This thesis discusses the understanding of the structural and electronic properties
of metal halide perovskite materials (e.g., CH3NH3PbI3) as well as the interaction with
additive materials at the atomic scale. Metal halide perovskite materials are currently at
the core of attention because of their high-efficiency attainable when utilized in solar cell
applications as well as lost fabrication cost. However, the stability of these perovskite
materials and perovskite-based solar cells is still a major challenge. The proposed
atomic-scale research helps address the thermal instability and ambient air induced
degradation issues commonly observed in perovskites. Incorporation of additive
materials into the perovskite was reported to be an efficient strategy to enhance the
stability. Cl and KI have been found to be eligible candidates but studies on the
fundamental processes at the atomic scale of these additives are scarce. Therefore, we
comprehensively investigated Cl incorporation in MAPbI3 at the atomic scale by
combining scanning tunneling microscopy (STM), X-ray photoelectron, ultraviolet, and
inverse photoemission spectroscopy. For the Cl concentration of 14.8 ± 0.6%, STM
images confirm the presence of Cl ions on the MAPbI3 (MAPbI2.59Cl0.21) surface leading
to the highest surface stability found from the viewpoint of both thermodynamics and
kinetics by density functional theory and molecular dynamics calculations. This study
evidence that Cl can substitute I ions of the surface structure and/or fill the surface I-
vacancies and further enhance the structural stability of MAPbI3. Upon the PbCl2
deposition on MAPbI3, only Cl ions were observed in MAPbI3 surface crystal structure
while Pb ions were not observed by LT-STM. Before investigating the PbCl2 interaction
with MAPbI3, the atomic structures of metallic-Pb and PbCl2 on Au (111) substrate need
be studied by LT-STM. For the PbCl2/Au (111) system, a myriad of structures such as
the intact form of PbCl2 as well as dissociated species in the forms of Cl ions, Cl-Cl
dimer, Pb, and Pb-Pb dimers were observed. Furthermore, the KI additive was
investigated by LT-STM, which reveals the I-I square atomic structure. After deposition
protocol optimization, KI was deposited on MAPbI3 and characterized by XPS
confirming K incorporation in MAPbI3. As a future plan, LT-STM studies can be
conducted to find the optimal concentration of KI for enhancing the MAPbI3 surface
stability. Also, it is important to point out that STM can be used to characterize mainly
the surface atomic structures of perovskite materials.
Exam Date
2021-04-30
Degree Conferral Date
2021-05-31
Degree
Doctor of Philosophy
Degree Referral Number
38005甲第75号
Degree Conferrral Institution
Okinawa Institute of Science and Technology Graduate University
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Scanning Probe Microscopy Studies of Metal Halide Perovskite Materials
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