Sensitizing dyes electron trapping

Two main mechanisms were proposed for the supersensitization effect. One is the hole-trapping mechanism in which the electron from SS fills the hole in the highest occupied molecular orbital (HOMO) of the excited sensitizing dye, since the HUMO level of SS is chosen to be higher than that of the sensitizer (Fig. 5) [2,10,11]. The resultant ionic state gives up an electron to the conduction band of silver halide with much higher quantum yield. 

The extent to which desensitization can occur by recombination of a photoelectron with a hole trapped by a dye is not known. This mode is theoretically possible, but it is also possible that hole trapping could increase sensitivity if the probability of reaction of a mobile hole with a trapped electron or silver atom is greater than the probability of loss of a mobile electron to a trapped hole or if the cross-section for recombination of an electron with a dye-trapped hole is less than with an intrinsically trapped hole. Sensitization by hole trapping dyes has been demonstrated by Leubner (269) who showed that, independently of Er, dyes with Eqx less than 0.5 V can chemically sensitize emulsions with about.constant efficiency. 

For Class 1 dyes, sensitization occurs by transfer of the electron from the state of the excited dye to the conduction band of the silver halide. The hole created by the transfer remains in the dye molecule. If the hole is not too deeply trapped, it may eventually escape into the valence band with the aid of thermal energy. These dyes provide few, if any, electron traps and desensitization by oxygen/moisture in their presence would equal that for the undyed emulsion. 

Bailes M., Cameron P. J., Lobato K. and Peter L. M. (2005), Determination of the density and energetic distribution of electron traps in dye-sensitized nano-crystalhne solar cells , J. Phys. Chem. B 109, 15429-15435. 

Because of local variations in the relevant energy levels, sensitizing dyes could also desensitize when their excited energy level is below the CB of the AgX, causing them to behave as an electron trap (Fig. 2c) this would possibly allow the electron to combine with oxygen and prevent it from taking part in the formation of metallic Ag. Also, a dye may trap a hole (Fig. 2d) by transferring an electron... 

FIGURE 2 Energy levels of AgX and of color-sensitizing dyes, showing sensitization by (a) electron- and (b) energy-transfer, and desensitization by (c) electron trapping and (d) hole trapping, e represents electron pathways. 

Gratzel M (1994) NanocrystaUine solar cells. Renew Energy 5 118-133 Peter LM, Ponomarev EA, Franco G, Shaw NJ (1999) Aspects of the photoelectrochemistry of nanocrystalhne systems. Electrochim Acta 45 549-560 Peter L (2007) Transport, trapping and interfacial transfer of electrons in dye-sensitized nanocrystalline solar cells. J Electroanal Chem 599 233-240... 

Season et al.52) reported that the IMPS response of dye-sensitized TiOz mesoporous film is described by a diffusion model and the diffusion coefficient in the film depends on light intensity, where the electron transfer process is explained by thermal excitation from trap sites of the particles. 

Although these reactions may contribute to the effect of oxygen, the decrease in sensitivity also could be explained by a direct reaction of the photoelectron with oxygen to form the superoxide ion O2 (83). An oxygen molecule diffuses to a site where an electron is trapped temporarily by a crystal defect, dye, or impurity center, or an electron from the conduction band is captured by a physically adsorbed oxygen molecule... 

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