Semiconductors spectral sensitization

Electron Level Position. One essential condition of spectral sensitization by electron transfer is that the LUMO of the dye be positioned above the bottom of the conduction band, eg, > —3.23 eV in AgBr or > —4.25 eV in ZnO (108). To provide the desired frontier level position respectively to the valence and conduction bands of the semiconductor, it is necessary to use a polymethine with suitable electron-donor abiHty (Pq. Increasing the parameter (Pq leads to the frontier level shift up, and vice versa. Chain lengthening is known to be accompanied by a decrease of LUMO energy and hence by a decrease of sensitization properties. As a result, it is necessary to use dyes with high electron-donor abiHty for sensitization in the near-ir. The desired value of (Pq can be provided by end groups with the needed topological index Oq or suitable substituents (112). 

Fig. 1. Spectral sensitivity ranges for undyed semiconductors. Lack of sensitivity in the visible and infrared regions necessitates the use of spectral...

Table 2. Spectral Sensitizers for Inorganic and Organic Semiconductors...

Diaz AF, Logan JA(1980)Electroactive polyanihne films. JElectroanalChem 111 111-114 Noufi R, Nozik AJ, White J, Warren LF (1982) Enhanced stability of photoelectrodes with electrogenerated polyanUine films. J Electrochem Soc 129 2261-2265 Noufi R, Tench D, Warren LE (1981) Protection of semiconductor photoanodes with photoelectrochemicaUy generated polypyrrole films. J Electrochem Soc 128 2596-2599 Jaeger CD, Fan FRF, Bard AJ (1980) Semiconductor electrodes. 26. Spectral sensitization of semiconductors with phthalocyanine. J Am Chem Soc 102 2592-2598 Gerischer H (1977) On the stability of semiconductor electrodes against photodecomposition. J Electroanal Chem 82 133-143... 

Kohtani S, Kudo A, Sakata T (1993) Spectral sensitization of a TiOa semiconductor electrode by CdS microcrystals and its photoelectrochemical properties. Chem Phys Lett 206 166-170... 

Mixed compositions are of interest mainly because they allow tuning of the semiconductor properties (most commonly bandgap and, therefore, spectral sensitivity). This is useful for various device applications. Photoconductive detectors, where a certain spectral sensitivity range is desired, is probably the main application that drove many studies on CD of ternary semiconductors. 

Two main models are usually discussed for the mechanism of the spectral sensitization. The excitation of the sensitizer by absorbed light and electron transfer from the excited sensitizer to the semiconductor is the first model. The alternative mechanism consists of the transfer of the excitation energy from the sensitizer to the semiconductor. This energy is used for photogeneration of the charge carriers in the sensitized photoconductor. In the first case the excited singlet level of the sensitizers has to be located above the conduction band of the semiconductor for realization of the electron transfer. For hole transfer the basic sensitizer level has to be located lower than the valence band of the sensitized photoconductor. The energy transfer mechanism does not need a special mutual location of the semiconductor and sensitizer levels. 

TABLE 2. SPECTRAL SENSITIZERS FOR INORGANIC AND ORGANIC SEMICONDUCTORS... 

Research spectral sensitization. fluorescence quenching, energy transfer between evened stales. Model membranes to mimic photosvnihctic systems. Modification of solid surluce properties. Examination of lipids, proteins and membrane phenomena organic semiconductors. 

Spectral Sensitization of Semiconductor by Mixture of J-aggregated Cyanine Dyes... 

The photoelectrochemical activity inherent in thin films of aggregated cyanine dyes permits them to act as the spectral sensitizers of wide bandgap semiconductors [69]. It is seen from Fig. 4.14 that the photoelectrochemical behaviour of semiconductor/dye film heterojunctions fabricated by deposition of 200 nm-thick films of cyanine dyes on the surface of TiC>2 and WO3 electrodes, bears close similarity to that of semiconductor electrodes sensitized by the adsorption of dye aggregates. Thus, both anodic and cathodic photocurrents can be generated under actinic illumination, the efficiency of the photoanodic and photocathodic processes and the potential at which photocurrent changes its direction being dependent on dye and semiconductor substrate [69]. 

The use of low bandgap polymers (ER < 1.8 eV) to extend the spectral sensitivity of bulk heterojunction solar cells is a real solution to this problem. These polymers can either substitute one of the two components in the bulk hetero junction (if their transport properties match) or they can be mixed into the blend. Such a three-component layer, comprising semiconductors with different bandgaps in a single layer, can be visualized as a variation of a tandem cell in which only the current and not the voltage can be added up. 

An alternate mechanism for spectral sensitization, not involving direct electron injection into the semiconductor, is energy transfer from the excited, adsorber dye to some unidentified acceptor state near the surface of the silver halide, followed by promotion of an electron into the conduction band. In an elegant series of... 

Watanabe T., Fnjishima A., Tatsuoki 0. and Honda K. (1976), pH dependence of spectral sensitization at semiconductor electrodes . Bull. Chem. Soc. Jpn. 49, 8-11. 

A hybrid nanosystem which consists of semiconductor nanoparticles and organic dye J-aggregate may be self-assembled in RMs (Fig. 5) [4], In this structure, the dye adsorbed to the nanoparticle surface operates as spectral sensitizer and nanocrystal size stabilizer simultaneously. The hybrid nanosystem of this kind may be a key element of solar cells. 

---