Dye-sensitised solar cell

The main competition to semi-condnctor photovoltaics for producing solar electricity commercially is coming from photoelectrochemical devices based on dye sensitisation. These devices use relatively inexpensive semi-conducting materials such as titanium dioxide, zinc oxide and tin oxide. Gratzel in Switzerland has carried out the seminal work in this area, following on from the initial observation by Fujishima and Honda in Japan that a titanium dioxide electrode could be used to spht water into hydrogen and oxygen. 

The photoactive component in these cells is a dye adsorbed chemically onto the surface of the semi-conductor. When light hits this surface, the dye (S) absorbs a photon and becomes excited (S ) in this state it transfers an electron into the TiOj semi-conductor (injection). The positively charged dye (S+) then passes its positive charge to a redox mediator in the bulk electrolyte. The oxidised mediator is attracted to the counter electrode where it is reduced back by electron transfer, thus completing the circuit. 

The dye-sensitised solar cell (DSSC) is constructed as a sandwich of two conducting glass electrodes filled with a redox electrolyte. One of the electrodes is coated, using a colloidal preparation of monodispersed TiOj particles, to a depth of a few microns. The layer is heat treated to rednce resistivity and then soaked in a solution of the dye until a monomolecnlar dispersion of the dye on the TiO is obtained. The dye-coated electrode (photoanode) is then placed next to a connter electrode covered with a conducting oxide layer that has been platinised , in order to catalyse the reduction of the mediator. The gap between the two electrodes is filled with an electrolyte containing the mediator, an iodide/triodide conple in acetonitrile. The structure is shown schematically in Fignre 4.29. 

Other workers have employed different sensitiser systems, e.g. duel sensitisation by a zinc porphyrin and copper phthalocyanine on TiOj, Eosin Y or tetrabro-mophenol blue on ZnO, and a ZnO/SnOj mixture with a ruthenium bipyridyl complex, to produce good energy conversion factors. 

Whilst producing energy conversion factors comparable with semi-conducting photovoltaics, the original Gratzel type DSSCs do suffer from some problems around the use of liquid electrolytes. These problems include the need for highly efficient 

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Explain the photochemical principles of the dye-sensitised solar cell and understand the importance of various physicochemical parameters to the overall performance of such a cell. 

Figure 11.5 Essential features of a dye-sensitised solar cell based on sensitiser-coated Ti02 nanoparticles...

Tesfamichael, T., Will, G. and Bell, 1. (2005). Nitrogen ion implanted nanostructured titania films used in dye-sensitised solar cells and photocatalyst. Appl. Surf. Sd. 245(1 -4), 172-178. 

In this chapter, I give an account of the historical development of semiconductor photoelectrochemistry and nanostructured photovoltaic devices in Section 1.2, and then Sections 1.3-1.6 provide a brief introduction to the major cell types discussed in the remainder of the book the ETA (extremely thin absorber) cell, organic and hybrid cells, dye-sensitised solar cells (Gratzel cells) and regenerative solar cells. 

Nanosized TiOi powders are of outstanding importance in this context. Aqueous suspensions of 30 nm particulate TiOi (mostly in the rutile form) are the active agent in many of the photocatalytic systems described by Serpone and Emetine in Chapter 5. Agglomerations of Ti02 nanoparticles into mesoporous films of pore size 2-50 nm which allow the penetration of liquid are the basis of the important dye-sensitised solar cell (DSSC) discussed by Grateel and Durrant in Chapter 8, as well as most of the hybrid devices described by Nelson and Benson-Smith in Chapter 7, and some of the ETA (Extremely Thin Absorber) cells described by Konenkamp in Chapter 6. The term eta-solar cell was actually introduced by Konenkamp and co-workers (Siebentritt et al., 1997), who had earlier used the term sensitisation cell for the same type of device (Wahi and Konenkamp, 1992). Precursors to ETA cells with liquid electrolytes as hole conductors were developed by Vogel et al. (1990), Ennaoui et al. (1992) and Weller (1993). Similar electrolytic cells with RuSa (Ashokkumar et al, 1994) and InP (Zaban et al, 1998) nanoparticle absorbers have also been demonstrated. 

