Study On The Moisture Degradation Mechanism Of Organic-inorganic Hybrid Perovskite Films And Perovskite Solar Cells

As an inexhaustible source of clean energy,solar energy has attracted intensive attentions of the researchers on developing high-efficiency solar cells,which benefit to convert solar energy into electricity.The photoelectric conversion efficiency（PCE）of hybrid perovskite solar cells（PSCs）has rapidly increased up to 25.2%in just a few years.However,the instability of organic-inorganic hybrid PSCs is usually a major factor to seriously impede its commercialization.In the traditional PSCs,both the metal electrode and the hole transport layer（HTL）are vulnerable to humidity corrosion as the presence of water molecules.Moreover,the intrinsic humidity instability of organic-inorganic hybrid halide perovskite layer is believed to further lead to the failure of PSCs.So far,the understanding of the evolution process of MAPbI₃ is still controversial.So,it urgently needs to give a comprehensive understanding of the moisture failure mechanism of PSCs in high humidity treatment.In this work,we comprehensively investigated the humidity degradation mechanism of organic-inorganic hybrid perovskite films and perovskite solar devices by using various surface and cross-sectional characterization techniques,including atomic force microscopy-infrared absorption spectrometry（AFM-IR）,focused ion beam-scanning electron microscopy（FIB-SEM）dual-beam system and advanced transmission electron microscopy（TEM）.The main results are as follows:（1）By controlling the heating temperature and annealing time of the perovskite films,the surface terminals of these films were modified and the two kinds of surface terminations were detected by AFM-IR.For the unannealed MAPbI₃ film,only the MAI terminations exist on the surface.However,after annealing at 100℃for 5 minutes,the volatile MAI molecules will escape from the surface to form the PbI terminated perovskite films.（2）By using XRD,FIB-SEM and TEM characterizations,we not only observed the moisture induced evolution of morphology,microstructure and element distribution of the perovskite films,but also explore the degradation mechanism of the MAI-terminated and PbI-terminated films.The results show that:for the MAI-terminated perovskite films,MAPbI₃ would transfer into hydrate MAPbI₃·H₂O firstly,and then into PbI₂.The penetrative water molecules initially adhere to the surface and grain boundary of polycrystalline perovskite film,and then form the hydrates layer by layer longitudinally through the diffusion between the MAPbI₃ lattice.For the PbI-terminated perovskite films,there was almost no hydrate during the whole degradation process,PbI₂ phase was the only detectable degradation product,which is consistent with the view of deprotonation process of MAPbI₃.Furthermore,the nucleation and growth of the PbI₂ grains are distributed not only at the grain boundaries but also in the inner grains.After PbI₂ nucleation and growth,the lamellar PbI₂ grains penetrating MAPbI₃films were formed.In addition,large number of voids formed near to the PbI₂ grains,which acted as the transport channels for water molecules and produced gases during the moisture degradation of PbI-terminated perovskite and accelerated the decomposition of MAPbI₃.（3）By using XRD,FIB-SEM,TEM and the I-V test system of perovskite solar cells,we analyzed the moisture induced performance changes of perovskite solar devices and observed the morphology and microstructure evolution of each layer in PSCs.The results show that water molecules will destroy the metal electrode of perovskite solar cell,meanwhile the porous structure of Spiro-OMe TAD will accelerate the permeation of water molecules into the perovskite layer.The degradation mechanism of the light-absorption layer（MAPbI₃）in the perovskite solar cell is the same as that of the PbI-terminated perovskite film,that is,water causes the deprotonation of MAPbI₃.As a conclusion,we explored the humidity degradation mechanism of organic-inorganic hybrid perovskite films and perovskite solar devices with the nano-scale resolution.The results confirmed that the surface terminations play a key role in dominating the degradation pathway of MAPbI₃ films.While,each layer of perovskite solar cells affected the failure mechanism of the devices.All of these discoveries will enrich the current understanding of the humidity degradation mechanism of perovskite materials and provide an important reference for the design of high stability perovskite solar cells.