Low-Recombination Thieno[3,4-b]thiophene-Based Photosensitizers for Dye-Sensitized Solar Cells with Panchromatic Photoresponses.

We report four NIR photosensitizers employing a low-recombination donor and a thieno[3,4-b]thiophene (3,4-TT) π bridge for use in dye-sensitized solar cells. The inclusion of electron rich π spacers red-shifts the dye absorbance with solution absorption onsets reaching 700 nm. Dyes were found to have suitable energy levels for rapid electron transfers using cyclic voltammetry and UV/Vis-NIR absorption spectroscopy. Computationally optimized ground-state geometries show an increased torsional angle between π spacer and π bridge brought about by an added alkyl chain. This results in a widened optical band gap and increased oxidation potentials owing to a weakening of the electron-accepting ability of 3,4-TT for solution-state measurements. Interestingly in terms of device parameters, the alkylated π spacer had a nearly identical incident photon-to-current conversion efficiency (IPCE) curve onset when compared to a non-alkylated analogue, suggesting more similar dye geometries on the surface of TiO2 . Elevated short-circuit current density (JSC ) values and comparable open-circuit voltage (VOC ) values were observed in the alkylated-π-spacer-dye-based devices with power conversion efficiencies up to 6.8 % observed with IPCE onsets exceeding 800 nm.