4-Tert-butylpyridine Free Organic Hole Transporting Materials for Stable and Efficient Planar Perovskite Solar Cells

4-Tert-butylpyridine (tBP) is an important additive in triarylamine-based organic hole-transporting materials (HTMs) for improving the efficiency and steady-state performance of perovskite solar cells (PVSCs). However, the low boiling point of tBP (196 °C) significantly affects the long-term stability and device performance of PVSCs. Herein, the design and synthesis of a series of covalently linked Spiro[fluorene-9,9′-xanthene] (SFX)-based organic HTMs and pyridine derivatives to realize efficient and stable planar PVSCs are reported. One of the tailored HTMs, N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-3′,6′-bis(pyridin-4-ylmethoxy) spiro[fluorene-9,9′-xanthene]-2,7-diamine (XPP) with two para-position substituted pyridines that immobilized on the SFX core unit shows a high power conversion efficiency (PCE) of 17.2% in planar CH₃NH₃PbI₃-based PVSCs under 100 mW cm⁻² AM 1.5G solar illumination, which is much higher than the efficiency of 5.5% that using the well-known 2,2′,7,7′-tetrakis-(N,N-di-p-methoxy-phenyl-amine)9,9′-spirobifluorene (Spiro-OMeTAD) as HTM (without tBP) under the same condition. Most importantly, the pyridine-functionalized HTM-based PVSCs without tBP as additive show much better long-term stability than that of the state-of-the-art HTM Spiro-OMeTAD-based solar cells that containing tBP as additive. This is the first case that the tBP-free HTMs are demonstrated in PVSCs with high PCEs and good stability. It paves the way to develop highly efficient and stable tBP-free HTMs for PVSCs toward commercial applications. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.