空气下光诱导有机金属卤化钙钛矿薄膜的降解机理

Photoinduced Degradation of Lead Halide Perovskite Thin Films in Air

近几年来钙钛矿材料作为新兴光伏材料取得了巨大的发展进步,但有机无机杂化钙钛矿较差的环境稳定性限制了它的大规模应用。因此深入研究钙钛矿材料的降解机制有助于开发更稳定的钙钛矿光伏器件。本文基于透射电子显微学的微观形貌观察、晶体结构及元素成分表征,详细研究了杂化钙钛矿CH3NH3PbI3薄膜在光照以及空气共同作用下的降解机理。研究发现,光诱导下CH3NH3PbI3薄膜会与空气中的氧气发生交互作用,同时生成六方晶态PbI2甚至氧化为非晶态化合物PbI2-2xOx (0.4 <x <0.6),而其衰减位点主要存在于薄膜与空气接触的表面。降解过程中,由于存在着挥发性分解产物(I2,CH3NH2)的大量丢失,薄膜的表面会产生许多小孔洞,继而形成一种蜂窝状的介孔衰竭通道。而这种衰竭方式主要与光照下钙钛矿中光生电子与氧气结合形成超氧根自由基(O2·-)有关,该基团诱导了CH3NH3PbI3向PbI2和非晶氧化态的转变。本文揭示了空气中光照诱导钙钛矿薄膜的降解机理,这将为未来设计和优化更稳定的钙钛矿太阳能电池提供全面的实验数据与理论支持。

As an excellent photoelectric material,metal halide perovskites have been rapidly developed in the photovoltaic field.The power conversion efficiency of solar cells based on perovskite materials now exceeds 24%,which is close to the conversion efficiency of silicon-based solar cells.However,organic-inorganic hybrid perovskite materials are sensitive to light,oxygen,and moisture,particularly when combined in the ambient environment,limiting their commercial application in perovskite devices due to their poor environmental stability.Therefore,a comprehensive understanding of the degradation mechanism is the key for development of an effective method to inhibit the degradation of perovskite materials.Herein,the photo-induced degradation process of CH3 NH3 PbI3 films in air was studied by conventional optical and structural characterization methods,including ultraviolet-visible(UV-Vis) absorption spectroscopy,X-ray diffraction(XRD) and advanced transmission electron microscopy(TEM) equipped with a probe spherical aberration corrector.The CH3 NH3 PbI3 films were first decomposed into hexagonal PbI2 and amorphous phase,and subsequently oxidized to the amorphous phase under the combined effects of light and oxygen.The molecular formula of the amorphous phase was further confirmed as Pbl2-2 xOx(0.4 <x <0.6) via X-ray energy dispersive spectroscopy(EDS) and electron energy loss spectroscopy(EELS).Further analysis showed that the film degradation is mainly related to superoxide(O2·-) formed by combination of oxygen molecules and photoelectrons in the perovskite film.The organic part of the CH3 NH3 PbI3 is oxidized by O2·- and CH3 NH3 PbI3 is decomposed to form volatile products,such as CH3 NH2 and I2, then degraded into PbI2,and oxidized to form the amorphous PbI2-2 xOx.Therefore,during the initial degradation of film under light soaking in air,the degradation sites are mainly located at the interface between CH3 NH3 PbI3 and air.Many pores were observed on the film surface due to the large loss of volatile decomposition products during the initial degradation.The films then converted to a honeycomb hollow morphology due to the continuous consumption of material under light soaking,reducing the mass of the film as well.Finally,the entire film was oxidized to form an amorphous structure.Herein,for the first time,we report that the formation of amorphous oxides is accompanied by the degradation of perovskite film.This study presents a new understanding of the photo-induced degradation mechanism of perovskite films in air and provides novel theoretical guidance to promote the long-term stability of perovskite solar cells.