Role of Electronic Structure of Ruthenium polypyridyl Dyes in the Photoconversion Efficiency of Dye - Sensitized Solar cells: A Semi-Empirical Investigation.

Abstract

Ruthenium polypyridyl complexes as dyes have shown the highest (~10%) photoconversion efficiency so far in Dye sensitized nanocrystalline TiO₂ based photoelectrochemical solar cells1. The dye in these cells performs the crucial role of absorption of light followed by electron injection into conduction band of TiO₂ with high efficiency. The oxidized dye is regenerated by electron transfer from the redox mediator present in the electrolyte. The lowest energy transition in these ruthenium complexes is believed to be a metal to ligand charge transfer transition2 (MLCT) from metal centred HOMO to ligand centred LUMO. Further enhancement in the photoconversion efficiency of these dyes has therefore been attempted through extension of the spectral response3 of the dye towards longer wavelengths by tuning the HOMO and LUMO levels involved in MLCT transitions of these metal complexes. However, while the substitution of 4,4′-dicarboxy-2,2′-bipyridine (abbreviated as 4dcb) in cis Ru(4dcb)₂(NCS)₂, the most efficient dye known so far, with 5,5’-dicarboxy-2,2’,-bipyridine (abbreviated as 5dcb) whose π* orbital is lower in energy compared to that of the 4dcb ligand, did extend the spectral response towards the red, it resulted in lower electron injection efficiency at shorter wavelengths4. Therefore, synthesis of dyes with lower LUMO energy alone as the criterion for obtaining efficient photoconversion may not be effective.