Photocurrent saturated

ZnS-CdS (bandgap = 2.3-2.4 eV) composite semiconductor photoelectrodes show a broad spectral response and n-type behavior, with saturation of the anodic photocurrent upon increasing anodic potential making the system suitable for use as a photoelectrochemical cell photoanode [72], Nanostructured ZnS-CdS thin film electrodes show that anodic photocurrent saturation can be attained with the application of a small, 0.1 V, bias [73], while hydrogen evolution is observed at the Pt cathode. The performance of the ZnS-CdS photoanodes appear strongly dependent upon the method of film preparation [72,73], with Zn rich films demonstrating superior photocurrent generation, and stability, in comparison to Cd rich films. 

When a reactively sputtered a-Si H is used for the photosensitive layer, the voltage at which the photocurrent saturates is about 15 V for undoped a-Si H. This saturation voltage decreased to 5 V when using nitrogen-doped a-Si H due to the increased electron mobility (Shimomoto et al., 1982). 

The ITO electrode is negatively biased relative to the Al electrode. Electron and hole pairs are generated by the incident light and electrons pass through the a-Si H. Since in undoped a-Si H the electron mobility is small, nitrogen is added to the sputtering atmosphere to increase the electron mobility. As a result, the photocurrent saturation voltage for the photodiode decreased to 5 V and the photoresponse time decreased to less than 500 psec (Shimomoto et al., 1982). 

Fig. 97. (a) Electrical equivalent circuit of an illuminated semiconductor electrolyte interface, (b), (c) Experimental impedance plots for n-GaAs/selenide under 22mWcnT2 illumination at different potentials, (b) V = -0.60V/SCE (in the photocurrent saturation region) (c) V = - 1.575 V/SCE (in the onset region). The circles are experimental points and the dotted curve is the best fit to (a). 

Figure 12.34 LMMRS response of p-Si in HF, showing that the semicircular response persists at negative potentials that are in the photocurrent saturation region. Note that the high-frequency intercepts in the plots are at zero. For further details, see Schlichthorl et at. (1995).

Figure 15-21 shows the dependence of the short circuit current and the photocurrent at -IV (reverse) bias as a function of the illumination intensity (514.5 nrn) the data show no indication of saturation al light intensities up to ca. 1 W/cm2. 

Figure 17. PMC behavior in the accumulation region, (a) PMC potential curve and photocurrent-potential curve (dashed line) for silicon (dotted with Pt particles) in contact with propylene carbonate electrolyte containing ferrocene.21 (b) PMC potential curve and photocurrent-potential curve (dashed line) for a sputtered ZnO layer [resistivity 1,5 x 103 ft cm, on conducting glass (ITO)] in contact with an alkaline electrolyte (NaOH, pH = 12), measured against a saturated calomel electrode.22...

In the experiment discussed (n-Si/0.6 M NH4F), the flatband potential (0.8 V vs. a saturated Hg-sulfate electrode) would have been immediately recognizable as the pronounced minimum between PMC and the photocurrent curve (Fig. 29). 

Photocells The basic construction of a photocell is illustrated in Figure 17. A photocurrent flows when the photocathode is illuminated, this is proportional to the intensity of illumination if the supply potential has been chosen to be higher than the saturation potential. A minimal potential is required between the photocathode and the anode in order to be able to collect the electrons that are emitted. The sensitivity is independent of frequency up to 10 Hz. The temperature sensitivity of evacuated photocells is very small. The dark current (see below) is ca. 10 " A[l]. 

Figure 1. Photocurrent/electrode potential curves for n-type TiOg single crystal with ohmic-indium contact on hack side. Potentials measured with respect to saturated KCl calomel electrode with a platinum black counter electrode. Light intensity increasing in order 3,2,1. Wavelength, 415 nm or less. Exposed surface of TiOg crystal ((Mil) (10).

ZnO-electrodes (band gap 3.2 eV) have in this connection been studied in great detail 30,31,47,48,49). We give here a few typical examples of the experimental results. Fig. 13 shows the dependence of the photocurrent on electrode potential for the dye rhodamine adsorbed at a ZnO-electrode with illumination in the wave length range of the absorption maximum of the dye (A=570 nm). The photocurrent reaches a saturation above a critical voltage which is about 0.25—... 

In Fig. 16 we show a current-voltage curve for >-type GaP in the presence of a dye in the dark and at illumination with light being only absorbed by the dye. We see again a saturation current, now however, in cathodic direction. The photocurrent spectrum is represented in Fig. 17. It corresponds fully with the absorption spectrum of the adsorbed dye. One sees a saturation current at a polarisation of more than 0.35 eV negative of the flat band potential. Some special features at GaP-electrodes seem to be caused by the existence of surface states with energies in the range of the band gap. It has been assumed that these surface states can... 

J.H model is, indeed, the common behavior. Figure 1 reports the degradation rate of phenol [36], a poorly adsorbed compound, and that of CHBA [35], a strongly absorbed compound, as a function of their initial concentration. A peaked reaction rate is observed, in contrast to the saturative LH model, also for CHCI3 and dodecane (see Fig. 2 in Ref. 37). For the photocurrents measured in photoelectro-chemical oxidation experiments of methanol and salicylic acid on anatase film electrodes, a saturation curve for the poorly adsorbed methanol and a peak at an intermediate concentration for strongly adsorbed salicylic acid were also observed as a function of the substrate concentration [38]. 

Figure 28.5 Current-potential curves for p-GaP under low- to moderate-intensity illumination a 1 M NaCl (pH = 1) electrolyte is employed. Illumination is from a 200-W high-pressure mercury lamp filtered with neutral density filter. Intensity is relative to the full lamp output. The H2/H+ redox potential is -0.3 V vs. SCE in this cell. Thus, this cell yields approximately 400 mV of open-circuit photovoltage. Note that increased illumination increases both the saturation photocurrent and the onset potential. Although the photocurrent is increased at higher light intensities, a calculation of the quantum yield for electron flow indicates that this parameter decreases with increased light intensity.

Sensitization of Ti02 nanosized particle films soaked in water was tried by dissolving a sensitizer and a sacrificial electron donor (EDTA) in the water phase (Fig. 19.7). Photocurrent was strongly dependent on the concentration of Ru(bpy)32+, reaching saturation at higher concentrations beyond 2 mM. By analysis of the photocurrent-vs.-concentration curve, a Langmuir-type adsorption of the dye was suggested. 

Photocurrent voltage curves have been studied with molybdenum selenide crystals of different orientation and different pretreatment. Figure 5 represents results for three typical surfaces of n-type MoSe (JJ+). An electrode with a very smooth surface cleaved parallel to the van der Waals-plane shows a very low dark current in contact with the KI containing electrolyte since iodide cannot directly inject electrons into the conduction band and can only be oxidized by holes. At a bias positive from the flat band potential U where a depletion layer is formed a photocurrent can be observed as shown in this Figure. This photocurrent reaches a saturation at a potential about 300 mV more positive than when surface recombination becomes negligible. 

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