Cyclometalated Ruthenium Complexes for Dye-Sensitized Solar Cells

The transition towards carbon-free sources of energy is vital for humankind. This process is feasible through the extensive use of photovoltaic technologies, which may convert solar energy into elec-trical and chemical energy. In the last half-century, many competitive technologies were developed. One promising type of solar cell is Dye-Sensitized Solar Cell (DSC), which differ from other solar cells in many ways. DSCs can be produced in different colors and used as constructing components of urban buildings. Additionally, DSCs have inner beauty related to their working principle. The pro-cesses of light absorption and charge transport are spatially separated in many ways reminiscent of water oxidation in Photosystem II in nature. Historically, DSCs were developed with an electrolyte based on an iodine-iodide redox shuttle. However, the overall performance of these solar cells is restricted due to the high loss of potential imparted by multistep dye regeneration by iodide. Shift to the outer-sphere one-electron redox couple, such as Co3+/2+ imine complexes, boosted the field, and new record efficiencies were achieved. Still, new redox mediators failed to provide reasonable power conversion efficiencies using the classic ruthenium sensitizers with isothiocyanate ligands. Poor performance is related to high charge recombination rates. To tackle this problem, I present my research on new cyclometalated ruthenium (II) complexes as sensitizers for DSCs that employ cobalt-based electrolytes. I introduce tris-heteroleptic cyclometalated Ru (II) complexes, which have various organic substituents. Optical and electrochemi-cal analyses are conducted before applying the new sensitizers in state-of-the-art solar cells. In this work, a record-high power conversion efficiency of 9.4 % was obtained with a ruthenium sensitizer and a cobalt-based redox shuttle. Transient absorbance spectroscopy, electrochemical impedance spec-troscopy, and transient photovoltage and photocurrent decay measurements were conducted to reveal the main factors that limit the performance of the solar cells. The role of dye-loading, which is generally ignored in the field, was shown to be crucial. Additionally, it was shown that organic substituents that are usually attached to improve the photophysical properties of sensitizers cause irreversible sensi-tizer electrochemical oxidation. Further, in this thesis, I discuss the role of sulfur atoms in the sensitizer structure on the performance of the solar cell. Then I introduce new bidentate ligands, which coordinate to the metal with both cyclometalation and N-heterocyclic carbenes. I investigate the potential of new complexes with presented ligands as sensitizers. In the last section, I discuss the extended anchoring ligands and their complexes with ruthenium, which have six-membered chelating rings. In this section I also introduce new binding mode for the pyridine-type of ligand. Finally, I briefly summarize the obtained results.


Advisor(s):
Nazeeruddin, Mohammad Khaja
Graetzel, Michael
Year:
2018
Publisher:
Lausanne, EPFL
Keywords:
Laboratories:
LPI




 Record created 2018-02-05, last modified 2018-12-05

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