Real-Space Mapping of Surface-Oxygen Defect States in Photovoltaic Materials Using Low-Voltage Scanning Ultrafast Electron Microscopy

Abstract

Ultrathin layers of native oxides on the surface of photovoltaic materials may act as very efficient carrier trapping/recombination centers, thus significantly affecting their interfacial photophysical properties. How ultrathin oxide layers affect the surface and interface carrier dynamics cannot be selectively accessed by conventional ultrafast transient spectroscopic methods due to the deep penetration depth (10s-100s nm) of the pump/probe laser pulses. Herein, scanning ultrafast electron microscopy (S-UEM) at a low voltage of 1 keV electrons was recently developed at KAUST to selectively map the ultrafast charge carrier dynamics of a few layers on the top surfaces of photovoltaic materials. Unlike high voltage 30 keV experiments, at 1 keV, the depth of detected secondary electrons (SEs) underneath the surface is significantly reduced five times, thus making it possible to visualize the dynamics of charge carrier from the uppermost surface of photoactive layers. More specifically, this new approach has been employed to explore and decipher the tremendous impact of native oxide layers and oxygen defect states on charge carrier dynamics in space and time simultaneously at sub-nm levels on several photovoltaic material surfaces, including Si, GaAs, CdTe and CdZnTe single crystals. Interestingly, the contrast in the SEs timeresolved difference images switched from a dark "energy-loss mechanism" to a bright "energygain mechanism" only by removing the layers of surface oxides. Moreover, the charge carrier recombination was estimated and found to be dramatically affected by the native oxide layers. The density functional theory (DFT) calculations demonstrate that the work function of oxygenterminated Si surface also increases slightly upon optical excitation and makes for less SEs intensity, providing another piece of evidence that the origin of the dark contrast observed on these material surfaces should be assigned to the surface oxide formation, mainly oxygen defect states in the bandgap, and/or work function increment. Our findings provide a new method and pave the way for evaluating the effect of surface morphology and defects, including but not limited to native oxide, on charge carrier dynamics and interfacial properties of photovoltaic absorber layers.

Access to Ultrafast Surface and Interface Carrier Dynamics Simultaneously in Space and Time

“…Schematic illustration of the UEM apparatus in (a) transmission mode (T-UEM), mapping the phase transition from the amorphous (blue) to crystalline (yellow) form in the laser-irradiated region and expansion to other regions of an amorphous GeTe sample, and in (b) scanning mode (S-UEM), tracking the surface charge carrier dynamics of Si single crystals, in which the photoexcitation sequential processes include charge carrier generation, as evident directly from the bright contrast of illuminated regions (the dashed ellipse). Note that to keep the schematic illustration concise, the schematic drawing does not present the fundamental IR femtosecond (fs) laser, the source for the generation of the probe and pump pulses.…”

“…Copyright 2013 AIP Publishing LLC. Snapshots of panel (b) are adapted from ref . Copyright 2020 American Chemical Society.…”

“…By this way, the difference between the irradiated and unirradiated areas by an optical pulse can be clearly seen, and the charge carrier dynamics can be extracted from even a single pixel. For instance, SEs difference images that reflect the photoinduced charge carrier generation process on a clean surface of a single-crystal Si (100) sample are shown in Figure b) …”

“…Therefore, the first generation of scanning ultrafast electron microscopy (S-UEM) was built and developed in 2010 by Zewail and co-workers. , Five years later, Mohammed and co-workers established the second generation of S-UEM in KAUST with better spatial and temporal resolutions. ,− Since then, S-UEM has demonstrated its superior capability for direct visualization of ultrafast dynamic events that occur near material surfaces and interfaces. With an adjustable AV in the range of 1–30 kV, the escape depth of secondary electrons (SEs) can be precisely controlled from sub-nanometer to several nanometers, , providing abundant dynamical information from the very top surface of the photoactive material. These are the unique capabilities of S-UEM that make it a very powerful surface technique compared to the conventional optical-pump optical-probe techniques, including TAS and TAM .…”

Light-Induced Anisotropic Morphological Dynamics of Black Phosphorus Membranes Visualized by Dark-Field Ultrafast Electron Microscopy

“…Stroboscopic measurements, with a series of a time delay (Δ t ) between the electron pulse and the optical pump pulse, produce a “movie” after montaging the time-framed micrographs, diffractograms, or spectra. This methodology has been successfully exploited for studies of ultrafast structural dynamics in materials at the nanoscale. − In this study, the optical pump pulse induces thermal stress in a BP membrane mechanically exfoliated and transferred to a holey silicon nitride (Si 3 N 4 ) support (Figure b,c) and functions as a clocklike pulse to enable us to follow the dynamics. The brief photoelectron packet is used to form a snapshot (micrograph or diffractogram) of the fleeting BP structure.…”