Photocurrent density, intensity

The photocurrent density (/ph) is proportional to the light intensity, but almost independent of the electrode potential, provided that the band bending is sufficiently large to prevent recombination. At potentials close to the flatband potential, the photocurrent density again drops to zero. A typical current density-voltage characteristics of an n-semiconductor electrode in the dark and upon illumination is shown in Fig. 5.61. If the electrode reactions are slow, and/or if the e /h+ recombination via impurities or surface states takes place, more complicated curves for /light result. 

The overall conversion efficiency (rj) of the dye-sensitized solar cell is determined by the photocurrent density (7ph) measured at short circuit, Voc, the fill factor (fif) of the cell, and the intensity of the incident light (7S) as shown in Equation (9). 

For current densities below JPS the photocurrent in aqueous HF is found to be increased by a factor of 2 or even up to a factor of 4 for small photocurrent densities [Br2, Mai, Pel]. This effect is shown in Fig. 4.13. For non-aqueous HF electrolytes factors between 2 and 3 are observed. For further reduction of the illumination intensity the multiplication factor approaches infinity, because of the illu-... 

Fig. 10.4 (a) Computed minority carrier generation rate in bulk silicon for different wavelengths of monochromatic illumination of an intensity corresponding to a photocurrent density of 10 mA crrf2. (b) Bulk minority carrier density for carrier collection at the illumi-... 

Indeed, when using ( )/( ) couples, mass transport limitations are evident from the lack of linearity of the photocurrent versus incident power plots (Fig. 17.33) in the presence of a spacer, the photocurrent density produced by the cobalt-mediated cell is very close to the iodide/iodine system at low irradiation intensities (0.018 and... 

The overall efficiency, rjsi0bai, of the photovoltaic cell depends on the integral photocurrent density, zph, which represents the overlap between the solar light envelope and the monochromatic current yield, the open-circuit voltage, Voc, the fill factor of the cell iff), and the light intensity, Is, as shown in the following ... 

The promising initial study of hematite nanorod initiated a study of hematite nanorods for the aim of water oxidation [29]. IPCEse and IPCEee in a three-electrode set-up were determined at 350 nm to 2.3 % and 1.0 %, respectively. The fact that IPCEse is twice the IPCEee reveals that the collection of the photogenerated electrons across the hematite film is poor. Nevertheless, the ratio in the quoted investigation [29] is significantly smaller compared to the nanosized isotropic random walk hematite particle system reported by Bjbrksten et al [44]. The photocurrent density at an intensity of 1 sun was in the (xA/cm2 scale in 0.1 M NaOH. 

Fig. 40. (a) Dependence of the open-circuit potential E x on the illumination intensity J, and (b) depen-dence of the (1) dark current and (2) photocurrent density jpb on the electrode potential E. Potentials given vs. Ag,AgCl electrode [172], Reproduced by permission of The Electrochemical Society, Inc. 

To compare these methods, in Fig. 40 we show (a) EIX vs. J and (b) /Ph vs. E plots for a CVD single crystal thin-film electrode. We see that with increase in illumination intensity J, the open-circuit potential E(X approaches a limit of 0.7 V, which is close to the photocurrent onset potential (0.75 V). [The photocurrent density squared vs. potential dependence for this electrode, although far from linear (unlike that of Fig. 38), by the extrapolation to yph -> 0 gives the potential value of approx. 0.65 V.] It is concluded that, on the whole, methods (i) and (ii) are in a good agreement and can be used in the determination of the flat-band potential. Similar results were obtained with HTHP single crystals. 

The overall power conversion PCE efficiency (if) of the photovoltaic cell (range 0 to 1, or percentage from 0% to 100%) can be calculated from the photocurrent density (/ph A m-2), the open-cell circuit photovoltage (Voc), the geometrical fill factor of the cell (FF), and the intensity of the incident light (typically Js = 1000Wm 2 = 0.1 Won-2) as... 

Figure 28. PEIS data for the p-InP-1 M H2SO4 interface at a constant illumination intensity and varying photocurrent densities and bias potentials as shown. Both data points and model fits are shown with the equivalent circuit employed shown in the inset. R, in our notation (refer to text). (Reproduced with permission from Ref. [91).)...

When performing a study of this type on the system n-GaP/Fe(CN), the research group to which the present authors belong found that s decreases with increasing light intensity, hence photocurrent density [48]. This dependence was subsequently also observed for most other systems involving III-V semiconductor electrodes [49-51] (some exceptions are discussed in ref. [52]). In all of these cases, the values of s were found to be independent of the rotation speed of the electrode, i. e., they are kinetically controlled. The i-dependence of s hence indicates that the competition between the six-equivalent photoanodic oxidation of the III-V semiconduc-... 

Halmaim was the first to report the photoelectrochemical reduction of CO2 in 1978, using a p-type GaP electrode T Since then, a number of studies have been carried out on this subject2.3, aimed at reducing CO2 to produce useful products with a low input energy. In these reports, however, the photocurrent densities for CO2 reduction have been limited to maximum values of about 10 mA cm-2. Even when the light intensity was increased, the partial current density of CO2 reduction reached saturation. This was explained in terms of CO2 mass transport limitations and competition with hydrogen generation. 

FIGURE 6.27. Open-circuit photovoltage versus solution redox potential for n-Si and p-Si photoelectrodes in l.OM KCl/CHjOH solution. The redox couples used were (A) cobaltocene ° (B) A,W -dimenthyl-4, 4 -bipyridinium dichloride (C) V,V -dibenzyl-4,4 -bipyridinium dibromide (D) decamethylfer-rocene (E) Al,AI,N, AI -tetramethylphenylenediamine (F) dimethylferrocene" (G) ferrocene " (H) octyl-ferrocene ", A tungsten-halogen bulb was used to provide light intensities which yielded short-circuit photocurrent densities of 25-30 mA/cm After Lewis. (Reproduced by permission of The Electrochemical Society, Inc.)... 

The i-V curve of n-Si under a high illumination intensity, when the reaction is no longer limited by the availability of photogenerated carriers, is identical to that for p-Si except for a shift along the potential axis. As for p-Si, formation of PS on n-Si occurs only below the critical current, Ji. The /-Vrelationship at a current density much lower than the saturation photocurrent density is similar to that observed onp-Si. When the saturation photocurrent density is lower than the critical current density, the current is limited by the saturation photocurrent. 

The occurrence of the two-layer PS under front illumination depends on light intensity and the amount of charge passed as illustrated in Fig. 8.43. There is a correlation between the occurrence of two-layer PS and the saturation photocurrent value. Only a single micro PS layer forms at photocurrent densities below the photosaturation value whereas a two-layer PS forms at current densities above the saturation current as shown in Fig. 8.44. Also, a macro PS layer forms only after a certain amount of... 

Figure 32. Photocurrent density vs light intensity for poly-86.

Figure 3.3 Photocurrent density-voltage characteristics under lOOmWcm" incident intensity of AMI.5 simulated sunlight and IPCE action spectrum for an N719-sensitized Ti02 ITO-PEN electrode. This cell exhibits 6.1% efficiency.

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