Photocurrent transient

Erley G, Gorer S, Penner RM (1998) Transient photocurrent spectroscopy Electrical detection of optical absorption for supported semiconductor nanocrystals in a simple device geometry. Appl Phys Lett 72 2301-2303... 

Figure 20 Logarithmic plots of short circuit transient photocurrents for (a) PD100, (b) PD17, and (c) PDO LB films excited at 525 nm laser.

Khan RUA, Poplavskyy D, Kreouzis T, Bradley DDC (2007) Hole mobility within arylamine-containing polyfluorene copolymers a time-of-flight transient-photocurrent study. Phys Rev B 75 035215... 

The IFTOF technique requires the current mode of operation in which the various Resistance-capacity product (RC) time constants in the bridge are much smaller than the transit time tj. If the resistance Ri is larger than Rj, then the signal across the amplifier will be simply Rj i(t), where i(t) is the transient photocurrent. 

Figure 4.16 shows a series of waveforms of transient photocurrent for various temperatures. 

Further, we have also shown that the transient photocurrents in a double-layer shucture can conveniently be used for spectroscopic purposes in situations where the use of a conventional TOF technique is limited. 

Transient Photoconductivity. A solution of neutral molecules in a polar solvent shows only ohmic conductivity, but if ions are formed by the action of the photolytic flash these charge carriers generate an additional current which is proportional to the ion concentration. The observation of such transient photocurrents is the most direct experimental evidence for the formation of free, solvated ions in electron transfer reactions. The quantum yield of ion formation can be obtained through proper calibration procedures and the kinetics of ion recombination can be determined. Figure 7.37 gives an example of such transient photocurrent rise and decay. 

Figure 7.37 (a) Diagram of a transient photocurrent detection system. Ev, Eq high voltage and virtual ground electrodes R, load resistance, (b) Oscillogram of the rise and decay of the photocurrent in an electron transfer reaction... 

Fig. 35 Transient photocurrent action spectra for holes and electrons in PTS (p-toluenesulphonate)-polydiacetylene single crystals. The dashed curve in the spectra for holes corresponds to the absorption spectrum of the polymer dispersed in KBr. (After Chance et al.,...

The key point of this discussion is that it shows that analysis of the transient photocurrent response allows separation of ktr and krec. The ratio /Pho.o(00)/yPho o(0) is given by kj(ktr + knc) and a plot of ln(/photo(0 - jpho. to(/infinity)) vs. t has a slope of -(ktr + krec). It is important to note that the analysis is only possible if recombination occurs. For krec— 0, the photocurrent simply follows the illumination step, and 4>(f) = 1. 

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. 

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. 

Figure 93 Transient photocurrent signals (i) for 8 pm thick Alq3 layers sandwiched between the ITO anode and A1 cathode. Light pulses entered the samples through the ITO anode. The change from the dispersive transport for Alq3 as-received from the supplier (circles) to the non-dispersive transport with purified (squares) Alq3 samples. It can be seen in both linear (a) and double logarithmic (b) plots. In the inset of part (a) electric field dependence of mobility is shown, in part (b) the TOF transient for as-received Alq3 exposed to ambient is added (diamonds). After Ref. 423. Copyright 2003 American Institute of Physics, with permission.

Transient photocurrent measurements canlbe performed under a wide range of experimental conditions. The measurements can also be made under conditions such that the photocurrents are significantly less ithan the dark currents. The basic limitation of the technique, as with all techniques based on current measurements, is that it is difficult to determine pihotogeneration... 

While transient photocurrent and photoinduced discharge techniques are the conventional methods for measuring photogeneration efficiencies, these cannot be readily employed in the presence of trapping. A further limitation is that it is difficult to separate the field dependence of the photogeneration process from field dependencies associated with an injection or interfacial process (Seki, 1970, 1972a). As such, it is difficult to apply the method to dispersions or two-phase materials. Experimental methods that may avoid these limitations are... 

Figure 42 Photogeneration efficiencies for a double-layer photoreceptor comprised of a MSQ generation layer determined by photoacoustic and transient photocurrent measurements. The transport layer was a hydrazone doped polymer.

Figure 26. Transient photocurrent data plotted in semilogarithmic format for n-WSe2 in 1.0 M KI at —0.30 V (vs. SCE) showing the rapid decay component at short times. (Reproduced with permission from Ref. [113].)...

The transient photocurrent method requires that the dielectric relaxation time be... 

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