The impact of fluorine atoms on a triphenylamine-based dopant-free hole-selective layer for perovskite solar cells

Abstract

For industrial use, perovskite solar cells (PSCs) require long-term stability and a cost-effective hole-transport layer (HTL). The PSC stability can be substantially improved via the rational design of a dopant-free HTL that possesses inherent electrical merits. Further, through molecular engineering, via placing fluorinated arms on an established triphenylamine core, superior stability can be achieved using cost-effective precursors and easy synthesis routes. Here we developed a fluorinated triphenylamine-based HTL, probed its structural and electro-optical properties, and demonstrated the practical utility of the HTL in a PSC in its pristine form. The designed fluorinated HTL, called tri(3-fluoro-4-methoxy-N-(4-methoxyphenyl)aniline) triphenylamine (FOMePh), gave a high power conversion efficiency (PCE) of 17.08%, which exceeded that of doped Spiro-OMeTAD (16.9%) while consuming 2.3-times less material than Spiro-OMeTAD during PSC fabrication. We carried out synthetic cost analysis, and the cost of FOMePh was calculated to be €80.19 g⁻¹ which is four-times lower than the cost of Spiro-OMeTAD. To investigate the impact of fluorine atoms, we synthesized molecules without fluorine atoms (OMePh). FOMePh and OMePh had hole mobilities of 5.9 × 10⁻⁴ and 3.59 × 10⁻⁵ cm² V⁻¹ s⁻¹, which are on par with doped Spiro-OMeTAD. The photovoltaic parameters suggest that the performance of OMePh dropped due to its poor film formation abilities compared with FOMePh, and this has a bearing on the performance and reliability of PSCs.