Excited States Dynamics at Pentacene/Perfluoropentacene Interfaces: A Femtosecond Time-Resolved Second Harmonic Generation Study.

Understanding the dynamics of excited states after optical excitation at donor-acceptor (D/A) interfaces is of paramount importance for improving the efficiency and performance of optoelectronic devices. Here, we studied the ultrafast excited state dynamics after optical excitation at interfaces between the electron donor (D) pentacene (PEN) and the electron acceptor (A) perfuoropentacene (PFP) as well as within the single compounds (PEN and PFP) using femtosecond (fs) time-resolved second harmonic generation (SHG). In the single compounds singlet fission is observed on a time scale of around 200 fs. In the bilayer systems a huge SHG intensity rise is observed due to the creation of charge transfer states at the interface and accordingly to formation of a local electric field within tens of picoseconds. The local electric field and therefore the SHG signal intensity from the interface of PEN/PFP bilayer is much more intense compared to the PFP/PEN system because the PFP and PEN intermixing at the PEN/PFP interface is higher. Accordingly a population of defect states on a time scale of 55±12 ps has been proposed for PEN/PFP. Our study provides important insights into D/A charge transfer properties, which is needed for the understanding of the interfacial photophysics of pentacene-based organic compounds.

Two-dimensional filtering in the Fourier domain of transient grating coherent artifacts in time-resolved spectroscopy.

Removal of coherent artifacts is important in the analysis of time and wavelength resolved spectroscopy data. By taking advantage of the strong correlation between spectra acquired sequentially in time, artifact removal can be formulated as a 2D problem for improved effectiveness. This paper proposes a 2D method to remove transient grating coherent artifacts from femtosecond time-resolved spectroscopy data based on filtering in the Fourier domain, leading to better estimation of the material parameters from the measured data. The method is simple, intuitive, and light on computation resources. The effectiveness of the method is demonstrated with experimental data acquired from a bare gold film with and without coherent artifacts using mutually parallel and perpendicular pump/probe polarizations, as well as with more complex samples (nanostructured gold film on a glass substrate and rhodamine fluorophores in solution). The proposed method is expected to be applicable to coherent artifact removal in other types of time and wavelength-resolved spectroscopy data.

Orbital Topology Controlling Charge Injection in Quantum-Dot-Sensitized Solar Cells.

Quantum-dot-sensitized solar cells are emerging as a promising development of dye-sensitized solar cells, where photostable semiconductor quantum dots replace molecular dyes. Upon photoexcitation of a quantum dot, an electron is transferred to a high-band-gap metal oxide. Swift electron transfer is crucial to ensure a high overall efficiency of the solar cell. Using femtosecond time-resolved spectroscopy, we find the rate of electron transfer to be surprisingly sensitive to the chemical structure of the linker molecules that attach the quantum dots to the metal oxide. A rectangular barrier model is unable to capture the observed variation. Applying bridge-mediated electron-transfer theory, we find that the electron-transfer rates depend on the topology of the frontier orbital of the molecular linker. This promises the capability of fine tuning the electron-transfer rates by rational design of the linker molecules.