A new push–pull dye for semi-transparent p-type dye-sensitized solar cells: tuning conjugation by sexithiophene chain engineering

We report on the synthesis of two new dyes to be employed as sensitizers in p-type dye-sensitized solar cells (DSCs). The design of the two new molecules under consideration has been inspired by the state-of-art dye PMI-6 T-TPA. In particular, a specific engineering of the thiophene-based central core is here considered to favour structural planarity between an oligothiophenic π-spacer (a sexithiophene), and the acceptor and donor units made by peryleneimide (PMI) and triphenylamine (TPA) moieties, respectively. This leads to a wide absorption in the NIR with stabilization of the HOMO energy level in the resulting dyes, as supported by TD-DFT simulations and spectroscopic characterization. When tested as sensitizers in NiOx-based p-type DSCs, A6D (with an Acceptor-π-Donor structure) outperforms both its counterpart with a Donor-π-Donor structure (D6D) and P1, a benchmark dye in the field of p-DSCs. With A6D dye-sensitizer the resulting DSC device presents the quite remarkable value of stabilized efficiency as high as 0.15 % when I-/I3- is employed as redox couple and nanostructured NiOx photocathode is thick less than 2 μm and does not contain any blocking layer. Notwithstanding the panchromatic feature of the sensitizer, A6D-based devices show an average visible transmittance (AVT) of 8 %. Such a result paves the way toward the application of these types of multifunctional dyes in semi-transparent solar cells.