Impact of Alkyl Chain Length on the Properties of Fluorenyl-Based Linear Hole-Transport Materials in <i>p-i-n</i> Perovskites Solar Cells
Zhe Sun, Zhengxu Liu, Jiajia Zhang, Jiajia Zhang, Chuanyu Zhou, Ziyin Chen, Lei Chen, Shuai Zhang, Xuguang Jia, Jing Zhang, Jing Zhang, Yi Zhou, Bo Song, Ningyi Yuan, Jianning Ding
Abstract
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.