Photocurrent density

It is clear that one of the major limitations of this analysis is the assumption of constant excited-state coverage. Deviations from the behavior described by Eq. (45) in the low frequency range have been observed at photocurrent densities higher than 10 Acm [50]. These deviations are expected to be connected to excited-state diffusion profiles similar to those considered by Dryfe et al. [see Eq. (38)] [127]. A more general expression for IMPS responses is undoubtedly required for a better understanding of the dynamics involved in back electron transfer as well as separation of the photoproducts. 

The photopontential also approaches to zero when the semiconductor photoelectrode is short-circuited to a metal counterelectrode at which a fast reaction (injection of the majority carriers into the electrolyte) takes place. The corresponding photocurrent density is defined as a difference between the current densities under illumination, /light and in the dark, jDARK ... 

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). 

Figure 5. Output characteristics and photocurrent density at maximum power point against time (inset) for an intrinsic a-Si H photoanode-based cell. (Reproduced from Ref. 5.)...

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-... 

Quite differently, Pleux et al. tested a series of three different organic dyads comprising a perylene monoimide (PMI) dye linked to a naphthalene diimide (NDI) or C60 for application in NiO-based DSSCs (Fig. 18.7) [117]. They corroborated a cascade electron flow from the valance band of NiO to PMI and, finally, to C60. Transient absorption measurements in the nanosecond time regime revealed that the presence of C60 extends the charge-separated state lifetime compared to just PMI. This fact enhanced the device efficiencies up to values of 0.04 and 0.06% when CoII/m and P/Ij electrolytes were utilized, respectively. More striking than the efficiencies is the remarkable incident photon-to-current efficiency spectrum, which features values of around 57% associated to photocurrent densities of 1.88 mA/cm2. 

Fig. 3.22 (a) Current - voltage characteristics of a p-type photocathode and that when it is replaced by a platinum shown to illustrate the efficiency calculation using the power saving approach. The shaded area represents the maximum power saving as a result of photoelectrolysis, (b) The efficiency at various photocurrent densities obtained useing the base graph. 

Ip is the photocurrent density in mA cm, AE the potential difference between working electrode and counter electrode under illumination minus the potential difference between the same electrodes without illumination (dark). That is, AE is the photovoltage with dark voltage subtracted from it. This equation is misleading and has no thermodynamics basis. AE does not necessarily represent the sample behavior but it depends upon the experimental conditions. Furthermore the hydrogen produced at current Ip can yield a power output higher than Ip AE. 

Ip( i) is the photocurrent density at wavelength X. IPCE becomes 100% when all photons generate electron-hole pairs. However, in practical situations IPCE is always less than 100% due to the losses corresponding to the reflection of incident photons, their imperfect absorption by the semiconductor and recombination of charge carriers within the semiconductor, etc. 

Ti. This leads to an increase in the concentration of the majority (electron) carriers and therefore the conductivity. The photocurrent densities sometimes vary due to incomplete dissolution as well as large dopant concentrations [10]. Raising the sintering temperature of Ti02 Nb to 1350°C and maintaining the doping concentration ND/[Ti j in the range of 0.05 at%, leads to an increase in photocurrent by a factor three [11]. 

Figure 5.38 illustrates the experimental setup for water photoelectrolysis measurements with the nanotuhe arrays used as the photoanodes from which oxygen is evolved. The 1-V characteristics of 400 nm long short titania nanotuhe array electrodes, photocurrent density vs. potential, measured in IM KOH electrolyte as a function of anodization hath temperature under UV (320-400 nm, lOOmW/cm ) illumination are shown in Fig. 5.39. The samples were fabricated using a HF electrolyte. At 1.5V the photocurrent density of the 5°C anodized sample is more than three times the value for the sample anodized at 50°C. The lower anodization temperature also increases the slope of the photocurrent—potential characteristic. On seeing the photoresponse of a 10 V 5°C anodized sample to monochromatic 337 nm 2.7 mW/cm illumination, it was found that at high anodic polarization, greater than IV, the quantum efficiency is larger than 90%.

Fig. 5.39 Variation of photocurrent density (in IM KOH solution) vs. measured potential vs. Ag/AgCl for lOV samples anodized at four anodization bath temperatures, 5, 25, 35 and 50°C. The samples were measured under 320-400nm lOOmW/cm illumination.

The short circuit photocurrent density (320-400 nm illumination, lOOmW/cm ) of the sample anodized in 1 1 DMSO and ethanol containing 4% HF solution, Fig. 5.43 curve (a), is more than six times the value for the sample obtained in a 1% hydrofluoric acid aqueous solution, Fig. 5.43 curve (b). 

Fig. 5.43 Photocurrent density versus applied potential in 1 M KOH solution under UV (320 nm to 400 nm) illumination (96 mW/cm ). Anodic samples prepared as (a Titanium foil anodized at 20 V for 70 h in DSMO and ethanol mixture solution (1 1) containing 4% HF. (b) H2O-HF electrolyte at 20 V for 1 h. Both samples were annealed at 550°C 6 h in oxygen atmosphere prior to testing. Dark current for each sample is shown in (c).

The photocurrent density of nanotube array samples fabricated in an electrolyte of 1.2 g of NH4F in a solution of 5 ml deionized water + 95 ml formamide at 35 V is shown in Fig. 5.46(a). The resulting nanotube array samples were 30 pm in length, with an outer diameter of 205 nm. The samples were annealed at 525°C and 580°C for 1 hour in oxygen prior to measurement. The 580°C annealed sample had an open circuit voltage Voc of -0.925 V (vs. Ag/AgCl) the 525°C annealed sample had an open circuit voltage... 

The photocurrent density of nanotuhe array samples fabricated in an ethylene glycol electrolyte, 0.25 wt % NH4F and 1% H2O at BOV for 6 hours is shown in Fig. 5.47(a). The resulting... 

Fig. 5.59 Photocurrent density versus potential in 1 M NaOH solution for annealed Ti-Fe-0 nanotube array samples, and a-FezOs nanoporous film, under AM 1.5 (100 mW/cm ) illumination.

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