Effect of Coordination Sphere Geometry of Copper Redox Mediators on Regeneration and Recombination Behavior in Dye-Sensitized Solar Cell Applications

The recombination of injected electrons with oxidized redox species and regeneration behavior of copper redox mediators are investigated for four copper complexes, [Cu(dmby)2]2+/1+ (dmby = 6,6′-dimethyl-2,2′- bipyridine), [Cu(tmby)2]2+/1+ (tmby = 4,4′,6,6′- tetramethyl-2,2′-bipyridine), [Cu(eto)2]2+/1+ (eto = 4-ethoxy-6,6′-dimethyl-2,2′-bipyridine), and [Cu- (dmp)2]2+/1+ (dmp = bis(2,9-dimethyl-1,10-phenantroline). These complexes were examined in conjunction with the D5, D35, and D45 sensitizers, having various degrees of blocking moieties. The experimental results were further supported by density functional theory calculations, showing that the low reorganization energies, λ, of tetra-coordinated Cu(I) species (λ = 0.31−0.34 eV) allow efficient regeneration of the oxidized dye at driving forces down to approximately 0.1 eV. The regeneration electron transfer reaction is in the Marcus normal regime. However, for Cu(II) species, the presence of 4-tert- butylpyridine (TBP) in electrolyte medium results in penta-coordinated complexes with altered charge recombination kinetics (λ = 1.23−1.40 eV). These higher reorganization energies lead to charge recombination in the Marcus normal regime instead of the Marcus inverted regime that could have been expected from the large driving force for electrons in the conduction band of TiO2 to react with Cu(II). Nevertheless, the recombination resistance and electron lifetime values were higher for the copper redox species compared to the reference cobalt redox mediator. The DSC devices employing D35 dye with [Cu(dmp)2]2+/1+ reached a record value for the open circuit voltage of 1.14 V without compromising the short circuit current density value. Even with the D5 dye, which lacks recombination preventing steric units, we reached 7.5% efficiency by employing [Cu(dmp)2]2+/1+ and [Cu(dmby)2]2+/1+ at AM 1.5G full sun illumination with open circuit voltage values as high as 1.13 V.

Phenoxazine Dyes for Dye-Sensitized Solar Cells and Relationship Between Molecular Structure and Electron Lifetime

Ulf Anders Hagfeldt, Xiao Jiang

Metal-free org. dyes with a core phenoxazine chromophore have been synthesized and tested as sensitizers in dye-sensitized solar cells. Overall conversion efficiencies of 6.03-7.40% were reached under std. AM 1.5 G illumination at a light intensity of 100 mW cm-2. A clear trend in electron lifetime could be seen; a dye with a furan-conjugated linker showed a shorter lifetime relative to dyes with the acceptor group directly attached to the phenoxazine. The addn. of an extra donor unit, which bore insulating alkoxyl chains, in the 7-position of the phenoxazine could increase the lifetime even further and, together with additives in the electrolyte to raise the conduction band, an open circuit voltage of 800 mV could be achieved. From photoelectron spectroscopy and x-ray absorption spectroscopy of the dyes adsorbed on TiO2 particles, the excitation is mainly of cyano character (i.e., on av., the dye mols. are standing on, and pointing out, from the surface of TiO2 particles).

2011

Solid hybrid dye-sensitized solar cells

Nathalie Rossier-Iten

Dye-sensitized solar cells (DSC), introduced by O'Regan and Grätzel in 1991, are a low cost alternative to conventional silicon photovoltaic cells, the latter requiring extremely pure starting materials and sophisticated production procedures. DSC's, based on an inorganic wide band-gap semiconductor (TiO2) coated by a ruthenium polypyridyl complex as dye, have been studied and improved over the last decade and have reached a considerable solar to electric conversion efficiency of 11 % over the standard air mass (AM) 1.5 spectrum (100 mWcm-2). Traditionally, a liquid electrolyte redox system is used to regenerate the photo-excitated dye and carry current through the cell. Practical advantages have been gained by replacing the liquid electrolyte with an organic solid hole-transporting material (HTM). These type of cells exhibit a record efficiency of 4 % with 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) as the HTM. Clearly this lags behind the performance features of liquid electrolyte dye-sensitized solar cells, as the research of the solid-state solar cells (SSD) is still in its infancy. The objective of the present work is to study the SSD performance limitations emerging from the cells and to find strategies for addressing these problems. Interfacial charge recombination is a significant loss mechanism in DSC. This is particularly true for solid-state devices, as the solid HTM is less efficient in screening the internal fields, which assist recombination. The primary, one electron-process, charge recombination occurs between injected electrons in the TiO2 and holes in the oxidized HTM. In this work, a strategy is developed to minimize interfacial charge recombination by using guanidinium carboxylic acid molecules as co-adsorbents to the dye molecules on the TiO2 nanocrystals. The charge-recombination rate of the injected electrons in the TiO2 with the holes in the HTM was retarded by an order of magnitude. The open-circuit voltage was also significantly improved by 14%. We attribute these effects to be due to a "space-filling" on the TiO2 surface and "columbic-screening" of the electrons in the TiO2 from the holes in the HTM. The dipolar nature of the co-adsorbent molecules is also likely to contribute to improvements in open-circuit voltage by further offsetting the energy levels between the n-type semiconductor and the HTM. In order to decrease the recombination of electrons, from the semiconductor, with holes in the HTM, the hole concentration must be reduced. For this reason, new hole-conductors, based on triphenyldiamine, with high hole-mobility have been investigated, avoiding the high degree of oxidation otherwise required in the standard HTM (less hole concentration). The 1,3,5-tris(N-(1-naphtyl)-N-[4-(1-naphthylphenylamino)-phenyl]-amino)-benzene (TTADB), having a very high hole-mobility, exhibits an extremely large open-circuit voltage on flat TiO2 films compared to the Spiro (30% higher). The 4,4',4''-tris(N-3-methylphenyl)-N-phenylamino)-triphenylamine (p-MTDATA) showed an improvement of 35% in the current density for very thin TiO2 films (0.5 µm). However these TPD molecules do not fill the nanocrystalline layer. Another nanoporous filling method has to be found for these HTM candidates to enable them to compete with the Spiro. In order to understand the weaknesses of the materials and interfaces during the aging process of solid DSC's, their long-term stability was studied under constant illumination. The experiments showed a destructive decrease of the shunt resistance and a rapid loss of the oxidized form of the HTM, Spiro-OMeTAD, during constant 0.75 sun illumination. Meanwhile, the short-circuit current of the cells increased during the first five hours of illumination. This phenomenon has already been observed and was attributed to a massive reduction of the Schottky barrier size, at the anode interface. This resulted in the generation of surface states and in a "pinning" of the Fermi level in the semiconductor at this junction. After the first 5 hours, however, the degradation of the solid cells is much more apparent than in the absence of UV light, where cells stabilized after approximately 20 hours with a loss of short circuit current of ∼ 50%. Strategies towards flexible solid-state DSC were investigated. For the flexible cell construction a metal foil was used as substrate and a semi-transparent gold layer as counter electrode, which allowed light transmission through back illumination.

EPFL2006

Solid hybrid dye-sensitized solar cells

Nathalie Rossier-Iten

EPFL2006