Influence of dye characteristics on the photoelectrochemical kinetics in dye-sensitized solar cells

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FalgenhauerJane_2018_02_09.pdf (15.24 MB)

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2017

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DOI:
http://dx.doi.org/10.22029/jlupub-9778

Abstract

In this work, the properties of dye sensitized solar cells after the application of different preparation parameters were characterized. The focus lies on solar cells based on electrodeposited ZnO, sensitized with four different indoline dyes and for three different sensitization times. The four indoline sensitizers included the established dye D149 with one carboxylic anchor group, and three less investigated indoline dyes containing a second anchor group with an increasing length of the alkyl spacer, DN91, DN216 and DN285. Additionally to current voltage curves the solar cells were characterized by different photoelectrochemical methods like impedance spectroscopy, which allowed the assignment of the observed changes in cell parameters e.g. to increased recombination due to molecular aggregates, or to a shift of the conduction band edge. For a given sensitization time and at a given density of states, indoline dyes with a longer alkyl spacer showed a slightly decreased recombination due to a better shielding of the ZnO surface against the electrolyte. For the variation of the adsorption time, intermediate sensitization times of 15 min led to the best cell parameters, as more dye was adsorbed compared to a sensitization for 1 min, and yet recombination was not increased by a higher extent as observed for a sensitization for 1 h. A sensitization of the indoline dyes without coadsorbate resulted in cells with an increased recombination due to an increase in aggregation, on the other hand the short circuit current increased because of the larger amount of adsorbed dye. The observed increase in the short circuit current density during a storage of cells in the dark was traced to a shift of the conduction band edge to lower energies, and thus an enhanced injection. For all cells, very different recombination was observed for measurements at different illumination intensities, which was explained by a decreased regeneration efficiency at high illumination intensities, leading then to increased recombination via oxidized dye molecules. ZnO-based cells sensitized by phthalocyanine, perylene and triphenylamine dyes, and TiO2-based cells sensitized by different indoline dyes served for a comparison, where e.g. perylene dyes showed very low power conversion efficiency due to high aggregation and a reduced injection because of low dye states. For cells based on TiO2 the power conversion efficiency and the short circuit current density ISC was low after preparation, however the cell values increased upon illumination with AM1.5 because of a considerable shift of the conduction band edge to lower energies as observed by impedance measurements. The decrease of the conduction band edge proved beneficial for the TiO2-based cells, as ISC increased by a factor of 2, and VOC decreased only slightly because of a decreased recombination. For the cells in this work, with the complete set of methods each change in cell parameters could be assigned to a specific cause. A simulation of the chemical capacitance Cµ of the cells by a combination of different capacitance contributions corresponded well with measured values, and helped to elucidate different influences on Cµ.

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