Stationary photocurrents

The stationary photocurrent is determined by the product of the ffee-ion mobility and their concentration /Vv specified in Eq. (3.362). With the notations made above, we can represent the latter in the following form... 

The silicon surface in nonfluoride and nonalkaline solutions is spontaneously passivated due to the formation of a thin native oxide film at a rate depending on many factors as discussed in Chapter 2. For n-Si samples in aqueous solutions under illumination the occurrence of passivation causes a decrease of the photocurrent as shown in Fig. 5.11. " In the absence of HF, photocurrent rapidly reduces to near zero due to the formation of an oxide film. The stationary photocurrent increases with increasing HF concentration. For a given light intensity, there is a HF concentration above which the photocurrent does not decrease from the initial value. The surface is free of oxide film at this HF concentration. 

If the dipole transition moment d is comparatively large and the value (d q)2/2E is of the order of / (recall that / (10 4 to 10 2)/cb where T 300 K), then expression (282) gives a considerable increase of the current. With this regard the stationary photocurrent for 8 < 108 V/m below room temperature is linear with the constant field 8 and quadratic with the amplitude of the variable field 8. ... 

Experimental measurements of photoemission currents are generally taken at far more positive potentials compared to the equilibrium potential of the electron electrode. Therefore, even when the solvated electrons are stable in the bulk of the solution, the electrode-emitter surface traps them effectively. For the electrode-to-solution transition of electrons to be irreversible (this is a necessary condition for measuring a stationary photocurrent), readily reducible substances — solvated electron acceptors (so-called scavengers) — are added to the solution. The electron level in a reduced acceptor (A ) is quite low, and this makes this state very stable trapping of electrons by a scavenger is the final transformation an emitted electron undergoes. 

Fig. 8.41 Thermally-stimulated electron current in a highly-pure anthracene crystal whose traps were previously filled at T= 70 K by means of a stationary photocurrent. From the temperatures at which the current attains its maximum values, the trap depths can be determined. After [44].

Fig. 3 and increases the dip, i.e., decreases the stationary photocurrent. The flat-band situation is reached at a bias of - 0.95 V as indicated by the disappearance of the initial photocurrent peak, labeled peak (filled squares in Fig. 5). Both the stationary photocurrent, labeled final (filled circles in Fig. 5), and the corresponding stationary recombination loss are linked to the stationary accumulation of holes at the interface. The stationary photocurrent disappears already at finite band bending of about 0.3 V with respect to the flat-band potential at - 0.95 V (Fig. 5).

An increase in the membrane conductivity induced by the uncoupler TTFB was accompanied by an enhancement of the stationary photocurrent for the (a) and (d)-... 

Barbara, Belcher, 2000), proteins (Bruchez, Moronne, Gin, Weiss, Alivisatos, 1998 Chan Nie, 1998 Mattoussi et al., 2000), and DNA (Mitchell, Mirkin, Letsinger, 1999 Pathak, Choi, Amheim, Thompson, 2001). Current bioconjugation methods are schematically illustrated in Figure 4.5 (Chan et al., 2002). For example, Lee et al. constructed a hybrid bR/(QDs) bionanosystem of thin films, and a stationary current is generated from the modified photocycle (Li, Li, Bao, Bao, Lee, 2007). They proposed a model to explain that QDs could act as nanoscaled light sources embedded in bR to assist its generation of a stationary photocurrent. 

Accordingly, the excited sensitizer was reduced by an electron donor before the electron transfer occurred. This process competes with the recombination given by Eq. (10.10). Such a supersensitization was never really proved. It is assumed that the increase of the stationary photocurrent upon addition of a reducing agent is entirely due to the regeneration of the adsorbed dye (Figure 10.20). 

Stationary microwave electrochemical measurements can be performed like stationary photoelectrochemical measurements simultaneously with the dynamic plot of photocurrents as a function of the voltage. The reflected photoinduced microwave power is recorded. A simultaneous plot of both photocurrents and microwave conductivity makes sense because the technique allows, as we will see, the determination of interfacial rate constants, flatband potential measurements, and the determination of a variety of interfacial and solid-state parameters. The accuracy increases when the photocurrent and the microwave conductivity are simultaneously determined for the same system. As in ordinary photoelectrochemistry, many parameters (light intensity, concentration of redox systems, temperature, the rotation speed of an electrode, or the pretreatment of an electrode) may be changed to obtain additional information. 

The fact that a potential-dependent lifetime peak for PMC transients has been found which coincides with the stationary PMC peak in the depletion region near the onset of photocurrents (Fig. 22) is very relevant since the stationary PMC peak is determined by the interfacial rate constants of charge carriers (Figs. 13 and 14) this should also be the case for the transient PMC peak. To demonstrate this correlation, the following formalism can be developed10 ... 

