Solar cell electrolytes: Into the mix (Nature)
by Gavin Armstrong, Nature Chemistry
The electrolytes of high-performance dye-sensitized solar cells (DSCs) have previously contained volatile solvents, which need robust encapsulation if they are to be used outdoors. Now, Michael Graetzel and co-workers from the Swiss Federal Institute of Technology and the Chinese Academy of Sciences in Changchun have developed stable and solvent-free ionic liquid electrolytes through the rational mixing of three solid imidazolium salts .
In the basic set-up for a DSC, an incident photon excites a dye molecule, initiating the transfer of one of its electrons to a porous TiO2 electrode. This leaves behind a 'hole' that must be removed to stop hole–electron recombination, and this is achieved through an iodide electrolyte that reacts with the oxidized dye to form triiodide, which can then diffuse to the cathode. The electrolyte must be highly conductive, a property that is inversely proportional to viscosity. Alkyl-substituted methylimidazolium iodides have previously been demonstrated as electrolytes in DSCs; their viscosity decreases with decreasing substituted alkyl chain length. The least viscous and most conductive of the series at room temperature is 1-propyl-3-methylimidazolium iodide (PMII), but decreasing the alkyl chain — and thus viscosity — any further is not possible because the shorter chain 1-ethyl- and 1-methyl- imidazolium iodides (EMII and DMII) are solids at room temperature.
After systematically studying the viscosity, the melting temperature and the conductivity of a series of hydrocarbon-substituted imidazolium iodides, Graetzel and colleagues created two ternary mixtures of EMII, DMII and either 1-allyl-3-methylimidazolium iodide (AMII) or 1-ethyl-3-methylimidazolium tetracyanoborate (EMITCB). The mixtures were liquid at room temperature and displayed the desired high-conducting properties of their constituents. DSC devices were made incorporating the electrolytes, and were stable for over 1,000 hours of testing. The EMII–DMII–EMITCB system displayed an efficiency of 8.2%, setting a benchmark for solvent-free electrolytes.
1. Bai, Y. et al. High-performance dye-sensitized solar cells based on solvent-free electrolytes produced from eutectic melts. Nature Mater. doi: 10.1038/nmat2224 (2008).