Molecular Orbital-Based Verification of Conductivity of Tetramethylammonium Pentaiodide and Pentaiodide-Based Electrolytes in Dye-Sensitized Solar Cells

Abstract

In order to verify the electrical conductivity of tetramethylammonium pentaiodide (TAPI, I5−(N(CH3)4+)) crystal, molecular orbital configurations and energy structures of the electron-carrying state TAPI packing unit (TAPI-6) are examined by density functional theory (DFT). The narrow energy gap of the electron-carrying state radical anion of [TAPI-6].− proves the semi-conductivity of TAPI together with the aligned configuration of singly occupied molecular orbital (SOMO) and the spin density distribution on SOMO. The molecular orbital analysis of the electron-accepted states of the vertical fragment unit of [TAPI-4].− and the coplanar fragment unit of [TAPI-3].− rationalize the anisotropic semiconducting property in TAPI. DFT-based molecular modeling for molecular integration of I5−N(CH3)4+ with I5− and with I5−N(CH3)4+ confirms the formation of I5− integrated complexes of respective I5−&I5−N(CH3)4+ and (I5−N(CH3)4+)2. The degenerate HOMO and LUMO in (I5−N(CH3)4+)2 suggest expanding integration of I5− net via van der Waals and coulomb interactions (vdW&Clb) in I5−-based electrolytes. The narrow energy gap (0.3 ∼ 0.35 eV) and the spin density on SOMO of the radical anions of [(I5−N(CH3)4+)2]− and [I5−&I5−N(CH3)4+]− explain that the I5−N((CH3)4+-derived self-integrated I5− cluster nets become semiconducting and the photo-formed electron is transported effectively through SOMOs on I5− units of the electron-carrying clusters of [I5−&I5−N(CH3)4+].− and [(I5−N(CH3)4+)2]−. The DFT-based molecular modeling verifies reasonably the semi-conductivity of I5−-based DSC electrolytes.