Sensitization hole injection mechanism

Figure 52. Hole-trapping mechanism for supersensitization. The super-sensitizer, SS, transfers an electron to the excited state of the dye to form the radical anion of the dye, before electron transfer occurs to the silver halide. The radical anion subsequently injects the electron to the CB [207].

Many studies based on the observed bulk photoelectrochemical effects and on direct probing of the processes occurring at the solid surface have provided firm evidence that the sensitizing mechanism involves as a primary step electron or hole injection by the electronically excited sensitizer molecule (S ) into the semiconductor (SC). 

According to the electron-transfer mechanism of spectral sensitization (92,93), the transfer of an electron from the excited sensitizer molecule to the silver haHde and the injection of photoelectrons into the conduction band ate the primary processes. Thus, the lowest vacant level of the sensitizer dye is situated higher than the bottom of the conduction band. The regeneration of the sensitizer is possible by reactions of the positive hole to form radical dications (94). If the highest filled level of the dye is situated below the top of the valence band, desensitization occurs because of hole production. 

The other is the aggregate-partitioning mechanism [2,15], where it is considered that the SS molecules work as J-aggregate partitioners, which decrease the size of the aggregates. This causes an increase in the fluorescence lifetime and/or an increase in the electron injection rate, and hence an increase in the quantum yield of the electron injection [2], In the present work the sensitization and supersensitization processes are described below by the hole-trapping supersensitization. 

Regensburger5 suggested that photosensitization by selenium involves hole generation and migration in selenium, injection across the interface and transport through the sensitized material. He also suggested that sensitization by dyes may involve the same type of mechanism. 

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