Flexible Substituted\nBenzo[1,2-B:4,5-B’]dithiophene-Cored\nD‑π‑D Hole-Transporting Materials for Perovskite\nSolar Cells

Here, four star-shaped D-π-D-π-D molecules\nS01–S04\nwere designed and synthesized, with thiophene (S01), bithiophene (S02),\n3,4-ethylenedioxothiophene (S03), and hexyloxy-substituted benzo[1,2-b:4,5-b’]dithiophene\n(S04) as the central core, alkenyl as the π-brigde, and methoxytriphenylamine\nas the end groups, respectively. S04 with alkenyl bridges endowed\nhole transport materials (HTMs) with a long conjugation length and\na deep highest occupied molecular orbital (HOMO) level. The π-bridge\nof the double bond can regulate the dihedral angles of the conjugate\nplane in the HTM molecules. Theoretical calculations show that a twisted\ndihedral angle between the central core and the carbon–carbon\ndouble bond is as low as 12.7° for S01, 9.4° for S02, 11.4°\nfor S03, and 10.9° for S04, respectively. S04 has a negative\ncharge on two O atoms instead of the S atom for S01–S03. The\nlong alkoxy side chains may more effectively reduce molecular aggregations\nin the spin-coating process. A S04 film has a smoother surface with\na lower root-mean-square (RMS) value (4.16 nm). As a result, S01,\nS02, S03 and S04 devices show photoelectric conversion efficiencies\n(PCEs) of 18.78%, 16.67%, 15.70%, and 21.07%, under simulated AM 1.5\nG irradiation (100 mW cm<sup>–2</sup>). The device performance\nof S04 is higher than that of Spiro-OMeTAD (19.65%). S01–S04\ndevices maintained more than 80% of the initial efficiency after 500\nh under a relative humidity (RH) of 30%.