使用有机无机杂化钙钛矿材料作为光吸收层的钙钛矿太阳能电池自进入人们的视野以来,其制备工艺和器件结构不断得到优化,短短几年内效率取得了非常可观的增长.与此同时,这种基于三维钙钛矿材料的电池的缺点也越来越突出,尤其是材料的不稳定性,严重阻碍了其发展.低维钙钛矿材料具有有机胺层与无机层(金属卤化物钙钛矿晶体)之间相互交替的低维(层状)结构,其中被有机胺隔开的独立钙钛矿层中八面体的层数 n越小,钙钛矿越接近二维结构.相比传统三维钙钛矿结构,低维钙钛矿材料应用于光伏器件具有两大优势:(1)耐湿性、光热稳定性大大增强;(2)可以通过改变 n和插入的有机胺的种类来实现光学及电学性质的可调性.然而,低维钙钛矿具有较大的光学带隙,有机胺的引入降低了载流子迁移率,导致低维钙钛矿电池的效率明显低于三维钙钛矿电池.因此,近三年来除研究钙钛矿层数对材料性质和器件性能的影响外,研究者们主要从选择合适的有机胺和优化薄膜制备工艺方面不断尝试,并取得了丰硕的成果,在充分发挥低维钙钛矿稳定性优势的同时大幅提升了器件效率.目前,低维钙钛矿太阳能电池的光电转换效率已由2014年的4.37% 跃升至13.7%.在较高效率的低维钙钛矿太阳能电池中已取得成功应用的有机胺类包括苯乙胺(PEA)、正丁胺(n-BA)、异丁胺(iso-BA)、聚乙烯亚胺(PEI)等.其中PEA应用得最早;n-BA是运用在目前为止最高效的低维钙钛矿电池中的有机胺;而PEI插层形成的低维钙钛矿拥有相对更小的光学带隙和更高的耐湿性,但载流子的传输会受到更大的限制.低维钙钛矿薄膜的制备起初主要采用简单的一步旋涂法,但此法所得的低维钙钛矿平行于基底生长,器件效率很低.近两年的研究工作将基底预热、浸泡、反溶剂滴加等手段引入到钙钛矿旋涂工艺中,实现了低维钙钛矿优先垂直基底生长,为突破低效率瓶颈提供了可能.此外,以三维钙钛矿为基础,以有机胺为添加剂,制得的二维和三维混合的钙钛矿结构,也可以实现器件效率和稳定性的双提升.本文归纳了低维钙钛矿光伏器件的研究进展,分别对低维钙钛矿的分子结构、插入的有机胺的选择、钙钛矿薄膜的制备方法等进行介绍,分析了低维钙钛矿太阳能电池面临的问题并展望其前景,以期为制备稳定和环境友好的新型钙钛矿太阳能电池提供参考.%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,w hile 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.
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