Solid-state dye-sensitized solar cell with a charge transfer layer comprising two ionic liquids and a carbon material

Abstract

A solid-state composite electrolyte, comprising two ionic liquids and a carbon material, was sandwiched between a dye-sensitized porous TiO₂ working electrode and its counter electrode to fabricate a solid-state dye-sensitized solar cell (DSSC); the ionic liquids were 1-ethyl-3-methylimidazolium iodide (EMII, ionic liquid crystal) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF₄, room temperature ionic liquid), and the carbon materials were carbon black (CB), multi-wall carbon nanotubes (MWCNT), and single-wall carbon nanotubes (SWCNT). A cell efficiency (η) of 0.41% was achieved by using the bare 1-ethyl-3-methylimidazolium iodide (EMII) as the charge transfer intermediate (CTI); an efficiency of 2.52% was achieved for a solid-state DSSC by the incorporation of carbon black (CB) in the same composite electrolyte. To further improve the cell efficiency, we utilized 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF₄), a crystal growth inhibitor, as an additive to the electrolyte. The binary CTI (EMII plus EMIBF₄) was in the form of a solid, until the weight percentage of EMIBF₄ in the binary CTI exceeded the level of 60%. A cell efficiency of 3.09% was obtained using an electrolyte containing the CB and the binary CTI; the binary CTI for this cell contained EMII and EMIBF₄ in the ratio of 8/2. When the CB was replaced with MWCNT and SWCNT, the cell efficiency could be improved to 3.53% and 4.01%, respectively. Long-term durability of the DSSC with SWCNT–binary CTI was found to be far superior to that of the cell with an organic solvent electrolyte, and in fact the durability was uninterrupted for at least 1000 h. Scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), differential scanning calorimetry (DSC), and laser-induced photo-voltage transients were used to substantiate the results.