Over the last 24 months we have witnessed a spike in interest in the study of perovskite-based solar cells, due to a surprisingly fast rise in their power conversion efficiency. Despite the excellent efficiency still little is known about their long term stability and degradation processes. In order to shed light on some of the mechanisms underlying the loss in performance of halide organic–inorganic perovskite based devices, we study the behaviour of an archetypical device before and after two months of air exposure in dark using a scanning transmission electron microscope (STEM) for imaging and compositional analysis with nanometer resolution.

Analytical transmission electron microscopy represents a powerful tool that allows probing nanoscale morphology and crystal structure, as well as local elemental composition. To further expand the EDX mapping capabilities of modern S/TEMs, we employ multivariate analysis, obtaining a remarkable improvement of the signal to noise ratio. This novel quantification procedure naturally optimally separates different compounds without introducing operator bias, and is particularly effective in the presence of complex compounds, such as those developed during solution-based perovskite synthesis in photovoltaic devices.

The solar cell under investigation was prepared according to a standard double step method processed in air: a fluorine doped tin oxide layer (FTO) on glass was coated first by a compact layer and then by a nanoporous TiO2 layer[1]. The TiO2 scaffold was infiltrated and capped by a methylammonium lead iodide layer. Spiro-OMeTAD was spin coated on the perovskite layer; Au contacts were deposited on top. FIB milling was used to extract cross sectional lamellae of the device, immediately after breaking the seal and after two months of air exposure. We studied the changes in chemical composition and morphology using the aforementioned STEM/EDX approach. The principal variations in local elemental composition concerned respectively the migration of lead and iodine into the HTL layer towards the Au electrode, resulting in a severe degradation of the photoactive layer. Additionally, HAADF (high-angle annular dark field) cross sectional images of the full device highlight the presence of gas bubbles within the hole transport layer. The observation of this phenomenon is particularly important since it might be considered as one of the main contributors for the loss in performance of the cell, promoting the entrance of moisture and air within the device and resulting in a reduced active area for carrier transport.

[1] F Matteocci et all. ACS applied materials&interfaces 7 (47), 26176-26183 (2015)