Performance of dye-sensitized solar cells (DSCs) was investigated depending on the compositions of the electrolyte, i.e., the electrolyte with a different cation such as Li(+), tetra-n-butylammonium (TBA(+)), or 1,2-dimethyl-3-propylimidazolium (DMPIm(+)) in various concentrations, with and without 4-tert-butylpyridine (tBP), and with various concentrations of the I(-)/I(3)(-) redox couple. Current-voltage characteristics, electron lifetime, and electron diffusion coefficient were measured to clarify the effects of the constituents in the electrolyte on the charge recombination kinetics in the DSCs. Shorter lifetimes were found for the DSCs employing adsorptive cations of Li(+) and DMPIm(+) than for a less-adsorptive cation of TBA(+). On the other hand, the lifetimes were not influenced by the concentrations of the cations in the solutions. Under light irradiation, open-circuit voltages of DSCs decreased in the order of TBA(+)> DMPIm(+) > Li(+), and also decreased with the increase of [Li(+)]. The decreases of open-circuit voltage (V(oc)) were attributed to the positive shift of the TiO(2) conduction band potential (CBP) by the surface adsorption of DMPIm(+) and Li(+). These results suggest that the difference of the free energies between that of the electrons in the TiO(2) and of I(3)(-) has little influence on the electron lifetimes in the DSCs. The shorter lifetime with the adsorptive cations was interpreted with the thickness of the electrical double layer formed by the cations, and the concentration of I(3)(-) in the layer, i.e., TBA(+) formed thicker double layer resulting in lower concentration of I(3)(-) on the surface of the TiO(2). The addition of 4-tert-butylpyridine (tBP) in the presence of Li(+) or TBA(+) showed no significant influence on the lifetime. The increase of V(oc) by the addition of tBP into the electrolyte containing Li(+) and the I(-)/I(3)(-) redox couple was mainly attributed to the shift of the CBP back to the negative potential by reducing the amount of adsorbed Li cations.

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A dye-sensitized solar cell fabricated using the room temperature molten salt, 1-hexyl-3-methylimidazolium iodide, iodine and a low molecular weight gelator as a quasisolid-state electrolyte showed a 5.0% light-to-electricity conversion efficiency under AM 1.5 irradiation, and high-temperature stability.

Dye-sensitized solar cells (DSCs) using ionic liquids, 1-alkyl-3-methylimidazolium iodide (alkyl:  C3−C9), were fabricated with and without a low molecular weight gelator. The highest energy conversion efficiency of 5.0% was obtained from a quasi-solid-state DSC using 1-hexyl-3-methylimidazolium iodide (HMImI). Gelation of these ionic liquids demonstrated better high-temperature durability without decreasing the solar cell efficiency. However, the short-circuit currents (J SC) obtained from these DSCs were about 70% of that obtained from DSCs using organic liquid electrolyte (OLE). To explain the difference of the J SC values between the DSCs using ionic liquid electrolyte (ILE) and OLE, four primitive processes in DSCs, that is, charge transport in the electrolytes, light absorption by I3 -, electron diffusion in a TiO2 electrode, and charge recombination, were examined. Viscosities of the ILE decreased with increasing I3 - concentration and alkyl chain length. In ILE, measured J SC values increased with increasing I3 - concentration up to 0.7−1.4 M, depending on the alkyl chain length. Measured J SC values showed the same tendency as that estimated using a calculation with a model in which the redox couple is transported by diffusion in electrolytes. These results suggest that the slower diffusion of I3 - to the counter electrode (CE) limits the J SC values and requires a larger amount of I3 - in ILE. However, increasing [I3 -] to more than 0.7−1.4 M resulted in the decrease of J SC. At the optimized concentration of I3 - in ILE, the influence of the absorption was estimated to be 13% of the decrease on photocurrent. To the estimate electron diffusion length in the TiO2 electrode, the electron diffusion coefficient (D e) and electron recombination lifetime (τ) were measured, showing faster D e and shorter τ in ILE than in OLE. The faster D e was caused by the higher concentration of cation in ILE. However, the shorter τ was caused by the higher concentration of I3 - and depended on the concentration. Thus, the electron diffusion lengths (L) in the DSC using ILE were shorter than that using OLE. Their shorter L also reduced the J SC in the ILE and with the increase of I3 - concentration. Among the ILE, the increase of alkyl chain length increased τ. This result should explain the highest efficiency observed in HMImI. During the durability test of the DSC at high temperature, a decrease of the efficiency of the cell using ILE was observed in 1000 h. Time course change of I3 - concentration measurements revealed that the gelation of the electrolyte depresses a decrease of I3 - concentration caused by sublimation of I2. Depression of sublimation of I2 is important to improve the high-temperature durability in nonvolatile ionic liquid electrolyte.

