Browsing by Author "Ahmed, Essraa Taha"

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    A near infrared femtosecond laser source for observation of charge transfer processes in semiconductors
    (Stellenbosch : Stellenbosch University, 2017-03) Ahmed, Essraa Taha; Schwoerer, Heinrich; Stellenbosch University. Faculty of Science. Dept. of Physics
    ENGLISH ABSTRACT : Near infrared femtosecond laser pulses were generated in the spectral range 900 - 2400 nm using the idler of a noncollinear optical parametric amplifier (NOPA). The amplitude and phase of four exemplary pulses were retrieved using frequency resolved optical gating (XFROG). The durations of these pulses were found to be ≤ 151 fs. The generated pulses were used as a probe to detect the charge dynamics within two types of organic solar cells: indoline DN216/ZnO dye sensitized solar cell (DSSC) and fluorinated Zinc Phthalocyanine FnZnPc/C60 (n = 0, 4, 8, 16) bilayer photovoltaic cell. The two organic cells utilize organic materials as light absorbers. In the DN216/ZnO DSSC, electrons are injected from the dye, the electron donor, into the conduction band of the inorganic semiconductor ZnO upon photoexcitation. The injection into the conduction band of ZnO was found to occur directly in 200 fs, via a neutral charge transfer state (NCT) in 2 ps or via an ionic charge transfer state (ICT) in 10 ps [1]. In the FnZnPc/C60 (n = 0, 4, 8, 16) cells, electrons are injected into the C60 layer after dissociation of excitons at the donor/acceptor heterojunction [2]. A C− 60 anion is formed due to injection of electrons into the C60 layer as well as by direct photoexcitation of the molecule. The C− 60 formation was found to occur instantaneously after photoexcitation of either the donor or the acceptor material, independent of the energetics at the donor/acceptor heterojunction. Charge recombinations of the C− 60 anion within the FnZnPc/C60, n = 0, 4, 8, 16 cells were found to occur in the bulk of the C60 layer in 4 ps ± 1 ps as well as at the surface of the C60 layer with air in 43 ps ± 9 ps.