Page - 15 - in Photovoltaic Materials and Electronic Devices

Materials 2016, 9, 137 2 of 37 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Years Terpyridine Quaterpyridine Figure 1. Publications concerning the use of terpyridines (blue) and quaterpyridines (red) in DSCs. Source: SciFinder (January 2016) [15]. While the general use of polypyridines in Ru complexes sensitizers has already been deeply reviewed in the past by Islam [16], Vougioukalakis [17], and Adeloye [18], or for the electrolytes by Bignozzi et al. [19], no insight about the specific structure–properties relationships of tpy and qtpy complexes in the same field have been provided. Thus, we drew our attention on these panchromatic sensitizers with a particular focus on cells performances and device investigation. For this reason works dealing only with computational investigation [20] will not be taken into consideration. The first use of tpy ligands in DSCs technology was pioneered by Nazeeruddin et al. [14], providing good performances owing to their broader absorption with respect to the standard bipyridine-based Ru complexes. The structure proposed in 1997 by the EPFL researchers was named N749 or Black Dye (BD), thanks to its panchromatic absorption (Figure 2, top) and represents a benchmark standard as tpy complex sensitizer. In this dye, ruthenium(II) is complexed by a tpy, the 4,4’,4’’-tricarboxy-2,2’:6’,2’-terpyridine (tctpy) and three isothiocyanate ancillary ligands. X-ray diffraction showed a slightly distorted octahedral coordination around the Ru atoms by the three nitrogen donors of tctpy and three nitrogen of isothiocyanate ligands. Very strong intermolecular bonds account for bidimensional arrays, in which the distance between the planes prevents π-stacking between the tpy rings (Figure 2, bottom) [21]. The final BD was prepared by titration with tetrabutylammonium hydroxide in order to deprotonate two of the three carboxylic functions, which proved to be a crucial feature for performances’ optimization. (a) (b) Figure 1. Publications concerning the use of terpyridines (blue) and quaterpyridines (red) inDSCs. Source: SciFinder (January2016) [15]. While the general use of polypyridines in Ru complexes sensitizers has already been deeply reviewed in the past by Islam [16], Vougioukalakis [17], and Adeloye[18],orfortheelectrolytesbyBignozzietal. [19],noinsightaboutthespecific structure–properties relationships of tpy and qtpy complexes in the same field have been provided. Thus, we drew our attention on these panchromatic sensitizers wit a particular focus on cells performances and d vice investigation. For this reasonw rksdealingonlywithcomputational investigation[20]willnotbe taken intoconsideration. The first use of tpy ligands in DSCs technology was pioneered by Nazeeruddin et al. [14], providing good performances owing to their broader absorption with respect to the standard bipyridine-based Ru complexes. The structure pr posed in 1997 by the EPFL rese rchers was named N749 or Black Dye (BD), thanks to its p nchromatic absorption (Figur 2, top) and represent a benchmark standard as tpy complex sensitizer. In this dye, ruthenium(II) is complexed by a tpy, the 4,4’,4”-tricarboxy-2,2’:6’,2’-terpyridine (tctpy) and three isothiocyanate ancillary ligands. X-ray diffraction showed a slightly distorted octahedral coordination around the Ru atoms by the three nitrogen donors of tctpy and three nitrogen of isothiocyanate ligands. Very strong intermolecular bondsaccount forbidimensionalarrays, inwhichthedistancebetweentheplanes preventspi-stacking between the tpy rings (Figure 2, bottom) [21]. The final BD was preparedby titrationwith tetrabutylammonium hydroxide in order todeprotonate 15