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Halos show the path to perfection : peripheral iodo-substituents improve the efficiencies of bis(diimine)copper(I) dyes in dye-sensitized solar cells
Journal
RSC advances
Volume
4
Number
89
Pages / Article-Number
48712-48723
Abstract
The homoleptic copper(I) complexes [CuL2][PF6] (L = 4,4'-bis(4-halophenyl)-6,6'-dimethyl-2,2'-bipyridine with halogen = F (2) and Cl (3)) have been prepared and characterized, and their absorption spectroscopic and electrochemical properties compared to that with L = 4,4'-bis(4-bromophenyl)-6,6'-dimethyl-2,2'-bipyridine (4). The synthesis of [CuL2][PF6] (L = 4,4'-bis(4-iodophenyl)-6,6'-dimethyl-2,2'-bipyridine, 5) resulted in a mixture of [Cu(5(2)][PF6] and [Cu(5)(MeCN)(2)][PF6]; variable temperature H-1 NMR spectroscopy confirmed that the complexes are in equilibrium in CD3CN solution. The structure of [Cu(5)(MeCN)(2)][PF6] was determined by single crystal X-ray crystallography, and confirms a distorted tetrahedral geometry for the copper(I) centre. The heteroleptic dyes [Cu(1)(2)](+), [Cu(1)(3)](+), [Cu(1)(4)](+) and [Cu(1)(5)](+) (1 = ((6,6'-dimethyl-[2,2'-bipyridine]-4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid)) have been assembled by ligand exchange between [CuL2](+) and TiO2 functionalized with the anchoring ligand 1, and the performances of the dyes in fully masked dye-sensitized solar cells (DSCs) have been measured and compared. On the day of DSC fabrication, the trend for the global efficiencies, eta, depends on the halo-substituent in the order I > F approximate to Br > Cl. Ripening of the DSCs occurs and after 7 days, the dependence of eta on the halo-atom is in the order I > Cl approximate to F approximate to Br; the highest eta is 3.16% for [Cu(1)(5)](+) compared to 7.63% for N719. Compared to the other halo-functionalized dyes, [Cu(1)(5)](+) shows an extended spectral response to longer wavelength, with enhanced electron injection. The results of DFTcalculations suggest that the better dye performance of [Cu(1)(5)](+) may be associated with improved electron transfer over the halogen of the aryl substituent from the reduced electrolyte. The assembly of anchored dye [Cu(1)(5)](+) by treating functionalized-TiO2 with a 1 : 1 mixture of [Cu(MeCN)(4)]+ and 5, yields a dye which gives a DSC performance that matches that made by ligand exchange using [Cu(5)(2)][PF6] and [Cu(5)(MeCN)(2)][PF6].
