabrication and aging^[3]. This section summarizes the potential causes of segregation formation across these processes. Perovskite films prepared via the solution method can generally be divided into three stages: the precursor liquid stage, gel stage, and solid film stage. The first two stages are part of the crystallization process.

The gel stage provides a unique perspective for understanding and modulating phase evolution during the crystallization process. The gel is a complex formed by interactions between perovskite precursor solutes and solvents, and its characteristics can be tailored by adjusting the precursor components and solvent types. Mixed cations and halides help prevent gel collapse by forming multiple phases, extending the gel duration^[7]. Prolonging the gel state benefits the broadening of the anti-solvent window, improving the reproducibility of devices prepared using the one-step method. In addition, for a given perovskite system, the duration of the gel state is determined by the solvent system. Therefore, perovskite crystallization can be regulated by modulating the interactions between solutes, which is achieved by adjusting solvent components and their concentrations. Furthermore, the solubility variation of cations is another critical factor influencing segregation. To address solubility-discrepancy-induced segregation, minimizing the solubility differences among cations has been reported. The cation cascade strategy, in particular, has been developed to resolve this issue effectively^[11]. Besides, the initial nanoscale compositional heterogeneity of MCPs layer is influenced by the thermal annealing conditions during the film processing, it is verified that optimizing these processing conditions contributed to improving the device performance stability (Fig. 2(b))^[12].

In recent years, the research advancements have highlighted the critical role of the A-site cation in determining the optoelectronic and physicochemical properties of organic−inorganic lead halide perovskites. Mixed-cation perovskites (MCPs) have been extensively used as absorber thin films in perovskite solar cells (PSCs), achieving high power conversion efficiencies (PCE) over 26%^{[1, 2]}. The incorporation of mixed cations has led to a more optimal tolerance factor for the crystal structure, enhancing structural stability and providing additional functionalities to improve the chemical stability of the absorber thin films. However, mixed-cation perovskite absorbers often experience element and phase segregation, which can reduce device efficiency and operational lifespan. This segregation is a widespread phenomenon observed across various types of MCPs, whether in 2D or 3D structures. Therefore, understanding the fundamental causes of non-uniformity and phase segregation, as well as effective nanoscale regulatory strategies, is essential for enhancing the performance of PSCs. The development of high-quality MCPs with highly uniform cation distribution and stable phases is critical for addressing the stability challenges in PSCs.