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

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 degrees C. Linear growth is achieved at 250 degrees 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 approximate to 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+.