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
