Molecular Insights into Early Nuclei and Interfacial Mismatch during Vapor Deposition of Hybrid Perovskites on Titanium Dioxide Substrate
JF Wang and LL Zhao and MC Wang and SC Lin, CRYSTAL GROWTH & DESIGN, 17, 6201-6211 (2017).
Theoretical understanding of the nucleus structures of hybrid perovskites, such as those of the prototypical methylammonium lead triiodide (MAPbI(3)), can greatly improve the deposited thin film quality and the resulting optoelectronic device performance. In this paper, we report a systematic molecular dynamics simulation study on nucleation and interfacial mismatch during the vapor deposition of MAPbI(3) on the TiO2 substrate under different ionic precursor (PbI2 and MAI salts) compositions and temperatures. Despite significant anisotropic lattice mismatches, small defects are observed at the TiO2/MAI interface due to intermediate electrostatic attractions between I and Ti atoms, while very strong electrostatic attractions between Pb and O atoms lead to significant defects at the TiO2/PbI2(0) interface. From the vapor deposition simulations, we identify PbI42- tetrahedra, PlA(5)(3-) pyramids, and PbI64- octahedra as dominant polyhedral building blocks of early MAPbI(3) nuclei. Specifically, the PbI53- pyramids dominate over other polyhedra and could be a good candidate for converting into PbI64- octahedra upon further crystallization. We further identify early MAPbI(3) nuclei built upon well-connected PbL polyhedral clusters and finally locate the efficient early MAPbI(3) nuclei based on sufficient amounts of surrounding MA+ cations. The populations of these early nuclei increase rapidly with increasing the MAI composition, suggesting that potential improvements in film quality could be introduced by depositing more MAI salts or MA+ cations, a finding consistent with experiments. Although the impact from temperature is weaker than that from composition, the optimal temperature for nucleation is found to decrease with increasing the precursor composition PbI2/MAI. Finally, the TiO2 substrate leads to layered structures of ionic species close to its surface, but such ordering does not seem to promote prenucleation, which poses a need for the new design of substrates that are more compatible with PbI64--based early nuclei.
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