Organic-inorganic halide perovskites are promising photovoltaic materials with excellent optoelectronics properties. Their extreme structural instability under electron beam however makes atomic-resolution electron microscopy characterization challenging. To address this problem, dose efficiency, image interpretation, and electron dose control are vital factors.

The does efficiency of phase contrast methods is advantageous, but conventional high-resolution transmission electron microscopy (HRTEM) is often hard to interpret. Ptychography provides an even more dose efficient and more easily interpretable method to reconstruct the phase of both heavy atoms and light atoms. Figure 1 shows high resolution image simulations for the organic-inorganic halide perovskite MAPbI3. Even in noise free conditions the organic methylammonium ions, composed of light carbon and nitrogen atoms, are essentially invisible in the high angle annular dark field signal (HAADF), whereas the single side band (SSB) ptychography reveals the molecules even at the very low 50 e-/Å2 dose required to image this material. HRTEM requires defocus to produce contrast and as the figure shows, the contrast changes significantly with the defocus chosen, which can often be misleading. However, in our example, the contrast is always lower than the ptychography.

4D STEM imaging with a focused probe enables simultaneous Z-contrast imaging with ADF, but requires a high time resolution camera to avoid the drift and high doses caused by slow scans at high resolution. By using an event driven camera, we can combine fast microsecond dwell time scans and low electron currents to achieve low doses. Figure 2 demonstrates experimental simultaneous sub-100 e-/Å2 HAADF and SSB ptychography imaging with MAPbI3. This combination of Z-contrast and dose efficient phase imaging with ptychography enables us to investigate more challenging structures such as defective regions and interface structures. We provide more insights into the structure of beam-sensitive organic-inorganic halide perovskites.