Surface treatment: Via Li-bis-(trifluoromethanesulfonyl) imide to eliminate the hysteresis and enhance the efficiency of inverted perovskite solar cells

Recently, inverted planar perovskite solar cells (PSCs) with a p-i-n configuration have drawn tremendous interest due to their high efficiency and reduced hysteresis. It has been demonstrated that p-i-n planar PSCs employing organic hole transport materials can obtain excellent photovoltaic performance, but their high cost and poor stability have restrained their further development. Herein, we employ Li-bis-(trifluoromethanesulfonyl) imide treated NiO_x (Li-Treated NiO_x) as an excellent and robust hole transport layer (HTL) for fabricating highly efficient and stable PSCs. Compared to the pristine NiO_x, the Li-Treated NiO_x exhibits enhanced electrical conductivity and better band alignment, enabling fast and efficient hole transport as well as exciton separation. The PSC with a Li-Treated NiO_x HTL shows a significantly improved fill factor and efficiency (from 0.75, 14.46% to 0.79, 17.09%) as well as less hysteresis with satisfactory long-Term stability. Furthermore, we obtain a champion efficiency of 18.03% via graded heterojunction engineering. Therefore, our results suggest Li-Treated NiO_x film is an effective and promising hole transport material for efficient PSCs.