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.