Insight into the effects of the anchoring groups on the photovoltaic performance of unsymmetrical phthalocyanine based dye-sensitized solar cells

Push-pull zinc phthalocyanine dyes bearing hexylsulfanyl moieties as electron donors and carboxyethynyl as mono- or di-anchoring groups have been designed, synthesized and tested as sensitizers in dye-sensitized solar cells (DSSCs). The effects of the anchoring groups on the optical, electrochemical and photovoltaic properties were investigated. The incorporation of a carboxyethynyl group in GT23 has a considerable effect on preventing dye aggregation due to its relatively non-planar structure. The mono-anchoring dye bearing a phenyl carboxyethynyl group, GT5, has a higher molar extinction coefficient and sufficient charge injection into the TiO2 conduction band. Therefore, GT5 achieved at least 90% higher power conversion efficiency than the di-anchoring dyes (GT31 and GT32). Time-dependent density functional theory (PBE0/6-31G(d,p)) was also used to calculate the electronic absorption spectra, which predicted very well the measured UV-Vis with an error of up to 0.11 eV for the Q bands and 0.3 eV for the B bands. The longest charge transfer bands are obtained in the visible light region and they correspond to a transfer phthalocyanine core -> substituent with a carboxyethynyl group where the absorptions of GT32 (465 nm) and GT31 (461 nm) are red-shifted compared to GT23 (429 nm) and GT5 (441 nm). The interaction energy between the phthalocyanine and a cluster of anatase-TiO2 (H4Ti40O82) was calculated using density functional theory. For all phthalocyanines, the interaction favored is monodentate and corresponds to -O(OH)MIDLINE HORIZONTAL ELLIPSISTi(TiO2), where the stronger interaction occurs for GT32 (-2.11 eV) and GT31 (-2.25 eV). This study presents the molecular combination of the anchoring groups in zinc phthalocyanine sensitizers, which is one of the effective strategies for improving the performance of DSSCs.