Boosting efficiency of planar heterojunction perovskite solar cells by a low temperature TiCl₄ treatment

It is well known that electron transport layer (ETL) plays an indispensable role in the planar heterojunction perovskite solar cells (PSCs). TiO₂ is widely used as an ETL material due to its excellent transport properties, however, the presence of defects in the TiO₂ layer diminishes the power conversion efficiency (PCE) of the devices. Herein, we introduce a method of low-temperature TiCl₄ treatment to deposit a TiO_x layer on the surface of TiO₂ film, which can effectively passivate trap states at the TiO₂ surface. Moreover, the treating process is optimized to be 30min using a 40mM TiCl₄ aqueous solution. Benefiting from this, we obtain the champion device with the highest PCE of 18.47%, which is mainly due to the reduction of surface defects and the deposition of the well-crystallized perovskite films. Besides, the modified PSCs exhibit an average PCE of 17.59%, which is much better than the control devices.