Abstract
The electrolyte is an important component and its properties have much effect on the conversion efficiency and stability of DSSCs. In this study, a protocol for shelf-life estimating was proposed. Accelerated degradation tests were performed at several elevated temperatures. The results were analyzed using the degradation model for the log ratio of efficiencies at different times to the initial efficiency. The model includes a log-linear degradation rate which may vary from unit-to-unit due to common-cause variations in assembly, and a Wiener stochastic process which account for random-walk effects between measurements. An Arrhenius-type acceleration factor was used to describe effects of temperature, which allows us to extrapolate and predict degradation cells efficiency at an ambient temperature. The procedure was demonstrated using three types of dye-sensitized solar cells: one using a volatile organic solvent 3-methoxy-propionitrile, one using a nonvolatile organic solvent polyethylene-glycol-dimethyl-ether, and one using an ionic liquid 1-ethyl-3-methyl- imidazolium tetra-cyanoborate. It is obvious that MPN-based DSSC shows the best efficiency among the three different system, but it also exists the fastest degradation, especially at elevated temperature. The PEGDME-based DSSC have slower degradation rates, but their initial efficiencies are low. The EMITCB-based have the slowest degradation rates, while their initial efficiencies are comparable to the MPN-based DSSC. The EMITCB-based electrolyte is the best and the most suitable one of these three different types of system. Use of ionic liquid not only reduces the mean degradation rate but also the unit-to-unit variations. If a residual efficiency limit of 90% was used, the mean shelf-life of cells using ionic liquid was predicted to be about 8900 hours, with 95.4% confidence limit that it is greater than 2900 hours. If a residual efficiency limit of 80% was used, the mean shelf-life of cells using ionic liquid was predicted to be about 18800 hours, with 95.4% confidence limit that it is greater than 8500 hours.