Imaging the interfacial charge carrier dynamics at axial p-n junction nanowires
To optimize the light harvesting, multiple junctions with different band gaps can be combined to match the solar spectrum, but lattice matching requirements severely limit the materials available for devices based on thin film growth. In general, nanowires (NWs) have emerged as building blocks for electronic and photonic technologies due to their distinct advantages over its bulk and planar counterparts. Multi-junction solar cells containing several p-n junctions which can be used to surpass the Shockley−Queisser efficiency limit in solar cells. However, the carrier dynamics at the interfaces of the p-n junctions are still not well understood. Being in this regime, the classical picture of photo-induced charge transfer at a heterojunction can be described as follows: right after excitation, electron-hole pairs are generated. This is followed either by electron-hole separation and drifting in opposite directions under the influence of the electric field of the p-n junction, or the electron-hole pairs can recombine radiatively by emitting a photon, or non-radiatively by carrier trapping or/and Auger recombination. However, where the exciton is localized and trapped, how the transfer occurs and how it depends upon the local environment is an important as yet unanswered question. The processes by which carriers transfer back across the interface and recombine, thereby returning to the equilibrium state, are highly dependent upon the local environment and are not fully understood and they cannot be catalogued using spectroscopic techniques. In other words, the accessibility of these dynamical processes by static imaging or steady-state and time-resolved spectroscopic techniques is very limited. The unique opportunity to visualize the carrier dynamics selectively on the material surface can only be accessed by 4D S-UEM with fs temporal and nm spatial resolutions. We will begin by utilizing the S-UEM technique to investigate charge dynamics in n-InGaN/p-GaN nanowires. For the S-UEM measurement, right after laser excitation, the time-resolved secondary electron images arising from the first few nm of the sample surface will be recorded, providing the image-contrast changes for mapping charge dynamics. Straightforwardly, bright or dark image contrast correspond to increase or decrease in the local electron density, telling us where the carriers (electrons/holes) are localized. In other words, bright and dark image contrast is interpreted as an increase in the local electron and hole densities, respectively. Moreover, charge separation and charge recombination can be directly accessed from the spreading out the bright contrast or from diminishing the dark contrast.
Physical Sciences and Engineering
Field of Study -
Electron imaging, interface dynamics, oproelectronic applications
Omar F. Mohammed
Associate Professor, Material Science and Engineering
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
With real-space imaging, we can examine the temporal behavior of a local surface area (i.e., pixel areas), providing direct information about the charge carrier dynamics including trapping and recombination on the surface and at the interface of inorganic single crystals, different structural compositions of InGaN nanowires and the electron-hole localization across the p-n junction based on GaN/InGaN nanowires. The results would help us in gaining valuable insights into the photo-physics of such materials and guide in the better designing of optoelectronic devices