Peak photocurrent excited

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. 

In order to check the conclusions of DW we repeated their transient photoconductivity experiment under conditions allowing for higher electric fields. We find that the peak photocurrent excited by a N2-laser pulse follows an ig Eif(E) relationship which results in a linear field-dependence of the drift velocity as expected for conventional transport. 

Pig. 7. Relative carrier yield versus excitation wavelength. All data are normalized to the same absorbed photon flux. The electric field was t300 V/cm oriented approximately in the polymer chain direction in a standard surface cell configuration. The peak photocurrent-to-dark current ratio was 300 at 300 °K and >600 at 120 °K. Also shown is the absorption spectrum of PTS. The solid portion of the curve refers to the polymer chain direction. The dashed portion is an extrapolation derived from absorption measurements for a dispersion of PTS in a KBr pellet24 ... 

Fig. 3. Intensity dependence of the peak photocurrent in an oriented PPV film at 300 K for excitation light parallel and perpendicular to the chain orientation at different bias fields. 

The authors included that the excited states created by the optical transition at peak absorption ( 620 nm) are not involved in carrier generation. Very interesting is the observation that photocurrents increased with decreasing temperature to the peak value at 175—225 °K. They proposed that carrier mobilities increase with decreasing temperature and that the increase is high enough to offset the exponential decrease in carrier separation and lifetime. 

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.

Sanda et al. (1988) measured thermally stimulated currents in DEH doped PC. A peak in the spectrum was observed at 0.50 eV. The peak was attributed to the thermal excitation of holes from DEH donor states. The activation energy is in good agreement with values later derived from conventional photocurrent transient measurements by Schein and Mack (1988), Mack et al. (1989), and Kitamura and Yokoyama (1991). A secondary peak was observed at higher energies and attributed to trap states. 

A peak in the photoconductivity observed in the vicinity of 700 nm in partially decomposed AgNa was attributed to optical excitation of electrons from colloidal silver into the conduction band [92,98]. The photocurrents may lead to the optical bleaching discussed above. Strong photocurrents were also observed in the 500-420 nm range in colored material and may be associated with the other bands reported including the colloidal band. Although photocurrents were not observed in uncolored material at these wavelengths by Bartlett et al. 

The generated excitons must be quenched because the density of the excitons in this area is probably very high. Consequently, the peaks of the photocurrent quantum efficiency appear at the edge of the absorptions, which establish bulk excitation in the whole film. 

Figure 1.64. Normalized fast component of the photocurrent in oriented Omham trans- C )x excited with 25 ps lasser pulses (532 nm) at room temperature. The energy of the incident light pulses with polarization perpendicular to the stretch direction is 0.95 / J at a sample area of 200 /xm 300 /xm. The integral under the peak corresponds to 1.5 x 10 electronic charges (17o = 100 V). The solid line represents the best fit to the curve obtained by convoluting the photoconductivity response with the overall system response (assumed to be Gaussian). (Reprinted with permission from ref. 149)...

Recently, Baumann et al.(43) have measured time-resolved photoconductivity in PDA-TS-6 crystals as well as polyacetylene excited by 25 ps pulses of a Nd YAG laser (ftU) = 2.3 eV). The response time of the detector was 200 ps. The transient signal shown in fig.5 reveals a fast initial peak with instrument-limited pulse-shape followed by a slower decaying tail. The field dependence of the peak height (fig.6) parallels that of the carrier generation process and is in accord with what Donovan and Wilson have found on a 20 ns time resolution. The quantum efficiency associated with the fast photocurrent peak is 1.5x10 times the dc-quantum efficiency measured at hu) = 2.7 eV. 

At high F concentrations, the anodic dark current is very low and independent of the potential because holes are required for the anodic dissolution process. The anodic current is increased by light excitation [8]. The situation is changed for lower F concentrations. The photocurrent still remains constant at low light intensities. At higher light intensities, however, a current peak was observed, similarly as in the case of p-type electrodes. In this case, the current is limited again by the diffusion of F ions toward the electrode [76]. 

The above consideration can readily be applied to the case of photocurrent sensitization by dopant/impurity excitation, the condition being that the HOMO/LUMO position of the dopant is such as to permit transfer of the hole/electron to the valence/conduction band of the host while its twin remains trapped at the dopants. This mechanism can explain the appearance of a peak in the photocurrent-acting spectra at the low-energy edge of absorption, observed, for instance, in crystals of polydiacetylene-bis-toluenesulfonate and, possibly, other conjugated polymers, as well. 

Figure 9.28. Temperature dependence of the peak transient photocurrents measured at hv = 2.0 eV in oxygen-free 50 (circles), partially oxygen-exposed C o (triangles) and fully oxygenated 50 (squares). The inset compares the transient photoconductivity at the peak (circles) and at 2 ns after excitation (triangles). (Reproduced by permission of the American Physical Society from ref 43.)...

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