Search

The curious phenomenon of cracking in organic-inorganic halide perovskite (OIHP) thin films for solar cells during scanning electron microscopy (SEM) can be seen in literally thousands of published SEM micrographs. Here we demonstrate, for the first time, the mechanisms responsible for this e -beam-induced damage in OIHP thin films, which is precluding their detailed SEM-characterization and understanding. The e -beam-induced rapid volatilization of the organic species from the OIHP surface in the SEM results in localized shrinkage and buildup of tensile stresses. These stresses drive grain-boundaries cracking, resulting in a 'mud-cracking' pattern that is influenced by the thin-film grain size. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Organic-inorganic halide perovskite (OIHP) thin films at the heart of the new perovskite solar cells (PSCs) are very brittle, limiting the mechanical reliability of PSCs. Here we show that fine-grained MAPbI 3 (prototypical OIHP) films with grain size (~290 nm) smaller than the typical film thickness (~500 nm) tend to fracture intergranularly, resulting in low toughness (0.41 J.m−2). In contrast, MAPbI 3 /substrate interfacial fracture occurs in films with grains larger (~730 nm) than the film thickness, resulting in much higher toughness (1.14 J.m−2). Thus, coarse-grained OIHP films are deemed desirable for not only improved PSCs performance and stability but also mechanical reliability. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Pure formamidinium lead triiodide (α-FAPbI 3) organic-inorganic halide perovskite (OIHP) semiconductor is very attractive for use as light absorber in the new thin-film perovskite solar cells (PSCs) technology. This is primarily because of its superior thermal stability, more suitable bandgap, and compositional simplicity. However, the existence of the photo-inactive non-perovskite δ-FAPbI 3 polymorph ('yellow' phase) is a major hurdle in the path towards the development of α-FAPbI 3 -based PSCs. Also, there is general consensus that the fine-grained nature of OIHP thin films is detrimental to the environmental stability and performance of the resulting PSCs. In this context, here we take advantage of the polymorphism in FAPbI 3 , and use solvent-vapor-assisted δ-to-α phase transformation to induce exceptional grain coarsening (up to 50-fold) in 0.3-μm thickness FAPbI 3 thin films, resulting in an unprecedented average grain size of up to ~9 μm. The underlying mechanisms are elucidated based on the results from a combination of some key experiments, which involve studying systematically the effects of time, temperature, initial grain size, and solvent polarity index (PI). The ultra-coarse-grained α-FAPbI 3 thin films show dramatically improved environmental stability over their medium-grained counterparts, which is explained based on grain-boundary density arguments. PSCs made using the ultra-coarse-grained α-FAPbI 3 thin films have improved photovoltaic (PV) performance, but it is somewhat modest. This is attributed to the underestimation of the effective grain size relevant to photocarrier dynamics. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Organic–inorganic halide perovskite (OIHP) thin films at the heart of the burgeoning thin-film perovskite solar cells (PSCs) technology possess poor mechanical properties, which is likely to limit the long-term reliability of PSCs as they are poised for commercialization. In an effort to address this issue, here we demonstrate that through-thickness cracks induced by bending-tension in thin films of the two prototypical OIHPs, methylammonium lead triiodide (MAPbI 3) and formamidinium lead triiodide (FAPbI 3), can be healed easily. This is through the application of either a moderate compressive stress (bending-compression) at room temperature or a simple heat-treatment at modest temperatures. The crack-healing process is found to be time-dependent, which indicates that facile mass-transport in OIHPs plays a key role in this phenomenon. An explanation for this phenomenon is provided, one based on the fundamentals of brittle fracture. This discovery has broad implications for the prevention and/or restoration of the overall performance, environmental stability, and mechanical reliability of PSCs, and other devices. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Organic-inorganic halide perovskites (OIHPs) are a remarkable family of hybrid materials at the heart of the emerging thin-film perovskite solar cells (PSCs) technology and other applications. In this Viewpoint article, in addition to some original experimental results, thermo-mechanical analyses of residual macro- and micro-stresses in OIHP thin films are presented in the context of fracture of both the OIHP layer itself and its interface with the substrate. The implications of mechanical behavior of OIHPs and interfaces on the reliability of PSCs, as well as opportunities for future research directions in this general area, are highlighted. [ABSTRACT FROM AUTHOR]

The lineal-intercept method is commonly used to estimate the average grain size, where the 'Mendelson' multiplying factor (1.60) that is typically used explicitly assumes tetrakaidecahedral grains. While this is appropriate for bulk equiaxed microstructures, grains in thin films where the grain size exceeds the film thickness, are closer to prismatic polyhedrons in shape. Thus, assuming log-normal distribution of hexagonal-prism-shaped grains, a new multiplying factor of 1.20 is derived which relates the average grain size and measured intercept size. This model is successfully validated using experimental measurements of three different coarse-grained thin films of halide perovskites used in solar cells. Image, graphical abstract [ABSTRACT FROM AUTHOR]