Perovskite solar cells: Continuing to soar.

845 news & views occur when deliberately using specific non-stoichiometric growth conditions. Until now, it had been generally thought that Ruddlesden–Popper phases grow in the same layer-by-layer order that their constituent monolayers are deposited. These latest results undermine this assumption, with dramatic consequences for the controlled production of high-quality thin films and their resulting properties. Understanding of the dynamic layer-rearrangement is a prerequisite for engineering atomically sharp interfaces at the desired location within the heterostructure. Based on these results, novel growth routes can now be envisaged, enabling the study of new interfacial phases containing any desired member of the Ruddlesden–Popper series, which can, in turn, lead to improved performance and new functionalities. Such growth routes are not only limited to layered oxides such as Ruddlesden–Popper phases, but can also be used to explore the structure and properties of oxide interfaces more generally. To study the fundamental physics of the dynamic rearrangement in detail, a strong degree of collaboration between experimental and theoretical solid-state scientists was necessary. Both teams used an oxide molecular beam epitaxy system with in situ growth-monitoring capabilities for the fabrication of Ruddlesden–Popper phases. Their crystalline properties, as well as the atomic stacking, were studied using state-of-the-art synchrotron X-ray scattering and high-resolution scanning transmission electron microscopy. Moreover, to support the experimental evidence for the rearrangement of the surface layer, density functional theory calculations were performed. It is by combining various experimental approaches with theory that provides the insight and mechanistic understanding that no individual research group or institution can hope to achieve alone. Taken as a whole, these two studies clearly underline the role of thermodynamic factors during the atomic-layer-by-layer growth of complex materials, and provide a clear route for expanding the base of materials that can be prepared by design, with precise interface control. The range of quantum phenomena accessible in oxide interfaces is already enormous 1. With an even richer variety of oxide interfaces to explore, this will continue to be fertile ground for fundamental studies in the future. T he emergence over the past four years of solar cells made by solution casting thin films of light-absorbing methylammonium lead halide perovskite semiconductors has captured the attention of materials scientists and researchers in renewable energy 1–3. The 15% efficiency values reported nearly a year ago ignited an explosion of research in this field (Fig. 1). Unsurprisingly, researchers quickly identified challenges that needed to be addressed for …