Nanoscale characterization of dye sensitized solar cells - Kelvin probe force microscopy in liquid

Dye Sensitised Solar Cells (DSSCs) are a recent solar cell type based on the sensitisation of a nanostructured wide band gap semiconductor such as titania with a light absorbing dye molecule. The sensitisation is necessary in order to exploit the energy content of the visible light, which is not absorbed by the titania. The nanostructuring of the semiconductor is fundamental for a highly efficient DSSC, since it multiplies the surface area and thereby the amount of adsorbed dye and thus of absorbed light. Despite the dependence of DSSCs on the nanostructuring of their active layer, most science treating DSSCs is concerned with entire cells, and studies examining the nanoscale properties such as the work function on the single nanoparticles are rare.
To fill this gap we performed surface potential measurements on bare and sensitised nanoporous titania layers for DSSCs in different environments including electrolyte solutions. The only recently described Open Loop Kelvin Probe Force Microscopy (OLKPFM) mode was employed, since it enables the measurement of the surface potential with nanoscale resolution without the necessity to apply a direct voltage (dc-voltage) and thus is not limited to measurements in air and non-polar solvents.
Both the fabrication of DSSCs and the surface potential measurement in a liquid environmenthad to be introduced to our workgroup as part of this work. Thus the relevant background information and the development of the necessary methods and expertise is the basis of this thesis. After the introduction of the general setup and working principle of DSSCs and a more detailed discussion of its key components and processes, the basics of surface potential measurement on the nanoscale are outlined. The necessary steps to fabricate reproducible DSSCs are treated in detail together with the identification of some dead-ends on the way there.
Surface potential measurements in air, water, sodium chloride solutions of different concentrations, and highly diluted electrolytes for DSSCs were performed on titania layers and a model sample. The measurements in air were compared to data recorded by traditional Kelvin Probe Force Microscopy (KPFM) measurements based on the compensation of the electrostatic force by the application of a dc-voltage between sample and tip. The comparison yielded equal contrast and distribution of the measured surface potential. All KPFM measurements are based on the modelling of tip and sample as the two plates of a plate capacitor. An excellent agreement of the measured data with the applied model used for the calculations of the surface potential was found not only for the measurements in air, but also in water. In combination with an excellent agreement between the data recorded in air and in water, the proof of the applicability of the basic model enabled the expansion of the application range of KPFM to polar liquids. In all the experiments performed in electrolyte solutions a strong non-linearity of the capacitance of the tip-sample system was observed. Possible reasons for the non-linear behaviour were identified and discussed. Examples are the diffusion of ions, shielding of electrical fields by charged layers and onsets of chemical reactions. The non-linearity impedes the application of the basic model, and thus the calculation of quantitative parameters from the measurement. However qualitative interpretation shows, that variations of the surface potential can still be detected and thereby valuable information about examined surfaces can be gained. Applied to the titania layers for DSSCs our measurements indicate a similar distribution of the dipoles on the titania surface upon sensitisation as was determined in air, and thus it shows the possibility to transfer findings made in air to the complete solar cell system and its electrolyte surrounding.