Solid hole conductors

For the QD-sensitised cell, QDs are substituted for the dye molecules, as shown in Fig. 3.19b they can be adsorbed from a colloidal QD solution (Zaban et al, 1998) or produced in situ (Weller, 1991 Liu and Kamat, 1993 Vogel et al, 1994 Hoyer and Konenkamp, 1995). Successful PV effects in such cells have been reported for several semiconductor QDs including InP, CdSe, CdS, PbS and InAs (Weller, 1991 Liu and Kamat, 1993 Vogel et al, 1994 Hoyer and Konenkamp, 1995 Zaban et al, 1998 Nozik, 2001a Robel et al, 2006 Yu et al, 2006 Tachibana et al, 2007). Possible advantages of QDs over dye molecules are the tunability of optical properties with size, better heterojunction formation with solid hole conductors, and the unique potential capability of the QD-sensitised solar cell to produce quantum yields greater than one by MEG (inverse Auger effect) (Nozik, 2002). 

Thus, measurement of the total conductivity together with the cell voltage allows the transport numbers of the ions to be determined (Fig. 8.17). The results show that at lower temperatures proton conductivity is of greatest importance, at middle temperatures oxygen ion conductivity becomes dominant, and at high temperatures the material is predominantly a hole conductor. Between these temperatures, at approximately 350°C the solid is a mixed H+ and O2- conductor while at approximately 650°C it is a mixed hole and O2- conductor. 

The electrostatic potential between a solid and an aqueous solution can be esplained in terms of a parallel plate condenser with a jiositiye excess charge on one phase and a negative excess charge on the other. The interfacial charge on the solid (electronic conductor) is usually carried by mobile excess electrons and holes, while it is carried by mobile excess hydrated ions on the side of aqueous solution (ionic conductor). 

For this reason (and others), it is not trivial to make a solid-state version of the dye cell. Initial attempts did not include a mobile electrolyte and thus had no way of neutralizing the Coulomb attraction between the photogenerated charge pairs [42,53]. The best results were achieved by Tennakone et al. [13] in a cell with solid Cul as the hole conductor—in which the ionic mobility of the Cul may have helped neutralize the Coulomb attraction. Later attempts included mobile electrolyte ions, which improved performance [9,54]. 

On the other hand, Tennakone and co-workers utilized a p-type semiconductor material, such as Cul (band gap,-3.1 eV), as a hole conductor and produced a solid-state DSSC [141,145,146]. Acetonitrile solution of Cul was dropped onto the surface of a dye-coated Ti02 film, which was heated up to approximately 60°C and then the solution penetrated into the film. After evaporation of the acetonitrile, Cul was deposited into a nanoporous Ti02 film. The Au-coated TCO substrate as the counterelectrode was pressed onto the surface of the Ti02/dye/ Cul film. In the system using the santalin dye photosensitizer, an efficiency of 1.8% was obtained under irradiation of 80 mW/cm2 [141] and the efficiency reached 4.5% for the Ti02/N3 dye/CuI/Au system. These results suggested that a highly efficient solid-state DSSC could be produced [145]. In these systems,... 

Cul could be partly in contact with Ti02 directly therefore, the efficiency decreased by the recombination of injected electrons with Cul. In order to increase cell performance, direct contact between the Ti02 film and Cul must be minimized. Solid-state DSSCs have been studied using other organic and inorganic hole conductor materials, such as p-type CuSCN [147,148], polypyrrole [149], and polyacrylonitrile [95]. 

Research on the solid state dye-sensitized solar cells (DSC) has gained considerable momentum recently as this embodiment is attractive for realizing flexible photovoltaic cells in a roll-to-roll production. The spzro-OMeTAD has been the most successful p-type organic conductor (hole transport material) employed. Its work function is about 4.9 eV and the hole mobility 2 x 10-4 cm2 s x. A schematic diagram of the solid sate DSC with the structure of this hole conductor is shown in Fig. 19. Reported first in 1998, the con-... 

Fig. 19 Cross-sectional view of a solid state dye-sensitized photovoltaic cell using the hole conductor spiro-OMeTAD, whose structure is indicated on the right...

A recent alternative embodiment of the DSSC concept is the replacement of the redox electrolyte with a solid-state hole conductor, which may be either inorganic (Tennakone et al, 1995 O Regan and Schwartz, 1998) or organic (Bach et al, 1998), thereby avoiding the use of a redox electrolyte. Such solid-state sensitised heterojunctions can be regarded as functionally intermediate between redox electrolyte-based photoelectrochetnical DSSCs and the organic bulk heterojunctions described in... 

Virkar, A.V. (2001) Transport of H2, O2 and H2O through single-phase, two-phase and multi-phase mixed proton, oxygen and electron hole conductors. Solid State Ionics, 140, 275-83. 

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