Low-dimensional Perovskites: a Novel Candidate Light-harvesting Material for Solar Cells that Combines High Efficiency and Stability

Since the emergence of organic-inorganic hybrid perovskite materials as light harvesters, the perovskite solar cells have attained a considerable efficiency improvement due to notable achievements in optimizing the fabrication process and device structure, while nevertheless been suffering increasingly serious challenges, especially instability. Layered (low-dimensional) perovskite materials are constructed based on a periodical (or quasi-periodical, or hybridized) structure which is composed of alternate layers of organic amines and metal halide perovskite crystals. The layered structure approximates to a two-dimensional structure whilst the number (n) of planes, which consist of the pyramids' squares of the perovskite octahedrons, within one separated perovskite layer approaches 1. For photovoltaic application, these low-dimensional perovskite structures have two advantages compared to their three-dimensional counterparts: I. remarkably enhanced moisture resistance and thermal stability; II. tunable optical and electrical characteristics by varying n and selecting different organic amines. On the other hand, poor carrier mobility (a consequence of the inhibition of out-of-plane charge transport by the organic amine cations) and wide band gap contribute to a far lower efficiency of low-dimensional perovskite solar cell than three-dimensional perovskite device. This urges intensive research endeavors to seek favorable organic amines and optimize perovskite film fabrication process, aiming at boosting photovoltaic efficiency while exploiting layered perovskite's stability. And in the past three years, impressive strides have been made in promoting the low-dimensional perovskite solar cells, with a giant leap in the reported power conversion efficiency (PCE) from 4.37% to 13.7%. Phenethylamine (PEA), n-butylamine (n-BA), isobutylamine (iso-BA), polyethylenimine (PEI), etc. have been found to be satisfactory as the hydrophobic amine interlayers for relatively-high-efficiency layered perovskite solar cells. PEA is the first to be involved in the attempts, and the n-BA-intercalated perovskite hold the currently highest efficiency of this new type of photovoltaic devices. PEI intercalation appears to result in narrower band gap and higher moisture resistance, but also leads to a larger inhibition to the carrier transport. Although one-step spin coating provides a facile route to obtain layered perovskite films, this method will cause the horizontal growth (i.e. interlayers parallel to substrate) of the layered structure and in consequence, an extremely low cell efficiency. Works in the past two years have established a new avenue to overcome the low-efficiency bottleneck, by introducing various techniques into the spin coating process, e.g. hot casting, immersion (in short-chain amines), antisolvent dripping, all of which have successfully achieved the preferential out-of-plane alignment of the inorganic perovskite layers. Besides, researchers also have demonstrated that the 2D-3D hybrid perovskite structures, which can be constructed on the basis of 3D perovskite with the presence of organic amine additives, can gain improvements in terms of both efficiency and stability. This review offers a retrospection of the research efforts with respect to the layered (low-dimensional) perovskite photovoltaic devices, and provides elaborate descriptions about the structure of low-dimensional perovskite, the selection of the intercalating organic amines, and the film fabrication process. We then pay attention to the problems confronting the current state-of-the-art low-dimensional perovskite solar cells. We have confidence that the low-dimensional perovskite solar cells have a bright future in the development and innovation of stable and environmental-friendly photovoltaic devices.