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Organic-inorganic hybrid perovskites are promising materials for the next generation photovoltaics and optoelectronics; however, their practical application has been hindered by poor structural stability mainly caused by ion migration and external stimuli. Understanding the mechanism(s) of ion migration and structure decomposition is thus critical. Here we observe the sequence of structural changes at the atomic level that precede structural decomposition in the technologically important Cs1-xFAxPbI3 using ultralow dose transmission electron microscopy. We find that these changes differ, depending upon the A-site composition. Initially, there is a random loss of FA+, complemented by the loss of I-. The remaining FA+ and I- ions then migrate, unit cell by unit cell, into an ordered and more stable phase with a √2 x √2 superstructure. Further ion loss is accompanied by A-site dependent octahedral tilt modes and associated tetragonal phases with different stabilities. These observations of the loss of FA+/I- ion pairs, ion migration, octahedral tilt modes, and the role of the A-cation, provide insights into the atomic-scale structural mechanisms that drive and block ion loss and ion migration, opening pathways to inhibit ion loss, migration and improve structural stability. Applications of perovskite solar cells have been hindered by ion migration and structural instability. Here, the authors use ultra-low dose transmission electron microscopy to reveal the process of ion migration and structural change at the atomic level in Cs1-xFAxPbI3. [ABSTRACT FROM AUTHOR]

Shape and size control of metal nanocrystals has become a powerful tool in tuning their physicochemical properties for applications, however, the growth mechanisms controlling shape and size are not fully elucidated. Gold nanocuboids provide a simple nanoparticle system for investigating nanocrystal growth mechanisms. Here, we study the control over size and shape anisotropy of gold nanocuboids by copper additives. We first optimize the synthesis and yield. We find that, in the presence of copper additives, symmetry is broken and anisotropic growth can occur, leading to nanobars, rather than nanocubes, accompanied by a significant reduction in particle size. We show that symmetry breaking is caused by a combination of rapid deposition on {111} facets coupled with the slow surface diffusion rate introduced by surface copper. This reveals a mechanism by which metal additives can cause symmetry breaking and control shape in nanoparticle growth. [ABSTRACT FROM AUTHOR]

Use of the guanidinium iodide (GAI) cation is widely recognized as an interface engineering technique for perovskite solar cells that deliver stability improvements via defect passivation on surfaces and grain boundaries. However, a comprehensive understanding of the relationship between the structural and photophysical properties is lacking. Herein, GAI‐induced perovskite structural modifications, including derivative phases and underlying transitions, are detected in GAI surface‐treated Cs0.07MA0.14FA0.79Pb(I0.83Br0.17)3 through an analysis of X‐ray and electron diffraction and microscopy data. An optimum GAI solution concentration at 10 mg mL−1 can eliminate excess PbI2, improve crystallinity, and increase grain size of the as‐prepared perovskite films. However, a further increase to 20–40 mg mL−1 induces new (FAPbI3)x(GA2PbI4)x phases and a reduction in crystallinity and grain size. In addition, from confocal photoluminescence imaging, it is observed that 10 mg mL−1 GAI also helps to remove the microscale spatial heterogeneities, demonstrating optimum device performance. These results show that understanding the impact on structure and microstructure of the selection and concentration of surface treatment agents is critical for the homogenization of perovskite optoelectronic properties and achieving efficient device. [ABSTRACT FROM AUTHOR]

The high‐power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) on lab‐scale devices trigger the need to develop scalable manufacturing processes to accelerate their commercialization transition. A roll‐to‐roll (R2R) vacuum‐free printing on flexible substrates allows for high‐volume and low‐cost manufacturing which is especially well‐suited for PSCs due to its solution processibility and low‐temperature annealing requirements. Herein, a facile hot deposition technique is reported to fabricate triple‐cation (Cs0.07FA0.79MA0.14Pb(I0.83Br0.17)3) perovskite films in an ambient environment using a R2R slot‐die coating method. This perovskite composition, whilst being most studied in lab devices due to its high efficiency and stability, has not been applied in R2R fabrication thus far. The demonstrated R2R slot‐die coated flexible PSCs achieve stabilized PCE reaching 12% at maximum power point in inverted "p‐i‐n" architectures, the highest efficiency reported to date for R2R inverted PSCs. To achieve this, the underlying hole transport layer (poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate) is modified with guanidinium iodide additive which leads to the formation of large millimeter‐sized perovskite clusters, improved perovskite crystallinity, and enhanced charge‐transfer efficiency. This study highlights the potential of the facile hot‐deposition method while providing critical insights into the role of interfacial engineering in eliminating performance losses and fabricating efficient printed flexible PSCs. [ABSTRACT FROM AUTHOR]