Practical and Thermal Atomic Layer Deposition of NiO as Hole‐Transporting Layers for Inverted Perovskite Solar Cells

The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has significantly improved through advancements in fabrication methods, which have primarily focused on the perovskite absorber layer. The significance of improving the charge transport layer as the next crucial step toward achieving highly stable and efficient PSCs has also been emphasized. In inverted PSCs (i‐PSCs), the selection of a suitable p‐type hole‐transporting layer (HTL) has been restricted to mainly organic materials due to the rarity of p‐type inorganics. The instability and inherent disadvantages of organics necessitate the use of stable p‐type oxides as HTLs for i‐PSCs. Herein, uniform, conformal, and practical, yet thermal atomic layer deposition (ALD) for NiO is demonstrated by employing two different oxidant, ozone (O 3 ) and hydrogen peroxide (H 2 O 2 ). Both ALD‐NiO films are characterized by X‐ray diffraction and X‐ray reflection. By conducting X‐ray photoelectron spectroscopy analysis of the ALD‐NiO surfaces, a correlation between the oxidation power of the oxidant during ALD and the surface oxidation state of the ALD‐NiO films is established. Finally, the relationship between the oxidation state of the surfaces with different oxidant and the i‐PSC performance is verified. The fabricated i‐PSCs exhibit a PCE exceeding 19%.