Spectroscopic Investigations of Surface-Ligand Binding during Perovskite Nanocrystal Growth
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Date
2022-02-18
Authors
Sadighian, James
Journal Title
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Publisher
University of Oregon
Abstract
Lead halide perovskite nanocrystals (NCs) are promising materials for a range of photovoltaic and optoelectronic applications due to their favorable properties and potential for low-cost, solution-based processing. The model of nucleation and growth proposed by Victor LaMer that is typically used to describe colloidal NC synthesis has guided our understanding of NC formation for over 70 years. However, LaMer's model does not account for the effects of surface ligand interactions on the availability of precursor, the resulting burst nucleation, or the growth of the NC. The large surface-area-to-volume ratio means that NC properties are heavily influenced by the surface. Thus, accurate characterization of surface-ligand interactions is critical to better understand the mechanisms by which these nanocrystals nucleate and grow and how their electronic structure evolves during these processes so that better materials may be made through rational design. Photophysical characterization that can probe the electronic structure and dynamics of a material is typically limited to structurally stable NCs owing to the long timescales required, preventing the accurate measurement of NCs during growth. This is a particular challenge for non-linear spectroscopies such as transient absorption. Here we report on work done to spectroscopically investigate nucleation and growth of methylammonium lead triiodide (MAPbI3) perovskite nanocrystals to better understand the role of surface ligands. MAPbI3 NCs are grown via a novel synthesis where reaction kinetics are mediated by the solubility of the solid precursors in a non-polar solvent. Use of a novel single-shot transient absorption spectrometer reveals that photogenerated charge carriers become localized at surface trap states during NC growth, producing a TA lineshape characteristic of the Stark effect. Observation of this Stark signal shows that the contribution of trapped carriers to the TA signal declines as growth continues, supporting a growth mechanism characterized by increased surface ligation toward the end of NC growth. This work opens the door to the application of time-resolved spectroscopies to NCs in situ during their synthesis, providing greater insight into their growth mechanisms and the evolution of their photophysical properties.
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Keywords
Nanocrystal, Perovskite, Transient Absorption, Ultrafast Spectroscopy