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

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 pi-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 similar to 1.9 eV bandgap. The polymers adsorb onto the surface of nanostructured TiO2 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 TiO2 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, the DSSC efficiency varies inversely with NI, 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 similar to 4000, and it exhibits a similar to 65% peak IPCE with J(sc) similar to 12.6 mA cm-2 under AM 1.5 illumination and an overall power conversion efficiency of similar to 3%.