Hole conductors liquid electrolyte

LSE, the classical electrochemistry, is concerned with electrochemical cells (ECs) based on liquid ionic-conductors (liquid electrolytes (LEs)). Solid-state electrochemistry is concerned with ECs in which the ionic conductor (electrolyte) is a solid. Both fields are based on common thermodynamic principles. Yet, the finer characteristics of ECs in the two fields are different because of differences in the materials properties, conduction mechanisms, morphology and cell geometry. Differences that come immediately to mind are (1) The lack of electronic (electron/hole) conduction in most LEs, while electronic conduction exists to some extent in all solid electrolytes (SEs). (2) In LEs both cations and anions are mobile, while in SEs only one kind of ions is usually mobile while the other forms a rigid sublattice serving as a frame for the motion of the mobile ion. An... 

While impressive progress has been made in the development of stable, non-volatile electrolyte formulations, the conversion yields obtained with these systems are presently in the 7-10% range, i.e., below the 11.1% reached with volatile solvents. Future research efforts will be dedicated to bridge the performance gap between these systems. The focus will be on hole conductors and solvent-free electrolytes such as ionic liquids. The latter are a particularly attractive choice for the first commercial modules, due to their high stability, negligible vapor pressure and excellent compatibility with the environment. 

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. 

Fig. 2 Scheme representing the general principle of a photoelectrochemical system. Electrons are photoexdted in the absorber. The electron and hole are selectively transferred to an electron conductor (usually a metal or a semiconductor) and to a hole conductor (a redox system in a liquid electrolyte). The photovoltage is the difference between the electrochemical potentials in the electron and hole conducting phases thus Mh-... 

Three-dimensional geometry Because of the structure of the junction, the geometry is three dimensional on the microscopic scale. The electrochemical junction gives rise to large local electric fields within the hole conductor at all points of the interface. These are typically a few run in extent in the case of liquid electrolytes [23, 24]. 

Although the most commonly used redox couple to act as a hole transport medium is the I3 /I this does not mean the couple is necessarily unique. Actually the space for improvement for DSSCs that use this redox mediator is mostly limited to improvements in better light harvesting dyes [41]. Corrosion, light absorption and diffusion limitations had been identified for the l3 /I pair and it has been replaced successfully by cobalt-based redox systems [14], as well as by organic hole conductors [42]. Difficulties in sealing to prevent evaporation and water diffusion into the cell led to research into the substitution of liquid redox pair electrolyte, replacing the liquid for solid or quasi-solid hole-conduction media, such as polymeric, gel [43], or solid electrolytes [44]. 

Solid-state redox mediators or hole conductor materials would make it possible to construct completely solid-state DSSCs that will probably have considerable added commercial value. One of the main difficulties in substituting liquid electrolytes is the need for an interpenetration of the sensitised metal oxide by the electrolyte, in order to have efficient contact between the sensitiser cation (the hole) and the mediator. Additionally, prospective solid hole collectors should have the following properties the valence band of the hole collector material must be located above the bottom of the sensitiser dye ground state it must be transparent throughout the visible spectrum, where the dye absorbs fight and the deposition of the solid material should be done without degrading the monolayer of sensitiser dye adsorbed on Ti02. 

The principal elements of the liquid-junction photovoltaic cell, as shown in Fig. 1, are the counterelectrode, the electrolyte, the semi-conductor-electrolyte interface, and the semiconductor. The distribution of charged species (ionic species in the electrolyte and electrons and holes in the semiconductor) is altered by the semiconductor-electrolyte interface, and an equilibrium potential gradient is formed in the semiconductor. The interfacial region may be... 

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