Correlation between Chemical and Electronic Properties of Solution-Processed Nickel Oxide

Abstract

Solution-processed nickel oxide (sNiO) is known to be an excellent charge-selective interlayer in optoelectronic devices. Its beneficial properties can be further enhanced by an oxygen plasma (OP) treatment. In order to elucidate the mechanism behind this improvement, we use infrared transmission and X-ray photoelectron spectroscopy to probe the bulk and surface properties of the sNiO. We find that increasing the annealing temperature of the sNiO not only increases the structural order of the material but also reduces the concentration of nickel hydroxide species present in the bulk and on the surface of the film. This results in a decrease of the work function, while an additional OP treatment raises the work function to between 5.5 and 5.6 eV. For all annealing temperatures investigated, the consequences of the OP treatment are identified as reactions of both NiO and β-Ni(OH) 2 to form thin β-NiOOH phases in the first atomic layers. Our results emphasize the importance of understanding the correlation between the preparation and resulting properties of sNiO layers and provides further insight into the interpretation of interface properties of NiO.

Rapid Aqueous Spray Fabrication of Robust NiOx: A Simple and Scalable Platform for Efficient Perovskite Solar Cells

“…A three‐component fit of the O1s signal which differentiates the contributions of NiO x , NiO x (OH), and Ni(OH) 2 indicates that the fraction of O bonded in stoichiometric NiO x is ≈18% higher for spray‐coated films from aqueous precursors compared to spin‐coated organic control samples. This is particularly relevant given recent findings that these hydroxide species increase the work function of NiO x to 5.5–5.6 eV and reduce transmittance, which can degrade its performance as an effective hole transport layer and result in devices with low PCE. However, additional lengthy postannealing of sprayed films for up to 60 min begins to increase the surface hydroxide content of sprayed NiO films, as shown in Figure S2 (Supporting Information), resulting in similar ratios of NiO bonded O to Ni(OH) 2 between spray and spin‐coated films.…”

Nanocomposite of nickel oxide nanoparticles and polyethylene oxide as printable hole transport layer for organic solar cells

“…Before and aer plasma treatment, the spectral distribution of the O 1s and the Ni 2p peaks ascribed to NiO x are not heavily affected, as in previously reported NiO x HTL lms prepared by sol-gel methods. 19,26 This suggests that the ratio between different oxidation states in the NiO x NPs remains unaltered aer plasma treatment. Interestingly, the Ni 2p peak is observed to increase in intensity for the blends while remaining constant for the neat NiO x layer.…”

“…S2 †). 19,26,33 In the same gure we plotted for comparison a device on bare plasma-cleaned ITO (i.e. no HTL), showing a clear improvement in device performance with NiO x as HTL, and a device with PEDOT:PSS, with a PCE of 3.00% that conrms the quality of the P3HT:PCBM active layer.…”

“…17 Recently, NiO x layers, obtained through a sol-gel process from nickel organic salts precursors, have been successfully utilized in organic (OPV) and perovskite photovoltaic devices, showing improved device performance and stability. [18][19][20][21][22][23][24] However, the used sol-gel process required a high temperature treatment (250-400 C) to induce the decomposition of the precursor into a crystalline NiO x nanoparticle (NP) layer followed by a plasma treatment which further oxidizes the nanoparticles. The latter treatment enhances the non-stoichiometry of mixed nickel oxidation states, and, in-turn, the layer's conductivity.…”

Robust Inorganic Hole Transport Materials for Organic and Perovskite Solar Cells: Insights into Materials Electronic Properties and Device Performance

“…[241] For instance, increasing the annealing temperature from 275 to 400 C (within the range used to anneal the NiO hole-selective layers for PSCs and OSCs) reduces the surface concentration of hydroxides together with the W F (by about 0.5 eV). [242] Moreover, the exposure to the strong oxidizing environment (such us in UV-ozone or oxygen-plasma treatment) induces the formation of a NiOOH superficial layer, which increases the W F and the conductivity of the material. [243] The Ni 3þ states are energetically localized within the bandgap of NiO [244,245] and might promote surface or interface recombination in PV devices.…”