Density Functional Theory-Based Molecular Modeling: Verification of Decisive Roles of Van der Waals Aggregation of Triiodide Ions for Effective Electron Transfer in Wet-Type N3-Dye-Sensitized Solar Cells

Density functional theory-based molecular modeling (DFT/MM) validates that KI and I 2 undergo exothermic van der Waals (vdW) aggregation in acetonitrile (AN) or in the presence of 4-tert-butylpyridine (TBP), forming potassium triiodide (KI 3 ) and, further mutual vdW aggregation leads to the formation of (KI 3 ) 2 and AN, (KI 3 ) 2 and (AN) 2 and (KI 3 ) 2 and TBP in the AN-based Dye sensitized solar cells (DSSC) electrolytes. All KI 3 aggregates have a very low energy gap, 0.17 eV, 0.14 eV and 0.05 eV of lowest unoccupied molecular orbital (LUMO) + 1 and LUMO, respectively, verifying efficient electron diffusion in μm-thick DSSC electrolytes. Hydrogen-bonding aggregation of anatase TiO 2 model, Ti 9 O 18 H and OH, with N3 (proton) dye is also validated by DFT/MM, and the energy structure verifies unidirectional electron flow from highest occupied molecular orbital (HOMO) on thiocyanide (SCN) groups of N3 dye to LUMO on the TiO 2 model at the aggregates. Further, DFT/MM for the aggregation of K + I 3 − with N3 verifies the most exothermic formation of the aggregate of N3 (proton) and K + I 3 − . The UV-Vis spectra of N3 (proton) and K + I 3 − is consistent with reported incident photocurrent efficiency (IPCE) action spectra ( λ = 450–800 nm) of N3-sensitized DSSC, verifying that the N3 dye of N3 (proton) and K + I 3 − becomes an effective sensitizer in the anode / TiO 2 / N3 (proton) / KI/I 2 / acetonitrile (AN) / cathode structured DSSC. The energy structure of LUMO and LUMO + 1 of the aggregates, Ti 9 O 18 H and OH and N3 (proton), N3 and K + I 3 − , (KI 3 ) 2 and AN and (KI 3 ) 2 and TBP verifies high IPCE photocurrent and effective electron diffusion via KI 3 -aggregates in the DSSC of Ti 9 O 18 H and OH and N3 (proton) and K + I 3 − .