Electron transport and recombination in dye-sensitized solar cells with ionic liquid electrolytes

The electron transport and recombination in dye-sensitized TiO2 solar cells with different electrolytes have been investigated. The electrolytes were based on iodine-doped ionic liqs. (diethylmethylsulfonium iodide, dibutylmethylsulfonium iodide or 3-hexyl-1-methylimidazolium iodide) or an org. solvent (3-methoxypropionitrile with LiI, I2 and 1-methylbenzimidazole). The most viscous electrolytes showed a clear limitation in photocurrent, which can be attributed to a low diffusion coeff. for the triiodide that transports pos. charge to the counter-electrode. No essential difference was found in the electron transport properties of the solar cells, which appear to be dominated by the properties of the nanostructured TiO2 film. The intensity-modulated photocurrent response was strongly affected by current satn. It is proposed that this is caused by recombination processes. The electron lifetime is depending on the type of cation used in the ionic liq., where bulky, less absorptive cations seem to give longer lifetimes.

Influence of π-Conjugation Units in Organic Dyes for Dye-Sensitized Solar Cells

Ulf Anders Hagfeldt, Peng Qin

Two org. dyes with the general structure donor-conjugated chain-acceptor (D-π-A) were studied as sensitizers for nanocryst. TiO2 solar cells. The electron donor and acceptor groups were pyrrolidine and cyano acrylic acid, resp. The conjugated chain of 2-cyano-3-{5-[2-(4-pyrrolidin-1-ylphenyl)vinyl]thiophen-2-yl}acrylic acid contains one Ph ring and a thiophene unit and is therefore denoted PT, while for 2-cyano-3-{5-[2-(5-pyrrolidin-1-ylthiophen-2-yl)vinyl]thiophen-2-yl}acrylic acid the Ph ring is replaced by a 2nd thiophene unit (TT). Solar-to-elec. energy conversion efficiencies under simulated AM 1.5 irradn. (1000 W m-2) of 2.3% were obtained for solar cells based on PT but of

2007

Copper Bipyridyl Redox Mediators for Dye-Sensitized Solar Cells with High Photovoltage

Yiming Cao, Marina Freitag, Fabrizio Giordano, Ulf Anders Hagfeldt, Ladislav Kavan, Jacques-Edouard Moser, Norman Pellet, Yasemin Saygili, Magnus Söderberg, Nikolaos Vlachopoulos, Shaik Mohammed Zakeeruddin

Redox mediators play a major role determining the photocurrent and the photovoltage in dye-sensitized solar cells (DSCs). To maintain the photocurrent, the reduction of oxidized dye by the redox mediator should be significantly faster than the electron back transfer between TiO2 and the oxidized dye. The driving force for dye regeneration with the redox mediator should be sufficiently low to provide high photovoltages. With the introduction of our new copper complexes as promising redox mediators in DSCs both criteria are satisfied to enhance power conversion efficiencies. In this study, two copper bipyridyl complexes, Cu(II/I)(dmby)2TFSI2/1 (0.97 V vs SHE, dmby = 6,6′-dimethyl-2,2′-bipyridine) and Cu(II/I)(tmby)2TFSI2/1 (0.87 V vs SHE, tmby = 4,4′,6,6′- tetramethyl-2,2′-bipyridine), are presented as new redox couples for DSCs. They are compared to previously reported Cu(II/I)(dmp)2TFSI2/1 (0.93 V vs SHE, dmp = bis(2,9-dimethyl-1,10-phenanthroline). Due to the small reorganization energy between Cu(I) and Cu(II) species, these copper complexes can sufficiently regenerate the oxidized dye molecules with close to unity yield at driving force potentials as low as 0.1 V. The high photovoltages of over 1.0 V were achieved by the series of copper complex based redox mediators without compromising photocurrent densities. Despite the small driving forces for dye regeneration, fast and efficient dye regeneration (2−3 μs) was observed for both complexes. As another advantage, the electron back transfer (recombination) rates were slower with Cu(II/I)(tmby)2TFSI2/1 as evidenced by longer lifetimes. The solar-toelectrical power conversion efficiencies for [Cu(tmby)2]2+/1+, [Cu(dmby)2]2+/1+, and [Cu(dmp)2]2+/1+ based electrolytes were 10.3%, 10.0%, and 10.3%, respectively, using the organic Y123 dye under 1000 W m−2 AM1.5G illumination. The high photovoltaic performance of Cu-based redox mediators underlines the significant potential of the new redox mediators and points to a new research and development direction for DSCs.

American Chemical Society2016

The effect of dye coverage on the performance of dye-sensitized solar cells with a cobalt-based electrolyte

Ulf Anders Hagfeldt

The effect of dye coverage of the mesoporous TiO2 electrode on the performance of dye-sensitized solar cells based on the cobalt tris(bipyridine) electrolyte and the D35 dye was studied in detail. The dye coverage was controlled by using a dye bath with different dye concentrations and containing an inert salt, LiClO4, which was found to promote equilibrium conditions in the dye adsorption process. The amount of adsorbed D35 dye on mesoporous TiO2 was reasonably fit using the Langmuir adsorption isotherm, with a binding constant of 55 000 M-1. Upon increasing the dye coverage on the TiO2 electrode, the electron lifetime in the dye-sensitized solar cell increased remarkably, demonstrating the blocking behavior of the D35 dye at the TiO2-electrolyte interface. Consequently, the solar cell efficiency increased dramatically with the D35 dye coverage. Â© 2014 the Owner Societies.

2014

Copper Bipyridyl Redox Mediators for Dye-Sensitized Solar Cells with High Photovoltage

American Chemical Society2016