Influence of brominated-TPA-stilbazole based ancillary ligand on the photocurrent and photovoltage in dye-sensitized solar cells

Two novel heteroleptic brominated-TPA-stilbazole based Ru (II)-sensitizers denoted as SD-11 and SD-11 mono were designed, synthesized and characterized for dye-sensitized solar cells (DSCs) in comparison to bench marked dye N719. The molecular structures of SD-11 and SD-11 mono were confirmed using FTIR, H-1-NMR and mass spectrometry. The aim was to systematically study the effect of mono (SD-11 mono) versus bis-brominated-TPA moiety (SD-11) on molar absorptivity, ground and excited state oxidation potentials, light harvesting efficiency (LHE), incident-photon-to-current conversion efficiency (IPCE), short-circuit photocurrent density (Jsc), and total solar-to-electric conversion efficiency (eta) for DSCs. Optical results revealed that SD-11 and SD-11 mono have higher molar extinction coefficients, narrower HOMO - LUMO gaps compared to N719. Hence, it was found that SD-11 exhibited remarkably greater Jsc than SD-11 mono and interestingly showed comparable extinction coefficients. However, in optimized conditions, SD-11 showed a higher Jsc of 22.1 mA cm(-2), Voc of 0.68 V with (% eta) of 8.7 compared to Jsc of 14.4 mA cm(-2), Voc of 0.63 V and with (% eta) of 5.8 for SD-11 mono whereas N719 attained a Jsc of 15.9 mA cm(-2), Voc of 0.74 V with (% eta) of 7.6 under the same experimental device conditions. It was concluded that SD-11 outperformed SD-11 mono in photovoltaic performance when anchored on TiO2, owing to better light-harvesting ability. These results clearly demonstrated that the enhancement of the photovoltaic performance in SD-11 can be attributed to the presence of more number of -Br group.