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

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. (C) 2014 Elsevier Ltd. All rights reserved.