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