DSSCS

Polo and Murakami Iha used anthocyanins extracted from jaboticaba (Myrciaria cauliflora Mart) and calafate (Berberis buxifolia Lam) as dyes for DSSCs. [46] The interaction between the dye molecules and Ti02 was identified by comparing the visible absorption spectra of the bare dye in solution with those acquired after dye absorption on the semiconductor a 15 nm red shift indicated the anchorage of the anthocyanin molecules on the Ti02 nanoparticles. The inorganic semiconductor layer was deposited on ITO and the electrolyte employed was I /I3 dissolved in acetonitrile. The photovoltaic cell obtained with the jaboticaba extract gave an IPCE value of 0.2 with a short-circuit current (/sc) of 7.2 mAcm 2, a Voc of 0.5 V and a fill factor of 54%. 

The influence of the extraction temperature on the performance of DSSCs was also investigated with the optimum temperature being 50 °C Js< —2.06mAcm 2, Voc = 433 mV, FF=0.59, t] = 0.52% for roselle. Higher temperatures lead to thermal degradation of the dye, and lower temperatures imply inferior solubility. 

Garcia et al. [48] used extracts from chaste tree and mulberry fresh fruits and cabbage palm pulp. In all cases the binding of the anthocyanins to Ti02 through hydroxyl and carbonyl groups caused a red shift of the absorption peak in comparison with solutions. The main parameters for DSSCs sensitized with chaste tree, mulberry, and cabbage palm fruits were Isc — 1.06, 0.86, 0.37 mA, VOC = 390, 422, 442 mV, Pmax= 198, 154, 99.3 iW, FF = 0.48, 0.43, 0.61, respectively. 

In general, the reports describing the performance of DSSCs based on mesoporous Ti02 by either method are consistent regarding the positive effect of applying nanostructured Ti02 films. 

Relatively little is understood in the presence of non planar-non ideal interfaces, where electronic levels located in the band gap region act as recombination centers. Colloidal materials, low cost polycrystalline materials and films, interpenetrating networks of absorber and charge collecting phases (e.g., as in the DSSC cells), and the presence of redox active adsorbing species, all give rise to... 

Figure 11.6 Schematic energy-level diagram relating to the principles of operation of the DSSC...

Thus electrical power is produced without any permanent chemical change. The overall performance of the DSSC depends on the energy levels of certain of the components of the cell, namely ... 

The following relationships exist between the various energy levels and the performance of the DSSC ... 

The maximum voltage of the DSSC is given by the energy gap between the Fermi level of the semiconductor electrode and the redox potential of the mediator. 

The DSSC differs substantially from the p-n-junction solar cell because electrons are injected from the photosensitiser into the CB of the semiconductor and no holes are formed in the VB of the semiconductor. 

Scientific interest in nanocarbon hybrid materials to enhance the properties of photocatalysts and photoactive electrodes has been growing rapidly [1-8]. The worldwide effort to find new efficient and sustainable solutions to use renewable energy sources has pushed the need to develop new and/or improved materials able to capture and convert solar energy, for example in advanced dye-sensitized solar cells - DSSC (where the need to improve the photovoltaic performance has caused interest in using nanocarbons for a better cell design [9,10]) or in advanced cells for producing solar fuels [11-13]. 

There are thus multiple effects by which the properties of the nanocarbon-semiconductor hybrid material can be different from the simple physical mixture of the two components [1] The nanocarbon offers an effective way for an efficient dispersion of the semiconductor, thus preventing agglomeration, but also providing a hierarchical structure [15] for efficient light harvesting and eventually easy access from gas/liquid phase components (in photocatalytic reactions) or electrolyte (in DSSC). 

Another effect involves charge transport resistivity at the semiconductor-semi-conductor interface. The charge transport of the Ti02 photoelectrode, limited by its poor conductivity (about 0.1 cm2 V-1 s-1) [94], is the rate-determining step for the power-conversion efficiency in DSSCs [95]. As mention above, an usual strategy to improve charge transport is to add CNTs to the DSSC photoelectrode. It could be expected that the effect is proportional to the conductivity of the CNT, but Guai et al. 

Fig. 16.7 Energy-band diagrams of DSSCs with incorporated (a) semiconducting (s-) SWCNTs and (b) metallic (m-) SWCNTs. The solid and dashed arrows represent desired charge transport and undesired recombination processes. Adapted from Guai etal. [95].

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