Dibenzoquinquethiophene- and Dibenzosexithiophene-Based Hole-Transporting Materials for Perovskite Solar Cells

Fused oligothiophene-based π-conjugated organic derivatives have been widely used in electronic devices. In particular, two-dimensional (2D) heteroarenes offer the possibility of broadening the scope by extending the π-conjugated framework, which endows enhanced charge transport properties due to the potential intermolecular π–π stacking. Here, the synthesis and characterization of two new small-molecule hole-transporting materials (HTMs) for perovskite solar cells (PSCs) are reported. The newly custom-made compounds are based on dibenzoquinquethiophene (DBQT) and dibenzosexithiophene (DBST) cores, which are covalently linked to triphenylamine moieties to successfully afford the four-armed tetrakistriphenylamine (TTPA) derivatives TTPA–DBQT and TTPA–DBST. The combination of these novel central scaffolds with the electron-donor TTPA units bestow the resulting HTMs with the appropriate energy levels and, therefore, good electronic contact with the perovskite for extracting the hole efficiently. TTPA–DBQT surpasses TTPA–DBST not only in terms of conductivity but also in light-to-energy conversion efficiency using conventional mesoscopic n–i–p perovskite devices, 18.1% and 14.3%, respectively. These results were systematically compared with the benchmark HTM, 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD). Additionally, scanning electron microscopy (SEM) hints that TTPA–DBQT forms high quality and fully homogeneous films, whereas TTPA–DBST leads to the formation of thinner films with pinholes, which explains its lower fill factor despite its better hole-extraction properties owing to its more planar π-extended scaffold.