Dye-sensitive solar cells

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 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... 

Purity is an indispensable requirement of any sensitizer in a dye-sensitized solar cell. While well worked out procedures exist for the efficient purification of metal complexes, we found that the isolation of the complexes at their isoelectric point, followed by column purification using Sephadex LH-20 gel, resulted in analytically pure samples. 

Very recently, even transparent conducting oxides (TCOs), such as indium-tin-oxide (ITO), have been prepared using suitable KLE templates.59 As one potential application, such porous TCOs (ZnO, etc.) are interesting for use in dye-sensitized solar cells. In general, such porous electrodes cover a variety of potential electro-optical applications, because they are both conducting and transparent. 

Zukalova, M. Zukal, A. Kavan, L. Nazeeruddin, M. K. Liska, P Gratzel, M. 2005. Organized mesoporous Ti02 films exhibiting greatly enhanced performance in dye-sensitized solar cells. Nano Lett. 5 1789-1792. 

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