Mapping Charge Carrier Dynamics of Photoactive Material Surfaces in Space and Time

Abstract

Understanding light-triggered charge carrier dynamics on photovoltaic-material surfaces and at interfaces has been a key element and one of the major challenges for the development of real-world energy devices. For this reason, studying charge carrier dynamics at photoactive material surfaces and interfaces have been among the most active areas of research in the last decade [1-4]. However, the ability to access carrier dynamics selectively on material surfaces with high spatial and temporal control in a photo-induced reaction is a particularly challenging task that can only be achieved by applying four-dimensional (4D) ultrafast electron microscopy (4D UEM) along with time-resolved laser spectroscopy. For this purpose, we established and developed the second generation 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 imaged using a pulsed primary electron beam as a probe pulse, generating secondary electrons, which are emitted from the 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 and photo-catalysis. For instance, we have clearly demonstrated in space and time how the surface morphology, surface passivation, thickness of the absorber layer, grains, surface defects and nanostructured features can significantly impact the overall dynamical processes on the surface of absorber layers including nanomaterials and single crystals [5-8]. Finally, charge carrier dynamics in semiconductor quantum dots and perovskite single crystal using femtosecond laser spectroscopy will be also presented and discussed. References M. Bakr, O. F. Mohammed., Science 355 , 1260 (2017).O. El-Ballouli, E. Alarousu, M. Bernardi, S. M. Aly, A. P. Lagrow, O. M. Bakr, O. F. Mohammed., J. Am. Chem. Soc. 136 , 6952 (2014).Begum, M. R. Parida, A. L. Abdelhady, B. Murali, N. Alyami, G. H. Ahmed, M. N. Hedhili, O. M. Bakr, and O. F. Mohammed., J. Am. Chem. Soc. 139, 731 (2017).F. Mohammed, D.-S. Yang, S. Pal, A. H. Zewail, J. Am. Chem. Soc. 133, 7708 (2011).Sun, V. A. Melnikov, J. I. Khan, O. F. Mohammed, J. Phys. Chem. Lett. 6, 3884 (2015).Bose, J. Sun, J. I. Khan, B. S. Shaheen, A. Adhikari, T. K. Ng, V. M. Burlakov, M. P. Parida, D. Priante, A. Goriely, B. S. Ooi, O. M. Bakr, O. F. Mohammed, Adv. Mater. 28, 5106 (2016). S. Shaheen, J. Sun, D-S Yang, and O. F. Mohammed, J. Phys. Chem. Lett. 8, 2455 (2017).Bose, A. Bera, M. R. Parida, A. Adhikari, B. S. Shaheen, E. Alarousu, J. Sun, T. Wu, O. M. Bakr, O. F. Mohammed, Nano Lett. 16 , 4417 (2016).