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 TiO 2 . Elevated short‐circuit current density ( J SC ) values and comparable open‐circuit voltage ( V OC ) 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.