Dopant-Free Two-Dimensional Hole Transport Small Molecules Enable Efficient Perovskite Solar Cells

Developing dopant-free hole transport materials (HTMs) to replace Spiro-OMeTAD is a challenging but urgent issue for commercialization of state-of-the-art n-i-p structured perovskite solar cells (PSCs). Here, this work proposes an effective two-dimensional conjugate engineering strategy to tune molecular stacking orientation and improve the hole mobility of dopant-free small molecule HTMs. For the first time, triphenylamine (TPA) groups are incorporated as side chains of benzo [1,2-b:4,5-b ']dithiophene (BDT) unit to extend the longitudinal conjugate, achieving two donor-acceptor-acceptor type 2D small molecules, namely XF2 and XF3, which show a dominant face-on orientation and better hole transport mobility than the linear small molecule XF1. The incorporation of alkoxy Lewis base groups makes XF3 a more effective defect passivator for perovskite surfaces. As a result, the PSCs using pristine XF3 HTM show a dramatically improved efficiency of 20.59% along with improved long-term stability compared to that of XF1 HTM (power conversion efficiency (PCE) = 18.84%). A champion efficiency of 21.44% is achieved through device engineering for dopant-free XF3-based PSCs. The results show that the building block with longitudinal conjugate extension in small molecules plays an essential role in the face-on orientation morphology and elucidates a key design rule for the dopant-free small molecule HTMs for high-performance PSCs.

Automated summary

This study proposes an effective two-dimensional conjugate engineering strategy to improve the hole mobility of dopant-free small molecule HTMs. The incorporation of triphenylamine (TPA) groups as side chains of benzo[1,2-b:4,5-b']dithiophene (BDT) unit extends the longitudinal conjugate and achieves two donor-acceptor-acceptor type 2D small molecules with dominant face-on orientation and better hole transport mobility. The incorporation of alkoxy Lewis base groups further enhances the defect passivation for perovskite surfaces. As a result, the perovskite solar cells using the developed HTM show significantly improved efficiency and long-term stability.