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

Abstract 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.