SCL Seminar by Veljko Jankovic

SCL seminar of the Center for the Study of Complex Systems, will be held on Friday, 31 March 2017 at 14:00 in the library reading room “Dr. Dragan Popović" of the Institute of Physics Belgrade. The talk entitled

"Origin of Space-separated Charges in Photoexcited Organic Heterojunctions on Ultrafast Time Scales"

will be given by Veljko Janković (Scientific Computing Laboratory, Center for the Study of Complex Systems, Institute of Physics Belgrade).

Abstract of the talk:

The promise of economically viable and environmentally friendly conversion of sunlight into electrical energy has driven vigorous and interdisciplinary research on organic photovoltaics based on the heterointerface between an electron-donating and an electron-accepting material. The crucial steps in the photocurrent generation are the dissociation of the electron-hole pairs (excitons) photogenerated in the donor material and their further separation. In these processes, the electron and hole have to overcome the Coulomb barrier whose depth is, mainly because of the low dielectric constant of organic materials, much larger than the thermal energy at room temperature. The findings of recent spectroscopic studies, which point towards the presence of separated charges on < 100-fs time scales, cannot be easily reconciled with the large depth of the Coulomb barrier, and the actual mechanism of free-charge generation on subpicosecond time scales remains elusive.

We investigate subpicosecond exciton dynamics in the lattice model of an all-organic heterointerface. Exciton generation, exciton dissociation, and charge separation are treated on equal footing and on a fully quantum level using the density matrix formalism combined with the dynamics controlled truncation scheme [1]. Our results indicate that the space-separated charges appearing on subpicosecond time scales following the photoexcitation are predominantly directly optically generated, in contrast to the usual viewpoint that they originate from ultrafast population transfer from initially generated excitons in the donor material [2]. The states of separated charges acquire nonzero oscillator strengths from donor excitons thanks to the strong resonant mixing between these two groups of exciton states. The interpretation of ultrafast pump-probe spectroscopy experiments, which is commonly in terms of exciton populations only, is hampered by exciton coherences, which cannot be disregarded on such short time scales [2].

[1] V. Janković and N. Vukmirović, Phys. Rev. B 92, 235208 (2015).
[2] V. Janković and N. Vukmirović, Phys. Rev. B 95, 075308 (2017).

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