Photocurrent-time transient

Figure 18. Photocurrent-time transients for 9.94 nM TBATPB. (Reprinted from Ref. 144 with permission. Copyright Elsevier Science Publishers, Amsterdam.)...

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 this section, we briefly consider the response of nanocrystalline semiconductor-electrolyte interfaces to either pulsed or periodic photoexcitation. Several points are worthy of note in this respect (a) the photocurrent rise-time in response to an illumination step is nonlinear. Further, the response is faster when the light intensity is higher, (b) The decay profiles exhibit features on rather slow time-scales extending up to several seconds, (c) The photocurrent decay transients exhibit a peaking behavior. The time at which this peak occurs varies with the square of the film thickness, d. (d) A similar pattern is also seen in IMPS data where the transit time, r, is seen to be proportional to d. ... 

Salvador, P., Gutierrez, C. Analysis of the transient photocurrent time behavior of a sintered n-SrTiOs electrode in water photoelectrolysis. J. Electroanal. Chem. 160, 117-130 (1984)... 

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. 

Relaxations in photoprocesses, which may be due to surface recombination, minority carrier diffusion, or capacitive discharges, are typically measured as transients of photocurrents or photoprocesses. An analysis of such processes in the time domain encounters some inherent problems. 

Although the reports by Brown et al. provided strong evidence of photoinduced heterogeneous ET responses [49], the rather slow rising times observed in the transients of Fig. 14(a) are difficult to rationalize on the basis of the mechanism in Fig. 11. Another puzzling point is the potential dependence of the photocurrent obtained under chopped... 

Quite differently, Pleux et al. tested a series of three different organic dyads comprising a perylene monoimide (PMI) dye linked to a naphthalene diimide (NDI) or C60 for application in NiO-based DSSCs (Fig. 18.7) [117]. They corroborated a cascade electron flow from the valance band of NiO to PMI and, finally, to C60. Transient absorption measurements in the nanosecond time regime revealed that the presence of C60 extends the charge-separated state lifetime compared to just PMI. This fact enhanced the device efficiencies up to values of 0.04 and 0.06% when CoII/m and P/Ij electrolytes were utilized, respectively. More striking than the efficiencies is the remarkable incident photon-to-current efficiency spectrum, which features values of around 57% associated to photocurrent densities of 1.88 mA/cm2. 

Fig. 6 Electron time-of-flight photocurrent transients of solution-cast film of EHO-OPPE (L=8 pm), measured at 295 K and an electric field of 2.5-10 V cm in (a) linear and (b) double logarithmic plots. Reproduced with permission from [61]...

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... 

A further application of the coplanar cell configuration showed in Fig. 3.1(c) concerns the study of the time dependence of the photocurrent following carrier excitation by means of a short pnlse of illnmination. This transient photodecay technique enables the examination of the interaction of initially free carriers with varions localized states. In principle, the decay of photocnrrent measured in this manner should (in the absence of recombination effects and phenomena associated with drift close to the surface of a thin film) correspond to the behavior in the initial pre-transit regime of a TOF pnlse. Becanse it allows measurements to be performed on very thin films under conditions appropriate to their nse in many device applications, and because the photocurrent may be examined over several decades of time withont the complications associated with carrier extraction, the techniqne has become rather popular over recent years. 

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. 

It can be seen that the 1 kV transient switching voltages, which would normally occur at the switching on and off times of the bias voltage, were eliminated down to level where they do not interfere with the photocurrent measurement. Figure 4.7... 

Fig. 4. Energy below the conduction band of levels reported in the literature for GaP. States are arranged from top to bottom chronologically, then by author. At the left is an indication of the method of sample growth or preparation liquid phase epitaxy (LPE), liquid encapsulated Czochralski (LEC), irradiated with 1-MeV electrons (1-MeV e), and vapor phase epitaxy (VPE). Next to this the experimental method is listed photoluminescence (PL), photoluminescence decay time (PLD), junction photocurrent (PCUR), photocapacitance (PCAP), transient capacitance (TCAP), thermally stimulated current (TSC), transient junction dark current (TC), deep level transient spectroscopy (DLTS), photoconductivity (PC), and optical absorption (OA).

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.

Figure 8.27 illustrates the theoretical electron density profiles and photocurrent transients calculated by Solbrand et al. The transients exhibit a maximum at a time fpeak = d2/6D. The inset in Fig. 8.26 shows that a plot of fpeak VS. d2 is linear as predicted (the authors use W rather than d to denote the film thickness), and the slope of the plot gives a value of 1.5 x 10-scm-2s-1 for the electron diffusion coefficient. 

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. 

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