Novel branched coumarin dyes for dye-sensitized solar cells: significant improvement in photovoltaic performance by simple structure modification

The construction of branched coumarin dyes based on a 2D-pi-A system is focused to optimize the light-harvesting capability and enhance the photovoltaic performance. Three novel coumarin dyes, coded as MS-C, MS-F, and MS-M, respectively, were obtained by incorporating different units (carbazole, fluorene and methyl groups) with different molecular volume and degrees of electron-donating capability into the pi-conjugation linker. Through molecular engineering, their light-harvesting capabilities, energy levels and photovoltaic performance were studied systematically. As expected, with the additional donor, especially utilizing strong electron-donating carbazole unit, the IPCE action spectrum of MS-C keeps a high plateau from 390 nm to 580 nm with nearly unity values. Along with the improvement of IPCE, the J(sc) values of the three dyes were increased by 77% from 7.61 (MS-M) to 13.47 mA cm(-2) (MS-C). Using DCA as the co-adsorbed material, the J(sc) of MS-M was increased while these values were decreased in the cases of branched MS-C and MS-F. This means that the twist structure arising from the branched donors could prevent pi-aggregation effectively. More importantly, the V-oc was also improved significantly by introducing the additional donor. While the branched 2D-pi-A system was constructed, the V-oc was sharply increased from 508 (MS-M) to 610 mV (MS-C). Comprehensively, a more than 100% enhancement in overall conversion efficiency (5.53%) was realized by branched MS-C compared to the linear MS-M (2.74%). Although the overall conversion efficiencies of these dyes are not very high, these results will still afford significant value for future development of efficient coumarin sensitizers with high open-circuit voltage.