Our research interests focus on the characterization of 3rd generation photovoltaic and photocatalytic systems and on the understanding of the mechanisms and dynamics of underlying phenomena. To investigate the dynamics of fundamental processes (such as photoinduced interfacial electron transfer, charge transport, energy transfer and relaxation), which prevail in nanostructured materials and their interfaces, we utilize state-of-the-art time-resolved techniques.
Identification of reaction intermediates and quantification of the kinetics of the underlying photoinduced processes are achieved by a combination of various spectroscopic methods. These include femtosecond-, picosecond- and nanosecond pulsed laser excitation, coupled to various fast transient absorbance, stimulated emission, diffuse reflectance, nonlinear second harmonic generation, and photoemission probing techniques. The time-resolution of current laser equipments allows for the monitoring of temporal domains extending over fifteen orders of magnitude, roughly from a few femtoseconds to seconds. All optical wavelengths in the UV, visible, and NIR spectral ranges can be used for pump and probe. In addition, terahertz time-domain spectroscopy (THz-TDS) (linear and time-resolved) enables us to study the dynamics of low frequency vibrations in molecules, solvents, solids, and supramolecular systems, as well as charge carrier mobility and transport mechanisms in nanomaterials.
Systems currently under investigation range from bandgap-irradiated and dye-sensitized nanodispersed metal oxides (in the form of transparent colloidal sols, mesoporous films, opaque electrodes, or powders) to quantum dots and perovskites light absorbers, metal oxide-organic hybrid systems, small molecules-based organic semiconductors, and polymer bulk heterojunctions.
Light-induced Interfacial Electron Transfer Dynamics Photoinduced electron transfer (PET) at molecular-bulk interfaces and donor-acceptor materials heterojunctions are pertinent to a variety of photovoltaic, photocatalytic and photoelectochemical systems. With the help of modern spectroscopic methods, such as fs transient absorption spectroscopy, fluorescence upconversion, and nanosecond laser flash photolysis, our research expands from studying molecular donor/acceptor conjugates over charge transport through molecular wires to PET processes in novel organic and inorganic-organic hybrid solar cell devices.
Primary ET Processes in Solid-State 3rd Generation Solar Cells Solid-state dye-sensitized solar cells and new meso-superstructured solar cells are important technologicaly as they could alleviate the problems related to leakage and corrosion occurring with liquid electrolyte-based systems. Charge separation and recombination processes have not been adressed in much detail in such devices containing organic hole-transport solid materials. As these systems rely on the kinetic competition between fast energy- and electron transfer, a thorough understanding of the dynamics of the latter processes is crucial for the optimization of photovoltaic cells performance.
Terahertz Time-Domain Spectroscopy Terahertz time-domain spectroscopy (THz-TDS) is a technique probing simultaneously far infrared radiation (1THz ~ 33 cm–1) absorption and dephasing. It provides a convenient method for determining the frequency-dependent complex permittivity of a sample. Combined with an ultrafast excitation pulse in an optical pump-THz probe (OPTP) scheme, the technique allows to studying photogenerated charge carriers and low-frequency molecular vibrations dynamics with sub-ps time-resolution, making it an ideal complementary tool for investigating primary light-induced events.
Small Molecules-Based Organic Photovoltaic Systems In the broad field of organic photovoltaics (OPV), small soluble molecules are gaining more and more interest. Compared to polymers, small molecules offer a large panel of advantages like ease of synthesis, monodisperse behavior as well as high purity, which is responsible for higher charge carrier mobility. The primary electron transfer (ET) processes are investigated via femtosecond transient absorption spectroscopy in small molecule donor-acceptor bilayers and in bulk heterojunctions. This technique enables the study of the elementary steps of charge separation required in complete solar cells.
Electromodulated Ultrafast Differential Absorption A number of fundamental aspects of the photophysics of OPV and perovskite solar cells remains to be understood. Open questions in particular are concerned with the mechanism of charge generation from optically created charge transfer states and excitons and transport within the active layer under an external electric field. Electromodulated ultrafast differential absorption technique is a method designed to optically probe electric field-induced changes of transient absorption (Stark effect). The method allows measuring carrier dynamics and mobility on a wide range of distances and is complementary to THz-TDS.
Transient Diffuse Reflectance Spectrometry Femtosecond and nanosecond time-resolved transient diffuse reflectance spectroscopies are used to monitor the rate of charge transfer in opaque nanostructured semiconducting electrodes upon bandgap photoexcitation or spectral sensitization by molecular dyes. One typical application of such systems is in opaque solid-state 3rd generation photovoltaic systems. In these cases, time-resolved diffuse reflectance spectroscopy is a perfect tool to determine the dynamics of photophysical processes and charge transfer reactions in complete working cells under potentiostatic control.