Transient current measurements

Figure 16 Schematic drawing of the system developed for transient current measurement using heavy-ion microbeam.

Current transformers used in transient current measurements must have a peak current rating at least equal to the maximum expected currents otherwise, current peaks are lost in the data due to saturation of the current probe. Figure 9.6 indicates how current probe saturation resulted in a flat-top current waveform and loss of vital information, making power quality analysis more difficult. 

B. DC Transient Current Measurement DGEBA oligomer (Tg=45 °C) Havriliak-Negami Williams-Landel-Ferry 92... 

Brohede U, Bramer T, Edsman K, et al. Electrodynamic investigations of ion transport and stractural properties in drug-containing gels dielectric spectroscopy and transient current measurements on catanionic carbopol systems. J Phys Chem B 2005 109 15250-15255. 

Figure 12. Schematic of the transient currents measured using the apparatus shown in Fig. 10 for a square pulse input voltage.

Model. The comparison of theory and experiment in Figure 9 indicates that the simplified model can be used to calculate transients which agree with experiment in gross trends. The model permits quantitative analysis of bulk and contact space-charge effects in PI transient current measurements. In particular, this model is sufficient to calculate measurement-history effects due to mobile ions and bulk electronic space-charge (9). The relaxation of space-charge upon removal of the bias is intrinsically slower than its accumulation. Thus, the sample history is stored in the space-charge distributions. These results will be demonstrated in a future publication. 

In the third subsection, the determination of ion density and mobility will be in focus, both by using impedance spectroscopy and transient current measurements. 

Figure 43 2. Diffusion coefficients far LA in crystalline (a) and disordered (i>) WO3 as obtained from electrochemical transient current measurements and from electrochemical impedance spectroscopy (Strpmme Mattsson [2000]). The diffusion coefficient obtained by impedance spectroscopy is the chemical diffusion coefficient relevant for a specific composition (specified on the upper x axis), while that obtained from the current measurements is an average diffusion coefficient for all compositions lower than the composition value specified on the upper x axis.

Impedance Spectroscopy. Impedance spectroscopy has been carried out on devices with WO3 as the cathodic electrochromic layer, counter electrodes of iridium oxide, polyaniline or Prussian blue, and polymers as electrolytes (Katsube et al [1986], Friestad et al [1997]). The equivalent circuit for a whole device becomes very complicated. In the works quoted above simplified, Randles-type circuits were used for the two electrochromic layers, while the ion conductor was modeled by a pure resistance, or neglected. Extraction of device parameters from the data fitting was reported. However, it is clear that in many cases it will be difficult to distinguish the contributions from the different layers in a device, in particular if the migration impedances, ion diffusion impedances, etc. are of the same order of magnitude. When it comes to characterizing electrochromic devices, impedance spectroscopy is a very time-consuming process, since a spectrum down to low frequencies should be taken at a number of equilibrium potentials. Thus we believe that transient current measurements in many cases offer a faster alternative that sometimes allows a simple determination of diffusion coefficients. 

Transient Ion Currents. Transient current measurements, according to Section 4.3.4.3, have been performed on laminated devices (Jonsson et al [2004]) as well... 

Much information about the kinetics of electrochromic devices can be obtained from impedance spectroscopy or transient current measurements. In particular, the transient current technique is a promising method for device testing. It is based on the well-known transient ion current technique used in studies of ion conduction and it is less time consuming than impedance spectroscopy. More efforts should be devoted to development of theory as well as to detailed comparison with experimental data. 

Transient electrical data collected under different bias conditions contain enough information to determine the density of states with the aid of a transient device model. One study has shown that by simultaneously fitting a number of transient current measurements, as well as steady-state device response, a non-trivial best fit for the densities of sub gap electron and hole states could be obtained [86]. In a further application of the transient device simulation it has been shown that in certain conditions the transient current extracted from a solar cell device accurately reflects the density of states of the carrier type with the greater density of deep trap states [160]. In those conditions where this approximation is true, then transient current data can be modelled with a simple expression for current due to thermally emitted charge carriers, and the current transient mapped onto a density of states function [51, 52]. However, the approximation fails in the limit of significant recombination or low electric fields. 

Heilmeier, G.H., Heyman, P.M. Note on transient current measurement in liquid crystals and related systems. Phys. Rev. Lett. 18, 583-585 (1967)... 

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