Charge injection process

Fig. 5.17 CdS-ZnO coupled semiconductor system (a) interaction between two colloidal particles showing the principle of the charge injection process and (b) light absorption and electron transfer on an electrode surface leading to the generation of photocurrent. (Reproduced from [330])...

Figure 5 Photoinduced direct and remote charge-injection process leading to a common interfacial charge-separated state.

One of the most convenient methods of studying the charge-injection process is by monitoring the radical species generated at the adsorbed layer. The rates of formation of these species then indicate the rate of charge injection, while their decay are generally indicative of back electron transfer. Ultrafast spectroscopic methods are described further in Chapter 3. 

As already discussed above, one of the principal reasons for the introduction of semiconductor surfaces in supramolecular assemblies is to achieve long-lived charge separation based on a fast electron injection into the substrate, coupled to a slow back-reaction. In this section, the charge-injection process will be considered in more detail by using some examples. 

A number of different approaches can be taken to investigate the charge-injection process. The first one, outlined in the last section, is based on the absorption rise observed at about 1200 nm which is associated with the presence of electrons in TiC>2. A second method is based on the measurement of the IPCE values for the assemblies in the presence of iodide, while the third approach is based on the intrinsic spectroscopic features of the sensitizer. In this present section, the focus is on the latter two approaches since the absolute rate for charge injection is not of direct interest but simply whether or not injection is taking place. To estimate the injection and charge-separation process, the transient absorption spectra of the sensitizer in solution are compared with those obtained in the interfacial supramolecular assembly. A typical example of this approach is shown in Figure 6.17 for the compound [Ru(dcbpy)2(bpzt)] [8], (see Figure 6.7 above for the structure). 

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. 

Microwave absorption and luminescence decay measurements have been independently carried out to monitor the charge injection from excited Ru(bpy)2-(dcbpy)2+ into Sn02, ZnO, and Ti02 nanocrystallites [245]. Since microwave conductivity arises as a result of mobile charge carriers within semiconductor particles it is possible to probe the charge injection process by monitoring the growth in the microwave absorption [178]. 

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. 

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. 

These observations of an excitation wavelength dependence of the charge injection process show that photoinduced interfacial electron transfer from a molecular excited state to a continuum of acceptor levels can take place in competition with the relaxation from upper excited levels. The rather slow growth of the injection... 

Furthermore, the results support that NO photodesorption and decay of a dimer itself should be competing channels because the introduced resonant orbital is the only one for a dimer which has anti-bonding character for both N—O and NO—NO. This is consistent with the experimental results that N20 desorption is also observed by substrate-mediated excitation, and its action spectrum exactly mirrors that for NO photodesorption. Again we emphasize our results and analyses are obtained by ah iniio calculations within the model of NEGF-DFT focusing on hot electron transport and a charge injection process. This is an example to show the importance and... 

The first picosecond time-resolved observations of a photosensitised charge-injection process in semiconductor particles were carried out by Moser et at. (1985). The rate of electron injection from the excited singlet state of the surface-adsorbed dye eosin, EO(Si), to the conduction band of colloidal Ti02 particles... 

Nanocrystalline semiconductor films of Ti02, ZnO, and Sn02 containing 18 adsorbed on the surface indicated that both the monomeric and the aggregated forms participate in the charge-injection process. 

Improved charge transfer capacity is commonly estimated by using a reversible charge injection process through either double layer capacitive reactions and reversible faradaic charge transfer reactions at the electrode/electrolyte interface as... 

As the frequency of voltage perturbation is increased, one may limit the penetration depth (D/jco) of a concentration wave generated by the redox reaction so that it is much smaller than the thickness of the electroactive polymer film. In this region, one measures the kinetics of the charge injection process at the surface (Region III). The impedance characteristic is a semicircle in the Zreal vs —Zjmag impedance plane plot. For the impedance measurement, one may obtain Rct, the charge transfer resistance, and the double layer capacitance Cdl- This procedure was used to calculate the... 

The photodiode device geometry is similar to that of an LED. The photoconductive polymer layer is sandwiched between two electrodes, one of them transparent to permit light to achieve the polymer. The term diode is used due to the rectification characteristics of devices made with materials of different work functions. These devices present rectification in the dark as well as under illumination, owing to the role of the interface in the charge injection process. 

Only those dye molecules adsorbed on the surface of the semiconductor participate in the sensitization process and hence information on the condition of the adsorbed dye (amount of dye adsorbed, degree of aggregate formation, adsorption isotherms,..) are essential to optimize the charge injection process. A number of other parameters such as nature of the light and its intensity, electrolyte and its concentration, surface roughness, solution pH also influence the charge injection process. 

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. 

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