FIB Sample Preparation and Low Dose STEM Characterisation Challenges of Hybrid Organic-inorganic Perovskite (HOIP) Solar Cells

Hybrid Organic-Inorganic Perovskites (HOIPs) are a class of photoactive semiconductor compounds which have garnered much attention in the last 2 decades as candidates for high conversion efficiency solar cells, and have shown a remarkable increase in conversion efficiencies up to 25.7% in 2021 with excellent stability parameters that approach industrial standards [1]. Further conversion efficiencies improvement will require in depth understanding of the Perovskite solar cell microstructure, interface composition along with atomic scale defects correlation to the photovoltaic properties which gave rise to high interest of electron microscopy community to HOIPs microstructure investigation over the last few years. Different research groups tried first to understand the electron beam sensitivity of HOIPs using a polycrystalline sample grown directly or single crystals samples drop casted on carbon grids. With electron diffraction and High-resolution TEM analysis, it has been shown that the perovskite structure can be damaged for a beam dose above 100 e/A2. Different degradation process has been proposed and all leads to a Perovskite conversion to PbI2 structure [2,3]. Few recent studies using full cell have been also investigating the double challenge of ion beam damage generated by the FIB lamella preparation and the electron beam damage for STEM/TEM observations. If some groups recommend using CryoFIB [4] to avoid HOIPs degradation to PbI2 generated by room temperature Ga FIB sample preparation, Others shows that the impact of room temperature Ga-FIB preparation can be minimised by optimising the preparation protocol and allow STEM atomic resolution defects investigation of full HOIPS solar cells [5]. In our case, using a CsFAMAPbI3 solar cell, we first optimised the FIB preparation process and focusing on the acceleration voltage effect on the final polishing. Two damage mechanisms are likely to dominate the effects of the ion-beam on the HOIPs - radiolysis and localized heating, followed by degradation of the organic cations [6]. Decreasing the final polishing voltage tend to increase the void and pores in the HOIP structure (Figure 1). Radiolysis effects tend to be amplified at lower accelerating voltages and thus the final low-voltage polishing steps commonly used in FIB preparation procedures appear to introduce a significant portion of the damage observed. The optimisation of the FIB lamella preparation, help us to achieve low dose atomic resolution STEM imaging and confirm the existence of high density of the PbI2 particles that affect the Grain domain morphology and cannot only by related to the sample preparation but also to the synthesis pathway regarding the presence of several species, competing crystalline phases may exist [7] and the nucleation pathway can be highly nontrivial (figure 2). Further investigation of these structure using Cryo-FIB is undergoing to minimise further the ion beam effect and confirm the high density of PbI2 particles formed under synthesis. STEM-HAADF images prepared at different final polishing 8 KV in (a), 5 KV in (b) and 2 KV in (c). Of note is the significantly smoother appearance of the grain domains when the milling and polishing is conducted at higher voltages. HR-STEM HAADF image of PbI2 array linking 2 grain domains and is responsible for the 3D-zigzag surface sample shape in (a) with atomic resolution STEM-HAADF images of PbI2 particles agglomeration in (b) and a mixture of perovskite structure (yellow square) and PbI2 (red square) in (c).