K. P. McKenna

Polycrystalline materials are ubiquitous in technology, and grain boundaries have long been known to affect materials properties and performance. First principles materials modeling and electron microscopy methods are powerful and highly complementary for investigating the atomic scale structure and properties of grain boundaries. In this review, we provide an introduction to key concepts and approaches for investigating grain boundaries using these methods. We also provide a number of case studies providing examples of their application to understand the impact of grain boundaries for a range of energy materials. Most of the materials presented are of interest for photovoltaic and photoelectrochemical applications and so we include a more in depth discussion of how modeling and electron microscopy can be employed to understand the impact of grain boundaries on the behavior of photoexcited electrons and holes (including carrier transport and recombination). However, we also include discussion of materials relevant to rechargeable batteries as another important class of materials for energy applications. We conclude the review with a discussion of outstanding challenges in the field and the exciting prospects for progress in the coming years.

Metal halide perovskite semiconductors have shown great performance in solar cells, and including an excess of lead iodide (PbI 2 ) in the thin films, either as mesoscopic particles or embedded domains, often leads to improved solar cell performance. Atomic resolution scanning transmission electron microscope micrographs of formamidinium lead iodide (FAPbI 3 ) perovskite films reveal the FAPbI 3 :PbI 2 interface to be remarkably coherent. It is demonstrated that such interface coherence is achieved by the PbI 2 deviating from its common 2H hexagonal phase to form a trigonal 3R polytype through minor shifts in the stacking of the weakly van‐der‐Waals‐bonded layers containing the near‐octahedral units. The exact crystallographic interfacial relationship and lattice misfit are revealed. It is further shown that this 3R polytype of PbI 2 has similar X‐ray diffraction (XRD) peaks to that of the perovskite, making XRD‐based quantification of the presence of PbI 2 unreliable. Density functional theory demonstrates that this interface does not introduce additional electronic states in the bandgap, making it electronically benign. These findings explain why a slight PbI 2 excess during perovskite film growth can help template perovskite crystal growth and passivate interfacial defects, improving solar cell performance.