Direct Estimation of the Electron Diffusion Length in Dye-Sensitized Solar Cells

The diffusion length is a key parameter that controls the electron collection efficiency in dye-sensitized solar cells (DSCs). In this work, we carry out a direct estimation of this parameter by means of the laser beam-induced current (LBIC) technique. The DSC devices are prepared on transparent conducting glass substrates, which were divided in two electrically isolated parts by means of a groove. The LBIC measurement is conducted by moving a highly focused laser spot over the DSC across the groove and monitoring the open-circuit voltage yielded by the solar cell. The resulting voltage profile can be fitted to a simple diffusion-recombination model such that the electron diffusion length can be extracted. Measurements carried out on DSC with various oxides (TiO₂/ZnO) and electrolytes (organic, ionic-liquid) yield diffusion lengths in the 10–35 μm range, with longer values found for higher illumination and for cells of better efficiency

Brookite-Based Dye-Sensitized Solar Cells: Influence of Morphology and Surface Chemistry on Cell Performance

The transport and recombination properties of dye-sensitized solar cells based on phase-pure anatase and brookite nanomaterials are compared as a function of the surface chemistry and morphology. Phase-pure brookite has been synthesized from amorphous TiO₂ using two different solutions at low and high pH, resulting in different size and morphology of brookite nanoparticles. The smaller short-circuit current density (<i>J</i>_SC = 6.6 mA cm^–2) for acidic brookite compared to anatase (9.8 mA cm^–2) was related to the light harvesting efficiency because of the lower amount of dye adsorbed. However, a larger open-circuit voltage for acidic brookite indicates the promise of the material. The basic brookite-based solar cells gave a very low <i>J</i>_SC (0.10 mA cm^–2), which increased dramatically by a factor of about 30 after an acid treatment of the films, illustrating the effect of surface chemistry. A combination of experiments shows that the improvement is related to an increase in injection efficiency. Electrochemical impedance and intensity-modulated photocurrent and photovoltage spectroscopies show that electron transport is faster in the acid-treated basic brookite nanomaterial, related to the larger feature sizes. However, the recombination kinetics is also significantly faster, with as net result a smaller diffusion length and hence smaller collection efficiency

High Capacity Na–O₂ Batteries: Key Parameters for Solution-Mediated Discharge

The Na–O₂ battery offers an interesting alternative to the Li–O₂ battery, which is still the source of a number of unsolved scientific questions. In spite of both being alkali metal–O₂ batteries, they display significant differences. For instance, Li–O₂ batteries form Li₂O₂ as the discharge product at the cathode, whereas Na–O₂ batteries usually form NaO₂. A very important question that affects the performance of the Na–O₂ cell concerns the key parameters governing the growth mechanism of the large NaO₂ cubes formed upon reduction, which are a requirement of viable capacities and high performance. By comparing glyme-ethers of various chain lengths, we show that the choice of solvent has a tremendous effect on the battery performance. In contrast to the Li–O₂ system, high solubilities of the NaO₂ discharge product do not necessarily lead to increased capacities. Herein we report the profound effect of the Na⁺ ion solvent shell structure on the NaO₂ growth mechanism. Strong solvent–solute interactions in long-chain ethers shift the formation of NaO₂ toward a surface process resulting in submicrometric crystallites and very low capacities (ca. 0.2 mAh/cm²_(geom)). In contrast, short chains, which facilitate desolvation and solution-precipitation, promote the formation of large cubic crystals (ca. 10 um), enabling high capacities (ca. 7.5 mAh/cm²_(geom)). This work provides a new way to look at the key role that solvents play in the metal–air system