Photocurrent spectrum

FIG. 16 Photocurrent spectra corresponding to the photo-oxidation of DCMFc by ZnYPPC" at the water-DCE interface under chopped illumination and lock-in detection. The main features of the spectra coincide with the onset of the Soret band and the Q-bands of the porphyrin ring. (From Ref. 73. Reproduced by permission of the Royal Society of Chemistry.)... 

Besides evaluating photoelectrodes for use in PECs, photoelectrochemical characterization can be used for other purposes. For example, photocurrent spectra of CD CdS has been used to measure the semiconductor bandgap (as f vs. hv), and agreement between the bandgap values measured by this method and by absorption spectroscopy for as-deposited and annealed films was found [78]. 

Fig. 10.6 Photoelectrochemical photocurrent spectra (in a selenosulphate electrolyte) of CD PbSe films deposited under various conditions with four different crystal sizes (see text). The cutoff at 1400 run is due to absorption by the water. (From S. Gorer, PhD dissertation, Weizmann Institute of Science, Rehovot Israel, 1996).

The close correspondence between the absorption spectrum in solution and the photocurrent spectrum of the adsorbed dye is by no means found in all cases. The adsorbed state can be different in structure from the solution state which is seen in a different photocurrent spectrum. It has been found e. g. that polymers are formed in the adsorbed state. This is a well known phenomenon for cyanine dyes 50-5b where polymer bands are found in the absorption spectrum of the adsorbed molecules. An example is given in the photocurrent spectra of Fig. 15. One sees that with increasing amount of adsorbed dye — no equilibrium adsorption was reached during this experiment — the polymer absorption band appears in the photocurrent. 

Fig. 15. Photocurrent spectra for -ZnO-electrode (at pH = 5) with different amounts of adsorbed cyanine dye, increasing in concentration from 1 to 3. For comparison the light absorption spectrum of the adsorbed dye on ZnO (dashed curve)...

Figure 10. Photocurrent spectra for p-polarized light at different angles of incidence, normalized to s-polarized light, represented by the spectrum at 0° (GaSe electrode with N, = 7 X 1016 cm 3 at V3ce = —0.7 V electrolyte /M H,SOk)...

Figure 9. Photocurrent spectra of two parts of the same n-WSes electrode (a) relatively smooth part and (b) heavily structured part. The arrows indicate the assumed...

Fig. 4.10. Photocurrent spectra with (curve 1) and without (curve 2) 10 3 M hydroquinone (QH2) for Dye II partially aggregated at the surface of WO3 electrode with the use of PD III and the spectral distribution of the relative variation in photocurrent upon insertion of QH2 in the electrolyte (curve 3). The inset shows the Stem-Volmer plots at 560 and 630 nm.

Photocurrent-potential curves and photocurrent spectra have been reported for DLC electrodes [61, 62, 170], Under illumination, the DLC demonstrates properties of an intrinsic wide-gap semiconductor or insulator. By treating the spectra in the above-described way, the mobility gap in this disordered carbon material was estimated. 

Photocurrent spectra of the CU2O / CuO,Cu(OH)2 duplex film suggest a smaller band gap of the CuO layer with respect to the value of Cu20 [97], However, the investigation of a simple anodic Cu(II) oxide layer was not possible. All trials to prepare this layer without a Q12O contribution were not successful. 

Photocurrent spectra of a p-type GaN epilayer have also been measured [7], From the onset of photoconductivity spectra, it was suggested that metastable centres at 1.1, 1.40, and 2.04 eV above the valence bandedge were responsible for the PPC in Mg-doped GaN, and that Ga vacancies may be responsible for PPC in n-type GaN [7], The spectral dependence of the optical cross section of the PPC related impurities in n-type epilayers has also been measured and the results are shown in FIGURE 4, from which an optical ionisation energy of about 2.7 eV was obtained [8], The correlation between the... 

The exact optical model describing multiple internal reflections, which are important for thin films where the penetration depth of the light 8 is larger than the layer thickness. Because of this part, iph depends on the layer thickness and the optical constants of both substrate and layer. This optical part mainly explains the observed variation in iph(df) found in the photocurrent spectra. 

Figure 1.8 Photocurrent spectra (quantum yield Q) ofTi02 passive films for different film thicknesses (formation potential, respectively). The measurements were carried out on three differently oriented Ti single grains. Electrolyte 0.5 M H2S04, anodic potential 2 V.

Photocurrent Spectra and iph(L/) Measurements on Single Ti/Ti02 Grains... 

Photocurrent Spectra Further correlation with the electronic structure of the films can be obtained by photocurrent measurements. The results are shown in Figure 1.32, with grains 1 and 2 as examples. As indicated from the photocurrent spectra (Figure 1.32a), the photocurrent quantum yield above the bandgap of 5.0 eVis... 

Figure 1.32 Photocurrent spectra (a) as a function of photon energy for grains 1 and 2 and (b) as a function of the formation potential 0.5 M H2SO4, 23 Hz, U = 3 V. The amorphous film on the grains exhibits a higher photocurrent in the sub-band regime due to the states within the mobility gap [17].

Figure 6.28 (a) Photocurrent spectra for a quantum dot in a PbS/Ti02 heterojunction film, showing... 

Figure 12.7 Photocurrent spectra for a series of Gai jAl cAs alloy samples, showing the shift in bandgap energy. Adapted from Hutton and Peter (1992).

Figure 12.9 Photocurrent spectra for anodic oxide films on bismuth. The left-hand spectrum is for the anodic photocurrent and the right-hand spectrum is for the cathodic photocurrent. Note the additional low-energy response due to internal photoemission at the Bi/Bi203 interface. Adapted from Castillo and Peter (1983).

Time-resolved measurements of electron transfer times for quantum well photoelectrodes which can be compared with hot electron relaxation times, have not yet been reported. Only some excitation spectra, i.e. photocurrent vs. photon energy for MQWs and single quantum wells (SQWs), have been published so far [2]. In both cases, the photocurrent spectra show distinct structures corresponding to transitions between the hole and electron wells as shown for SQW electrodes in Fig. 9.32. The... 

A very interesting example was published by Arden and Fromherz [34] who studied the performance of a multilayer carbocyanine dye structure. They used two types of cyanine dyes, A and D, incorporated into the multilayer structure as shown schematically in the upper part of Fig. 10.14. Dye A absorbs at around 420 nm and dye D around 370 nm. The photocurrent spectra presented in Fig. 10.14 exhibited the following features. If only one dye was used (curves I and II), the typical photocurrent was observed. In the case of curve I, the photocurrent was relatively low because the dye was separated from the electrode by an inert layer of arachidate molecules which inhibited the electron transfer process. If both dyes were used, in a structure given by III, the corresponding photocurrent exhibited a spectrum which was determined by both dyes. It should be noted that the spectrum of dye D was then much more pronounced compared with curve I. This effect was interpreted as energy transfer from D to A. The complete reaction scheme is then given by... 

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