Owing to the superior performance in spectral efficiency, connectivity, and flexibility, nonorthogonal multiple access (NOMA) is recognized as the promising access protocol and is now undergoing the standardization process in 5G. Specifically, dozens of NOMA schemes have been proposed and discussed as the candidate multiple access technologies for the future radio access networks. This paper aims to make a comprehensive overview about the promising NOMA schemes. First of all, we analyze the state-of-the-art NOMA schemes by comparing the operations applied at the transmitter. Typical multiuser detection algorithms corresponding to these NOMA schemes are then introduced. Next, we focus on grant-free NOMA, which incorporates the NOMA techniques with uplink uncoordinated access and is expected to address the massive connectivity requirement of 5G. We present the motivation of applying grant-free NOMA, as well as the typical grant-free NOMA schemes and the detection techniques. In addition, this paper discusses the implementation issues of NOMA for practical deployment. Finally, we envision the future research challenges deduced from the recently proposed NOMA technologies.

Fabricating perovskite films with a dominant crystal orientation is an effective path to realizing quasi‐single‐crystal perovskite film, which can eliminate the fluctuation of the electrical properties in films arising from grain‐to‐grain variations, and improve the performance of perovskite solar cells (PSCs). Perovskite (FAPbI3) films based on one‐step antisolvent methods usually suffer from chaotic orientations due to the inevitable intermediate phase conversion from intermediates of PbI2•DMSO, FA2Pb3I8•4DMSO, and δ‐FAPbI3 to α‐FAPbI3. Here, a high‐quality perovskite film with (111) preferred orientation ((111)‐α‐FAPbI3) using a short‐chain isomeric alcohol antisolvent, isopropanol (IPA) or isobutanol (IBA), is reported. The interaction between IPA and PbI2 leads to a corner‐sharing structure instead of an edge‐sharing PbI2 octahedron, sidestepping the formation of these intermediates. With the volatilization of IPA, FA+ can replace IPA in situ to form α‐FAPbI3 along the (111) direction. Compared to randomly orientated perovskites, the dominantly (111) orientated perovskite ((111)‐perovskite) exhibits improved carrier mobility, uniform surface potential, suppressed film defects and enhanced photostability. PSCs based on the (111)‐perovskite films show 22% power conversion efficiency and excellent stability, which remains unchanged after 600 h continuous working at maximum power point, and 95% after 2000 h of storage in atmosphere environment.

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