Publication
In this work, we describe a new class of non-fused 3D asymmetric compounds (named 1, 2, 3, and 4) as low-cost organic hole-transporting materials (HTMs) for perovskite solar cells (PSCs). The fundamental understanding of the influence of the methylthio and methoxy group substitutions on the fluorene moiety has been analyzed, as well as the position of methoxy groups in the aromatic rings of triphenylamine pending groups (para or meta). Experimental results demonstrate that the position of the methoxy group in the triphenylamine pending group influences decisively the thermal properties and the amplitude of the electronic bandgap, hydrophobicity, film formation, and thermal stress stability. The presence of methylthio or methoxyl substituents in the 2,7-positions of the fluorene moiety mainly affects the electrochemical properties, hole mobility, and morphology of the hole-transporting layer (HTL). Thus, maxima sunlight-to-electricity power conversion efficiencies (PCEs) of 17.7 and 17.8% have been obtained in PSCs with methoxy groups in the fluorene moieties (1 and 3), respectively. Consequently, compound 1-based PSCs exhibit a better stability than the other three materials and the standard HTM-spiro-OMeTAD-based devices.