Structure-property relationships: Influence of number of anchoring groups in triphenylamine-carbazole motifs on light harvesting and photovoltaic performance for dye-sensitized solar cells

Three novel organic sensitizers were molecularly designed and synthesized based on triphenylamine (TPA), as the core donor (D_TPA), linked to one, two or three carbazole (D_Carb) moieties with different number of anchoring groups (AG) to generate: D_TPA-π-D_Carb-A (DE1), D_TPA(π-D_Carb-A)₂ (DE-2) and D_TPA(π-D_Carb-A)₃ (DE-3). The primary goal of this study was to investigate the influence of bridging different number of D_Carb and AG, through π-spacer, with TPA on the photovoltaic performance in DSSCs. The molar extinction coefficient increased with increasing the number of D_Carb and A. DE-3 showed the greatest light harvesting in solution and when anchored onto TiO₂. Hence, the increase in the number of D_Carb and AG led to better light harvesting and stronger electronic coupling with TiO₂, which reflected positively on the photovoltaic properties, and this is the first systematic study of its kind involving TPA-π-carbazole motif. DE-3 achieved the best efficiency of 5.44% and showed IPCE of 74% at 470 nm. The V_oc increased with increasing the number of anchoring groups. Equilibrium molecular geometries and frontier molecular orbitals studied by DFT and TD-DFT calculations were in excellent agreement with experiments, which showed enhanced electronic coupling with TiO₂ and photovoltaic performance as the number of anchoring groups increased.