Photocurrent transport controlled

Equation (10.27) indicates that the charge transfer becomes the rate-limiting step under the condition when kcr (kS[ + ) The term in large brackets is a function of transport control of the photocurrent. If the electrode potential is sufficiently negative in a cathodic reaction at a p -type semiconductor, CT (kSI + kbr) and interfacial charge transfer control is lost. Eventually, control passes to transport within the semiconductor (although it is affected by recombination). 

Light is switched off at +0.5 V vs. SCE for the cathodic sweep. In (a) there is no added reductant (b), (c), and (d) contain 0.5mM ferrocene, 1, l -dimethylferrocene, and acetyl-ferrocene, respectively. Acetylferrocene does not attenuate the surface ferricenium surface ferrocene wave since it is not a sufficiently powerful reductant. Ferrocene and 1, l -dimelhylferrocene both attenuate the surface ferricenium - surface ferrocene wave. But l,l -dimethylferrocene is more effective under identical conditions despite the fact that the same, mass transport-limited, steady-state photocurrent is found for these two reductants. These data suggest that after the light is switched off the reduction of surface ferricenium is controlled partially by mass transport and partly by the electron transfer rate (see text). 

M. A. Butler, /. Appl. Phys. 44 1914 (1977). Theory of photocurrents in terms of energy gap and flatband potential transport in rate control. 

In photoelectrochemical work, it is usual to plot only the highest values of the photocurrent as a function of potential. This is necessarily an S-shaped curve, with the highest values of the photocurrent eventually being controlled by diffusion of carriers inside the semiconductor. (A thermally activated electrode reaction on a metal has a similar shape near the limiting current caused by transport of ions to the electrode.)... 

Photosystem II. Spinach and pea PSII particles coated on different Ti02 based electrodes were used for photocurrent measurements in the presence of PSII electron acceptor DMBQ. In all experiments, addition of DMBQ resulted in an increase in photocurrent which remained constant for long periods. In control experiments with no deposition of PSII on the electrodes, there was no change in the photocurrent pattern on addition of DMBQ. Addition of the PSII oxygen evolution inhibitor DCMU caused an immediate fall in photocurrent, suggesting that the electron transport to the Ti02 electrode is linked to water photolysis. 

Fig. 8.26. Experimental photocurrent transients for pulsed excimer laser excitation of nanocrystalline Ti02 electrodes of differing thicknesses taken from Ref. [78], Illumination from the electrolyte side (200 mJ, 30 ns, A 308 nm). Electrolyte 0.7 mol dm- 1 LiCI04 in ethanol. The insert shows that time rpcuk at which the current peak occurs depends on the square of the film thickness (VV), as expected for diffusion controlled electron transport.

Electron transfer kinetics are extremely rapid so that the photocurrent is under either mass transport or photochemical kinetic control. 

Yuh et al. (1987) investigated hole transport in TPD doped PC containing low concentrations of 5-(p-diethylaminophenyl)-l-phenyl-3-(p-diethylamino-styryl)-2-pyrazoline (DEASP). Time-of-flight photocurrent measurements showed a transition from nondispersive to dispersive and back to nondispersive behavior as the DEASP concentration increased from 0 to 1%. The results were described by Schmidlin s (1977) single-trap-controlled model. The attempt-to-escape frequency and trap depth were reported as 4 x 1012 s-1 and 0.56 eV. 

Peak photocurrents excited In a polymer of bis ( -toluene-sulfonate) of 2,4-hexadlyne-l,6-dlol (PTS) by N2-laser pulses vary superquadratically with electric field. The ratio ip(E)/((i(E), where ()i denotes the carrier generation efficiency, increases linearly with field. This indicates that on a 10 ns scale the carrier drift velocity is a linear function of E. Information on carrier transport kinetics in the time domain of barrier controlled motion is inferred from the rise time of photocurrents excited by rectangular pulses of A88 nm light. The intensity dependence of the rate constant for carrier relaxation indicates efficient interaction between barrier-localized carriers and chain excitons promoting barrier crossing. 

A field effect is also seen in the photocurrent decay time at high light intensity, interpreted in terms of bimolecular carrier recombination. Finally we discuss the rise time of a photocurrent following a rectangular light pulse. The latter results pertain to the time domain where transport is barrier controlled and bear out the importance of photo-induced barrier crossing processes. 

The role of trap states during the electron transport is very nicely reflected in potentiostatically controlled and time-resolved photocurrent measurements in Ref. 75. Figure 26 shows such photocurrent profiles and, in the inset, the experimental setup of this measurement. The nanoporous ZnO electrode was mounted in a photoelectrochemical cell and the photocurrent was measured after excitation with a 308-nm flash of an excimer... 

In the steady state the left-hand side of Eq. (13) is equal to 0. Measurables are the steady state photocurrent, photovoltage, light and dark current voltage characteristics, and the quantum efficiency spectrum (or IPCE). If the physical origin of in is known, the dependence of such DC measurements on variations in intensity, wavelength, and bias, deliver the parameters controlling (for example, the values of the diffusion length and diffusion constant in the case of diffusion limited transport). 

For the case of PPV the dispersion of the photocurrent is too large for transit time determination. It can be concluded that even at room temperature the release times from the deepest traps, which control the transport properties, are in the range of 1 s or less. This corresponds to an effective mobility of less than 10 cm A s [69]. Based on TSC measurements from different groups [70], we conclude that this behaviour is primarily caused by the existence of grain boundaries. 

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