Polyiodides formation in solvent based Dye Sensitized Solar Cells under reverse bias stress

In this work we investigate electrolyte degradation mechanisms in a Dye Sensitized Solar Cell (DSSC), when stressed under forced reverse bias (RB) conditions. During the stress test, we observe a gradual and visually evident cluster shaped browning of platinised counter-electrode in contact with electrolyte solution; Raman spectroscopy confirms that the observed phenomena is due to formation of polyiodide ions and reveals an arose marked fluorescence background, stemming from new chemical species induced by RB stress test. Raman and fluorescence measurements on RB stressed model electrolyte solutions reveal that photoluminescence emission is mainly related to degradation mechanisms involving the I^-/I<inf>3</inf>^- redox couple. In fact, due to the RB stress, the redox couple is unbalanced and the formation of various associated structures between 1-methyl-3-propyl imidazolium iodide (PMII) ions is favored. This can be detected by observing the Red Edge Effect (REE) in fluorescence emission spectra of stressed solutions. Thus, polyiodides formation in RB stressed DSSCs could be added to the several depletion channels of triiodide anions and should be taken into account in designing new stable and efficient electrolytes.