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 (DTPA), linked to one, two or three carbazole (DCarb) moieties with different number of anchoring groups (AG) to generate: DTPA-π-DCarb-A (DE1), DTPA(π-DCarb-A)2 (DE-2) and DTPA(π-DCarb-A)3 (DE-3). The primary goal of this study was to investigate the influence of bridging different number of DCarb and AG, through π-spacer, with TPA on the photovoltaic performance in DSSCs. The molar extinction coefficient increased with increasing the number of DCarb and A. DE-3 showed the greatest light harvesting in solution and when anchored onto TiO2. Hence, the increase in the number of DCarb 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 Voc 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.