Application of photoexcited states of Re (I) metal complexes to solar energy conversion

Abstract

Dye sensitized solar cells (Grätzel Cell) have attracted significant attention as promising next generation photovoltaic devices. In a photovoltaic cell, the dye sensitizers anchored to mesoporous thin films made of the broad band gap semiconductor TiO2, absorbs the incident photons and the excited electrons transfer into the conduction band of TiO2. The efficiency of a dye sensitized solar cells (DSSC) could potentially be improved by changing the dyes used as sensitizers. Toward this end, a series of Re (I) complexes, Re(dcbpy)(CO)3X, Re(dcbpz)(CO)3X, [Re(dcbpz)(CO)3(py)]PF6, [Re(9-cphe)(CO)3Cl] and [Re(dafo)(4-Isonicotinic-py)(CO)3]PF6 (where X = Cl, Br, I, CN, SCN; dcbpy = 4,4'-dicarboxyl-2,2'-bipyridine; dcbpz = 5,5'-dicarboxyl-2,2'-bipyrazine; py-pyridine; 5-cphe = 5-carboxyl-1,10-phenanthroline, dafo = 4,5-diazofluorene-9-one) have been synthesized, and their photophysical properties characterized. The band at lower energy shows the typical metal-to-ligand charge transfer (MLCT) due to the dπ → π* transitions consistent with Re(I) dicarboxylated-bipyridine systems located in the 390 nm to 436 nm region, and these compounds have long life times in solution at room temperature (τ < 50 ns). The dye efficiencies were calculated by anchoring the dicarboxylated complexes to the wide band gap semiconductor TiO2. The efficiency of the DSSC was near 1% compared to about 9 % for the best known N3 dye. To increase the efficiency, we plan to synthesize dyes that absorb energy at higher wavelengths in the visible region of the spectrum by changing the metal centers and ligands attached to them.