[institut] Reminder - SCL Seminar: Veljko Janković, Friday, 31 March, 14:00

[Nenad Vukmirovic]

Dear colleagues,

You are cordially invited to the SCL seminar of the Center for the Study 
of Complex Systems, which 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).

Best regards,

-- 
Dr Nenad Vukmirovic,
Research Professor,
Scientific Computing Laboratory,
Institute of Physics Belgrade,
Serbia.
URL: http://www.scl.rs/nenad