1,255 research outputs found
Beyond Vibrationally Mediated Electron Transfer: Coherent Phenomena Induced by Ultrafast Charge Separation
Wave packet propagation succeeding electron transfer (ET) from alizarin dye
molecules into the nanocrystalline TiO2 semiconductor has been studied by
ultrafast transient absorption spectroscopy. Due to the ultrafast time scale of
the ET reaction of about 6 fs the system shows substantial differences to
molecular ET systems. We show that the ET process is not mediated by molecular
vibrations and therefore classical ET theories lose their applicability. Here
the ET reaction itself prepares a vibrational wave packet and not the
electromagnetic excitation by the laser pulse. Furthermore, the generation of
phonons during polaron formation in the TiO2 lattice is observed in real time
for this system. The presented investigations enable an unambiguous assignment
of the involved photoinduced mechanisms and can contribute to a corresponding
extension of molecular ET theories to ultrafast ET systems like alizarin/TiO2.Comment: This work was supported by the German Research Foundation (DFG) (Hu
1006/6-1, WA 1850/6-1) and European Union projects FDML-Raman (FP7 ERC StG,
contract no. 259158) and ENCOMOLE-2i (Horizon 2020, ERC CoG no. 646669
Ultrafast photoinduced electron transfer in coumarin 343 sensitized TiO2-colloidal solution
Photoinduced electron transfer from organic dye molecules to semiconductor nanoparticles is the first and most important reaction step for the mechanism in the so called “wet solar cells” [1]. The time scale between the photoexcitation of the dye and the electron injection into the conduction band of the
semiconductor colloid varies from a few tens of femtoseconds to nanoseconds, depending on the specific electron transfer parameters of the system, e.g., electronic coupling or free energy values of donor and acceptor molecules [2–10]. We show that visible pump/ white light probe is a very efficient tool to investigate the electron injection reaction allowing to observe simultaneously the relaxation of the excited dye, the injection process of the electron, the cooling of the injected electron and the charge recombination reaction
Spatio-Temporal Dynamics of Free and Bound Carriers in Photovoltaic Materials
Charge transfer and subsequent separation into free carriers are key processes that govern the efficiencies of third generation solar cell technologies based on donor-acceptor heterojunctions. As these processes typically occur on picosecond to femtosecond timescales, it is necessary to employ ultrafast spectroscopic techniques to further our understanding of these processes in order to provide vital information that can aide in furthering material design. Within the framework of the National Centre of Competence in Research “Molecular Ultrafast Science and Technology” (NCCR MUST), we have developed and utilized a suite of different ultrafast spectroscopic techniques to study charge generation, separation and recombination in a variety of small molecule based organic solar cells and lead halide perovskites. Here, we provide an overview of the main techniques used in our laboratory and the recent results obtained using these spectroscopic techniques
Excitation-Wavelength Dependence of Photoinduced Charge Injection at the Semiconductor-Dye Interface: Evidence for Electron Transfer from Vibrationally Hot Excited States
Heterodyad systems composed of a redox photosensitizer adsorbed on the surface of a wide band gap semiconductor were designed in a way that the ν'=0 energy level of the electronically excited state of the dye lies below the bottom of the conduction band of the solid. Under these conditions, the quantum yield of the charge injection from the sensitizer into the conduction band of the solid was found to depend upon the excitation photon energy. This observation provides an evidence that interfacial charge transfer is occurring prior to nuclear relaxation of the sensitizer's excited state. It allows the use of a simplified kinetic model and offers an easy experimental path to the determination of the electronic coupling that controls the rate of the ultrafast injection process
Perovskite photovoltaics: Slow recombination unveiled
One of the most salient features of hybrid lead halide perovskites is the extended lifetime of their photogenerated charge carriers. This property has now been shown experimentally to originate from a slow, thermally activated recombination process
Molecular Photovoltaic Devices Mimic Photosynthesis
Photoinduced charge-transfer processes involving molecules adsorbed at interfaces are a fascinating topic which is presently attracting wide attention. Our investigations have focused on the identification of the factors that control the dynamics of such processes. The goal is to design
molecular electronic devices that achieve efficient light-induced charge separation. Applications of similar systems in photochromic and electrochromic devices appear also feasible
Exciton and Carrier Dynamics in 2D Perovskites
Two-dimensional Ruddlesden-Popper hybrid lead halide perovskites have become
a major topic in perovskite optoelectronics. Here, we aim to unravel the
ultrafast dynamics governing the evolution of charge carriers and excitons in
these materials. Using a combination of ultrabroadband time-resolved THz (TRTS)
and fluorescence upconversion spectroscopies, we find that sequential carrier
cooling and exciton formation best explain the observed dynamics, where
exciton-exciton interactions play an important role in the form of Auger
heating and biexciton formation. We show that the presence of a longer-lived
population of carriers is due to these processes and not to a Mott transition.
Therefore, excitons still dominate at laser excitation densities. We use
kinetic modeling to compare the phenethylammonium and butylammonium organic
cations while investigating the stability of the resulting films. In addition,
we demonstrate the capability of using ultrabroadband TRTS to study excitons in
large binding energy semiconductors through spectral analysis at room
temperature
On the kinetics and mechanism of light-induced electron transfer at the semiconductor/electrolyte interface
Transient photoconductivity of dye-sensitized TiO2 nanocrystalline films probed by optical pump-THz probe spectroscopy
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