Abstract
Among the hybrid organic–inorganic perovskites MAPbX3 (MA: methyl-ammonium CH3–NH3+, X = halogen), the triiodide specimen (MAPbI3) is still the material of choice for solar energy applications. Although it is able to absorb light above its 1.6 eV bandgap, its poor stability in humid air atmosphere has been a major drawback for its use in solar cells. However, we discovered that this perovskite can be prepared by ball milling in a straightforward way, yielding specimens with a superior stability. This fact allowed us to take atomic-resolution STEM images for the first time, with sufficient quality to unveil microscopic aspects of this material. We demonstrated full Iodine content, which might be related to the enhanced stability, in a more compact PbI6 framework with reduced unit-cell volume. A structural investigation from neutron powder diffraction (NPD) data of an undeuterated specimen was essential to determine the configuration of the organic MA unit in the 100–298 K temperature range. A phase transition is identified, from the tetragonal structure observed at RT (space group I4/mcm) to an orthorhombic (space group Pnma) phase where the methyl-ammonium organic units are fully localized. Our NPD data reveal that the MA changes are gradual and start before reaching the phase transition. Optoelectronic measurements yield a photocurrent peak at an illumination wavelength of 820 nm, which is redshifted by 30 nm with respect to previously reported measurements on MAPbI3 perovskites synthesized by crystallization from organic solvents.
Highlights
Among the hybrid organic–inorganic perovskites M APbX3 (MA: methyl-ammonium CH3–NH3+, X = halogen), the triiodide specimen ( MAPbI3) is still the material of choice for solar energy applications
We and other authors[28,29] found that MAPbI3 specimens prepared by ball milling exhibit a superior stability, showing no signs of degradation after several months exposed to humid air[28,29]
It is interesting to consider the profile of I versus N, indicating that I concentration increases much faster when shifting into the inner regions of the crystal: I composition reaches saturation beyond 20 nm whereas N is still increasing up to 60 nm or more. This suggests that the iodide sublattice is fully stoichiometric in the bulk material, with negligible number of vacancies beyond the surface, whereas, for the organic methylammonium molecule, the existence of some bulk sub-stoichiometry is conceivable. It seems that the robustness of the crystal strongly relies on the PbI6 framework, and this is solidly built by mechano-chemical synthesis
Summary
The crystal structure of M APbI3 has been studied several times and reported in the tetragonal symmetry at ambient temperature. This work explores the reasons for the increased stability found in the ball-milled specimens, based on a detailed investigation of the crystal structure by neutron powder diffraction (NPD) techniques. The crystal structure of MAPbI3 was refined in the tetragonal centrosymmetric I4/mcm space group, from the neutron powder diffraction patterns collected at RT and 180 K, as previously reported[34,35,39,40].
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