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The intriguing functionalities of emerging quasi-two-dimensional (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-FAPbI3 surface is passivated by 2D spacer molecules, 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 could be beneficial for optoelectronic applications. Together, this study uncovers the optimal 2D:3D compositional range for complex quasi-2D MHP systems, realizing desired optoelectronic performances and stability. The automated experimental workflow significantly accelerates materials discoveries and processing optimizations while providing fundamental insights into complex materials systems.

The intriguing functionalities of emerging quasi-two-dimensional (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 – results in the 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-FAPbI3 surface is passivated by 2D spacer molecules, exclusively emerge at the compositional range (35-55% of FAPbI3) deviating from the stoichiometric considerations. 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 could be beneficial for optoelectronic applications. Together, our work clearly uncovers the optimal 2D:3D compositional range for complex quasi-2D MHP system realizing desired optoelectronic performances and stability. The automated experimental workflow significantly accelerates the materials discoveries and processing optimizations while providing fundamental insights into the complex materials systems.

The certified power conversion efficiency of state-of-the-art organic–inorganic hybrid perovskite solar cells has surpassed 25%, showing promising potential for commercialization. Compared with vol...

Atomic layer deposition (ALD) allows for a great level of control over the thickness and stoichiometry of materials. ALD provides a suitable route to deposit lead halides, which can further be converted to perovskite for photovoltaics, photoemission, and photodetection, among other applications. Deposition of lead halides by ALD has already begun to be explored; however, the precursors used in published processes are highly hazardous, require expensive fabrication processes, or contain impurities that can jeopardize the optoelectronic properties of metal halide perovskites after conversion. We sought to deposit lead iodide (PbI2) by a facile ALD process involving only two readily accessible, low-cost precursors and without involving any unwanted impurities that could act as recombination centers once the PbI2 is later converted to perovskite. Crystalline PbI2 nanocrystals were grown on soda-lime glass (SLG), silicon dioxide support grids, and silicon wafer substrates and provide the groundwork for further investigation into developing lead halide perovskite processes by ALD. The ALD-grown PbI2 was characterized by annular dark field scanning transmission electron microscopy (ADF-STEM), atomic force microscopy (AFM), and x-ray photoemission spectroscopy (XPS), among other methods. This work presents the first step to synthesize lead halide perovskites with atomic control for applications such as interfacial layers in photovoltaics and for deposition in microcavities for lasing.

One of the organic component in the perovskite photo-absorber, the methylammonium cation, has been suggested to be a roadblock to long-term operation of organic-inorganic hybrid perovskite-based solar cells. Methylammonium-free perovskites thus represent a possible direction for more stable photo-absorbers that are also compatible with multijunction solar cells. However, most work on methylammonium-free perovskites involves cesium and formamidinium as the A-site cations, which are thermodynamically less stable than the methylammonium-based materials. In this work we systematically explore the crystallographic and optical properties of the compositional space of mixed cation and mixed halide lead perovskites, where formamidinium (FA+) is gradually replaced by cesium (Cs+), and iodide (I-) is substituted by bromide (Br-), i.e., CsyFA1–yPb(BrxI1–x)3. The crystal phases, which could be tuned by changing the tolerance factor for mixed perovskite alloys, are qualitatively determined and the composition–structure relationship is established in the CsyFA1–yPb(BrxI1–x)3 compositional space. We find that higher tolerance factors lead to more cubic structures, whereas lower tolerance factors lead to more orthorhombic. We also find that while some correlation exists between tolerance factor and structure, tolerance factor does not provide a holistic understanding of whether a perovskite structure will fully form. Given the wide range of bandgaps produced by this compositional space, an empirical expression is devised to predict the optical bandgap of CsyFA1–yPb(BrxI1–x)3 perovskites – which changes as a function of composition –, conducive to the design of absorbers with bandgaps tailor-made for specific tandem and single-junction applications. By screening 26 solar cells with different compositions, we find that Cs1/6FA5/6PbI3 delivers the highest efficiency and long-term stability among I-rich compositions. This work sheds light on the fundamental structure-property relationships in the CsyFA1–yPb(BrxI1–x)3 compositional space, providing vital insight to the design of durable perovskite materials. Our approach provides a library of structural and optoelectronic information of this compositional space.

The impact on device stability of the bulky cation-modified interfaces in halide perovskite solar cells is not well-understood. We demonstrate the thermal instability of the bulky cation interface layers used in some of the highest performing solar cells to date. X-ray photoelectron spectroscopy and synchrotron-based grazing incidence X-ray scattering measurements reveal significant changes under thermal stress in the chemical composition and structure at the surface of these films. The changes impact charge carrier dynamics and device operation, as shown in transient photoluminescence, excitation correlation spectroscopy, and solar cells. The type of cation used for passivation affects the extent of these changes, where long carbon chains provide more stable interfaces and thus longer durability (more than 1000 hrs at 55ºC). Such findings highlight that annealing the treated interfaces before characterization is critical to enable reliable reporting of performances and to drive the selection between different cations.

Lead halide perovskites (LHPs) are promising semiconductors for optoelectronic applications. In LHP solar cells, the focus thus far has been mainly on compositional optimization of the MHP layer, without much understanding of the effects of compositional mixing on structure and texture. This is a serious gap in our knowledge because research has shown that texture underlies the mechanisms of charge carrier and ionic transport. Therefore, it is essential to understand the mechanisms that drive changes in texture in LHPs. This work examines the effect of moisture and composition on the structure and texture of LHPs and their impacts on optoelectronic properties. Exposure to moisture is shown to induce a crystallographic reorientation in the polycrystalline films, which is also dependent on the amount of organic cation material present at the surface. For films with an excess of organic halide, moisture was shown to induce texture in the (001) plane contributing to the enhancement of photocurrents and long-Term device stability. This work shows the importance of texture for the electronic properties of LHPs with a special emphasis on charge carrier extraction in optoelectronic devices.