Rediscovering a Key Interface in Dye-Sensitized Solar Cells: Guanidinium and Iodine Competition for Binding Sites at the Dye/Electrolyte Surface

Abstract

We propose a new mechanism by which the common electrolyte additive guanidinium thiocyanate (GdmSCN) improves efficiency in dye-sensitized solar cells (DSSCs). We demonstrate that binding of Gdm(+) to TiO2 is weak and does not passivate recombination sites on the TiO2 surface as has been previously claimed. Instead, we show that Gdm(+) binds strongly to the N719 and D131 dyes and probably to many similar compounds. The binding of Gdm(+) competes with iodine binding to the same molecule, reducing the surface concentration of dye-I2 complexes. This in turn reduces the electron/iodine recombination rate constant, which increases the collection efficiency and thus the photocurrent. We further observe that GdmNO3 can increase efficiency more than the current Gdm(+) source, GdmSCN, at least in some DSSCs. Overall, the results point to an improved paradigm for DSSC operation and development. The TiO2/electrolyte surface has long been held to be the key interface in DSSCs. We now assert that the dye layer/electrolyte interaction is at least, and probably more, important.

Theoretical description of efficiency enhancement in DSSCs sensitized by newly synthesized heteroleptic Ru complexes

“…69 The rate of electron capture by these iodine molecules depends, firstly, on their concentration near the surface which, in turn, increases by the concentration of the dye molecules, 70 and secondly, depends on their relative orientation. 69 The rate of electron capture by these iodine molecules depends, firstly, on their concentration near the surface which, in turn, increases by the concentration of the dye molecules, 70 and secondly, depends on their relative orientation.…”

High efficiency flexible fiber-type dye-sensitized solar cells with multi-working electrodes

“…Generally, the photoanode is a fluorine-doped tin oxide (FTO) glass substrate covered with a dye-sensitized titanium dioxide (TiO 2 ) film and the counter electrode is a platinized conductive glass [3]. The eventual performance of a DSSC is a synergistic effect of all these components [4][5][6][7][8][9][10][11].…”

Quasi-solid electrolyte with polyamidoamine dendron modified-talc applied to dye-sensitized solar cells

“…As a consequence, the entire conduction band of the TiO 2 film shifts to higher energy levels with the surface potential and, hence, results in an enhancement in V oc for quasisolid DSSCs with PAMAM-talc materials [4]. The phenomenon was also observed in a study of the addition of 4-tert-butylpyridine [47,49] or guanidinium [48] in liquid electrolytes and addition of montmorillonite in a gel electrolyte [4]. Additionally, the primary amine (pKa ¼ 7e9) [50] at the surface of the PAMAM dendron are strongest basic groups than the tert-butylpyridine molecule (pka ¼ 6) [51].…”

“…Usually, the surface of TiO 2 can be more or less protonated, depending on the pH of the electrolyte. Adding a base in the electrolyte deprotonates the surface of TiO 2 and results in a variation of surface potential, which is the origin of band bending [48]. However, in DSSCs, the nanoparticles are too small to support a significant band bending.…”