SCL Seminar by Veljko Jankovic

SCL seminar of the Center for the Study of Complex Systems, will be held on Wednesday, 6 June 2018 at 14:00 in the library reading room “Dr. Dragan Popović" of the Institute of Physics Belgrade. The talk entitled

"Some like it hot, others prefer it cold: Mechanism of free-charge generation in organic photovoltaics"

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 fundamental physical mechanisms governing light-to-charge conversion in photoexcited donor/acceptor (D/A) organic solar cells have recently been heavily debated. Having in mind that a photoexcitation of OSCs initially generates strongly bound excitons, experimental evidence demonstrating that almost all of the absorbed photons are converted to free carriers in a process that exhibits weak dependence on the internal electric field and temperature [1] seems surprising. Results of ultrafast spectroscopic studies point towards a subpicosecond time scale of free-charge generation, which occurs by virtue of high-energy ("hot") delocalized interfacial charge transfer (CT) states [2]. On the other hand, there are suggestions that free carriers are predominantly generated on much longer time scales out of the lowest-energy ("cold") CT state, which is strongly bound and localized [3]. Our comprehensive investigation of exciton dynamics at a model D/A heterojunction reveals that, on a picosecond time scale following the photoexcitation, the majority of photogenerated carriers remain bound in form of donor or CT excitons [4]. In this talk we will present our latest results that concern charge separation on longer time scales out of the aforementioned bound CT and donor excitons, which demonstrate that the synergy between carrier delocalization and moderate disorder can explain quite high and weakly field- and temperature-dependent free-charge yields observed in experiments [5].

[1] S. H. Park et al., Nat. Photonics 3, 297 (2009).
[2] G. Grancini et al., Nat. Mater. 12, 29 (2013).
[3] K. Vandewal et al., Nat. Mater. 13, 63 (2014).
[4] V. Janković and N. Vukmirović, J. Phys. Chem. C 121, 19602 (2017).
[5] V. Janković and N. Vukmirović, J. Phys. Chem. C 122, 10343 (2018).

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