Manipulating charge separation dynamics of zinc phthalocyanine based TiO2 films through asymmetrical push-pull structures

Zinc phthalocyanine (ZnPc) based dye-sensitized solar cells (DSSCs) that exhibit light absorption in the near IR and excellent chemical stability show great potential for efficient light harvesting and light to electrical energy conversion. However, they have exhibited poor device efficiencies (<; 1%) for a long time . Only recently efficiencies up to ~5% have been realized. An important tool in the development towards higher efficiencies involves replacing a symmetric phthalocyanine by an asymmetric equivalent with one electron pulling group (anchoring onto the TiO<;sub>2<;/sub>) and three electron pushing groups. This clearly suggests that such an asymmetrical push-pull structure supports light-induced forward electron transfer (from the ZnPc core into the TiO<;sub>2<;/sub>) and prevents back electron transfer. However, ultrafast photodynamics studies have not been reported so far. This paper reports on a systematic study to investigate the impact of modification of the anchoring electron-pulling group on the ultrafast dynamics of the forward and back electron transfer using femtosecond pump-probe spectroscopy.