Implementierung kohlenstoffreicher Farbstoffe und einwandiger Kohlenstoffnanohörner in Farbstoffsensibiliserte Solarzellen

Abstract

Given the emerging field of nanostructured electronic devices, a multitude of different concepts for further improvements is discussed in the scientific community. Especially in the field of solar cells, vast varieties of approaches for improvements are proposed. However, in some cases the impact on the environment during the assembly procedure is left aside. The thrust of this thesis was to tackle the concepts of improving the performance, long-term stability, and ecological footprint of dye-sensitized solar cells (DSSCs). This was accomplished by employing novel, carbon-rich photosensitizers and implementing single-walled carbon nanohorns (SWCNHs) into different parts of TiO2-based DSSCs. To this end, two sensitizing schemes were adsorbed onto mesoporous TiO2 networks, namely i) benzoporphyrins (BPs) were introduced as a new dye class for DSSCs, and ii) carbon nanodots (CNDs) were implemented as novel nanocarbon photosensitizer for low-cost solar cell applications. The former exhibits a unique adsorption behavior depending on the metalation of the porphyrin unit, the TiO2-particle size, and the nature of the solvent. This enabled a selective adsorption onto different parts of the TiO2-network. As a consequence, unfavorable energy transfer processes between the two different chromophores could be circumvented. In contrast to the conventional dye-TiO2 binding scheme, novel CNDs were tested as potential replacement for complex and, as a consequence, expensive dyes, which are synthesized via multi-step procedures or contain heavy metal elements. CNDs are, however, synthesized in a one-step procedure in water. With this approach, the costs of the photosensitizer and the environmental impact of the overall process of DSSC assembly could be minimized. As another strategy, SWCNHs were implemented into different parts of the DSSC. Firstly, SWCNHs were employed as replacement for the standard TiCl4 treatment, which constitutes a health hazard and represents an increasing production of inorganic waste due to its fast degradation. To this end, nanometer-sized buffer layers of SWCNHs were deposited onto fluorine-doped tin oxide (FTO) transparent glass slides. A similar efficiency compared to the TiCl4-treated DSSCs could prove that SWCNHs are a sound alternative to TiCl4. Secondly, SWCNH layers were tested to replace the rare earth element Platinum (Pt), which is normally used as a counter electrode (CE) material. Due to their excellent catalytic activity towards electrolyte regeneration, SWCNH-based CEs featured similar efficiencies compared to Pt-based ones. Furthermore, no high temperature sintering of the SWCNH-based films was necessary, which facilitates the assembly process and lowers the energy footprint for the device assembly. Finally, since SWCNHs feature remarkable properties, such as their catalytic reduction of I3- to I-, and their good miscibility with common organic solvents and ionic liquids, SWCNH-based solid- and quasi solid-state ionic electrolytes were tested in DSSCs. Implementing SWCNHs introduced a higher ionic conductivity and a catalytically activity into the electrolyte, which improved both the performance and long-term stability of DSSCs. For investigating the novel photosensitizer schemes and SWCNH-based DSSCs, a broad range of electrochemical, spectroscopic, and microscopic techniques was employed. In particular, current density vs. voltage (J-V) measurements, electrochemical impedance spectroscopy (EIS), incident photon-to-current efficiency (IPCE), and Raman experiments were at the forefront for characterizing the dynamic processes involved in DSSCs. Several techniques such as steady-state absorption and fluorescence spectroscopy, time-resolved femtosecond laser absorption spectroscopy, X-Ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscopy (TEM) complemented the investigations in order to unravel the mechanisms and working principles involved in SWCNH-based DSSCs