Study On The Degradation Mechanism Of Organic-inorganic Hybrid Perovskite Films And Solar Cells Under Light Illumination

In recent years,organic-inorganic hybrid perovskites have become one of the most popular optoelectronic materials,which is expected to be with great application prospects due to their excellent photoelectric properties and simple preparation processes.And now,the photoelectric conversion efficiency of thin-film solar cells based on perovskite materials has exceeded 24%.But unfortunately,the instability of the device,especially the unavoidable light-induced instability,limits its further development and commercial application.At present,there are few studies foucing on the degradation mechanism of perovskite films and solar cells,especially with the observation at micro-nano or even atomic scales.In this paper,the morphology,crystal microstructure and elemental composition of perovskite films and devices before and after light illumination were intensively studied by using advanced Cs-corrected transmission electron microscopy?TEM?.The experimental results are concluded as follows:?1?The degradation process of organic-inorganic hybrid perovskite CH₃NH₃PbI₃?MAPbI₃?film under light illumination was studied in detail by TEM.The results showed that light illumination would induce the migration of Pb and I element in the film,while the grain boundary was the initial degradation site where the ion migration would induce the formation of lead-rich nanoclusters.Furthermore,the aggregation of Pb element was segregated as a uniform core-shell nanostructure with lead-metal core and amorphous-crystalline coexisted lead?II?iodide?PbI₂?shell?Pb@PbI₂?.Then the interface between the MAPbI₃ and Pb@PbI₂ clusters would act as the degradation sites to keep the degradation of film,which would leave degradation channels behind the moving nanoparticles.?2?We further studied the degradation process of MAPbI₃ film under combined effect of air and light in detail by TEM.It was found that MAPbI₃ film was firstly decomposed into hexagonal PbI₂ and amorphous phase PbI_2-2xO_x?0.4<x<0.6?,and continued to be completely oxidized into amorphous phase.The degradation sites were mainly triggered at the interface between MAPbI₃ and air.During the degradation process,many small holes appeared on the surface of the film,and they eventually formed a honeycomb mesoporous degradation channel.Further studies have found that degradation process was mainly related to the formation of superoxide radical?O₂^-???3?By JV curve test of perovskite solar cells?PSCs?and AC-TEM to characterize the morphology and elemental composition of the cross-section samples of the PSCs,the PSCs with MAPbI₃ as the light absorbing layer were studied in details.The results showed that the light woud induce the migration of Au element of electrode and I element of perovskite.The migration of these elements caused the interface between the hole transport layer as well as the perovskite layer,the interface between the electron transport layer and the perovskite layer to be destroyed,then the transport and separation of carriers were hindered,resulting in an increase in recombination rate and decresase of short-circuit current density and conversion efficiency.Further analysis suggested that the migration of Au may be due to the chemical reaction of Au with the degradation product of perovskite to form an ionic complex,so it can migrate under the driving of photogenerated electric field.?4?The degradation process of perovskite solar cells under ultraviolet irradiation was further studied by replacing the light source with ultraviolet light source.It was found that the phenomenon of ultraviolet radiation and white light irradiation was similar.The migration of Au and I elements appeared,but the damage of interface between the hole transport layer and the perovskite layer and the interface between the electron transport layer and the perovskite layer were more serious.And further analysis,indicated that the increase of Au migration was due to the fact that ultraviolet radiation exacerbated the degradation at the grain boundary of the perovskite layer,which broadened the grain boundary and provided a migration channel for I^-,Au-containing ionic complexes to accelerate the migration of elements.All above results have further provided the new understanding of the light-induced degradation mechanism of organic-inorganic hybrid perovskite films and solar cells which will help to develop more stable perovskite solar cells in the future,and all these results would be of great significance for the commercialization of perovskite solar cells.