Synthesis of sterically hindered phthalocyanines and their applications to dye-sensitized solar cells

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Abstract
Phthalocyanines with high peripheral substitutions and free from potential contamination by regioisomers have been synthesized and evaluated as photosensitizers for dye-sensitized solar cell applications. Each of the sterically hindered precursor compounds was accomplished by Suzuki–Miyaura cross-coupling reactions with the arylchloride and corresponding boronic acids. Metal free phthalocyanine-sensitized solar cells showed no photocurrent generation due to its low excited singlet state (LUMO) compared with the conduction band of TiO2. Upon zinc metalation, the LUMO level of the phthalocyanine was pushed up, and this variation afforded an exergonic free energy change for electron injection. The zinc phthalocyanine-sensitized solar cell displayed 0.57% power conversion efficiency (η) and 4.9% maximal IPCE in the near infrared region. More importantly, the cell prepared with and without the presence of chenodeoxycholic acid revealed no difference in the power conversion efficiency. This implies that the well-known aggregation tendency of phthalocyanines that is considered to enhance the self-quenching of the phthalocyanine excited singlet state is effectively suppressed by the high degree of substitutions. The significance of the driving force for electron injection and the distance between the dye core and the TiO2 surface is also highlighted for devising high performance phthalocyanine photosensitizers.