Transient photocurrent behaviour

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. 

Thus, it can be seen that a study of the steady state photoelectrochemistry of colloidal semiconductors with the ORDE can provide information relating to the energy distribution of the particle surface states, the photogenerated carrier density and the quantum efficiency of carrier generation. The next section describes how to obtain information pertaining to intraparticle charge carrier dynamics from a study of the behaviour of transient photocurrents at the ORDE. 

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. 

In this paper we report on the photoconductivity of fully oriented Durham/Graz-polyacetylene and of fully converted, highly oriented polyphenylenevinylene (PPV) films. Though the two materials are different in several aspects - e.g. polyacetylene is adegenerate ground state system whereas PPV is the non-degenerate one, which has important consequences for the kind of possible excitations - they both show similar behaviour, when looking at the transient photocurrent response. 

The electrical conductivity of hydrophilic polyamides " and the photoconductivity of nylon-6,6" are reported. The time dependence of two transient photocurrents suggest the possible formation of a space charge in the polymer. Time-dependent effects in the form of creep measurements have also been used to examine the influence of moisture on the behaviour of nylon-6,6. Other low molecular weight molecules, whose effects on the properties of this polymer have been reported are surfactant and both acid and disperse dyes. Also with a textile connotation was a paper with more general application describing the determination of amino-acid N groups in nylon-6 and -6,6. ... 

Nevertheless, we were not able to carry out TOP measurements with calamitic monomers, because the dark currents in the liquid crystalline phases were too high. As mentioned before, such measurements are possible with the cyclic tetramers. The transient photocurrents of the tetrasiloxanes 19, 21, 22 illustrate that the carrier transport is totally dispersive, i. e. dominated by deep traps in which the charge carriers are captured. This is a typical behaviour of amorphous polymers. No transit time could be detected and no statements about the carrier mobility can be made for the rod-like mesogens. 

In addition to a.c. techniques as described above, photocurrent transients have also been investigated extensively as a means of providing additional evidence in favour of models of the sort discussed above. The most typical type of behaviour, observed at relatively long time constants ( 100 ms),... 

There are several possible fundamental causes for the observation of transient behaviour in the photocurrent. 

Under these circumstances, the height of the transient will depend on the light intensity and the decay constant again only on the electron concentration at the surface. As an example of this behaviour, consider the photocurrent transients for depopulation of a set of surface states on p-GaP, which are shown in Fig. 80 [152]. The statistics are poor but show an approximate linear dependence of the reverse transient on light intensity, 0, and also... 

Therefore transient measurements in the spectral range from 1.3 to 2.5 eV [129,130], which are mostly determined by the features of the neutral excitations, cannot be compared with the decay behaviour of the photocurrent. Nevertheless the creation and decay times of the neutral excitation itself can be studied. Shank et... 

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