Non‐Planar and Flexible Hole‐Transporting Materials from Bis‐Xanthene and Bis‐Thioxanthene Units for Perovskite Solar Cells

Two new hole-transporting materials (HTMs), BX-OMeTAD and BTX-OMeTAD, based on xanthene and thioxanthene units, respectively, and bearing p-methoxydiphenylamine peripheral groups, are presented for their use in perovskite solar cells (PSCs). The novelty of the newly designed molecules relies on the use of a single carbon-carbon bond ‘C−C’ as a linker between the two functionalized heterocycles, which increases the flexibility of the molecule compared with the more rigid structure of the widely used HTM spiro-OMeTAD. The new HTMs display a limited absorbance in the visible region, due to the lack of conjugation between the two molecular halves, and the chemical design used has a remarkably impact on the thermal properties when compared to spiro-OMeTAD. BX-OMeTAD and BTX-OMeTAD have been tested in ([(FAPbI3)0.87(MAPbBr3)0.13]0.92[CsPbI3]0.08)-based PSC devices exhibiting power conversion efficiencies of 14.19 and 16.55 %, respectively. The efficiencies reached, although lower than those measured for spiro-OMeTAD (19.63 %), are good enough to consider the chemical strategy used as an interesting via to design HTMs for PSCs.