紫外光辐照下CH3NH3PbI3基钙钛矿太阳能电池失效机制

随着光伏产业的不断发展，有机无机杂化钙钛矿太阳能电池的研发成为科学与工业界广泛关注的焦点。到目前为止，其光电转换效率已经提高到了25.2%，成为替代硅基太阳能电池的核心方案之一。然而，钙钛矿太阳能电池的稳定性较差，容易受到环境中氧气、水分、温度甚至光照的影响，这严重制约了其大规模推广与应用。大量科学研究表明，如何避免紫外辐照下有机无机杂化钙钛矿太阳能电池的性能衰减，对于提高钙钛矿太阳能电池的光照稳定性至关重要。然而到目前为止，仍然没有系统的工作来对紫外辐照下钙钛矿太阳能电池性能以及微结构演化过程进行详细的表征与分析。本文中，我们利用聚焦离子束-扫描电子显微分析(FIB-SEM)以及球差校正透射电子显微分析(TEM)等技术，全面地研究了紫外辐照过程中有机无机杂化钙钛矿太阳能电池性能变化规律以及电池微结构演化特征。实验结果表明，紫外辐照过程中太阳能电池内部会形成0.5–0.6 V的内建电场，钙钛矿中的I^-离子在电场的驱动下向金属Au电极和空穴传输层2, 2’, 7, 7’-四N, N-二(4-甲氧基苯基)氨基]-9, 9'-螺二芴(Spiro-OMeTAD)一侧迁移；随后，空穴传输层与金电极的界面处，碘离子与光生空穴一起与金电极发生反应，将金属态Au氧化成离子态Au⁺。而Au⁺离子则在内建电场的驱动下反向迁移穿过钙钛矿MAPbI₃层，直接被SnO₂和MAPbI₃界面处的电子还原形成金属Au纳米团簇。除此之外，紫外辐照过程中钙钛矿太阳能电池性能降低的同时，往往伴随着Spiro-OMeTAD与钙钛矿界面处物质迁移、钙钛矿薄膜内晶界展宽以及Au纳米颗粒周围MAPbI₃物相分解等现象。以上各种因素的协同作用，共同导致了紫外光照下有机无机杂化钙钛矿太阳能电池光电转换性能(PCE)、开路电压(V_oc)以及短路电流(J_sc)等性能参数的急剧下降。

Degradation Mechanism of CH3NH3PbI3-based Perovskite Solar Cells under Ultraviolet Illumination

With the development of photovoltaic devices, organic-inorganic hybrid perovskite solar cells (PSCs) have been promising devices that have attracted significant attention in the fields of industrial and scientific research. Currently, the photoelectric conversion efficiency (PCE) of PSCs has been improved to 25.2%, and they are considered to be the primary alternative to silicon-based solar cells. However, the environmental stability of PSCs is unsatisfactory; they are prone to degradation under exposure to moisture, oxygen, elevated temperature, or even light illumination, which restricts their wide application in industrial production. Previous studies have elucidated that understanding the ultraviolet (UV)-induced degradation mechanism of organic-inorganic PSCs is of great importance for the improvement of light stability in PSCs. However, until now, there has been almost no comprehensive investigation on the decay process of PSCs under UV light illumination nor on the corresponding evolution of their microstructure. In this study, focused ion beam scanning electron microscopy (FIB-SEM) and aberration-corrected transmission electron microscopy (TEM) were used to comprehensively study changes in the performance and the evolution of the microstructure of PSC devices. The experimental results show that a built-in electric field developed under UV light illumination, which drove the diffusion of iodide ions (I^-) from the CH₃NH₃PbI₃ (MAPbI₃) layer to the hole transfer layer (HTL, Spiro-OMeTAD). Together with the photo-excited holes in the HTL, the I^- ions reacted with the Au electrode, and the Au atoms were oxidized into Au⁺ ions. Furthermore, Au⁺ ions preferred to diffuse across the HTL and the perovskite layer into the interface between the SnO₂ and MAPbI₃ layers. SnO₂ is known to be a good electron transfer layer (ETL), which should collect the photo-excited electrons to reduce the Au⁺ ions into metallic Au clusters; this is why the Au electrode was destroyed and Au clusters aggregated at the SnO₂-MAPbI₃ interface under the UV light illumination. Meanwhile, the Au clusters would accelerate the degradation of the perovskite. In addition, as the PSC performance declined (as determined by the PCE, open-circuit voltage (V_oc), and short-circuit current (J_sc)), the decomposition of tetragonal MAPbI₃ into hexagonal PbI₂ was observed at the interface between Spiro-OMeTAD and MAPbI₃, along with a widening of the grain boundaries in the perovskite layer. All of these factors play critical roles in the UV-induced degradation of PSCs. This is the first study to elucidate the light-induced migration of Au from the metal electrode to the interface between SnO₂/MAPbI₃, which reveals that the UV-induced degradation of PSCs may be mitigated by finding new ways to restrain the interdiffusion of Au⁺ and I^- ions.