No abstract

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.

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.

Diffusion coefficients of electrons in mesoporous TiO2−electrolyte systems are determined by laser pulse induced photocurrent measurements in the presence of a wide range of concentration of Li+, Na+, Mg2+, tetrabutylammonium cation (TBA+), or dimethylhexylimidazolium cation (DMHI+) in electrolytes. The total amount of photocharge generated by a laser pulse increases in the order of DMHI+< TBA+ < Na+ < Li+. The diffusion coefficients (D Amb) increase with increasing the cation density. In the case of TBA+, the increase of the diffusion coefficients is interpreted with ambipolar diffusion mechanism. In the case of Li+, Na+, Mg2+, and DMHI+, each diffusion coefficient increases to some extent with the cation density as in the case of TBA+, but does not fit well with ambipolar diffusion mechanism with the assumption of constant diffusion coefficient and electrons at the high density of the cations. Electron diffusion in the systems is discussed in terms of the surface cation density arising from the adsorption of cations on the films.

The transport of photogenerated electrons in nanoporous TiO2 electrodes is examined in a lithium perchlorate electrolyte solution by laser-induced photocurrent transient measurements. The diffusion coefficients of the photoelectrons are found to depend not only on electron density but also on the lithium ion concentration in the solution. The behavior of diffusion coefficients depending on lithium ion concentration is interpreted with an ambipolar diffusion mechanism. Diffusion coefficients at a high photoelectron density were observed to level off at a low lithium ion concentration, which is attributed to the effect of an ambipolar diffusion mechanism.

No abstract

No abstract

No abstract