It is interesting to note that independent, direct calculations of the PMC transients by Ramakrishna and Rangarajan (the time-dependent generation term considered in the transport equation and solved by Laplace transformation) have yielded an analogous inverse root dependence of the PMC transient lifetime on the electrode potential.37 This shows that our simple derivation from stationary equations is sufficiently reliable. It is interesting that these authors do not discuss a lifetime maximum for their formula, such as that observed near the onset of photocurrents (Fig. 22). Their complicated formula may still contain this information for certain parameter constellations, but it is applicable only for moderate flash intensities. 

This relation shows that the lifetime of PMC transients indeed follows the potential dependence of the stationary PMC signal as found in the experiment shown in Fig. 22. However, the lifetime decreases with increasingly positive electrode potential. This decrease with increasing positive potentials may be understood intuitively the higher the minority carrier extraction (via the photocurrent), the shorter the effective lifetime... 

Equation 10-48 is obtained by excluding the photocurrent ipb from the reaction current of Eqn. 10-44. In the stationary state, the total ciurent i in Eqn. 10-48 equals the transfer current of cathodic redox holes across the electrode interface. 

IET serves as a theoretical basis not only for fluorescence and photochemistry but also for photoconductivity and for electrochemiluminescence initiated by charge injection from electrodes. These and other related phenomena are considered. The kinetics of luminescence induced by pulse and stationary excitation is elucidated as well as the light intensity dependence of the fluorescence and photocurrent. The variety and complexity of applications proves that IET is a universal key for multichannel reactions in solutions, most of which are inaccessible to conventional (Markovian) chemical kinetics. 

Transient photoelectrochemical behaviour of colloidal CdS The experiments described in this section are performed by recording light-on transient photocurrents from aqueous dispersions of 2-12 nm radii CdS particles (prepared as above) at a stationary optical rotating disc electrode. However, to be able to interpret the results from these experiments, it was first necessary to model the time-dependent behaviour of the mass transport limited photocurrent at the ORDE. 

Theory of the transient photocurrent behaviour at the ORDE. The stationary optical disc electrode is assumed to be uniformly illuminated by parallel light which is switched on at time t = 0, and which produces a measurable concentration of photogenerated electrons on the particles denoted by c. The differential equation for the generation and transport of these electrons to the electrode surface, with concurrent homogeneous back reaction is set up with the following assumptions. 

Experimental values of (ihv)J(ihv) are larger than predicted theoretical values at high t, which is attributed to the deposition of particles on the electrode surface. As the electrode is stationary and there exists no convection to sweep the deposited particles away from the electrode surface, this residual current persists after illumination is stopped, distorting the form of the observed light-off current-time transient. Consequently, theoretical analysis of the transient was not attempted. The rate constant k may be obtained from (//, ) and the rotation speed dependence of the photocurrent. From equations (9.101) and (9.76), it can be seen that ... 

In the following, an approach is used that has been developed for the quantitative description of combined stationary excess microwave conductivity experiments and photocurrent spectroscopy [45]. The model uses analytical expressions for the... 

The efficiency of photoelectrochemical devices is based on effective charge transfer while suppressing surface recombination and corrosion. While the photocurrent is a direct measure of the irreversibly transferred electrons, it is not trivial to obtain a measure for the losses at surfaces due to recombination. As will be shown in Section 2.3.1, stationary microwave reflectivity is a method that measures the integral of the excess minority carrier profile. Such profiles are shown in Figures 2.3-2.6. The simultaneous recording of photocurrent and excess microwave reflectivity in an electrochemical cell allows the assessment of the relative contributions of kr and Sr for well-defined systems. These parameters are defined as follows ... 

Figure 2.21 shows a schematic of the setup for simultaneous measurement of the stationary light-induced excess minority carrier microwave reflectivity and the photocurrent at the semiconductor-electrolyte contact. The sample is illuminated from the front side and photoelectrochemistry is performed using the standard... 

Figure 2.21 Experimental arrangement for simultaneous in situ stationary light-induced excess microwave reflectivity and photocurrent measurements LD, laser diode (A = 830nm) L, collimating lens C, chopper...

A fairly linear dependence of the photocurrent with pH is observed in the range of 4.5-9.5. The dark current is in the range of lO" " A. Upon illumination, the current increases sharply and becomes stationary after some 10 to 100 s. The photocurrent is in the range of nA. The results are reversible and reproducible. The dependence of the current on pH is shown in Figure 3.14. 

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