Absorption and photovoltaic properties of organic solar cell sensitizers containing fluorene unit as conjunction bridge

A novel series of organic dyes containing a fluorene unit as the conjunction bridge (S1-S4) have been designed and synthesized for a potential application in dye-sensitized solar cells (DSSCs). Their absorption spectra, electrochemical and photovoltaic properties have been investigated in detail. The tuning of the HOMO and LUMO energy levels can be conveniently realized by alternating the donor moiety. As demonstrated, the indoline unit can exhibit a stronger electron donating ability, realizing a broader absorption spectrum when coated onto TiO2. The incorporation of octyl-substituted fluorene is highly beneficial to prevent close pi-pi aggregation, thus favorably suppressing charge recombination and intermolecular interaction. In both the HOMO and LUMO orbitals, the electron density located at the conjunction bridge (fluorene unit) is quite low, suggesting that the incorporated fluorene unit plays quite a small role in the electron delocalization between the donor and acceptor units due to the twist conformation. The overall conversion efficiencies (eta) of DSSCs based on these dyes are in the range of 3.04 to 4.26% and highly dependent upon their donor moiety. Among these dyes, S3, which utilizes a methyl-substituted indoline moiety as an electron donor, has shown a maximum monochromatic incident photon to current efficiency (IPCE) of almost 90%, with a relatively high open circuit voltage (V-oc) of 727 mV.