Hole conductors solid-state

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... 

A limited number of inorganic solid-state hole conductors have been considered. These need to be p-type semiconductor materials, which can be grown within the pores of the Ti02 film. The most successful materials reported to date are copper (I) compounds such as copper iodide and copper thiocyanate [80, 81]. 

PEDOTrPSS has also been employed as a coating on the counter electrode and as a solid-state hole conductor in dye-sensitized solar cells (DSSCs) forming its own class of organic photovoltaic devices. 

The loading of the QDs on the nanostructured film can easily be quantified by UV-visible absorption spectroscopy. The spectrum of the sensitised film should match that of the QDs in solution, superimposed on the spectrum of the substrate, if the native quantisation property of the CdSe nanocrystals is maintained after assembly on the TiOi surface. The electrodes can be sandwiched with a counter electrode and filled with a redox mediator or assembled with a solid-state hole conductor to make devices in the same way as dye-sensitised analogues (Figure 3.109). 

Fig. 19. Schematic illustration of the mechanisms of (a) hole hopping and (b) iodine radical transport through CBZ-IMDZ-I solid-state ionic conductors (Midya et al, 2010).

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...

Solid state detectors consist of three layers, a layer of pure silicon sandwiched between a p-type and an n-type conductor. We recall that an example of an n-type conductor is germanium to which is added P or As, an impurity. The extra electron in the phosphorus or arsenic atoms is thought of as being in an energy level close to the conduction band. These electrons are readily thermally excited into the conduction band increasing the conductivity. A p-type semiconductor may be silicon to which a trivalent element such as boron or aluminum is added as an impurity. This creates holes close to the valence band. Electrons are readily promoted to these holes leaving positive holes in the valence band that provide for a conduction pathway. 

The p- and n-type conductors are necessary in the solid state detector because silicon crystals are very difficult to obtain pure. Usually they contain impurities such as boron, which would make it a p-type semiconductor. This situation would allow thermal electrons to cross the silicon energy gap to the low lying hole state. However, we want only electrons that are excited by X-ray photons to act as conductors. Interference of thermal electrons is avoided by a process called drifting in which a small amount of lithium is added, usually by thermal diffusion, to one side of the Si crystal and a larger amount to the opposite side. The lithium ionizes within the silicon... 

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