Optical Spectroscopy on Organic-Inorganic Hybrid Structures

Organic Functionalization of Inorganic Substrates: Exciton Formation in Organic Layers on Inorganic Substrates The organic molecules pentacene and phthalocyanine both are widely used for functionalization of semiconductors. Different phases form up when growing thin layers at the interface which...

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Bibliographic Details
Main Author: Meyenburg, Ingo
Contributors: Heimbrodt, Wolfram (Prof. Dr.) (Thesis advisor)
Format: Dissertation
Language:English
Published: Philipps-Universität Marburg 2020
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Summary:Organic Functionalization of Inorganic Substrates: Exciton Formation in Organic Layers on Inorganic Substrates The organic molecules pentacene and phthalocyanine both are widely used for functionalization of semiconductors. Different phases form up when growing thin layers at the interface which have to be considered when discussing organic-inorganic hybrids. There remain open questions about exciton formation and optical properties. A thorough investigation of the influence of the substrate on the organic properties is necessary. Different crystalline phases of pentacene and phthalocyanines with an axial anchor including a metal atom (Cu-Pc, TiO-Pc) have been prepared and studied by X-ray diffraction and atomic force microscopy (Group Witte). Optical absorption spectroscopy and photoluminescenc spectroscopy as a function of temperature between room temperature and 10 K have been performed. Peculiarities by growing pentacene on inorganic substrates are discussed. Accessing the Interface - Pentacene on Different Inorganic Substrates The polarization-resolved absorption spectra are determined for different pentacene polymorphs, both, for thin films grown on ZnO as well as for free-standing single crystals. A clear interrelation between the Davydov splitting of the lowest-energy singlet-exciton type transitions and the herringbone angle of the molecules in the unit cell is found, by this the herringbone angles can be determined by optical means. The variation in oscillator strength of the individual excitonic Davydov components with temperature is explained by a variation of this herringbone angle. The extraordinarily strong variation of the herringbone angle for Campbell phase pentacene films grown on ZnO substrates is attributed to interface-mediated strain due to the different thermal expansion coefficients of the organic and inorganic constituents. Charge Transfer in Organic-Inorganic Hybrid Structures We studied the electron transfer from excitons in adsorbed indoline dye layers across the organic-inorganic interface. The hybrids consist of indoline derivatives on the one hand and different inorganic substrates (TiO2, ZnO, SiO2(0001), fused silica) on the other. We reveal the electron transfer times from excitons in dye layers to the organic-inorganic interface by analyzing the photoluminescence transients of the dye layers after femtosecond excitation and applying kinetic model calculations. A correlation between the transfer times and four parameters have been found: (i) the number of anchoring groups, (ii) the distance between the dye and the organic-inorganic interface, which was varied by the alkyl-chain lengths between the carboxylate anchoring group and the dye, (iii) the thickness of the adsorbed dye layer, and (iv) the level alignment between the excited dye pi*-level) and the conduction band minimum of the inorganic semiconductor. Optimization of Solar Cells: Revealing Level Alignment and Cluster-Formation An efficient charge transfer from absorbing dye across the organic-inorganic interface into the semiconductor is crucial for high external quantum efficiencies in dye sensitized solar cells (DSSC). We use voltage dependent time-resolved photoluminescence in order to determine independently the charge transfer time from relaxed excited dye states into the conductive band minimum of the semiconductor, in consequence giving access to the level-alignment between dye and semiconductor. We can distinguish between excitons reaching the interface and excitons recombining due to quenching in the dye layers, which is especially important to detect unwanted agglomeration in DSSC. The advantages and disadvantages of panchromatic cells will be discussed.
Physical Description:226 Pages
DOI:https://doi.org/10.17192/z2021.0104