Elucidating the electronic properties of single-wall carbon nanohorns

Single-walled carbon nanohorns are an allotrope of carbon with promising properties for a variety of applications. Despite their promise, the majority carrier type (i.e. electrons or holes) that defines the electronic properties of this novel semiconductor is poorly understood and so far only indirect measurements have been employed to arrive at contradictory results. Here, we directly determine the majority carrier type in single-wall carbon nanohorns for the first time by means of thermopower measurements. Using this direct method, we show that SWCNH films exhibit a positive Seebeck coefficient indicating that SWCNHs behave as p-type semiconductors. This result is further corroborated by intentionally tuning the hole or electron concentrations of SWCNH layers via redox doping with molecular electron acceptors and donors, respectively. These results provide a framework for both measuring and chemically tuning the majority carrier type in this emerging nanocarbon semiconductor

Implementierung kohlenstoffreicher Farbstoffe und einwandiger Kohlenstoffnanohörner in Farbstoffsensibiliserte Solarzellen

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

Hydrogen bonding mediated orthogonal and reversible self-assembly of porphyrin sensitizers onto TiO2 nanoparticles

We report on the orthogonal, highly directional and reversible self-assembly of porphyrins onto TiO2 nanoparticles by means of hydrogen bonding interactions. Unifying the stable covalent surface attachment of tailored, synthesized Hamilton receptors with the advantages of a non-covalent supramolecular immobilization of porphyrin cyanurates resulted in a redox- and photo-active nanohybrid. The latter was successfully implemented into a new type of supramolecular dye-sensitized solar cells

Substituting TiCl4-carbon nanohorn interfaces for dye-sensitized solar cells

5siAn easy-to-implement strategy to introduce carbon nanohorns as interlayers in the photoelectrode of a dye-sensitized solar cell without affecting its overall performance is presented. This provides a clean, easy, and ecofriendly alternative to achieve highly efficient cells.nonenoneCasillas, Rubén; Lodermeyer, Fabian; Costa, Rubén D.; Prato, Maurizio; Guldi, Dirk MCasillas, Rubén; Lodermeyer, Fabian; Costa, Rubén D.; Prato, Maurizio; Guldi, Dirk M

Carbon nanohorn-based electrolyte for dye-sensitized solar cells

For the first time, carbon nanohorns were implemented into solid-state electrolytes for highly efficient solid-state and quasi-solid-state DSSCs. They feature an effective catalytic behavior towards the reduction of I3 - and enhance the I3 - diffusivity in the electrolyte. In a final device, solar cells with 7.84% efficiency at room temperature were achieved. As a matter of fact, this is the highest reported efficiency for nanocarbon-based electrolytes up to date

Novel nanographene/porphyrin hybrids-preparation, characterization, and application in solar energy conversion schemes

Four novel nanographene/porphyrin hybrids were prepared, characterized, and probed in solar energy conversion schemes. Exfoliation of graphite by means of immobilizing four different porphyrins onto the basal plane of graphene is accompanied by distinct electronic interactions in both the ground and the excited states. In the ground state, a strong loss in oscillator strength goes hand-in-hand with a notable broadening of the porphyrin transitions and, as such, attests to the shift of electron density from the electron donating porphyrins to nanographene. In the excited state, a nearly quantitative quenching of the porphyrin fluorescence is indicative of full charge transfer. The latter is corroborated by femtosecond transient absorption measurements, which reveal the generation of the one-electron oxidized radical cation of the porphyrins with absorption maxima at 490 and 625 nm in the visible region and conduction band electrons in nanographene with features at 890 and 1025 nm in the near infrared region. We have demonstrated the applicability of the new nanographene/porphyrin hybrids in, for example, solar cells. In this regard, the presence of flakes is crucial in terms of influencing the injection processes, preventing aggregation, and reducing recombination losses, which are commonly encountered in porphyrin-based DSSCs. 2013 The Royal Society of Chemistry