Short circuit photocurrents

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... 

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). 

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.

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]...

A typical time response for a short-circuited photocurrent in the presence of hydroquinone ( Q) as an added solution redox species is shown in Figure 9. These photocurrents were stable for several hours. In the absence of in the electrolyte, the photocurrent also increased rapidly upon the onset of illumination, but subsequently decayed exponentially to 70% of its initial value in a half-decay time of ca. 25 s. This behavior is similar to that observed for chlorophyll monolayers deposited on SnC (12). Photocurrents under potentially-controlled conditions were also stable upon illumination, but exhibited slower decay characteristics when the light was turned off. This effect is unusual and is currently under further investigation. 

Figure 9. Short-circuited photocurrent vs. time for a monolayer of ZnTOAPP (ZnTOAPP.SA, 1 4) directly on a SnOt OTE (0.1U KCl, pH 7.0, 0.05M HSQ, Ns purged)...

Two devices are prepared. In the case of the device A, the incident photon-to-collected electron conversion efficiency (IPCE) exceeds 80% from 410 to 590 nm, reaching the maximum of 93% at 530 nm. The short-circuit photocurrent density (/sc), open-circuit photovoltage (Voc), and fill factor (FF) of device A with an acetonitrile-based electrolyte under an irradiance of AM 1.5 G full sunlight are 14.33 mA cm-12, 734 mV, and 0.76, respectively, yielding an overall conversion efficiency (jf) of 8.0%. The photovoltaic parameters of device B with a solvent-free ionic liquid electrolyte are 14.06 mA cm 12, 676 mV, 0.74, and 7.0%, respectively. 

The incident monochromatic photon-to-current conversion efficiency (IPCE) is plotted as a function of excitation wavelength. The IPCE value in the plateau region is 80% for complex 2, while for complex 25 it is only about 66%. In the red region, the difference is even more pronounced. Thus, at 700 nm the IPCE value is twice as high for the fully protonated complex 2 as compared to the deprotonated complex 25. As a consequence, the short circuit photocurrent is 18-19 mAcrn-2 for complex 2, while it is only about 12-13 mA cm-2 for complex 25. However, there is a trade-off in photovoltage, which is 0.9 V for complex 25, as compared to 0.65 V for complex 2. Nevertheless, this is insufficient to compensate for the current loss. Hence, the... 

Liu prepared a sandwich type coordination compound (103) from porphyrin and phthalocyanine with the assistance of a microwave. The resulted compounds showed good solubility in conventional organic solvents. The photoelectric conversion properties have been tested with a Gratzel type cell. The results revealed that the sandwich type compound showed better photo-electric conversion efficiency than the corresponding monomeric porphyrin or phthalocyanine precursors. The short-circuit photocurrent of the solar cell with this sandwich type compound as sensitizer, was, as high as 691.31 A cm-2, which was much better, than those of porphyrin or phthalocyanine monomers [100]. 

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