Metal halide perovskites are of fundamental interest in the research of modern thin‐film optoelectronic devices, owing to their widely tunable optoelectronic properties and solution processability. To obtain high‐quality perovskite films and ultimately high‐performance perovskite devices, it is crucial to understand the film formation mechanisms, which, however, remains a great challenge, due to the complexity of perovskite composition, dimensionality, and processing conditions. Nevertheless, the state‐of‐the‐art in situ grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) technique enables one to bridge the complex information with device performance by revealing the crystallization pathways during the perovskite film formation process. In this review, the authors illustrate how to obtain and understand in situ GIWAXS data, summarize and assess recent results of in situ GIWAXS studies on versatile perovskite photovoltaic systems, aiming at elucidating the distinct features and common ground of film formation mechanisms, and shedding light on future opportunities of employing in situ GIWAXS to study the fundamental working mechanisms of highly efficient and stable perovskite solar cells toward mass production.

The bulk morphology of the active layer of organic solar cells (OSCs) is known to be crucial to the device performance. The thin film device structure breaks the symmetry into the in-plane direction and out-of-plane direction with respect to the substrate, leading to an intrinsic anisotropy in the bulk morphology. However, the characterization of out-of-plane nanomorphology within the active layer remains a grand challenge. Here, we utilized an X-ray scattering technique, Grazing-incident Transmission Small-angle X-ray Scattering (GTSAXS), to uncover this new morphology dimension. This technique was implemented on the model systems based on fullerene derivative (P3HT:PC71BM) and non-fullerene systems (PBDBT:ITIC, PM6:Y6), which demonstrated the successful extraction of the quantitative out-of-plane acceptor domain size of OSC systems. The detected in-plane and out-of-plane domain sizes show strong correlations with the device performance, particularly in terms of exciton dissociation and charge transfer. With the help of GTSAXS, one could obtain a more fundamental perception about the three-dimensional nanomorphology and new angles for morphology control strategies towards highly efficient photovoltaic devices.

The growth of high-quality tin-based perovskite films remains a grand challenge due to uncontrollable crystallization kinetics. Here, we report a facile strategy to realize an epitaxial-like growth of highly oriented tin-based perovskite films with the assistance of perovskite quantum dots (PQDs). Synchrotron-based in situ X-ray scattering results reveal that PQDs can act as nucleation centers to promote the growth of highly oriented perovskite crystals for both FASnI3 and MASnI3 systems. Remarkably, the degree of lattice strain can be readily modulated by tuning the lattice mismatch between various PQDs and bulk perovskites, thus reducing defect density and improving efficiencies. The efficiency of MASnI3 PSCs with PQDs has been pushed to 12.49%, which is the highest of this type reported so far. Furthermore, the film and device stability are enhanced owing to the improved film quality and the protection of hydrophobic ligands from PQDs.