Sensitized Solar Cells

DSSGs can also be seen as electrochemical cells, bearing an intrinsic capacitance. Though they are of course bad batteries or capacitors, they have many things in common, for example, the electrodes and electrolytes. Other than the semiconductor-based photovoltaic, they can be interpreted as a technical version of photosynthesis, because molecules instead of solids are excited by sunlight. 

The second important interface is found at the cathode, where the triiodide is reduced to iodide. While older cell types applied platinized indium tin oxide (ITO)-coated glass, novel concepts are using polypyrrol-coated ITO [7], because the catalytic activity of platinum led also to slow, but nonnegUgible degradation of the electrolytes. 

The use of ILs as lubricants was suggested for the first time in 2001 by Liu et al. [8]. The authors reported that the IL l-methyl-3-hexylimidazoliumtetrafluoroborateled in terms of friction reduction, anti wear performance, and load carrying capacity to interesting results. From this initial point, many publications followed [9]. 

the role of ILs in the field of lubricants can be diverse instead of using them as a neat compound, they also can be used as an additive for other base oils, enhancing their friction, and wear behavior, but may also introduce novel properties such as conductivity. 

Finally, if the tribochemistry is also brought into consideration, speciflc ILs containing elements, for example, like phosphorous, sulfur, or boron, lead also to some interesting results. Though tribochemistry of ILs is still an early scientific field, it is in terms of research dominated by companies, disclosing most of the more detailed information. On the other hand the industrial interest in this field 

FIGURE 1. The operating principles of dye-sensitized solar cells. 

However, the presence of liquid electrolyte has the problems of leakage, robust sealing, and device stability, thus results in limited commercialization. Quasi-solid-state and solid-state DSSCs based on nonvolatile ionic liquid or organic hole-conducting material/polymer as the electrolyte are, therefore, developed to circumvent the sealing problem. 

The prototype DSCs used liquid electrolytes, typically L/I2 in an organic solvent such as propylene carbonate. The electron generation/collection problem in this cell has been discussed analytically with the help of intensity-modulated photocurrent and photovoltage spectroscopy [314]. A particularly challenging issue has been the replacement of the liquid electrolyte with a solid charge-transport material 

Application of the above techniques has enabled the synthesis of a range of nanostructured materials with tunable composition, physical properties, and 

Hyun et al. [345] prepared PbS Q-dots in a suspension and tethered them to Ti02 nanoparticles with a bifunctional thiol-carboxyl linker molecule. Strong size dependence due to quantum confinement was inferred from cyclic voltammetry measurements, for the electron affinity and ionization potential of the attached Q-dots. On the basis of the measured energy levels, the authors claimed that pho-toexcited electrons should transfer efficiently from PbS into T1O2 only for dot diameters below 4.3 nm. Continuous-wave fluorescence spectra and fluorescence transients of the PbS/Ti02 assembly were consistent with electron transfer from small Q-dots. The measured charge transfer time was surprisingly slow ( 100 ns). Implications of this fact for future photovoltaics were discussed, while initial results from as-fabricated sensitized solar cells were presented. 

Meissner D (2002) Photoelectrochemical solar energy conversion. In Luther J, Nast M, Norbert Fisch M, Christoffers D, Pfisterer F, Meissner D, Nitsch J (Article) Solar Technology. Ullmanns Encyclopedia of Industrial Chemistry, Wiley VCH (electronic release) 

Gratzel M (2007) Photovoltaic and photoelectrochemical conversion of solar energy. Phil Trans R Soc A 365 993-1005 

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Figure 38. Decay of PMC transients measured with a TSO -based nanostructured sensitization solar cell (ruthenium complex as sensitizer in the presence of 0.1 M TBAP in propylene carbonate). The transients are significantly affected by additions of iodide.40 (a) no I", (b) 2 mM r, (c) 20 mM r. (d) 200 mMT.

ZnO instead of T1O2 because ZnO provides a 220 times higher mobility for photoinjected electrons, which would allow reduction of the exciting laser intensity. The slow PMC decay of TiOrbased nanostructured sensitization solar cells (the Ru complex as sensitizer), which cannot be matched by a single exponential curve and is influenced by a bias illumination, is strongly affected by the concentration of iodide in the electrolyte (Fig. 38). On the basis of PMC transients and their dependence on the iodide concentration, a kinetic mechanism for the reaction of photoinjected electrons could be elaborated.40... 

