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Seminar | X-Ray Science Division

Investigation of the Electronic Structure and Electron Dynamics of Semiconductor Thin Films by Electron Spectroscopies

XSD/MM Special Presentation

Abstract: Organic semiconductors are emerging alternative materials applied in electronic devices due to their electronic properties, sustainability, low-cost preparation, and flexibility. Diverse π-conjugated semiconductor polymers can be employed in field-effect transistors, sensors, among other devices where charge transfer plays an important role. In this way, it is crucial to investigate their electronic structure and electron dynamics using complementary spectroscopic techniques.

For example, the electronic transition between the ground state and the excited state is observed in absorption spectroscopies (e.g. XAS), while the electronic decay products are detected in emission spectroscopies (e.g. PES). In this way, the combination of absorption and emission spectroscopies gives a complete picture of chemical systems’ electronic structure and dynamics. Synchrotron radiation in the so-called tender Xray domain, which ranges from around 2 keV to 12 keV, enables the core-shell excitation/ionization of chemical systems. Since these core-excited/ionized states are quite unstable, the system decays. Such states own ultrashort lifetime on the time scales of femtoseconds (10-15 s). The relaxation pathways for the decay to a more stable state can occur via the ejection of an electron or emission of a photon.

In my Ph.D. research project, we applied the core-hole clock spectroscopy (CHCS) to investigate electron dynamics in two thiophenebased polymer thin films: polythiophene (PT) and poly[3-hexylthiophene] (P3HT) deposited on ITO substrate. These semiconductors have exciting applications as OPVs (organic photovoltaics) and solar cells, where charge transfer plays an important role in the device’s performance. Using resonant Auger electron spectroscopy (RAES), we provided an indication of from which molecular orbital the spectator electron delocalizes. We found that the electron delocalization in both films is related to a resonant excitation to the unoccupied σ∗ orbital, contrary to the expected role of the π* orbital in this process. Such results represent a step forward to reveal the charge transfer mechanism in those chemical systems.

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