The technology of ETA cells is not yet mature, and future improvements in their efficiency can reasonably be expected. Modelling calculations (Taretto and Rau, 2005) indicate that 15% efficient CdTe ETA cells are possible even at electron diffusion lengths as low as 10 nm, provided that the built-in voltage is optimised to restrict recombination over the working bias range of the cell. If efficiency improvements of this order can be achieved and fabrication methods can be satisfactorily scaled up, ETA cells could offer a low-cost, stable alternative to traditional photovoltaic cells and dye-sensitised solar cells. 

Figure 1.7 Dye-sensitised solar cell (a) cell architecture (b) electronic energy levels. The placement of the semiconductor band-edge energy and the solution Fermi levels S/S, S /S and 1713 on the same scale, the vacuum scale of electronic energy, is explained in Appendix lA at the end of this chapter.

The present understanding of electron transport in these nanostructured oxides has improved significantly with the many detailed investigations in the electrolytic dye-sensitised solar cell. The accepted picture explains the surprisingly efficient electron... 

Concepts for tandem arrangements have been suggested for dye-sensitised solar cells using dye sensitisation at both electrodes and an electrolyte that provides the coupling between the two dyes (He et al, 2000). A similar concept appears feasible also for solid-state devices, althongh it has not been explored yet in the context of ETA cells. 

Karuppuchamy S., Nonomura K., Yoshida T., Sugiura T. and Minoura H. (2002), Cathodic electrodeposition of oxide semiconductor thin films and their application to dye-sensitised solar cells . Solid State Ionics 151, 19-27. 

Eenzmann F., Krueger J., Bnrnside S., Gratzel M., Gal D., Riihle S. and Cahen D. (2001), Hot electron injection in dye-sensitised solar cells surface photovoltage spectroscopic evidence , J. Phys. Chem. B 105, 6347-6352. 

Olea A. and Sebastian P. J. (1998), (Zn,Cd)S porous layers for dye-sensitised solar cell application . Solar Energy Mat. Solar Cells 55, 149-156. 

Palomares E., Clifford J. N., Haque S. A., Lutz T. and Durrant J. R. (2003), Connol of charge recombination dynamics in dye-sensitised solar cells by the use of conformally deposited metal oxide blocking layers , J. Am. Chem. Soc. 125, 475-482. 

Tennakone K., Bandara J., Bandaranayake P. K. M., Kumara G. R. A. and Konno A. (2001), Enhanced efficiency of a dye-sensitised solar cell made from MgO-coated nanocrystalline Sn02 , Jap. J. Appl. Phys. 40, L732-L734. 

Turkovic A. and Crnjak Z. (1997), Dye-sensitised solar cell with Ce02 and mixed Ce02/Sn02 photoanodes . Solar Energy Mat. Solar Cells 45, 275-281. 

Figure 8.1 Schematic of a liquid electrolyte dye-sensitised solar cell. Photoexcitation of the sensitiser dye is followed by electron injection into the conduction band of the mesoporous oxide semiconductor, and electron transport through the metal oxide film to the TCO-coated glass working electrode. The dye molecule is regenerated by the redox system, which is itself regenerated at the platinised counter electrode...

Figure 8.3 Chemical structure of the N719 ruthenium complex used as a charge-transfer sensitiser in dye-sensitised solar cells. The N3 dye has the same structure, except that all four carboxylates are protonated.

Hinsch A., Kroon J. M., Kern R., Uhlendorf L, Holzbock J. and Meyer A. (2001), Long-term stability of dye-sensitised solar cells , Progr. Photovoltaics 9, 425-438. 

Nagai H. and Segawa H. (2004), Energy-storable dye-sensitised solar cell with polypyrrole electrode , Chem. Comm., 974-975. 

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