A new fire-retardant, diethyl ethyl phosphate (DEEP), was tested as a nonflammable electrolyte solvent for dye-sensitized solar cells (DSSCs). Electrochemical measurements demonstrated that the DEEP electrolyte has a wide potential window (>5 V), sufficient ionic conductivity (3.5 × 10−3 S cm−1 at 25 °C), and electrochemical activity for the 
Dye-sensitized solar cells (DSSCs) have attracted growing attentions as a future generation of cost-effective photovoltaic cells since the first report of about 11% overall light-to-electric energy conversion efficiency by Grätzel et al. in 1991 [1]. The original DSSC used an organic liquid electrolyte, which helped to establish a record conversion efficiency [2, 3], but also brought about a problem of long-term instability due to the leakage and volatilization of the electrolyte. To solve this problem, many attempts have been made to replace the liquid electrolyte by polymer electrolyte [4], hole-transporting materials [5–7], or other solid state ionic conductors [8, 9]. However, the conversion efficiencies of these DSSCs using the solid electrolytes were considerably smaller, obviously due to lower ionic conductivity of the solid electrolytes and imperfect contact at the electrolytes/electrodes interface [10]. To obtain a compromise between the flexible processability and high photoconversion efficiency, a number of studies were devoted to develop gel polymer electrolytes for DSSCs [11], in which organic liquid electrolyte was embedded to help ionic transport. To our understanding, it is possible to construct a stable DSSC with a liquid electrolyte if the liquid electrolyte was thermally and photochemically stable.
