Short-circuit photocurrent

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. 

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 fill factor is obtained by dividing the product of current and voltage measured at the power point by the product of short-circuit current and the open-circuit voltage. The power point is the maximum product of the cell voltage and the photocurrent obtained on the I V plot (see Section 9.16.4.5). The open-circuit voltage is the potential of the illuminated electrode, where the short-circuit current (7SC) is zero. 

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). 

In order to obtain high conversion efficiencies, optimization of the short-circuit photocurrent and open-circuit potential of the solar cell are essential. The conduction band of the Ti02 is known to have... 

Figure 20 Logarithmic plots of short circuit transient photocurrents for (a) PD100, (b) PD17, and (c) PDO LB films excited at 525 nm laser.

Semiconductor deposits thicker than the depletion layer (-10" cm) are required for optimum performance. The CdSe deposits were estimated to range up to 10" cm. The deposits were not doped chemically, but the heat treatment vaporizes Se leaving excess Cd which may act as a dopant. A marked increase in short circuit photocurrents was observed after heat treatments. 

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). 

Recently, photovoltaic cells that use a narrow band conjugated polymer PDDTT 196 as the electron donor and fullerene derivative 197 as the electron acceptor were developed. These cells show a short circuit density (J c) of 0.83mAcm , an open current voltage (Fqc) of 0.35 V, a fill factor (FF) of 38.6% under AM.5 simulator (lOOmWcm ) and unprecedented photocurrent response wavelengths up to llOOnm <2006APY081106>. 

Fig. 22, Short-circuit photocurrent of naked (—) and polypyrrole covered (.. . ) polycrystalline n-type Si electrode in aqueous 0.15 M FeS04,0.15 M FeNH4(S04)2 12 H,0 and 0.1 M Na2S04 at pH 1 under tungsten-halogen illumination at 143 mW/cm2. Air was not excluded.

In the case of a short circuit (V =0), the photocurrent density can be given as... 

Fig. 12. Relation between the short-circuit photocurrent in the system malachite green (n) + merocyanine FX 79 (p) 8 and light intensity /phot = f . Note A = 6200 A. = 100% corresponds to 3.7- 1015 quanta/cm2 sec Cell dimensions ImIO 4 cm illuminated area 1 cm2...

Fig. 13. Dependence of the p—n photovoltaic effect of dye/inorganic photoconductor systems on light intensity, (a) Short-circuit photocurrent of the system CdS/merocyanine A 10 7 = 5580 A 1 HK (Hefner unit) =94.7 //W/cm2 (b) Photovoltages of the systems 1. CdS/ merocyanine A 10 7 = 5580 A 2. Agl/rhodamine = 6000 A...

In this equation, isc is the short-circuit photocurrent, ff is the fill factor of the cell, and I is the intensity of the incident light. 

This reaction has been studied in some detail [2,4,31,32] and will be considered only briefly here. It is a remarkably slow process (microseconds to milliseconds) at short circuit and, thus, does not limit the short-circuit photocurrent density, Jsc. However, the rate of reaction (3) [33] and of the other recombination reactions increases as the potential of the substrate electrode becomes more negative [e.g., as the cell voltage charges from short-circuit (0 V) to its open-circuit photovoltage, Voc, (usually between —0.6 V and —0.8 V versus the counterelectrode)]. At open circuit, no current flows and the rate of charge photogeneration equals the total rate of charge recombination. 

The open-circuit photovoltage, short-circuit photocurrent, and fill factor obtained using the silanization reaction were superior to those obtained with the... 

In order to obtain high conversion efficiencies, optimization of the short-circuit photocurrent (z sc) and open-circuit potential (Voc) of the solar cell is essential. The conduction band of the TiO is known to have a Nernstian dependence on pH [13,18], The fully protonated sensitizer (22), upon adsorption, transfers most of its protons to the TiO surface, charging it positively. The electric field associated with the surface dipole generated in this fashion enhances the adsorption of the anionic ruthenium complex and assists electron injection from the excited state of the sensitizer in the titania conduction band, favoring high photocurrents (18-19 inA/cm ). However, the open-circuit potential (0.65 V) is lower due to the positive shift of the conduction-band edge induced by the surface protonation. 

On the other hand, the sensitizer (8), which carries no protons, shows a high open-circuit potential compared to complex 22, due to the relative negative shift of the conduction-band edge induced by the adsorption of the anionic complex, although, as a consequence the short-circuit photocurrent is lower. Thus, there should be an optimal degree of protonation of the sensitizer in order to obtain optimum short-circuit photocurrent and open-circuit potential, which determines tile power conversion efficiency of the cell. 

Fig. 24. Short circuit photocurrent (1) of the electrochemical cell with polythiophene and its absorption spectra (2) [194]...

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