Flexible Substituted Benzo[1,2-B:4,5-B’]dithiophene-Cored D-π-D Hole-Transporting Materials for Perovskite Solar Cells

Here, four star-shaped D-π-D-π-D molecules S01–S04 were designed and synthesized, with thiophene (S01), bithiophene (S02), 3,4-ethylenedioxothiophene (S03), and hexyloxy-substituted benzo[1,2-b:4,5-b']dithiophene (S04) as the central core, alkenyl as the π-brigde, and methoxytriphenylamine as the end groups, respectively. S04 with alkenyl bridges endowed hole transport materials (HTMs) with a long conjugation length and a deep highest occupied molecular orbital (HOMO) level. The π-bridge of the double bond can regulate the dihedral angles of the conjugate plane in the HTM molecules. Theoretical calculations show that a twisted dihedral angle between the central core and the carbon–carbon double bond is as low as 12.7° for S01, 9.4° for S02, 11.4° for S03, and 10.9° for S04, respectively. S04 has a negative charge on two O atoms instead of the S atom for S01–S03. The long alkoxy side chains may more effectively reduce molecular aggregations in the spin-coating process. A S04 film has a smoother surface with a lower root-mean-square (RMS) value (4.16 nm). As a result, S01, S02, S03 and S04 devices show photoelectric conversion efficiencies (PCEs) of 18.78%, 16.67%, 15.70%, and 21.07%, under simulated AM 1.5 G irradiation (100 mW cm–2). The device performance of S04 is higher than that of Spiro-OMeTAD (19.65%). S01–S04 devices maintained more than 80% of the initial efficiency after 500 h under a relative humidity (RH) of 30%.