E. Vella | Searcharxiv

arXiv2016

Do ultrafast exciton-polaron decoherence dynamics govern photocarrier generation efficiencies in polymer solar cells?

All-organic-based photovoltaic solar cells have attracted considerable attention because of their low-cost processing and short energy payback time. In such systems the primary dissociation of an optical excitation into a pair of photocarriers has been recently shown to be extremely rapid and efficient, but the physical reason for this remains unclear. Here, two-dimensional photocurrent excitation spectroscopy, a novel non-linear optical spectroscopy, is used to probe the ultrafast coherent decay of photoexcitations into charge-producing states in a polymer:fullerene based solar cell. The two-dimensional photocurrent spectra are interpreted by introducing a theoretical model for the description of the coupling of the electronic states of the system to an external environment and to the applied laser fields. The experimental data show no cross-peaks in the two-dimensional photocurrent spectra, as predicted by the model for coherence times between the exciton and the photocurrent producing states of 20\,fs or less.

cond-mat.mtrl-sci↗

arXiv2010

Temperature dependence of reflectivity of amorphous silicon dioxide: Evidence of delocalized excitons weakly scattered by phonons

We studied the reflectivity spectra of amorphous silicon dioxide detected under vacuum UV synchrotron radiation as a function of temperature between 10 and 300 K. Kramers-Kronig dispersion analysis of reflectivity spectra allowed us to determine the absorption coefficient in the range from 8 to 17.5 eV. Spectra show four main peaks, the spectral positions of which are consistent with literature data. An appreciable dependence of the line-shape on temperature is observed for the first two peaks only. We demonstrate the exciton peak at 10.4 eV to have a very good Lorentzian band-shape at all the examined temperatures. Based on existing theoretical models, this allows to argue excitons in SiO2 to be weakly scattered by phonons, thus retaining their mobility properties notwithstanding the effects of exciton-phonon coupling and of intrinsic structural disorder of amorphous SiO2. Moreover, the observed temperature dependence of the peak position together with the features of the Urbach absorption tail and of self-trapped exciton emission allow us to estimate the main parameters ruling exciton dynamics in SiO2. The features of the intrinsic Urbach absorption tail can be satisfactorily explained as a consequence of those of the first excitonic peak, supporting the interpretation of the Urbach tail in SiO2 as a consequence of the momentary self-trapping of the 10.4 eV exciton. Finally, the characteristics of the other energy peaks are discussed and an excitonic origin also for the 11.6 eV peak is put forward. On the whole, our results show that exciton dynamics accounts for all optical properties of pure silicon dioxide from 8 up to 11 eV.

cond-mat.dis-nn↗