Cyclopentadithiophene and Fluorene Spiro-Core-BasedHole-Transporting Materials for Perovskite Solar Cells

New three-dimensional hole-transporting materials (HTM) based on either 9,9’-spirobifluorene (SBF) or Spiro-[cyclopenta[1,2-b:5,4-b’]dithiophene-4,9’-fluorene] (SDTF) core have been synthesized. All three HTMs, namely SBFCz2, SDTFCz2 and SDTCz2F, are end-capped with two peripheral 3,6-dimethoxydiphenylaminyl-carbazole (CzDMPA) units. The HTMs behave as molecular glasses with glass transition temperature (Tg) close to or higher than that of the reference HTM Spiro-OMeTAD. Thermal and opto-electronic properties strongly depend upon the nature of the bridging core unit between the two CzDMPA units. The two fluorene-bridged molecules SBFCz2 and SDTFCz2 exhibit similar properties. On the contrary, SDTCz2F, where the CzDMPA units are bridged to the cyclopentadithiophene ring, displays lower HOMO/LUMO energy levels and smaller band gap. Upon doping of the HTM layer in perovskite solar cells (PSC), in spite of a much lower hole mobility, SDTCz2F leads to the highest power conversion efficiency (16.4% compared wit...