Principles of Dye Sensitization

The electronically excited state of the dye S then injects electron into the conduction band (equation 2)  

The oxidized dye (S+l is subsequently reduced back to ground state S by an electron donor and the latter can be an added molecule ( supersensitizer ) or the mediating redox couple in a regenerative cell. The injected electron flows through the semiconductor to arrive at the back contact and then through the external load to the counter electrode where the reverse reaction (reduction of the oxidized donor) takes place. The overall effect of the light irradiation is 

The sensitization process is often probed via stationary photoelectro-chemical measurements such as examination of the dependence of the photocurrent as a function of the excitation light ( action spectrum ). A strong resemblance of the action spectrum with the absorption spectrum of the adsorbed dye is taken as evidence in favour of the charge injection from the excited state of the dye. Correlation of the photocurrent onset with the location of the band-edge of the semiconductor, determined by other methods is a good indication that the ejected electron indeed has fallen in the conduction band of the semiconductor. 

The efficiency of the sensitization process is in tersm of incident photon-to-current conversion efficiency (IPCE), defined as the number of electrons generated by light in the external circuit divided by the number of incident photons (monochromatic). The IPCE values have been found to vary greatly with the chemical nature of the dye, the mediating redox electrolyte and the nature of the semiconductor and the interface with the electrolyte. 

In this chapter, we will be dealing mainly with sensitization of n-type semiconductor electrodes where oxidation of the dye takes place through transfer of an electron from the photoexcited dye to the conduction band of the semiconductor. The semiconductor is used as an anode and the charge transfer process monitored as anodic photocurrent. Though not discussed here, reduction of photoexcited dyes is also known to occur through a valence band mechanism. This can only be observed in compounds with a high hole mobility such as SiC or GaP. 

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FIGURE 1. The operating principles of dye-sensitized solar cells. 

Two dry film resists designed to address the needs of the laser direct-write applications for PCBs and PWBs are Riston LUV and Riston LV , developed by Dupont, and are reported to be sensitive to the UV and visible emission of argon lasers. In particular, Riston LV is based on the principles of dye-sensitized polymerization (presented earlier). 

The Working Principles of Dye-Sensitized Solar Cells (DSSCs)... 

Figure 3 Operating principles and energy-level diagram of dye-sensitized solar cell. S, and S are the Sensitizers in the ground, oxidized, and excited states, respectively. R/...

A final imaging process which involves electron transfer in the family of semiconductor materials marries the concepts of dye sensitization of silver halides discussed earlier (Sect. 3) and the electron donor properties of n-type semiconductor particles presented immediately above. Levy [21] has found that intimate contact of semiconductor particles (PbO and related materials) with silver halide crystals can sensitize the photographic action of silver halides to wavelengths absorbed by the semiconductor. Using this method photographic systems of modest speed and remarkably low silver content can be produced. The principle is identical... 

Bradford, M. M. (1976) A rapid and sensitive method for the quantification of microgram quantities of protein utilizing the principle of dye-bmding, Anal. Biochem. 76, 248-254. 

Labat F, Ciofini I, Hratchian HP, Frisch M, Raghavachari K, Adamo C (2009) First principles modeling of eosin-loaded ZnO films a step toward the understanding of dye-sensitized solar cell performances. J Am Chem Soc 131(40) 14290-14298... 

Mosconi E, Selloni A, De Angelis F (2012) Solvent effects on the adsorption geometry and electronic structure of dye-sensitized Ti02 a first-principles investigation. J Phys Chem C 116(9) 5932-5940... 

J. Xia and S. Yanagida. 2009. Strategy to improve the performance of dye-sensitized solar cells Interface engineering principle. Solar Energy, doi 10.1016/j. solener.2009.10.005. 

A less accurate but more colorful way to measure pH uses a universal indicator, which is a mixture of acid-base indicators that shows changes in color at different pH values (Figure 13.5, p. 359). A similar principle is used with pH paper. Strips of this paper are coated with a mixture of pH-sensitive dyes these strips are widely used to test the pH of biological fluids,... 

Fig. 20a. 10. Schematic of the sensing principle of a urea optical sensor based on an ammonium-sensitive membrane employing anionic dye and neutral carrier.

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