Ultrafast Dynamics Group
Frédéric Laquai's Group


Ultrafast Transient Absorption Spectroscopy

Broadband Ultrafast Transient Absorption Spectroscopy (pump-probe spectroscopy)

The experimental setup that we have developed for broadband TA spectroscopy allows the excited state absorption of molecules in solutions, thin films and in working devices to be measured over a broad wavelength range from 500 - 2200 nm, while covering a time range from 100 fs to 1 ms. We use home-built non-collinear optical parametric amplifiers (NOPAs) to generate fs pump pulses of tuneable wavelength, and supercontinuum white light generation in sapphire windows to create a well-defined supercontinuum probe light either in the visible to near-infrared (500 - 1100 nm) or in the NIR (1000 - 2200 nm) spectral range. In our setup the probe light is passed through the sample and then dispersed in a spectrograph and detected by home-built silicon photodiode (400 - 1100 nm) or InGaAs (1000 - 2200 nm) array detectors. These detectors record the entire probe light spectrum for each time delay chosen. The combination of such a broad wavelength and time range allows a variety of materials to be studied with this technique.

Scheme of the transient absorption (pump-probe) setup used for studies of charge generation and recombination in donor-acceptor systems. For clarity only the most important elements are shown, while most of the optics are omitted.

Efficiency-limiting processes in organic solar cells 

Despite extensive study by several research groups, the mechanism of free charge carrier generation in donor:acceptor bulk heterojunction blend films (i.e., whether charge carriers are generated by an ultrafast exciton dissociation process or by the field-dependent splitting of intermediate and interfacial charge transfer (CT) states), efficiency-limiting processes such as geminate and non-geminate recombination, and their dependence on molecular structure and morphology, are not yet well understood. Using broadband visible (VIS) to near-infrared (NIR) transient absorption (TA) spectroscopy to directly monitor charge generation and recombination in prototypic photovoltaic blends, we have recently shown that ultrafast exciton dissociation rather than field-dependent splitting of interfacial CT-states is the origin of free charge carrier generation in typical polythiophene:fullerene systems. These observations contribute to a better understanding of the fundamental processes of charge generation in these systems.