Thiocyanate-free ruthenium(ii) sensitizers with a bi-imidazole ligand in dye-sensitized solar cells (DSSCs)

Abstract

In this study, tris-heteroleptic Ru(II) sensitizers denoted as SD-15 and SD-16 with a 4,4′,5,5′-tetramethyl-1H,1′H-2,2′-bis-imidazole moiety as the ancillary ligand were designed, synthesized and characterized using FTIR-ATR, ¹H-NMR and UV-vis spectrophotometry. Structural modulations resulted in a red shift in the low-energy metal-to-ligand charge transfer (MLCT) absorption peak. The novel bis-imidazole based Ru(II) sensitizer was adsorbed on a TiO₂ surface via –COOH anchoring groups. The band gap, and the ground and excited state oxidation potentials were measured. The photovoltaic performance of the as-fabricated solar cells was measured and showed that the presence of bis-imidazole provided more negative free energy for dye regeneration (−ΔG_regeneration) and less driving force for electron injection (−ΔG_injection) into the conduction band edge of TiO₂. Therefore, the use of SD-15 and SD-16 resulted in lower J_SC values of 10.20 and 3.52 mA cm⁻², respectively, than N719. The open-circuit photovoltage (V_OC) of SD-15 and SD-16 was 0.58 and 0.52 V, respectively, less than N719 under the same experimental conditions due to the decreased eTiO₂ lifetime of 0.71 ms compared to 8.8 ms of N719 for the injected electrons that was calculated using impedance measurements. Our efforts in this research program led to the development of more stable Ru(II) complexes compared to N719 NCS-based complexes; however, the conversion efficiency was lower. Hence, the fine-tuning of bi-imidazole is needed to identify more efficient NCS-free Ru(II) sensitizers for higher efficiency DSSCs.