Ruthenium complex-cored dendrimers: Shedding light on efficiency trade-offs in dye-sensitised solar cells

Abstract

A series of first generation dendrimers provide important insight into the performance of dye-sensitised solar cells (DSSCs). The dendrimers are comprised of a substituted [ cis-di(thiocyanato)-bis(2,2′-bipyridyl)ruthenium(II) complex, first generation biphenyl-based dendrons, and either four, eight, or twelve 2-ethylhexyloxy surface groups. The dendrimers were bound to the titanium dioxide of the DSSCs via carboxylate groups on one of the bipyridyl moieties in a similar manner to the ‘gold standard’ [ cis-di(thiocyanato)-bis(4,4′-dicarboxylate-2,2′-bipyridyl)]ruthenium(II) 1 (N3). Exchanging one pair of the carboxylate groups on one bipyridyl ligand of N3 with styryl units to give [ cis-di(thiocyanato)-(4,4′-dicarboxylate-2,2′-bipyridyl)-(4,4′-distyryl-2,2′-bipyridyl]ruthenium(II) 2 resulted in an improvement in device performance (7.19% ± 0.11% for 2 versus 6.94% ± 0.12% for N3). Devices containing the dendrimers also had good efficiencies but the performance was found to decrease with the increasing number of surface groups, which gives rise to an increase in the molecular volume of the dye. The device containing the dendrimer with four surface groups, 3, had a global efficiency of 6.32% ± 0.13%, which was comparable to N3 (6.94% ± 0.12%) in the same device configuration. In contrast, the dendrimer with twelve surface groups, 5, had an efficiency of 3.69% ± 0.19%. Complex 2 and all three dendrimers have the same core chromophore, which absorbs more light than N3. The decrease in efficiency with increasing molecular volume was therefore determined to be due to less dye being adsorbed. Hence molecular volume and molar extinction coefficient are both first order parameters in achieving high conversion efficiencies and must be taken into account when designing new dyes for DSSCs.