Abstract
Fused oligothiophene-based pi-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 pi-conjugated framework, which endows enhanced charge transport properties due to the potential intermolecular pi-pi 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 fourarmed 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 pi-extended scaffold.