High‐Performance Hole‐Extraction Layer of Sol–Gel‐Processed NiO Nanocrystals for Inverted Planar Perovskite Solar Cells

Hybrid organic/inorganic perovskite solar cells have been rapidly evolving with spectacular successes in both nanostructured and thin‐film versions. Herein, we report the use of a simple sol–gel‐processed NiO nanocrystal (NC) layer as the hole‐transport layer in an inverted perovskite solar cell. The thin NiO NC film with a faceted and corrugated surface enabled the formation of a continuous and compact layer of well‐crystallized CH 3 NH 3 PbI 3 in a two‐step solution process. The hole‐extraction and ‐transport capabilities of this film interfaced with the CH 3 NH 3 PbI 3 film were higher than those of organic PEDOT:PSS layers. The cell with a NiO NC film with a thickness of 30–40 nm exhibited the best performance, as a thinner layer led to a higher leakage current, whereas a thicker layer resulted in a higher series resistance. With the NiO film, we observed a cell efficiency of 9.11 %, which is by far the highest reported for planar perovskite solar cells based on an inorganic hole‐extracting layer.