Different Degradation Mechanism of CH3NH3PbI3 Based Perovskite Solar Cells under Ultraviolet and Visible Light Illumination

Abstract

In recent years, the photoelectronic conversion efficiency (PCE) of organic-inorganic halide perovskite solar cell (PSC) devices has been improved greatly. However, these devices are not very stable, and it is hard to avoid the effect of visible or ultraviolet (UV) light on the performance decay of the organic-inorganic halide perovskite devices, and there is rare report on the evolution process of the microstructure of PSCs under the light illumination, let along discussing on the different degradation mechanism of PSCs between the UV and visible light soaking. To address these scientific issues, in this study, we compared the performance evolution of CH3NH3PbI3 (MAPbI3) based PSCs during the UV and visible light irradiation. The experimental results show that the perovskite layer has been photodegraded from MAPbI3into an amorphous phase under the white light LED soaking. Meanwhile, the migration of Au element from the electrode into the interface between MAPbI3 and SnO2 layers can also be captured. As comparing the kinetics of redox reaction of Au, we found that the formation rate of Au nanoparticle mass in PSCs under UV light irradiation is almost 30 times higher than that under visible light illumination. Considering on the different characteristics of microstructure evolution in PSCs under the UV and visible light irradiation, and theoretically analyzing the energy level of each functional layers in the device, the results confirm that UV light is easy to be adsorbed by electron transportation layer (ETL) of SnO2 to excite the electron-hole pairs, while the photo-excited holes have a low energy level of -8.4 eV, which could oxidize the iodide ions (I -) into atomic iodine (I atom). The I atoms would diffuse into the spiro-OMeTAD layer and metal electrode interface. Due to its strong oxidation property, the I atom would not only destroy the spiro-OMeTAD layer, but also oxidize the Au metal electrode into AuI, which accelerated the generation of Au+. However, under the illumination of visible light, it is hard to excite the electron-hole pairs in SnO2, which prevents the damage on the functional interfaces, and the transportation energy barrier is unchanged. So, the open circuit voltage (Voc) has a long-term photo-stability. However, the short-circuit current density (Jsc) decreased rapidly under visible light illumination, which is mostly ascribed to the changes of charge mobility resulting from the migration of Au element and photodecomposition of MAPbI3 layer. All these results give a new insight to understand the photo-instability of PSC.