Laser spectroscopic assessment of a phthalocyanine-sensitized solar cell as a function of dye loading

Silicon phthalocyanines with bulky axial ligands are interesting materials for dye-sensitized solar cells (DSSCs) because they possess a strong absorption in the red region (~670 nm) and their ligands hinder the stacking of their rings. This suppresses the formation of aggregates, a common tendency of dyes on the surface of dye-sensitized metal-oxide layers. Herein, the novel silicon phthalocyanine, Pc 61, which has ligands terminating in carboxylic acid groups has been synthesized, characterized and used in studies on the harnessing of photons in the long-wavelength visible light range. To investigate the relation between dye loading and performance of DSSCs, a series of Pc 61-sensitized TiO2 films and DSSC devices with different dye immersion times have been prepared and characterized through optical, electrical and laser spectroscopic studies. To our knowledge, this is the first time that injection rates of phthalocyanine-based DSSC performance have been assessed as a function of dye immersion time. The results of the incident photocurrent conversion efficiency (IPCE) and power conversion efficiency (ηeff) studies reveal that solar cells fabricated with a 120 min Pc 61 immersion time have the best performance among the tested ones. This is corroborated by time-resolved emission lifetime data. Our studies demonstrate that the short photoluminescence lifetime of Pc 61-TiO2 films results from efficient electron injection by excited-state Pc 61 into the TiO2 film. This agreed with both experimentally obtained higher IPCE and higher ηeff.