Donor–acceptor interaction between non-aqueous solvents and I 2 to generate I −3, and its implication in dye sensitized solar cells

Abstract

The spectrophotometric properties of I −, I 2 and the I −/I 2 mixture were studied in 1,2-dichloroethane (DCE), acetone (AC), acetonitrile (ACN), ethanol (EtOH), methanol (MeOH), tertiary-butanol (t-BuOH), dimethylformamide (DMF), propylenecarbonate (PC), 3-methoxypropionitrile (MePN), dimethylsulfoxide (DMSO), dioxane (DIO) and pyridine (PY) solutions. From the investigation it has been realized that in DCE, I −, I 2 and I −/I 2 mixture have the same absorption peak at 500 nm. I − gives rise to the absorption spectra at about 220, 290 and 360 nm in t-BuOH and in PY solutions. However, in all other solvents the I − generates peaks only around 220 nm. Similarly I 2 and the I −/I 2 mixture in all solvents except DCE have indicated similar absorption peaks around 220, 290 and 360 nm. On the other hand, except in PC and DMF, I 2 shows the additional peaks in the range of 380–500 nm which are assigned to the formation of a I 2–solvent complex. The peaks around 290 and 360 nm indicate the presence of I − 3 and around 220 nm is the peak of I −. The spectral shift of the I 2 solutions in the visible region is interesting and is the core of this report. It points to the importance of donor–acceptor interaction between solvents and iodine. The data obtained in these solvents were well correlated to the donor number (DN) of the solvents. From this correlation the DN of MePN was estimated to 14.6. The absorption peak of I 2 in DCE(DN=0.0) is 500 nm and in PY(DN=33.1) is 378 nm. This peak shift due to solvent effects corresponds to an energy difference close to 0.8 eV. The absorption peak shift due to addition of the 0.0080 vol%. PY(1 mM) in 1 mM I 2-ACN solutions corresponds to ca. 0.6 eV. The blue shift of I 2 absorption in basic solvents indicates the tendency to form a complex. The increase of the efficiency of the dye-sensitized solar cell by addition of PY to I −/I − 3 ACN solution is suggested to be due to the formation of the dipyridine complex, PY 2I +. Such complex formation decreases the amount of I 2 which is expected to be an electron scavenger. We also propose that the more bulky complex, PY 2I + has a slower kinetics with the conduction band electrons, and thus decrease the losses of photocurrent and photopotentials in the solar cell.