Laboratoire Francis PERRIN
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Ultrafast non-radiative processes in new materials for solar energy conversion

Solar energy conversion is today a fact. Traditionally dominated by crystalline silicone, new alternative technologies are emerging, in particular based on organic photovoltaics (OPV), hybrid solar cells (dye-sensitized solar cells) or new inorganic materials such as perovskites and nanomaterials. The efficiency of a photo-active material depends on the charge separation process that is itself largely dependent on the structural properties of the material. Charge carriers can be trapped and other competing non-radiative relaxation processes may decrease the efficiency. The initial processes preceding the charge separation are studied in DICO using time-resolved spectroscopy.

This topic is being developed in two directions :


1. Dye Sensitized Solar Cells, also known as Grätzel cells, are very promising owing to their potential efficiency as well as being cheap. However, further improvements are necessary, particularly on the photocathode side. This requires a more detailed understanding of the molecular and electronic dynamics. To this purpose we have undertaken a study of the photophysics and the primary processes occurring after light absorption using femtosecond luminescence up-conversion (project funded by the CEA programme DSM Energy).

A PhD student, Valentin Maffeis, co-directed by Drs. Thomas Gustavsson and Bruno Jousselme (DRF/IRAMIS/NIMBE/LICSEN) is working on this subject (2015-2018).

R. Brisse, et al., A red to blue series of push-pull dyes for NiO based p-DSSCs, Sustain. Energ. Fuels, 2 (2018) 648-654.


Figure 1. Time-resolved fluorescence spectra of a new push-pull type molecule I solution. The excited charge transfer state formed by light absorption is strongly stabilized by the polar environment. The charge separation (and ultimately, the charge injection) occurring in the excited state is in dynamic competition with the energy stabilization which leads towards a non-radiative internal conversion (leading to electron back transfer). It is therefore important to characterize the different processes involved in order to optimize the "wanted" reaction paths (charge separation) with regards to the "unwanted" reaction paths (internal conversion). 


2. Hybrid organic-inorganic structures are a new class of materials that combine the advantages of organic and inorganic systems. Among these materials, hybrid perovskites are currently attracting an enormous interest. This new research program aims to examine in detail the ultrafast photophysical processes occurring in hybrid perovskites using the combination of femtosecond luminescence up-conversion and two-dimensional (2D) electronic spectroscopy. This project is supported by an ANR ACHN grant (2016-2019) coordinated by Elsa Cassette.

A PhD student, Carolina Villamil-Franco, co-directed by Drs. Thomas Gustavsson and Elsa Cassette is working on this subject (2017-2020).


Figure 2. Principle and schematic view of a two-dimensional (2D) electronic spectroscopy measurement in the "boxcar" configuration. Also shown are typical "2D-plots" correlating the excitation and detection frequencies, showing the presence of off-diagonal coupling terms.