Department of Physics

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Browsing Department of Physics by Subject "Absorption spectroscopy"

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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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    Development of VUV tunable laser spectroscopy techniques for characterizing calcium fluoride
    (Stellenbosch : Stellenbosch University, 2014-12) Matindi, Tresor; Steenkamp, Christine M.; Rohwer, Erich G.; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: The large band gap (approximately 11.5-12.1 eV) and high transmission of calcium fluoride (CaF2) crystal in the ultraviolet (UV) and vacuum ultraviolet (VUV) region makes it an important material for optics for laser applications in UV. However, CaF2 degrades during long exposure to UV irradiation due to defect generation. The formation of selftrapped excitons (STE) is considered the first step in defect generation. In this project the possibility of observing STE states in CaF2 using a narrow bandwidth tunable VUV laser source is investigated. This is the first spectroscopy study of an alkaline earth fluoride using VUV tunable laser radiation instead of a fixed wavelength laser. The use of a VUV tunable laser source has potential for determining the energies of the STE states, which are unknown. Our main objective is addressed by developing techniques to measure absorption spectra of pure and doped CaF2 samples, using a VUV scanning monochromator and a tunable VUV laser, and by doing a literature study. The results obtained with the scanning monochromator show absorption features in 126-180 nm range of all our samples. These vary for different samples and correlate with information from the supplier on the samples’ fluorescence spectra. Total absorption of the VUV light by CaF2 in the 115-126 nm range is observed. With the narrow bandwidth tunable laser light, absorption spectra were obtained in the range of 143-146.7 nm of all our CaF2 samples. No significance peaks which can be related to the STE states in CaF2 were observed in the VUV laser absorption spectra, but the results are valuable to improve the technique. The conclusion is that either a different spectral range or fluorescence detection can be investigated in future.