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Abstract The intriguing functionalities of emerging quasi‐2D metal halide perovskites (MHPs) have led to further exploration of this material class for sustainable and scalable optoelectronic applications. However, the chemical complexities in precursors—primarily determined by the 2D:3D compositional ratio—result in uncontrolled phase heterogeneities in these materials, which compromises the optoelectronic performances. Yet, this phenomenon remains poorly understood due to the massive quasi‐2D compositional space. To systematically explore the fundamental principles, herein, a high‐throughput automated synthesis‐characterization workflow is designed and implemented to formamidinium (FA)‐based quasi‐2D MHP system. It is revealed that the stable 3D‐like phases, where the α‐FAPbI₃surface is passivated by 2D spacers, exclusively emerge at the compositional range (35–55% of FAPbI₃), deviating from the stoichiometric considerations. A quantitative crystallographic study via high‐throughput grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) experiments integrated with automated peak analysis function quickly reveals that the 3D‐like phases are vertically aligned, facilitating vertical charge conduction that can be beneficial for optoelectronic applications. Together, this study uncovers the optimal 2D:3D compositional range for complex quasi‐2D MHP systems, realizing promising optoelectronic functionalities. The automated experimental workflow significantly accelerates materials discoveries and processing optimizations that are transferrable to other deposition methods, while providing fundamental insights into complex materials systems. 

Tuning broad emission in 2D Pb–Sn halide perovskites (HPs) is essential for advancing optoelectronic applications, particularly for color‐tunable and white‐light‐emitting devices. This broad emission is linked to structural factors, such as defects and phase segregation of the Pb component within the Pb–Sn system, which are strongly influenced by the molecular structure and chemical properties of spacer cations. Atomic tuning of the spacers via halogenation opens up a new way to fine‐tune the molecular properties, enabling further augmentations of HP functionalities. Nevertheless, the distinct broad emission's sensitivity to spacer chemistry remains underexplored. Here, halogenation's influence is systematically investigated on 2D HP emission characteristics using a high‐throughput workflow. These findings reveal that the F‐containing phenethylammonium (4F‐PEA) spacer narrows the broadband PL, whereas Cl broadens it. Through a correlative study, it is found that 4F‐PEA reduces not only the local phase segregation but also the defect levels and microstrains in 2D HPs. This is likely attributed to the manifestation of less lattice distortion via stronger surface coordination of the dipole‐augmented 4F‐PEA. These results highlight halogenation as a key factor in modulating phase segregation and defect density in 2D Pb–Sn HPs, offering a promising pathway to tune the emission for enhanced optoelectronic performance. 

Cesium‐based quasi‐2D halide perovskites (HPs) offer promising functionalities and low‐temperature manufacturability, suited to stable tandem photovoltaics. However, the chemical interplays between the molecular spacers and the inorganic building blocks during crystallization cause substantial phase complexities in the resulting matrices. To successfully optimize and implement the quasi‐2D HP functionalities, a systematic understanding of spacer chemistry, along with the seamless navigation of the inherently discrete molecular space, is necessary. Herein, by utilizing high‐throughput automated experimentation, the phase complexities in the molecular space of quasi‐2D HPs are explored, thus identifying the chemical roles of the spacer cations on the synthesis and functionalities of the complex materials. Furthermore, a novel active machine learning algorithm leveraging a two‐stage decision‐making process, called gated Gaussian process Bayesian optimization is introduced, to navigate the discrete ternary chemical space defined with two distinctive spacer molecules. Through simultaneous optimization of photoluminescence intensity and stability that “tailors” the chemistry in the molecular space, a ternary‐compositional quasi‐2D HP film realizing excellent optoelectronic functionalities is demonstrated. This work not only provides a pathway for the rational and bespoke design of complex HP materials but also sets the stage for accelerated materials discovery in other multifunctional systems. 

