Asymmetric zinc phthalocyanines with large steric hindrance as efficient red/near-IR responsive sensitizer for dye-sensitized solar cells

Abstract

Asymmetric ZnPc derivatives with two carboxyl and six diphenylphenoxy or diphenylthiophenol groups were synthesized as dye of DSSCs. Those ZnPcs exhibit strong red/near-IR light absorption, and Zn-tri-PcNc-5 with six diphenylthiophenol groups shows obvious redshift in the Q-band and enhanced absorbance compared to Zn-tri-PcNc-4 with six diphenylphenoxy groups, while Zn-tri-PcNc-4 yielded a 3.22% efficiency in sensitizing TiO2-based solar cell, much higher than that (1.30%) of the S-substituted analog (Zn-tri-PcNc-5). The decreased efficiency of Zn-tri-PcNc-5 is due to the molecular orbital shift to negative direction, stemmed from S atoms instead of O atoms in the six substituents of Zn-tri-PcNc-4, which leads to insufficient driving force for the electron injection. The present results demonstrate that the optimization of molecular orbital levels of ZnPcs via changing the substituents’ “push–pull” effect is an effective approach to improve the ZnPc-sensitized cell performance.