Charge injection process rate constant

The sensitizer molecules adsorbed on Ti02 surface have a significantly shorter fluorescence lifetime than in the homogeneous solution and this decrease in lifetime has been attributed to the charge injection process [83,181-183,186-188, 197,218,225,236-239]. Heterogeneous electron transfer rate constants in the range of 107—1011 have been reported in these studies. 

They neglected the reverse rate governed by b (assuming an exothermic initial step) and assumed rapid nonradiative relaxation of nascent D BA (governed by rate constant e). The steady-state result once again identifies the charge injection process as the rate-determining step (i.e., k -t = a) under the conditions d e. 

Monitoring of the fluorescence of the dye could provide a direct evidence for operation of such a mechanism [49-59]. Muenter [49], for example, measured the fluorescence lifetime and quantum yields of two carbocyanine dyes in a photographically inert medium such as gelatin and in the adsorbed state on silver chloride, silver bromide microcrystals. Both the fluorescence lifetime and yield showed a large decrease upon change of the dye environment from that of gelatin to silver halides. Rate constants in the rate of 109-10 0 s-i were determined for the charge injection process. 

Figure 6 An energy diagram of the charge-transfer process at an n-type semiconductor/metal interface when an external potential (F) is applied across the semiconductor electrode. The applied potential changes the electric potential difference between the semiconductor surface and the bulk region. This perturbs the concentration of electrons at the surface of the semiconductor (ns), and a net current flows through the semiconductor/metal interface. The forward reaction represents the transfer of electrons from the semiconductor to the metal and the reverse reaction represents the injection of electrons into the semiconductor from the metal. The width of the arrows indicates schematically the relative magnitude of the current, (a) The reverse bias condition for an n-type semiconductor (V > 0). The forward reaction rate is reduced relative to its equilibrium value, while the reverse reaction rate remains constant. A net positive current exists at the electrode surface, (b) The forward bias condition (V < 0), the forward reaction rate increases compared to its equilibrium value, while the reverse reaction rate remains unaffected. A net negative current exists at the electrode surface...

Figure 15 schematizes the energetics and dynamics of processes that take place after charge injection from a molecular excited state to the acceptor levels of a semiconductor. Thermalization and trapping of hot injected carriers is known to occur typically with a rate constant kth = 10 s [69-71]. Reverse transfer of a hot... 

As the next step of the conversion of light into electrical current, a complete charge separation must be achieved. On thermodynamic grounds, the preferred process for the electron injected into the conduction band of the titanium dioxide films is the back reaction with the oxidized sensitizer. Naturally this reaction is undesirable, since instead of electrical current it merely generates heat. For the characterization of the recombination rate, an important kinetic parameter is the rate constant kb- It is of great interest to develop sensitizer systems for which the value of feinj is high and that of kb low. 

Table 21-3 gives a smnmary of the interfacial charge injection and recombination rate constants of some organic and metal cyano compounds on semiconductor surfaces. Although the data is not directly comparable, as different semiconductor preparations, solvents, electrolytes, pH, time-scales and kinetic models were used, it provides a basis for understanding these important interfacial electron transfer processes. 

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