Impact of Alkyl Chain Length on the Properties of Fluorenyl-Based Linear Hole-Transport Materials in p-i-n Perovskites Solar Cells

To develop low-cost and highly efficient hole-transport materials (HTMs), a series of linear HTMs based on a fluorenyl core were synthesized with different alkyl side chains substituted for the middle carbon atom, from methyl (FMT-M), and 2-ethylhexyl (FMT-EH) to dodecyl (FMT-D). Together with the hexyl substituent one (FMT), the photophysical and electronic properties, and hole-mobility of the compounds were investigated to determine the influence of the alkyl side chain on the performance of the material as dopant-free HTMs in p-i-n perovskite solar cells (pero-SCs) with the structure of ITO/HTM/MAPbI3-xClx/C60/BCP/Ag. All the linear molecules exhibit promising HTM properties including high transmittance, hole-mobility, and optimal energy level alignment with perovskites (MAPbI3-xClx). As the alkyl chain gets longer, the highest efficiency of corresponding p-i-n pero-SCs based on FMT-M, FMT-EH, FMT, and FMT-D increases from 16.68 and 17.60 to 18.57 and 19.99%, respectively. Through the investigation, it is determined that the alkyl side chain will affect the thermal stability, the melting point, and the intermolecular stacking of the linear molecules; thus, the corresponding HTMs obtain different transparency, hole-transport mobility, energy level, and film morphology, which together will affect the p-i-n pero-SCs performance.