Doctor of Philosophy
School of Chemistry
Zhao, Long, Di-chromophoric porphyrins: exploring new strategies for enhanced light harvesting and increased photovoltage of dye-sensitized solar cells, Doctor of Philosophy thesis, School of Chemistry, University of Wollongong, 2016. http://ro.uow.edu.au/theses/4630
Although numerous improvements have been achieved in the field of dye-sensitized solar cells (DSSCs), there are still some potential strategies for further development in this technology. In that regard, the most straightforward strategy is to enhance the dye light harvesting properties, in particular, by using low band gap dyes. While such approach would in principle increase the photocurrent, low band gap sensitizers typically yield relatively low photovoltage. This phenomenon is caused by strong intermolecular forces, inherent of low band gap materials. Therefore, to simultaneously increase the photocurrent and photovoltage, new concept of the dye design is required. For instance, a multi-chromophoric dye structure, which is basically a single molecule containing several independent light absorbing units.
Thus, the present thesis concerns with the investigation of different di-chromophoric dyes in DSSCs. As will be shown, these novel dyes can overcome the typical limitations of low band gap dyes in photovoltage, while increasing the photocurrent as a result of the increased light absorption. They not only enhance light absorption and overcome dispersion forces, but also feature other benefits owing to the tri-dimensionality. This is for instance the addition of bulky groups, which can hinder non desired processes such as recombination. The electron lifetimes and consequently the photovoltage of DSSCs increase as a result. Aspects related to the arrangement of the dyes on the photoanode are also benefited from such groups, which enhance the electron injection efficiency and photocurrent. Therefore, in some cases, co-adsorbers such as chenodeoxycholic acid are no longer needed in the di-chromophoric dye sensitization process. The utilization of the cobalt-based electrolyte in DSSCs using di-chromophoric dyes is also investigated in this thesis in terms of photovoltaic performance and dye regeneration. In a specific case, a microsecond component of partial intramolecular hole transfer is observed in a di-chromophoric dye for the first time, and dye regeneration kinetics in this dichromophoric dye is favoured in respect with the single chromophore of similar driving force.
Attaching an organic chromophore with tuned band gap to extend light absorption of the dyad towards the red may introduce competing electron injection pathways, probing a limitation of di-chromophoric dye using low band gap chromophore. Although the latter effect reduces the photocurrent of DSSCs, the electron lifetimes and photovoltage are increased. A comparison between di-chromophoric dye and co-sensitization approaches has been carried out. Utilizing intermolecular forces to enhance the electron lifetimes in DSSCs using di-chromophoric dye will be reported for the first time.
Although the power conversion efficiencies of DSSCs using the di-chromophoric dyes still lag behind compared to that of traditional dyes, this thesis provides new prospects in electron transfer mechanisms at the dye-sensitized interfaces. Thus, some of the findings of this work, such as the utilization of dispersion forces and the enhanced dye regeneration kinetics without free energy losses in di-chromophoric dyes, provide new strategies in the further multi-chromophoric dye design. These new insights into the multi-chromophoric dyes behaviour would lead the way to the further development of photovoltaic technology.