Browsing by Author "Minda, Iulia"

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    Charge dynamics in hybrid and organic-inorganic light harvesting thin films followed with femtosecond transient absorption spectroscopy
    (Stellenbosch : Stellenbosch University, 2017-12) Minda, Iulia; Schwoerer, Heinrich; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT : In order to bridge the gap between traditional sources of electricity and the increasing global demand for it, as a society we must move towards renewable sources of energy such as solar radiation. Photovoltaic devices (PVs) harness solar power and convert it to electrical power. In order to be commercially viable, they need to be efficient, cost effective, simple to fabricate and environmentally friendly. To address these requirements, the class of emerging PVs arose, which includes dye sensitised solar cells (DSSCs) and perovskite solar cells. Femtosecond transient absorption spectroscopy (TAS) is an experimental technique which allows us to follow the ultrafast photoinduced charge dynamics in real time in light harvesting thin films and PVs. By assigning time and rate constants to various processes governing the charge generation and extraction in solar cells, we construct charge dynamics models, and therefore learn the fundamental photophysics reasons behind what makes the power conversion efficiencies (PCEs) of some solar cells superior. In particular, this study focused on the charge transfer processes in indoline dye (DN216) sensitised electrodeposited ZnO solar cells, and the charge recombination dynamics in FA0.85MA0.15PbI2.55Br0.45 perovskite thin films. To construct the simplest fully consistent charge dynamics models, we match the visible and near-infrared spectroscopic signals of our samples to the allowed electronic transitions, and follow their temporal evolutions on the femtosecond and picosecond time scales. From our measured time and rate constants we observed that ZnO based DSSCs are less efficient than their TiO2 counterparts because the electron injection from the photoexcited indoline dye into the ZnO CB doesn’t just occur directly (< 200 fs), but also stepwise via neutral (∼ 2 ps) and ionic (∼ 10 ps) intermediate charge transfer states, resulting from surface trap states characteristic of electrodeposited ZnO. Moreover, FA0.85MA0.15PbI2.55Br0.45 is an excellent hybrid photoabsorber in record efficiency perovskite solar cells because even at high charge carrier densities of 1019 cm−3 , the third order non-radiative Auger recombination mechanism is not dominant. Furthermore we determined the associated geminate, non-geminate and Auger recombination rate constants as A = 5 × 109 s −1 , B = 10−10 s −1 cm3 and C = 50 × 10−32 s −1 cm6 .
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    Photoinduced charge dynamics in indoline-dye sensitised solar cells
    (2014-12) Minda, Iulia; Schwoerer, Heinrich; Schlettwein, Derck; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: The demand for renewable energy sources has grown out of the humanity’s increasing need for electricity as well as depleting fossil fuel reserves. Organic-dye sensitised solar cells were developed as a green, cost-effective alternative to the market-dominating silicon solar cell technology. The field of photovoltaic devices and organic-DSSCs is interesting because we want to develop better, more efficient cells at lower costs using environmentally friendly materials. By studying the fundamental physics and chemistry processes occurring during and after the interaction of light with these devices, we create a window into the mechanism of photosynthesis. Our DSSCs were prepared by sensitisation of highly porous ZnO with different indoline dyes containing the same chromophore, but different alkyl chain lengths bonded to one of two carboxyl anchors as: DN91 (1 C) < DN216 (5 C) < DN285 (10 C). The role of the dye molecules is to absorb photons and donate electrons to the ZnO which acts as the charge acceptor, at the dye|ZnO interface. Through photoelectrochemical characterisation it was found that the structure of the dyes has an effect on the maximum current (JSC) produced by the cells: the shorter the alkyl chain, the higher the JSC. This macroscopic investigation was complimented by microscopic measurements in the form of transient absorption spectroscopy. This allows us to follow, in real time, the photoinduced oxidation of the dye and its regeneration occurring through desired and undesired pathways. It was found that the injection efficiencies of the dye molecules were directly responsible for the trend in the short circuit currents.