Importance of terminated groups in 9,9-bis(4-methoxyphenyl)-substituted fluorene-based hole transport materials for highly efficient organic-inorganic hybrid and all-inorganic perovskite solar cells

Hole-transport materials (HTMs) play a crucial role in determining the photovoltaic performance and long-term stability of perovskite solar cells (PSCs), because they not only efficiently facilitate hole-extraction and transfer, but also act as a barrier to protect the perovskite from moisture and oxygen. So far, the power conversion efficiencies (PCEs) over 20% in PSCs have been mostly achieved by employing a Spiro-OMeTAD-based HTM. However, it suffers from some drawbacks such as relatively low hole-mobility, complicated synthesis and difficult purification, which hamper its potential commercial applications. Here, for the first time, two new easily accessible 9,9-bis(4-methoxyphenyl)-substituted fluorene-based HTMs comprising H (YT1) and methoxyphenyl-fluorene (YT3) as the terminated groups have been synthesized for use in organic-inorganic hybrid and all-inorganic PSCs. The (FAPbI₃)_0.85(MAPbBr₃)_0.15 and CsPbI₂Br PSCs based on YT3 yield very impressive PCEs of 20.23% and 13.36%, respectively, both of which are higher than that of Spiro-OMeTAD (19.18% and 12.30%). More encouragingly, the YT3-based PSC displays good long-term stability for 600 hours. These results confirm that different terminated groups in HTMs show a significant effect on the energy levels, hole extraction and transfer, thin-film surface morphology and photovoltaic performance. Our findings could provide a useful insight for future rational design of HTMs for highly efficient and stable PSCs.