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Dye Sensitized Solar Cell (DSSC) Materials
The following figure 1 shows the structure of dye-sensitized solar cells (DSSCs), consisting of photoanode, electrolyte and pair electrode composition. The porous photoanode is composed of conductive substrate, nanocrystalline porous semiconductor film and photosensitive dye. Oxygen chemical and reduction electrolytes generally contain I3-/I- redox electric pairs, and the counter electrode is generally made of glass as the substrate, above the conductive substrate on which a conductive film is deposited is loaded with platinum. Conductive substrate also has plastic, metal and other flexible materials.
Fig. 1 Structure of DSSCs[1]
When the DSSCs work, its internal workings are as follow (fig. 2): under sunlight irradiation, take TiO2 semiconductor as an example:
Fig. 2 Illustration of the thermodynamically favorable interfacial electron transfer processes in DSSCs[2]
- The electron (e-) in the dye adsorbed on the nanocrystalline porous semiconductor film is excited by the transition to the excited state S*:
S0→S*
- Then inject into the conduction band (CB) of the semiconductor:
S*→e-(CB)
- The electrons injected into the semiconductor conduction band reach the conductive glass by diffusion:
e-(CB)→e-(FTO)
- Flow through the external circuit to reach the counter electrode:
e-(FTO)→e-(CE)
- Dye molecules in the oxidized state S*accept the I- ions in the electrolyte (as redox agents) and return to the dye ground state S0:
2S*+3I-→2S0+I3-
- At the same time, the I3- in the electrolyte is transferred to the counter electrode, which accepts the e- in the counter electrode (from the external circuit) and is reduced to I-. Form a complete loop loop:
I3-+2e-(CE)→3I-
Materials
- Dye sensitizer
Dye sensitizer is a very important component and a key factor in the formation and injection. Its function is to absorb sunlight to excite the ground state electrons, transition to the excited state, and transfer to the conduction band of the semiconductor.
Inorganic dyes: At present, the inorganic dyes used are mainly polypyridine complexes, phthalocyanines, metal porphyrins and so on. Among them, polypyridine ruthenium complex dyes have very high chemical stability and strong redox ability, so they are considered as the most promising dyes.
Organic dyes: Transition metal complexes with organic materials derived from polypyridine, porphyrin and phthalocyanine as ligands have been proved to be the best choice for sensitizers in all organic compounds. These ligands containing nitrogen heterocycles have large π bonds or aromatic rings capable of hybridizing with various metal ions. The pyridine derivatives are generally based on Ru2+ mononuclear or multinuclear complexes; For porphyrin or phthalocyanine derivative metal hybrids, suitable metal ions are those with full or half-full outer D electrons such as Zn2+, Mg2+, AI3+.
- Redox electrolytes
Different redox electrolytes also have a great influence on the electron transport and recombination process of DSSCs. Liquid electrolytes are widely used in DSSCs because of their characteristics such as fast ion diffusion rate, high photoelectric conversion efficiency, easy to design and adjust composition, and good permeability to nanoparticle membranes. The redox electric pair commonly used in DSSCs is I3-/I-. The dyes used in the solid state are generally: natural products (such as anthocyanins), synthetic compounds such as ruthenium complexes, organic dyes and so on.
References
- D. Ccvab, C. Sm, A. Da, et al. Bio-sensitized solar cells built from renewable carbon sources [J]. Materials Today Energy. 2022, 23: 100910.
- A. Haucha, A. Georgb, et.al. Diffusion in the electrolyte and charge-transfer reaction at the platinum electrode in dye-sensitized solar cells. Electrochimica Actc, 2001, 46:3457-3466
