Among electrochromic materials, nickel oxide, NiO, has received significant attention due to its anodic coloration leading to neutral color devices when combined with WO₃ [1]. In the past few years, few authors pointed out the key role plays by the film stoichiometry on its electrochromic (EC) properties [2, 3]. In particular in our group, we reported a strong correlation between electrohromic properties and the nickel/oxygen content in Ni_1-xO thin films deposited by RF Sputtering [4]. Especially a departure from the stoichiometry leads to unexpected EC properties in electrolytes with no small cations. Such non stoichiometry, leading to unusual properties can also be induced by dopant addition.

Herein, NiO based thin films were fabricated by sol-gel method combined with spin coating technique onto ITO/Glass substrate. The influence of molarity, solvents, stabilizing and the nature of the final treatment on the structural, morphological, electrical and optical properties of NiO based thin films was studied and correlated to their EC behavior. Overall films are poorly crystallized. The films synthesized by methanol as solvent, triton 100X as stabilizer and treated by UV light thin film exhibit the highest capacity in KOH based electrolyte and the highest optical contrast. Lithium addition was investigated for improved p-type conductivity. Finally, the integration of sol-gel NiO thin films in full devices (WO₃/Li-based electrolyte/NiO) will be discussed.

[1] H. Moulki, D.H. Park, B.K. Min, H. Kwon, S.J. Hwang, J.H. Choy, T. Toupance, G. Campet, A. Rougier, Electrochimica Acta, 2012, 74, 46

[2] R.T. Wen, C.G Granqvist, G.A. Niklasson, ChemElectroChem 2016, 3(2) 266.

[3] X. Song, G. Dong, F. Gao, Y. Xiao, Q. Liu, X. Diao, Vacuum 2015, 111, 48.

[4] M. Da Rocha, A. Rougier, Electrochromism of nonstoichiometric NiO thin film: as single layer and in full device, Applied Physics A, 2016, 122(4), art. n° 370, 7.