The atomic-scale microstructure of metal halide perovskites elucidated via low-dose scanning transmission electron microscopy

Session

Stream 1: EMAG - Energy and Energy Storage Materials

Authors

Dr Mathias Uller Rothmann (2, 1), Dr Judy Kim (1, 3, 4), Dr Juliane Borchert (2), Kilian B. Lohmann (2), Dr Colum M. O'Leary (1), Alex A. Sheader (1), Dr Laura Clark (1), Professor Henry J. Snaith (2), Professor Michael B. Johnston (2), Professor Peter D. Nellist (1), Professor Laura M. Herz (2)

Affiliations

1. University of Oxford, Department of Materials
2. University of Oxford, Department of Physics
3. ePSIC, Diamond Light Source
4. Rosalind Franklin Institute

Keywords

Perovskite solar cells, atomic-resolution, STEM, low-dose

Abstract text

Understanding the atomic-scale crystallographic properties of photovoltaic semiconductor materials such as silicon, GaAs, and CdTe has been essential in their development from interesting materials to large-scale energy conversion industries. However, studying photoactive hybrid perovskites by transmission electron microscopy (TEM) has proved particularly challenging due to the large electron energies typically employed in these studies.[1] In particular, the very close structural relationship between a number of crystallographic orientations of the pristine perovskite and lead iodide has resulted in severe ambiguity in the interpretation of EM-derived information, severely impeding the advance of atomic resolution understanding of the materials.

Here, we successfully image the archetypal CH(NH₂)₂PbI₃ (FAPbI₃) and CH₃NH₃PbI₃ (MAPbI₃­) hybrid perovskites in their thin-film form with atomic resolution using a carefully developed protocol of low-dose STEM.[2] Our images enable a wide range of previously undescribed phenomena to be observed, including a remarkably highly ordered atomic arrangement of sharp grain boundaries and coherent perovskite/PbI₂ interfaces, with a striking absence of long-range disorder in the crystal. These findings explain why inter-grain interfaces are not necessarily detrimental to perovskite solar cell performance, in contrast to what is commonly observed for other polycrystalline semiconductors. Additionally, we observe aligned point defects and dislocations that we identify to be climb-dissociated, and confirm the room-temperature phase of CH(NH₂)₂PbI₃ to be cubic. We further demonstrate that degradation of the perovskite under electron irradiation leads to an initial loss of CH(NH₂)²⁺ ions, leaving behind a partially unoccupied, but structurally intact, perovskite lattice, explaining the unusual regenerative properties of partly degraded perovskite films. Our findings thus provide a significant shift in our atomic-level understanding of this technologically important class of lead-halide perovskites.

References

[1] Adv. Mater. 2018, 30, 1800629

[2] Science 370, eabb5940 (2020)