Molecular Engineering of Near Infrared Absorbing Thienopyrazine Double Donor Double Acceptor Organic Dyes for Dye-Sensitized Solar Cells.

The thienopyrazine (TPz) building block allows for NIR photon absorption in dye-sensitized solar cells (DSCs) when used as a π-bridge. We synthesized and characterized 7 organic sensitizers employing thienopyrazine (TPz) as a π-bridge in a double donor, double acceptor organic dye design. Donor groups are varied based on electron donating strength and sterics at the donor-π bridge bond with the acceptor groups varied as either carboxylic acids or benzoic acids on the π-bridge. This dye design was found to be remarkably tunable with solution absorption onsets ranging from 750 to near 1000 nm. Interestingly, the solution absorption measurements do not accurately approximate the dye absorption on TiO2 films with up to a 250 nm blue-shift of the dye absorption onset on TiO2 . This shift in absorption and the effect on electron transfer properties is investigated via computational analysis, time-correlated single photon counting studies, and transient absorption spectroscopy. Structure-performance relationships were analyzed for the dyes in DSC devices with the highest performance observed at 17.6 mA/cm2 of photocurrent and 7.5% PCE for a cosensitized device with a panchromatic IPCE onset of 800 nm.