Abstract Metal halide perovskites based on formamidinium (FA), or FA‐rich compositions have shown great promise for high‐performance photovoltaics. A deeper understanding of the impact of ambient conditions (e.g., moisture, oxygen, and illumination) on the possible reactions of FA‐based perovskite films and their processing sensitivities has become critical for further advances toward commercialization. Herein, we investigate reactions that take place on the surface of the FA_0.7Cs_0.3, mixed Br/I wide bandgap perovskite thin films in the presence of humid air and ambient illumination. The treatment forms a surface layer containing O, OH, and N‐based anions. We propose the latter originates from formamidine trapped at the perovskite/oxide interface reacting further to cyanide and/or formamidinate—an understudied class of pseudohalides that bind to Pb. Optimized treatment conditions improve photoluminescence quantum yield owing to both reduced surface recombination velocity and increased bulk carrier lifetime. The corresponding perovskite solar cells also exhibit improved performance. Identifying these reactions opens possibilities for better utilizing cyanide and amidinate ligands, species that may be expected during vapor processing of FA‐based perovskites. Our work also provides new insights into the self‐healing or self‐passivating of MA‐free perovskite compositions where FA and iodide damage could be partially offset by advantageous reaction byproducts. image 

Abstract Unlike single‐component 2D metal halide perovskites (MHPs) exhibiting sharp excitonic photoluminescence (PL), a broadband PL emerges in mixed Pb‐Sn 2D lattices. Two physical models –self‐trapped exciton and defect‐induced Stokes‐shift – are proposed to explain this unconventional phenomenon. However, the explanations provide limited rationalizations without consideration of the formidable compositional space, and thus, the fundamental origin of broadband PL remains elusive. Herein, the high‐throughput automated experimental workflow is established to systematically explore the broadband PL in mixed Pb‐Sn 2D MHPs, employing PEA (Phenethylammonium) as a model cation known to work as a rigid organic spacer. Spectrally, the broadband PL becomes further broadened with rapid PEA₂PbI₄phase segregation with increasing Pb concentrations during early‐stage crystallization. Counterintuitively, MHPs with high Pb concentrations exhibit prolonged PL lifetimes. Hyperspectral microscopy identifies substantial PEA₂PbI₄phase segregation in those films, hypothesizing that the establishment of charge transfer excitons by the phase segregation upon crystallization at high‐Pb compositions results in distinctive PL properties. These results indicate that two independent mechanisms—defect‐induced Stoke‐shifts and the establishment of charge transfer excitons by phase segregation—coexist which significantly correlates with the Pb:Sn ratio, thereby simultaneously contributing to the broadband PL emission in 2D mixed Pb‐Sn HPs. 

Abstract Quasi‐2D metal halide perovskites (MHPs) are an emerging material platform for sustainable functional optoelectronics, but the uncontrollable, broad phase distribution remains a critical challenge for applications. Nevertheless, the basic principles for controlling phases in quasi‐2D MHPs remain poorly understood, due to the rapid crystallization kinetics during the conventional thin‐film fabrication process. Herein, a high‐throughput automated synthesis‐characterization‐analysis workflow is implemented to accelerate material exploration in formamidinium (FA)‐based quasi‐2D MHP compositional space, revealing the early‐stage phase growth behaviors fundamentally determining the phase distributions. Upon comprehensive exploration with varying synthesis conditions including 2D:3D composition ratios, antisolvent injection rates, and temperatures in an automated synthesis‐characterization platform, it is observed that the prominentn= 2 2D phase restricts the growth kinetics of 3D‐like phases—α‐FAPbI₃MHPs with spacer‐coordinated surface—across the MHP compositions. Thermal annealing is a critical step for proper phase growth, although it can lead to the emergence of unwanted local PbI₂crystallites. Additionally, fundamental insights into the precursor chemistry associated with spacer‐solvent interaction determining the quasi‐2D MHP morphologies and microstructures are demonstrated. The high‐throughput study provides comprehensive insights into the fundamental principles in quasi‐2D MHP phase control, enabling new control of the functionalities in complex materials systems for sustainable device applications.