ABSTRACT
In the current economic, environmental and social climate, creating a revolutionary low-cost photovoltaic system suitable for large scale power generation is of utmost urgency. Dye-sensitized solar cells (DSCs) are receiving considerable academic and industrial attention for this purpose, since they promise to convert solar to electrical energy at a fraction of the cost of traditional semiconductor-based photovoltaics. As has been discussed in detail in previous chapters, the state-of-the-art DSCs are electrochemical solar cells, incorporating an iodide/triiodide redox couple, and have verifi ed solar-to-electrical power conversion effi ciencies in excess of 11 %. Although initial steps to commercialization have been taken, it is industrially much more attractive to remove the liquid phase of the device and realize an effi cient all solid-state version of the dye-sensitized solar cell. One only has to look at the short lifetime and leakage issue with disposable batteries to understand that this is a real issue, especially if lowcost sealing is required. Substantial research efforts involve using gel and solid-state electrolytes. These show great promise and are likely to be a key component of the fi rst commercially viable DSC concepts. An alternative, which furthermore negates the corrosive nature of the electrolyte, is to use a solid-state hole conductor. Amongst the possible inorganic p-type semiconductors, copper-based compounds appear to be suitable alternatives. These hole-transporters can be cast from solution or vacuum deposition to fully interpenetrate the mesoporous TiO2. A solid-state device based on CuI was fi rst demonstrated by Tennakone et al. [6.1], and a power conversion effi ciency as high as 4.7 % has been reported [6.2]. Though CuI is a very promising material, the cells are generally unstable, which is attributed to a stoichiometric excess of iodine molecules
adsorbed at the CuI surface, acting as hole-trapping sites [6.3]. DSCs incorporating CuSCN as the hole-transporter have demonstrated improved stabilities; CuSCN does not decompose to SCN-and there is no indication of excessive SCN-creating surface traps. However, these DSCs exhibit slightly lower effi ciencies of ∼2 % under 1 sun illumination [6.4-6.6]. Crystallization of the copper compounds in the pores is also thought to contribute to the cell degradation. Another class of materials constituting effective hole conductors are organic molecular and polymer semiconductors. Though the charge mobilities in organic hole-transporters are generally lower than in copper-based compounds, the mobilities are still suitably high. Moreover, the molecules are solution processable, rendering them well suited for mesopore infi ltration, they should be cheap, and innumerable variations can be conceived and created. For all these reasons, organic semiconductors are highly versatile. The fi rst solid-state DSCs incorporating polymeric and molecular organic hole transporters were realised in 1997 and 1998 respectively [6.7, 6.9]. The best performing solid-state DSCs incorporating organic hole transport materials deliver over 5 % solar-to-electrical power conversion effi ciency [6.8].
