King Abdullah University of Science and Technology
Mapping Surface and Interface Carrier Dynamics of Solar Cell Materials Using FourDimensional Electron Microscopy and Ultrafast Laser Spectroscopy
Light-generated charge-carrier collection in all optoelectronic devices is limited by the losses and ambiguous dynamics at surfaces and interfaces,1-4 which are ultrafast processes that can only be visualized in space and time by scanning ultrafast electron microscopy (the sole technique capable of surface-selective visualization of light-triggered carrier dynamics at nanometer scale). In additional to our state-of-the-art femtosecond electronic and vibrational spectroscopic techniques, we established and developed the second generation of scanning ultrafast electron microscopy (4D S-UEM) and demonstrate the ability to take time-resolved secondary electrons images (snapshots) of material surfaces with 650 fs and 4 nm temporal and spatial resolutions, respectively. In this method, the surface of the photoactive materials is excited by a clocking optical pulse and the photo-induced changes will be directly imaged using a pulsed electron beam as a probe pulse, generating secondary electrons with a couple of electron volts energy, which are emitted from the very top surface of the material in a manner that is extremely sensitive to the localization of the electron and hole on the surface and at the donor-acceptor interfaces. This method provides direct and controllable ultrafast dynamical information in many photoactive materials commonly used in solar cells, light-emitting diodes and photocatalysis. For instance, we have clearly demonstrated in space and time how the surface morphology, surface passivation, native oxide, thickness of the absorber layer, surface defects and nanostructured features can significantly impact the overall dynamical processes of photo-generated charge carriers on the surface of absorber layers.5-9 Finally, charge carrier dynamics in at the interface of semiconductor quantum dots and perovskite single crystal using femtosecond laser spectroscopy will be also presented and discussed.
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