Photovoltage, open circuit, increase

Tbpy increases the open-circuit photovoltage via suppression of the back recombination. With such a treatment, FT O/T i 02/dy e/P E D OT-P E D /FT sandwich cells afforded efficiencies of the order of 2.6%, one of the highest results so far recorded with solid-state DSCs based on hole conducting polymers (Fig. 17.46). 

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. 

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.

MSP titania was also used to make electrodes, which were tested in a dye-sensitized solar cell [116]. The short-circuit photocurrent, open-circuit photovoltage and fill factor increased with increasing sintering temperature, having a performance threshold at 450 °C, showing that the more ordered structures are required for high solar cell conversion efficiencies. 

The power characteristics of a photocell can be constructed from the individual current voltage curves of the photoelectrode and the counter electrode. This is shown in Fig. IV.4, again for an n-type semiconductor. In this figure the influence of the counter electrode is indicated. If the redox reaction there is slow, i.e. if it needs large overvoltages, the power characteristics are very much worsened. A similar effect has the photoelectrode if the photocurrent does not increase very steeply below the open circuit photovoltage and does not reach the saturation current very soon. 

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