Tetraphenylethene-based hole transporting material for highly efficient and stable perovskite solar cells

2,2,7,7-Tetrakis-(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (Spiro-OMeTAD) has been identified as the most widely used and effective hole transporting material (HTM) in perovskite solar cells (PSCs). However, the complicated multistep synthesis and low intrinsic hole mobility of Spiro-OMeTAD limit its commercialized application. Therefore, developing highly efficient HTMs with less synthetic steps becomes increasingly important. Moreover, understanding hot carriers transfer dynamics at the interface of perovskite layer and hole transport layer is crucial for further enhancing PSCs performance towards Shockley–Queisser limit, which still lacks full investigation to date. Herein, a new HTM based on tetraphenylethene (WP1) was successfully synthesized by a simple one-step reaction process. It was found that WP1-based HTM exhibits more matched energy level, higher hole mobility and conductivity than those of the control Spiro-OMeTAD. The femtosecond transient absorption results reveal that the transfer rate of hot holes in perovskite/WP1 sample is four times higher than that of perovskite/Spiro-OMeTAD, thereby helping enhance the device performance. Consequently, the efficiency of PSCs is enhanced to 24.04% (WP1) from 22.85% (Spiro-OMeTAD). Moreover, the un-encapsulated device prepared with WP1 exhibits better long-term stability, retaining 87% of its initial PCE value after storing for 72 days under air environment, while the reference device shows 76% of its initial value. This work indicates that simple tetraphenylethene-based organic small molecule could be a very promising HTM candidate for highly efficient PSCs, and gives some significant insights for understanding intrinsic hot carriers transfer dynamics in device.