Open-circuit photovoltage

Figure 19 Photovoltage (open circuit) response of dye modified electrode (OTE/TiOj/ 17) to illiunination in a photoelectrochemical cell containing Pt foil as counterelectrode and aqueous 1.5 M KCl and electrolyte. (From Ref. 64.)...

Besides improving stability, a practical goal of surface modification has been to utilize redox reactions, otherwise not applicable, to yield better electrical characteristics such as higher open-circuit photovoltage, and to promote high conversion efficiencies. 

Under open circuit conditions the photovoltage of the interface can be expressed as ... 

The photopotentials of dye-gas-metal systems have reached an order of 0.1 mV 55> and those of alkali metal-aromatic junctions 0.2 to 1.0 V 53). Whereas the photo-emf of noble metal-aromatic junctions were of the order 1—15 mV 53>, thin tetracene films sandwiched between two different evaporated metal electrodes (Au, Al) showed photovoltaic effects with an open-circuit photovoltage up to... 

Hence, a logarithmic relation exists between the open-circuit photovoltage and light intensity (Fig. 4). 

The best results were observed using a comediator/Co(II) ratio of 1 2 with 0.15 M Co(DTB)32+ in acetonitrile. Addition of 0.5 M Li+ and 0.1M Tbpy generally improved the open-circuit photovoltage. In the presence of the PTZ/Co(II) mixture cell IPCE% reached more than 80% (inset Fig. 17.27), a value well comparable to the best I /I cells. Under white light irradiation, the /sc was also comparable, with the advantage of a better fill factor and higher Uoc for the cobalt-based cell (Fig. 17.27). 

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 7 The open-circuit photovoltage plotted versus the difference between the work function of the substrate in vaccum, 8 , v.ac> and the solution redox potential, 4>redox- The work function of the substrate in the solution, 8 , the quantity of interest, is difficult to measure directly, but it is related to 8 , vac see the discussion in Ref. 12. The four types of substrates are, from left to right, ITO, Sn02, Au and Pt the filled diamonds are for 0.5 M Lil solution, the open circles are for 0.05 M ferrocene in 0.1 M LiCI04 solution, and the filled triangles are for 0.05 M hydroquinone in 0.1 M LiCI04 solution. The theoretical line shows the behavior predicted by the junction model. (Data from Ref. 12.)...

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

D. Incident Photon to Current Efficiency and Open-Circuit Photovoltage... 

The total efficiency of the solar cell depends on the current, voltage, and the fill factor of the cell. Many groups have focused on the development of new sensitizers, which exhibit visible spectral response that matches the solar spectrum [15,20,35,47,50,82,84-86]. The open-circuit photovoltage is one of the key fac-... 

Photovoltaic volume effects have been investigated for the ferroelectric polymer-poly vinylidene fluoride [84,85]. The photovoltage was of the order 4 x 104 V for open circuit. The addition of dyes shifts the photosensitivity to the longer wavelength. 

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.

Dye sensitization of a nanometer-sized Ti02 powder film soaked in an organic medium containing iodine/iodide redox electrolytes successfully generated open circuit photovoltage (Foe) 0.68 V, Jsc 11.2 mAcm-2, Fill factor (FF) 0.68, and... 

Several of the reversible redox couples could, in fact be used in a cell for electricity generation. However, the output photovoltage, Ey, associated with these n-type Si/ferricenium/-ferrocene photoanodes is only in the 0.3-0.4 V range at open-circuit. Such is likely too low to be useful in practical schemes for solar energy conversion. The approach of derivatization, though, may be applied to other photoanode materials to realize improved efficiency and durability. 

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. 

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