Hybrid organic/inorganic perovskite solar cells have been rapidly developed with spectacular success in both nanostructured and thin-film versions. Herein, we report a simple solvothermal method using WCl6 and NH3 center dot H2O as precursors in ethanol solvent at 200 degrees C for 24 h to fabricate WO3 nanocrystals (NCs) as the hole transport layer in an inverted perovskite solar cell. The thin WO3 NC film with a smooth and compact surface enabled the formation of a continuous and compact layer of well-crystallized CH3NH3PbI3 in a two-step solution process. A cell efficiency of 7.68% was obtained, which is a promising planar perovskite solar cell based on an inorganic hole extraction layer.