Efficiency light-harvesting

The light-harvesting efficiency of an electrode covered with an adsorbed sensitizer can be expressed in the form similar to Eq. (5.10.28) ... 

For a typical monomolecular coverage, T — 10 10 mol/cm2, an electrode roughness factor r = 1000 and an extinction coefficient ads = 107 cm2/mol, the light-harvesting efficiency is, in comparison to the preceding case, very high, intimate contact with the semiconductor surface, hence the conditions for charge injection from S into the semiconductor are almost ideal (q9j—>100 per cent). 

LHE is the light harvesting efficiency or absorptance, defined as LHE = 1-10" where A is the absorbance, ( )inj is the quantum yield of charge injection, and r is the efficiency of transporting injected electrons in to the external circuit. Equation (3.6.26) can be written as ... 

The naturally occurring Zn-bchl a and Mg-bchl a show large structural similarities and have very similar physicochemical characteristics. Likewise, Zn-chl a exhibits features similar to Mg-chl a with regard to redox potential and absorption maxima in organic solvents. The light-harvesting efficiency of Zn-chl a and Mg-chl a are very similar allhough... 

The parameter that directly measures how efficiently incident photons are converted to electrons is the IPCE. The wavelength-dependent IPCE term can be expressed as a product of the quantum yield for charge injection ( ), the efficiency of collecting electrons in the external circuit (17), and the fraction of radiant power absorbed by the material or light harvesting efficiency (LHE), as represented by Equation 17.8 ... 

The light harvesting efficiency, or absorption factor, is related to the dye molar extinction coefficient ( ( ), L/mol/cm) and to the surface coverage ( , mol/cm2)... 

Figure 5 Light absorption properties of N3 dye, black dye, and their Ti02 photoelectrodes represented by absorbance and light-harvesting efficiency.

Besides color, sensitization, and light-harvesting efficiency, the actual reactivity of the excited states populated is therefore a crucial property. Some aspects of relevance for bioinorganic and biomimetic systems will be discussed in the following chapters. 

X 10 cm one obtains LHE = 99.8%. On a flat surface the N3 dye would have only absorbed 0.3% of the incident 530-nm light. The dramatic difference of the light harvesting efficiency is illustrated by the deep coloration of the nanocrystalline Ti02 layers shown in Fig. 4, despite of the fact that they are covered only by a monolayer of sensitizer. On a flat surface the N3 dye would have remained invisible to the eye. A film exhibiting ordered mesoporous structure, such as shown in Fig. 5 has an even higher internal surface area than one that is composed of randomly associated nanoparticles [24]. Because more sensitizer is adsorbed for the same film... 

Here, LHE(A) is the light harvesting efficiency for photons of wavelength X, 0mj is the quantum yield for electron injection from the excited sensitizer in the conduction band of the semiconductor oxide and r]coii is the electron collection efficiency. Having analyzed above the light harvesting efficiency of dye-loaded mesoscopic films we discuss now the other two parameters. 

In view of the flexibility of the perovskite stracture, many modifications are possible. The band gap of these phases and thus the absorption spectrum of the perovskite and its light-harvesting efficiency can be tuned by doping the organic and the halogen sites. The perovskites CH3NH3Pb(Ij Br )3 have a band gap variation given by... 

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