*ZOOM* [PhD Thesis Presentation] - "Scanning Probe Microscopy Studies of Metal Halide Perovskite Materials" by Ms. Afshan Jamshaid
Presenter: Ms. Afshan Jamshaid
Supervisor: Professor Yabing Qi
Unit: Energy Materials and Surface Sciences Unit
Title: Scanning Probe Microscopy Studies of Metal Halide Perovskite Materials
So far very little atomic-scale research has been done to understand the influence of additives on the structural and electronic properties of halide perovskite materials (e.g., MAPbI3=CH3NH3PbI3). Additives have been suggested to solve the thermal instability and ambient air induced degradation problems of perovskites. Among studied additives, Cl and KI were found to be eligible candidates for improving the power conversion efficiencies (PCEs) and degradation issues in perovskite solar cells. Instabilities and degradation occur in perovskite materials due to the interaction of water, oxygen, light, and temperature stimuli during solar cell operation. To solve instability and degradation problems in perovskite materials, it is imperative to study the fundamental processes of the instabilities and degradation of perovskite at the atomic scale. In this thesis, scanning tunneling microscopy (STM) and density functional theory (DFT) calculations provide a fundamental understanding of the origin of the Cl interactions with MAPbI3 and provide useful hints for the design of stable and high-performance perovskite solar cells. My results provide information about different Cl and (KI) concentrations that play an important role in MAPbI3. (1) I found that at low coverage, Cl does not incorporate in MAPbI3 surface lattice (corroborated by X-ray photoelectron spectroscopy measurements). (2) However, with increasing Cl concentration, it is possible to visualize Cl ions on MAPbI3 surfaces. With a concentration of [Cl]:[I] ~ 18% (MAPbI2.54Cl0.46), I observed that Cl ions were mostly incorporated close to the grain boundaries, (3) Additionally, Cl incorporation was also observed in the center regions of grains that have different structures (namely, the dimer and zigzag structures). (4) My study evidence that Cl can substitute I ions of the surface lattice structure and/or fill the surface I-vacancies in MAPbI3. (5) Furthermore, Cl incorporation alters the bandgap of MAPbI3 from 1.45 eV to 1.65 eV which was characterized by ultraviolet and inverse photoemission spectroscopy (UPS/IPES). (6) Finally, the Cl incorporation ratio was verified experimentally by performing Fourier transform infrared (FTIR) and XRD experiments on thick films, and I determined the optimal Cl incorporation ratio that leads to enhanced Cl doped MAPbI3 perovskite stability. It is important to point out that STM can be used to characterize mainly the surface (and possibly the subsurface) atomic structures of perovskite materials. Although it is challenging for STM to provide a complete picture (especially regarding the bulk structures of perovskites), my study does offer valuable insights into the surface/interface properties. For example, (i) Cl diffusion into the perovskite film (ii) beneficial effects of Cl at the surface/interface on charge extraction (iii) passivation of surface defects by Cl.