Substituent effect in direct ring functionalized squaraine dyes on near infra-red sensitization of nanocrystalline TiO2 for molecular photovoltaics

We have synthesized a series of novel blue colored symmetrical squaraine sensitizers with variable alkyl chain length for their application towards the fabrication of dye-sensitized solar cells (DSSCs). It has been found that an increase in the alkyl chain length substituted at N-position of indole ring exhibits enhanced electron diffusion length and electron life-time resulting in better passivation of nanocrystalline TiO2 surface leading to enhancement in the cell performance. Based on HOMO and LUMO energy measurement of squaraine dyes, it has been demonstrated that about 0.16 eV energy barrier is sufficient for electron injection from LUMO of dye to TiO2 conduction band and dye regeneration after photo-excitation. Performances of DSSCs using model squaraine dyes indicate that dodecyl alkyl substituent is optimum giving highest Voc and use of chenodeoxycholic acid along with the dye SQ-4 shows a photoconversion efficiency of 3.5% under AM 1.5 irradiation.