How do you optimize the dye loading and injection efficiency in DSSCs?

1 Dye loading

Dye loading refers to the amount of dye molecules that are adsorbed on the surface of the TiO2 electrode. The dye molecules act as light harvesters and inject electrons into the TiO2 conduction band when they are excited by photons. Therefore, the more dye molecules there are, the more light can be absorbed and converted into electricity. However, there is a limit to how much dye can be loaded on the TiO2 surface, as too much dye can cause aggregation, self-quenching, or blocking of the electron transport pathways. To optimize the dye loading, you need to consider the following factors: the dye structure, the TiO2 morphology, the dye adsorption time and temperature, and the dye desorption treatment.

2 Injection efficiency

Injection efficiency refers to the probability that an electron injected by a dye molecule into the TiO2 conduction band will reach the external circuit without being lost to recombination. Recombination is the process where an electron in the TiO2 recombines with a hole in the electrolyte or the dye, resulting in wasted energy and reduced current. To optimize the injection efficiency, you need to consider the following factors: the energy alignment between the dye and the TiO2, the electron lifetime and diffusion length in the TiO2, the electrolyte composition and concentration, and the use of additives or co-adsorbents.

3 Dye structure

The structure of the dye molecule affects both the dye loading and the injection efficiency. The dye should have a high molar extinction coefficient, a broad absorption spectrum, and a good match with the TiO2 band gap. The dye should also have a suitable anchoring group, such as a carboxylic acid, that can form strong bonds with the TiO2 surface. Additionally, the dye should have a low recombination rate with the electrolyte or the dye itself, and a high stability under light and heat exposure. Some examples of commonly used dyes are ruthenium complexes, organic dyes, and natural dyes.

4 TiO2 morphology

The morphology of the TiO2 electrode affects both the dye loading and the injection efficiency. The TiO2 electrode should have a large surface area, a high porosity, and a good electrical conductivity. The surface area and the porosity determine how much dye can be adsorbed on the TiO2 and how well the electrolyte can infiltrate the electrode. The electrical conductivity determines how fast the electrons can be transported from the TiO2 to the external circuit. The TiO2 morphology can be controlled by varying the synthesis method, the particle size and shape, the annealing temperature and time, and the layer thickness and structure.

5 Dye adsorption and desorption

The dye adsorption and desorption processes affect the dye loading and the injection efficiency. The dye adsorption process involves immersing the TiO2 electrode in a dye solution for a certain time and temperature. The dye adsorption process should be optimized to achieve a high and uniform dye coverage on the TiO2 surface, without causing dye aggregation or degradation. The dye desorption process involves rinsing the dye-coated TiO2 electrode with a solvent to remove any excess or unbound dye molecules. The dye desorption process should be optimized to avoid losing too much dye or damaging the TiO2-dye interface.

6 Electrolyte composition and additives

The electrolyte composition and additives affect the injection efficiency and the stability of the DSSC. The electrolyte is usually a liquid or a gel that contains a redox couple, such as iodide/triiodide or cobalt complexes, that can regenerate the oxidized dye molecules and complete the circuit. The electrolyte should have a high ionic conductivity, a low viscosity, and a low volatility. The electrolyte should also have a low recombination rate with the TiO2 or the dye, and a high stability under light and heat exposure. To improve the electrolyte performance, additives or co-adsorbents can be used to modify the TiO2-dye interface, the electrolyte-dye interface, or the electrolyte itself. Some examples of additives or co-adsorbents are chenodeoxycholic acid, 4-tert-butylpyridine, guanidinium thiocyanate, and graphene oxide.