Supporting Information cm0349708"Novel Phenyl-conjugated Oligo-ene Sensitizers for TiO 2 Solar Cells" by Kitamura, T., et al., Synthetic Procedure and Spectral Data of Oligo-ene Dyes 2-Cyano-3-(4-N,N-dimethylanilino)-trans-acrylic acid (1a): 4-Dimethylaminobenzaldehyde (1.5 g, 10 mmol), methyl cyanoacetate (1.2 g, 12 mmol), and piperadine (0.1 g, 1.2 mmol) were dissolved in 20 mL ethanol and refluxed for 2 h. After the reaction, the precipitated ester was filtered and washed with ethanol. The ester was hydrolyzed in 5% KOH ethanol by refluxing for 2 h. The reaction mixture was diluted by water and the solution pH was adjusted to 6 by adding 10% HCl at r.t. The precipitate was then filtered and washed by ethanol. The precipitate was purified by column chromatography (hexane/ethyl acetate) and the recrystallization from ethanol solution gave pure 1a as yellow powder (1.5 g, 69%).

Nanoporous electrode films are prepared from different sized anatase TiO 2 nanoparticles, of which the average diameter is 14, 19, and 32 nm. Dye-sensitized solar cells are prepared from the films. Diffusion coefficients of electrons (D) in the solar cells are estimated by photocurrent transient measurements using a small-intensity laser pulse under continuous irradiation of bias light. Electron recombination lifetimes (τ) in the solar cells are measured by intensity-modulated photovoltage measurement (IMVS). It is found that the D increases and τ decreases with the increase of the particle size up to 32 nm. The increase of the D is interpreted with the decrease of the film surface area, where the charge trap sites are likely to exist, and the condition of grain boundary. The decrease of τ is discussed with the change of surface area and D with the particle size.

Quasi-solid-state dye-sensitized solar cells were fabricated using low-molecular-weight gelators. They showed comparable photoenergy conversion efficiencies to the liquid cell at high illumination intensity up to AM 1.5 (1 sun). Conductivity measurements of the electrolyte phases revealed that the gelation does not affect the conductivity of the electrolyte and that the conductivity increased with an increase of iodine in both gel electrolytes and liquid electrolyte. The formation of polyiodide ions, such as I3 - and I5 -, caused by addition of iodine was confirmed by Raman spectroscopic measurement. The self-diffusion of iodide species in the gel electrolyte was found about a quarter of that of I- in acetonitrile. The formation of less-mobile polyiodide ions in electrolyte increased the conductivity in the mesoporous phase, which should be rationalized as due to the Grotthuss-type electron exchange mechanism caused by rather packed polyiodide species in the electrolytes. The optimized quasi-solid-state cell showed the values of 0.67 V for open-circuit voltage, 12.8 mA cm-2 for short-circuit photocurrent density, and 5.91% for photoenergy conversion efficiency under AM 1.5 irradiation with higher durability.

Performances of dye-sensitized solar cells prepared from nanoporous TiO 2 films with different TiO 2 nanoparticle preparation methods and different annealing temperatures are studied with various film thicknesses. The results show that the solar cells prepared at higher annealing temperatures have higher energy conversion efficiencies. Regarding film thickness, thin film electrode solar cells annealed at low-temperature show comparable efficiencies with those of the cells annealed at high temperature. The difference of the efficiency between the cells with the film annealed at different temperatures increases with the film thickness. To explain the observations, the surface area of the films, the amount of the adsorbed dye, and the diffusion coefficient and lifetime of electrons are measured. The amount of adsorbed dye per unit area is found to be independent of annealing temperature, while the diffusion coefficient and lifetime increase with the temperature. With trapping models, the measured increases of the diffusion coefficient with annealing temperature are interpreted with the change of the charge-trap density and neck growth between particles, which are suggested by transmission electron microscope and surface area measurements of the films. Diffusion lengths of electrons for each solar cell estimated from the diffusion coefficient and lifetime increase with annealing temperature. From the comparison between the short circuit currents of the solar cells with the diffusion lengths, it is concluded that the higher efficiencies of the solar cells prepared from high-temperature annealed films are attributed to their longer diffusion lengths of electrons.