Low-Bandgap Donor-Acceptor Conjugated Polymer Sensitizers for Dye-Sensitized Solar Cells

摘要

A set of two donor-acceptor type conjugated polymers with carboxylic acid side groups have been synthesized and utilized as active materials for dye-sensitized solar cells (DSSCs). The polymers feature a π-conjugated backbone consisting of an electron-poor 2,1,3-benzothiadiazole (BTD, acceptor) unit, alternating with either a thiophene—fluorene—thiophene triad (2a) or a terthiophene (3a) segment as the donor. The donor—acceptor polymers absorb broadly throughout the visible region, with terthiophene—BTD polymer 3a exhibiting an absorption onset at approximately 625 nm corresponding to a ～1.9 eV bandgap. The polymers adsorb onto the surface of nanostructured TiO_2 due to interaction of the polar carboxylic acid units with the metal oxide surface. The resulting films absorb visible light strongly, and their spectra approximately mirror the polymers' solution absorption. Interestingly, a series of samples of 3a with different molecular weight (M_n) adsorb to TiO_2 to an extent that varies inversely with M_n. DSSCs that utilize the donor—acceptor polymers as sensitizers were tested using an I~-/I_3~- electrolyte. Importantly, for the set of polymer sensitizers 3a with varying M_n, the DSSC efficiency varies inversely with M_n, a result that reflects the difference in adsorption efficiency observed in the film absorption experiments. The best DSSC cell tested is based on a sample of 3a with M_n ～ 4000, and it exhibits a ～65％ peak IPCE with/_(sc) ～12.6 mA cm~(-2) under AM1.5 illumination and an overall power conversion efficiency of ~3％.