SFX as a low-cost 'Spiro' hole-transport material for efficient perovskite solar cells

Four spiro[fluorene-9,9′-xanthene] (SFX) derivatives, SFX-TAD, SFX-TCz, SFX-TPTZ and SFX-MeOTAD have been synthesized for use as hole-transport materials and fully characterized by ¹H/¹³C NMR spectroscopy, mass spectrometry, XRD and DSC. Their thermal, optical and electrochemical properties were investigated. The use of different substituents affects the highest occupied molecular orbital (HOMO) energy level proving the versatility of the central core towards the facile and low-cost preparation of spiro-hole-transport materials. Among the synthesized SFX derivatives, SFX-MeOTAD was employed as hole-transporting material (HTM) and the performance tested in perovskite solar cells with device architecture glass/FTO/compact TiO₂/mesoporous Al₂O₃/CH₃NH₃PbI_3-xCl_x/HTM/Au. SFX-MeOTAD gave a comparable efficiency to spiro-MeOTAD, demonstrating great potential for use of the SFX family as cost effective and versatile hole conductors facilitating commercialization of perovskite solar cells (PSCs).