Proceedings of 13th Conference on Hybrid and Organic Photovoltaics (HOPV21)
Publication date: 11th May 2021
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.
We thank the David Cockayne Centre for Electron Microscopy,
University of Oxford, for access and support in the use of the JEOL
ARM200F instrument (proposal number EP/K040375/1) and
additional instrument provision from the Henry Royce Institute
(grant reference EP/R010145/1). We also thank Diamond Light
Source for access and support in use of the electron Physical
Science Imaging Centre (E02, MG21734) that contributed to the
results presented here. L.M.H. and M.B.J. thank the Humboldt
Foundation for research awards. Funding: Supported by the UK
Engineering and Physical Sciences Research Council (EPSRC)
through grant EP/P033229/1 and through the EPSRC CDT for New
and Sustainable Photovoltaics.
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