Real-Space Imaging of Charge Carrier Dynamics in Photoactive Materials by 40 Scanning Ultrafast Electron Microscopy

  • Basamat S. Shaheen

Student thesis: Doctoral Thesis

Abstract

Surfaces of photoactive materials play a pivotal role in determining the interfacial properties and the photoconversion efficiency of optoelectronic devices. On the other hand, the fundamental photophysical processes of photo-generated carriers and their transport and recombination occur at extremely short timescales ranging from femtoseconds to nanoseconds. In order to provide a complete picture about the best working conditions of photoactive materials to improve their device performance, it is very essential to explore and decipher the ultrafast surface dynamics at nanoscale or even atomic levels. Four-dimensional scanning ultrafast electron microscopy (4D S-UEM) is the sole technique capable of surface-selective visualization of light-triggered carrier dynamics at nanometer scale. Herein, 4D S-UEM is used to investigate the effect of several key factors on the surface charge carrier dynamics of a variety of photoactive materials: (1) surface passivation in lnGaN nanowires, (2) deposition method in PbS quantum dots,(3) thickness in CdSe thin films, (4) crystal orientation in CdTe single crystals and (5) native oxide layer in Si wafers. Besides the visualization of surface charge carrier dynamics in these materials, new surface features were discovered such as the superior charge carrier diffusion on the surfaces of CdTe single crystals ≈ 10^4 times larger than that in their crystal's bulk. Furthermore, utilizing 4D S-UEM at low accelerating voltage of 1 kV enables monitoring the diffusion from underneath the surface region and discovering the reason behind the energy loss mechanism and ultrafast carrier recombination of surface charge carriers in solar cell materials, unlocking their interfacial behaviors at the nanoscale level. These new findings are believed to provide the foundation for potential applications of 4D S-UEM to be the method of choice in studies of surface dynamics in chemistry, materials science, and other disciplines. Furthermore, the work presented here provides the key to unlocking further optimizations of the surfaces and interfaces of photoactive materials, thus paving the way for more efficient optoelectronic devices.
Date of AwardApr 2019
Original languageEnglish (US)
Awarding Institution
  • Physical Science and Engineering
SupervisorOmar Mohammed Abdelsaboor (Supervisor)

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