Recently, room temperature ionic liquids (RT-ILs) have attracted much attention for their excellent properties, e.g., wide temperature range of liquid phase, ultra-low vapor pressure, chemical stability, potential as green solvents, and high heat capacities [64,65]. These properties make them good candidates for the use in many fields, such as thermal storage [66], electrochemical applications, homogeneous catalysis [67], dye sensitized solar cells [68], and lubricants [69,70]. 

Interfaces between two different media provide a place for conversion of energy and materials. Heterogeneous catalysts and photocatalysts act in vapor or liquid environments. Selective conversion and transport of materials occurs at membranes of biological tissues in water. Electron transport across solid/solid interfaces determines the efficiency of dye-sensitized solar cells or organic electroluminescence devices. There is hence an increasing need to apply molecular science to buried interfaces. 

Li, B., Wang, L., Kang, B., Wang, P., and Qiu, Y. (2006) Review of recent progress in solid-state dye-sensitized solar cells. Solar Energy Materials 11 Solar Cells, 90 (5), 549-573. 

Figure 9.1 Representation of a TiOj-sensitized solar cell with a platinum counter-electrode.

Figure 2 Schematic representation of the cross-section of a dye-sensitized solar cell.

Figure 4 Operating principles and energy level diagram of a dye-sensitized solar cell. S/S+/S = Sensitizer in the ground, oxidized and excited state, respectively. R/R = redox mediator (I3 / I-).

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). 

The optimal sensitizer for the dye-sensitized solar cell should be panchromatic, i.e., it should absorb visible light of all colors. Ideally, all photons at wavelengths shorter than a threshold of about 920 nm (see Section 9.16.1.1) should be harvested and converted to electric current.1,2 In addition, the sensitizer should fulfill several other demanding conditions ... 

An important aspect of dye-sensitized solar cells is water-induced desorption of the sensitizer from the surface. Extensive efforts have been made to overcome this problem by introducing hydrophobic properties in the ligands. Complexes that contain hydrophobic ligands ((48)-(53)) have several advantages compared to r/,v-dithiocyanato- u(2,2,-bipyndyl-4,4,-dicarboxylatc)ruthcnium(II) (22) ... 

Figure 11 Illustration of the interfacial CT processes in a nanocrystalline dye-sensitized solar cell. S / S+/S represent the sensitizer in the ground, oxidized and excited state, respectively. Visible light absorption by the sensitizer (1) leads to an excited state, followed by electron injection (2) onto the conduction band of Ti02. The oxidized sensitizer (3) is reduced by the I-/I3 redox couple (4) The injected electrons into the conduction band may react either with the oxidized redox couple (5) or with an oxidized dye molecule (6).

This type of sensitizer opens up new avenues for improving the near-IR response of dye-sensitized solar cells. In addition, important applications can be foreseen for the development of photovoltaic windows transmitting part of the visible light. Such devices would remain transparent to the eye, while absorbing enough solar energy photons in the near IR to render efficiencies acceptable for practical applications. 

Apart from recapture of the injected electrons by the oxidized dye, there are additional loss channels in dye-sensitized solar cells, which involve reduction of triiodide ions in the electrolyte, resulting in dark currents. The Ti02 layer is an interconnected network of nanoparticles with a porous structure. The functionalized dyes penetrate through the porous network and adsorb over Ti02 the surface. However, if the pore size is too small for the dye to penetrate, that part of the surface may still be exposed to the redox mediator whose size is smaller than the dye. Under these circumstances, the redox mediator can collect the injected electron from the Ti02 conduction band, resulting in a dark current (Equation (6)), which can be measured from intensity-modulated experiments and the dark current of the photovoltaic cell. Such dark currents reduce the maximum cell voltage obtainable, and thereby the total efficiency. 

Using the tri-iodide/iodide redox couple and the sensitizers (22) and (56), several groups have reported up to 8-10% solar cell efficiency where the potential mismatch between the sensitizer and the redox couple is around 0.5 V vs. SCE. If one develops a suitable redox couple that decreases the potential difference between the sensitizer and the redox couple, then the cell efficiency could increase by 30%, i.e., from the present value of 10% up to 13%. Towards this goal, Oskam et al. have employed pseudohalogens in place of the triiodide/iodide redox couples, where the equilibrium potential is 0.43 V more positive than that of the iodide/iodide redox couple.17 Yamada and co-workers have used cobalt tris-phenanthroline complexes as electron relays (based on the CoII/m couple) in dye-sensitized solar cells.95... 

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