Elemental mapping of perovskite solar cells using STEM and multivariate analysis

Over the last few years, the interest in perovskite based solar cells has boomed, due to a surprisingly fast increase in terms of their efficiency that has now reached values comparable with established photovoltaic technologies. Nevertheless, the understanding of the optoelectronic properties of such nanostructured materials is still an open problem and issues related to their stability and degradation pathways represent the current hot topic in this research area.\r\n\r\nOrganic-inorganic solar cells present a complex composition as well as a composite structure that are strongly related to device fabrication. In this work four processing methods of the organic-inorganic halide perovskite have been investigated, varying the deposition method (single step or double step) and the atmospheres in which the synthesis has been carried out1. We compared interface quality, morphology, chemical composition and efficiency of the resulting cells. A fluorine doped tin oxide (FTO) glass layer was coated first by a compact (hole blocking) TiO2 layer and then by a nanoporous TiO2 layer. The TiO2 scaffold was infiltrated and capped by a methyl-ammonium lead iodide. Spiro-MeOTAD, acting as hole transport layer was spin coated on the perovskite layer; Au contacts were deposited on top. The devices were analysed using several complementary characterisation techniques: Scanning Transmission Electron Miscroscopy (STEM) used in conjunction with EDX analysis, time of flight secondary ion mass spectrometry (ToF-SIMS) and X-rays photoelectron spectroscopy (XPS). In particular, the use of FIB specimen preparation, combined with analytical transmission electron microscopy, represents a powerful and versatile tool for the characterization of devices based on hybrid composites with nano- and micro-scale structural and chemical features. EDX maps were then treated using multivariate analysis in order to optimise signal-to-noise ratio and obtain high quality EDX maps. The application of this method plays a key role in the analysis of data acquired with low electron doses to minimise specimen damage, and is unique in allowing the identification of different materials present in the sample as compounds rather than individual elements.\r\n\r\nHaving fully characterised the devices, we investigated the different degradation processes that affect the perovskite based solar cell. Air exposure and temperature were proven to be responsible for the drastic reduction in device performance. STEM-EDX analysis was thus used in conjunction with the in situ heating, bringing the cells to 250 °C2. The main result was the direct observation of elemental migration, particularly evident in iodine maps, and the simultaneous formation of metallic Pb precipitates, resulting in the depletion of the initial perovskite region.\r\n\r\nLastly, we studied the changes in chemical composition and morphology after 2 months of air exposure in dark3. In this case the principal variations in local elemental composition and in morphology 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 and the physical formation of bubbles in the Spiro-OMeTAD.