Quantum yields charge injection

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 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 ... 

Figure 26. Quantum yield for charge injection as a function of the redox potential of the hole transport molecule for a TiOPc-based photoreceptor. (Reprinted with permission from Ref [37t].)...

Charge injection is slow enough compared with the vibrational relaxation of the dye excited state (k/ kj). In this event, electron transfer would be able to take place only from the lowest excited state v — 0) (Eq. (35)), and the injection quantum yield would be simply controlled by the kinetic competition between the electron injection (Eq. (35)) and the decay of the excited state (Eq. (36)) ... 

Charge injection is fast compared with nuclear relaxation of the excited state (k k,). In this case, interfacial charge transfer would take place from the prepared hot vibronic level (Eq. (34)) and the quantum yield for the primary injection process would be close to unity = 1). For both limiting cases, k[ kr and k[ kr, relation (30) would be relevant, provided electron transfer is nonadiabatic. 

Earlier studies on dye-sensitized Ti02 reported nanosecond time constants for the injection kinetics [16, 40-42]. These results were obtained indirectly from the measurement of the injection quantum yield and implicitly assumed that the interfacial electron transfer reaction was competing only with the decay of the dye excited state. Other studies were based on the same assumption but used measurements of the dye fluorescence lifetime, which provided picosecond-femtosecond time resolution [43-45]. Direct time-resolved observation of the buildup of the optical absorption due to the oxidized dye species S+ has been employed in more recent studies [46-51]. This appears to be a more reliable way of monitoring the charge injection process as it does not require any initial assumption on the sensitizing mechanism. 

H. J. Lewerenz, J. Stunper, and L. M. Peter, Deconvolution of charge injection steps in quantum yield multiplication on silicon, Phys. Rev. Lett. 61(17), 1989, 1988. 

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