Theoretical studies of 1:1 charge-transfer complexes between nitrogen-containing heterocycles and I2 molecules, and implications on the performance of dye-sensitized solar cell

Ab initio molecular orbital (MC) methods were used to examine the monomer and inter-molecular charge-transfer 1:1 complexes of six different nitrogen-containing heterocycles with I-2. The calculations at the MP2(full)/LANL2DZ* levels in acetonitrile revealed that the sigma* orbital of diiodine interacts with the N lone pair in the heterocyclic ring. The open-circuit photovoltage (V-oc) values of a Ru(II) complex dye-sensitized nanocrystalline TiO2 solar cell with an I-/I-3(-) redox electrolyte in acetonitrile using N-containing heterocyclic additives were compared to the computational calculations of the intermolecular interaction between the heterocycles and I-2. The Gibbs free energy changes, optimized geometries, frequency analyses, Mulliken population analyses, and natural bond orbital (NBO) analyses indicated that the V-oc value of the solar cell is higher when the N-containing heterocyclic compounds have a stronger interaction with I-2. In addition, the V-oc increases as the energy of the HOMO level increases and becomes closer to the LUMO level of the I-2 molecule. Therefore, the intermolecular charge-transfer interaction between the heterocyclic additives and the iodine redox electrolyte is an important factor for dominating dye-sensitized solar cell performance. (c) 2005 Elsevier B.V. All rights reserved.