New Mixed Ligand Co(II) Complexes as Possible Electrolytes for Dye-Sensitezed Solar Cells - A Combined Experimental and Computational Approach

We report the synthesis and characterization of a new series of mixed ligand Co(II) complexes, [Co(AA)(BB)2]X2, where AA and BB are derivatives of 1,10-phenanthroline and 2,2'-bipyridil, X=Cl-, TFSI- (TFSI= bis(trifluoromethanesulfonyl)imide), as possible redox electrolytes for dye-sensitized solar cells (DSSC).  Compared to the I3-/I- system, the cobalt(III/II) polypyridyl complex redox shuttles have low visible light absorption, and the ease of tuning their redox potentials is a very significant aspect for accomplishing a photovoltage enhancement.  In particular, our goal is to increase the open circuit voltage and the overall photovoltaic conversion efficiency of the device.  The synthetic strategy of such heteroleptic compounds is a challenge due to the tendency of the Co(II) ion to form tris chelates compounds or to disproportionate and  includes in the first step the synthesis of [Co(AA)Cl2] and [Co(BB)2Cl2] complexes and then the addition of the second aromatic diamine and TFSI anion.  The compounds were characterized by spectral (IR and UV-VIS spectroscopy) and structural (X-ray diffraction and DFT calculation) analysis. We also performed molecular modeling of the complexes synthesized based on density functional theory (DFT) calculations.  The structures of all complexes with tris chelates and a pseudo-octahedral geometry were optimized for the low spin state, based on DFT calculations. The IR spectra were simulated using the B3LYP/ECP-LANL2-DZ method.  Optical absorption spectra of all complexes were simulated using the TD-DFT method, both in vacuum and in acetonitrile solvent.  DFT simulated optical and vibration spectra compare well with the experimental data, allowing for a reliable assignment of the various transitions.