Photoinduced Interfacial Electron Injection Dynamics in Dye-Sensitized Solar Cells under Photovoltaic Operating Conditions

We report a pump–probe spectroscopy study of electron injection rates in dye-sensitized solar cell (DSSC) devices. We examine the case of working devices employing an N719 ruthenium sensitizer and an iodide electrolyte. Electron injection is found to occur mainly on a sub-100 fs time scale, followed by a slower component with a lifetime of 26.9 ps, in accordance with previous reports on model samples. The amplitude of this latter component varies with electrolyte composition from 25 to 9%. The appearance of slower components in the electron injection dynamics may be attributed to an aggregated or weakly bound state of the surface-adsorbed N719 sensitizer. Further measurements are reported varying the cell light bias and load conditions, revealing no influence on electron injection dynamics. No other electron injection event is found to occur up to 1 ns. These results show no evidence for a slowdown of electron injection under working conditions compared to model systems for the electrolytes examined in this study.

Influence of Iodide Concentration on the Efficiency and Stability of Dye-Sensitized Solar Cell Containing Non-Volatile Electrolyte

Jacques-Edouard Moser, Shaik Mohammed Zakeeruddin

Dye-sensitized solar cells based on nanocrystalline TiO2 have been fabricated with an amphiphilic ruthenium sensitizer NaRu(4-carboxylic acid-4'-carboxylate) (4,4'-dinonyl-2,2'-bipyridine)(NCS)2, coded as Z-907Na, and a series of non-volatile 3-methoxyproprionitrile (MPN)-based electrolytes with different concentration of 1-methyl-3-propylimidazolium iodide (PMII). The short-circuit photocurrent density increases with increasing iodide concentration until at 1.5M practically quantitative dye regeneration is achieved as proved by time-resolved laser experiments. Devices containing 1.0M PMII electrolyte show excellent stability during long-time thermal aging at 80 degrees C and under light soaking at 60 degrees C.

Wiley-Blackwell2009

Cyclometalated Ruthenium Complexes for Dye-Sensitized Solar Cells

Sadig Aghazada

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.

EPFL2018

Cyclometalated Ruthenium Complexes for Dye-Sensitized Solar Cells

Sadig Aghazada

EPFL2018