Electron‐Blocking and Oxygen Evolution Catalyst Layers by Plasma‐Enhanced Atomic Layer Deposition of Nickel Oxide

• Published in: Advanced materials interfaces Vol. 5; no. 16
• Main Authors: Hufnagel, Alexander G., Henß, Ann‐Kathrin, Hoffmann, Ramona, Zeman, Otto E. O., Häringer, Sebastian, Fattakhova‐Rohlfing, Dina, Bein, Thomas
• Format: Journal Article
• Language: English
• Published: Weinheim John Wiley & Sons, Inc 23-08-2018
• Subjects: Aspect ratio; Atomic layer epitaxy; blocking layers; Contact potentials; electrocatalysis; Electrolytic cells; Energy conversion; Energy storage; Film growth; Growth rate; Nickel; nickel oxide; Nickel oxides; Oxygen evolution reactions; Oxygen plasma; Photovoltaic cells; Pinholes; plasma‐enhanced atomic layer deposition; Precursors; Silicon substrates; Solar cells; Surface roughness; Thick films; Thin films
• ISSN: 2196-7350; 2196-7350
• DOI: 10.1002/admi.201701531

A plasma‐enhanced atomic layer deposition (ALD) process is presented, capable of producing thin conformal films of nickel(II) oxide (NiO) on various substrates. Nickelocene (NiCp2) is used as an inexpensive metal precursor with oxygen plasma as the oxidant. The film growth rate saturates with both nickel precursor and plasma exposure. An ALD window is observed between 225 and 275 °C. Linear growth is achieved at 250 °C with a growth rate of 0.042 nm per cycle. The thickness is highly uniform and the surface roughness is below 1 nm rms for 52 nm thick films on Si(100). Substrates with aspect ratios up to 1:10 can be processed. As‐deposited, the films consist of polycrystalline, cubic NiO, and are transparent over the entire visible range with an optical bandgap of 3.7 eV. The films consist of stoichiometric NiO and contain ≈1% of carbon impurities. Two promising applications of these films are showcased in renewable energy conversion and storage devices: The films are pinhole‐free and exhibit excellent electron blocking capabilities, making them potential hole‐selective contact layers in solar cells. Also, high electrocatalytic activity of ultrathin NiO films is demonstrated for the alkaline oxygen evolution reaction, especially in electrolytes containing Fe3+.
Plasma‐enhanced atomic layer deposition of transparent crystalline nickel(II) oxide can be used to prepare thin films with great uniformity and control over thickness. The films show very promising electron‐blocking behavior as well as oxygen evolution catalysis in alkaline media, making them attractive for a wide range of renewable energy harvesting and conversion devices.