Investigation of Charge-Carrier Dynamics in Organo-halide Perovskite and Colloidal Quantum Dot Semiconductors

Open Access
Stewart, Robert J
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
June 27, 2016
Committee Members:
  • John B Asbury, Dissertation Advisor
  • John B Asbury, Committee Chair
  • Raymond Edward Schaak, Committee Member
  • Mark Maroncelli, Committee Member
  • Noel Christopher Giebink, Outside Member
  • physical chemistry
  • spectroscopy
  • semiconductor
  • solar cell
  • quantum dot
  • organo-halide perovskite
Crystallite surfaces often dominate the optical and electronic properties of nanocrystalline semiconductors because the fraction of atoms at the surface experience different crystal field environments than bulk atoms, and as a consequence, surfaces influence the energetic landscape of the entire crystallite. Contributing electronic states that are either isoenergetic or below the semiconductor band edges, surface states mediate charge conduction and recombination - two critical processes in optoelectronic device performance. Utilizing a combination of inorganic synthesis, surface characterization, and time-resolved optical spectroscopy, the research presented herein begins to identify the link between charge carrier dynamics and the underlying surface chemistry in two emerging, yet promising, nanocrystal semiconductor systems: organo-halide perovskites and colloidal quantum dots (CQD). Notably, my research provided one of the first reports that charge recombination centers in lead halide perovskite films are localized almost exclusively on the surface of crystallites. Passivation of these nanocrystal surfaces with small molecules that contain strongly coordinating functional groups caused charge-carrier lifetimes in perovskite thin-films to approach the bulk radiative limit reported for single crystal analogues. Likewise, my research contributed to an understanding that surfaces in lead sulfide (PbS) CQDs produce electronic energy levels that are sufficiently delocalized to provide charge conduction pathways in CQD thin-film arrays. Given the strong coupling to the QD surface, charge carrier diffusion lengths were shown to be highly sensitive to the character of surface-bound ligands. My PhD research highlights the importance of understanding the interplay between surface chemistry and nanocrystal semiconductor photophysics as well iv as the importance of selecting surface treatment strategies capable of passivating diverse surfaces to eliminate energetic inhomogeneity while simultaneously allowing strong electronic coupling across interfaces.