Real-Space Imaging of Perovskite Single Crystals Using 4D Electron Microscopy


Project Description

​While charge carrier dynamics in the bulk of semiconductor materials are well understood, charge transport including ejection and diffusion on surfaces and interfaces is still one of the prime challenges facing the communities of solar cells and surface sciences. Unfortunately, time-resolved laser spectroscopies that have been commonly used to understand the dynamics of photo-generated carriers in condensed matter are limited by the large penetration depth (100s of nanometers) of their pump and probe pulses, making them only sensitive to the bulk properties of the investigated materials. Only rare, bespoke techniques based on ultrafast electron microscopies offer the surface sensitivity needed to track light-triggered carrier dynamics in real-time and space, such as four-dimensional scanning ultrafast electron microscopy (4D-SUEM) which has emerged recently as a powerful tool to investigate real space-time dynamics selectively, on top surfaces of various materials with high temporal and spatial resolutions. In 4D-SUEM, an optical laser pulse at 515 nm is used to excite the material surface; a pulsed primary electron beam is then generated through a delayed UV excitation pulse at 343 nm from the cooled Schottky field-emitter tip, emitting secondary electrons from the surface of the specimen in a manner that is extremely sensitive to the local electron/hole density at the surfaces and interfaces. Time-resolved secondary electrons images produced from the excited surface are detected and then analyzed, pixel-by-pixel. In this study, we will use 4D-SUEM to selectively map surface dynamics including carrier diffusion length of the MAPbI3 single crystals with different facets and different surface treatment. We will also try to image the interface of solar cells base on these single crystals including the charge carrier ejection to the electron and hole transporting layers as well as the impact of the oxide layers on the carrier dynamics and device performance​  ​​
Program - Chemical Science
Division - Physical Sciences and Engineering
Field of Study - ​Surface Dynamics Characterization

About the

Omar F. Mohammed

Associate Professor, Material Science and Engineering

Omar F. Mohammed
Professor Mohammed's research is directed towards fundamental understanding of carrier dynamics in a variety of solar cell systems, including semiconductor quantum dots, polymers and perovskite solar cells with the aid of cutting-edge nanotechnology, ultrafast laser spectroscopy and four-dimensional electron imaging.

Desired Project Deliverables

​The finding of this project may offer a clear view of the extreme carrier diffusion behavior as a result of facet termination and surface treament. The results will be useful to address device performance bottle-necks, opening a new avenue to create MAPbI3 single crystal-based optoelectronic devices that take advantage of previously unknown, extreme surface behaviors.​