Different 2D and quasi-2D perovskite materials have demonstrated significant improvements in the device stability compared to 3D perovskites due to their increased hydrophobicity and suppressed ion migration. However, fundamental investigations of these materials have been scarce and consequently detailed understanding of the processes responsible for experimental phenomena are often lacking despite huge interest in these materials. Even more importantly, there have been a limited number of structure-property studies for different material compositions, and research is generally by trial and error rather than by design. Here we discuss different stability issues in these materials and identify questions which need to be answered to design materials with further stability improvements.

Ruddlesden–Popper halide perovskite (RPP) materials are of significant interest for light‐emitting devices since their emission wavelength can be controlled by tuning the number of layers n, resulting in improved spectral stability compared to mixed halide devices. However, RPP films typically contain phases with different n, and the low n phases tend to be unstable upon exposure to humidity, irradiation, and/or elevated temperature which hinders the achievement of pure blue emission from n = 2 films. In this work, two spacer cations are used to form an RPP film with mixed cation bilayer and high n = 2 phase purity, improved stability, and brighter light emission compared to a single spacer cation RPP. The stabilization of n = 2 phase is attributed to favorable formation energy, reduced strain, and reduced electron–phonon coupling compared to the RPP films with only one type of spacer cation. Using this approach, pure blue light‐emitting diodes (LEDs) with Commission Internationale de l'éclairage (CIE) coordinates of (0.156, 0.088) and excellent spectral stability are achieved.

Formamidinium (FA)‐based perovskites exhibit great potential for photovoltaics since they enable the achievement of power conversion efficiency (PCE) over 22%. The bandgap of FA‐based perovskite is lower than that of the methylammonium‐based one, while the larger ionic radius and dual‐ammonia group of FA ions restrain their movement in close‐packing [PbI6]4− cages, leading to improved stability. Here, the structure and properties of FAPbI3− and FA‐based mixed cation perovkites are discussed. In particular, the issues of polymorphism and stabilization of the desired low‐bandgap crystal phase of FAPbI3 are considered. FAPbI3 exhibits polymorphisms with a photovoltaically unfavorable δ‐phase that is stable at room temperature, and, thus, it is difficult to prepare continuous and compact FAPbI3 with the desired crystal structure, namely, the pure α‐phase. Hence, overcoming the limitations of phase transitions is the critical issue in obtaining high‐quality FA‐based perovskite films, which are a prerequisite for solar cells with high PCEs. Here, the focus is on the fabrication methods of FA‐based perovskite films, namely, additive engineering, intermolecular exchange, interfacial engineering, and chemical vapor deposition. A comprehensive overview of the fabrication methodology for the FA‐based perovskite films is provided to facilitate understanding of the underlying mechanisms.

Hybrid organic–inorganic perovskites have attracted great attention as the next generation materials for photovoltaic and light-emitting devices. However, their environment instability issue remains as the largest challenge for practical applications. Recently emerging two-dimensional (2D) perovskites with Ruddlesden–Popper structures are found to greatly improve the stability and aging problems. Furthermore, strong confinement of excitons in these natural quantum-well structures results in the distinct and narrow light emission in the visible spectral range, enabling the development of spectrally tunable light sources. Besides the strong quasi-monochromatic emission, some 2D perovskites composed of the specific organic cations and inorganic layer structures emit a pronounced broadband emission. Herein, we report the light-emitting properties and the degradation of low-dimensional perovskites consisting of the three shortest alkylammonium spacers, mono-ethylammonium (EA), n-propylammonium (PA), and n-butylammonium (BA). While (BA)2PbI4 is known to form well-oriented 2D thin films consisting of layers of corner-sharing PbI6 octahedra separated by a bilayer of BA cations, EA with shorter alkyl chains tends to form other types of lower-dimensional structures. Nevertheless, optical absorption edges of as-prepared fresh EAPbI3, (PA)2PbI4, and (BA)2PbI4 are obviously blue-shifted to 2.4–2.5 eV compared to their 3D counterpart, methylammonium lead iodide (MAPbI3) perovskite, and they all emit narrow excitonic photoluminescence. Furthermore, by carefully optimizing deposition conditions, we have achieved a predominantly 2D structure for (PA)2PbI4. However, unlike (BA)2PbI4, upon exposure to ambient environment, (PA)2PbI4 readily transforms to a different crystal structure, exhibiting a prominently broadband light from ∼500 to 800 nm and a gradual increase in intensity as structural transformation proceeds.

The use of mixed spacer cations in quasi-2D Dion–Jacobson perovskites results in changes in film phase composition and efficient funneling for optimal composition. Optimal composition devices achieve a maximum EQE of 12.85% with TPPO passivation.

Functionalization with same functional group results in varied lead adsorption performance for different nanostructured carbon materials.

While the constraints on the choice of organic cations are greatly relaxed for layered two-dimensional perovskites compared to three-dimensional perovskites, the shape of the spacer cation is still subject to limitations due to the size of the inorganic pocket between four adjacent corner-sharing octahedra. To investigate the effect of the spacer cation branching on the formation of Ruddlesden−Popper (RP) structures, we performed a comprehensive investigation of structures formed using tert-butyl ammonium (t-BA). We demonstrate that in contrast to pure bromides and pure iodides, the use of mixed halides enables the formation of the t-BA 2 PbBr 2 I 2 RP perovskite structure with the specific ordering of the bromide and iodide anions. The t-BA spacer, despite its branched and bulky shape that prevents its deeper penetration, is able to form significant H-bonds that lead to the stabilization of the RP assembly if the inorganic pocket is designed in such a way that the bromide anions occupy terminal axial positions, while the iodides occupy equatorial positions. We obtain excellent agreement between experimentally determined and theoretically predicted structures using global optimization via a minima hopping algorithm for layered perovskites, illustrating the ability to predict the structure of RP perovskites and to manipulate the perovskite structure by the rational design of the inorganic pocket.

Perovskite solar cells represent one of the most promising emerging photovoltaic technologies due to their high power conversion efficiency. However, despite of the huge progress made not only in terms of the efficiency achieved, but also fundamental understanding of relevant physics of the devices and issues which affect their efficiency and stability, there are still unresolved problems and obstacles on the path towards commercialization of this promising technology. In this roadmap, we aim to provide a concise and up to date summary of outstanding issues and challenges, and progress made towards addressing these issues. While the format of this article is not meant to be a comprehensive review of the topic, it provides a collection of the viewpoints of the experts in the field which covers a broad range of topics related to perovskite solar cell commercialization, including those relevant for manufacturing (scaling up, different types of devices), operation and stability (various factors), and environmental issues (in particular the use of lead). We hope that the article will provide a useful resource for researchers in the field and that it will facilitate discussions and moving forward towards addressing the outstanding challenges in this fast developing field.