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| Dissertation / PhD Thesis | PUBDB-2021-00192 |
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2020
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Please use a persistent id in citations: doi:10.3204/PUBDB-2021-00192
Abstract: In recent years, the need for a more sustainable economic development contributed to the increasing interest in renewable energy sources. With encouraging trends on power conversion efficiencies and manufacturing costs, photovoltaics is expected to be the workhorse for the production of green energy in the future. Silicon is currently the dominant photovoltaic technology but, in the past decade, novel solutions based on organic semiconductors became attractive for their potential of overcoming the Shockley-Queisser limit and offering unmatched efficiencies by exploiting singlet fission. The latter is an exciton multiplication process in which, for a certain class of materials, a singlet exciton splits into two triplet excitons, thus potentially doubling the charge carriers. Significant work is still necessary to fully benefit of singlet fission in photovoltaics; in particular, a higher degree of control over exciton transport and dissociation mechanisms at hetero-organic interfaces is required for efficiently harvesting triplet excitons. To the aim of better understanding such processes, at the ANCHOR-SUNDYN endstation of the ALOISA beamline at Elettra we developed an experimental setup for time-resolved X-ray spectroscopies, in which the exciton dynamics in organic films can be characterized by X-ray photoemission and absorption spectroscopies with a 100 ps resolution. Here, we can combine time-resolved measurements with standard X-ray and UV spectroscopies for a more detailed analysis of the samples.We apply this approach to donor/acceptor interfaces, the prototypical architectures of organic photovoltaic devices; we investigate triplet excited states in pentacene by means of time-resolved X-ray absorption, which displays a pump-induced feature with a 0.3±0.2 ns lifetime below the LUMO resonance, that we associated to molecules in the triplet state. On the picosecond time scale, measurements performed at the FLASH free-electron laser reveal a photoelectron response that we deem related to the triplet exciton dissociation at the interface with the underlying C60 film. A similar effect is also observed in pump-probe photoemission spectra of tetracene / copper phthalocyanine interfaces. On this second system, we tuned the pump wavelength to selectively excite the two materials and examined the different behavior of the photogenerated excitons; the presence of a transient field in the microsecond time scale suggests that triplet excitons are involved in the charge transfer that occurs from tetracene to copper phthalocyanine, in agreement with previous studies. The results presented here demonstrate that time-resolved X-ray spectroscopies can provide valuable information for the characterization of exciton dynamics in hetero-organic interfaces.
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