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> 1.123 _{Whilst a higher density of nickel vacancies increases the work-function of the films, which}

may in turn improve the VOC of perovskite devices, this must be balanced with the density of

defect states at the NiOx|perovskite interface which act as non-radiative interfacial recombination

centres. Annealing metal oxides in metal rich environments has been demonstrated to fill metal vacancies. For example, Iza et al. demonstrated that annealing ZnO in a Zn-rich environment reduced the intensity of the PL peak attributed to zinc vacancies.294 _{This inspired experiments}

annealing NiOx in a nickel rich environment with the aim to reduce the density of nickel vacancy

defects.

20 nm NiOx was deposited onto sputtered Ni films as the source of nickel. Initially films

were annealed in an argon furnace, however residual oxygen within the furnace resulted in complete oxidation of the stack to form transparent NiOx. To prevent this, films were instead

annealed face down onto Ni foil in a vacuum chamber for 3, 10 and 20 minutes at 700 ºC, 1 x 10- 3 _{mbar. XPS was used in two ways to determine whether nickel vacancies were filled during the}

annealing; (i) by analysing the ratio of Ni2+_/Ni3+ _{using the O 1s core level spectrum and (ii) by}

studying the position of the Fermi level before and after annealing.

As mentioned previously, charge compensation on the formation of nickel vacancies results in Ni3+ _formation.123 _{Electrons in oxygen bound to Ni}3+ _{in the lattice have a different}

binding energy (531 eV)295 _{to those in oxygen bound to Ni}2+ _{(529 eV).}123 _{Thus, the ratio of the}

two peaks indicates the relative doping of the samples. Figure 10.16 a and b show the O 1s spectra for as deposited NiOx and after annealing for 20 minutes, whilst Figure 10.16c shows the fraction

of the O 1s signal corresponding to Ni2+ _{and Ni}3+ _{against annealing time. The Ni}3+ _{content of the}

films increased relative to Ni2+ _{with annealing, from 41 % before annealing to > 50 % for all}

annealing times. This means that the density of nickel vacancies increased with annealing. The aim of this experiment was to fill nickel vacancies; therefore, the experiment was unsuccessful.

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Figure 10.16 a) The XPS O 1s core level spectra of Ni|NiOx samples a) as deposited and b) after

annealing face-down on Ni foil under vacuum for 20 minutes. Figure c) shows the fraction contribution of the peaks at 529 eV (blue, corresponding to oxygen bound to Ni2+_{) and 531 eV}

(green, corresponding to oxygen bound to Ni3+_).

Evaluation of the Fermi level position could also be used to study changes in the density of nickel vacancies in NiOx; a reduction in the density of nickel vacancies would be expected to cause the

Fermi level to move closer to mid-gap corresponding to a lower level of p- type doping. The valence band spectra for as deposited and annealed samples were measured, and the valence band- Fermi level offset was approximated from the intercept of the gradient of the leading edge with the background. The values are presented in Table 10.3.

a) _b)

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Table 10.3 The valence band to Fermi level offset for Ni|NiOx samples annealed face down on

Ni foil for 0 – 20 minutes, determined from the intercept of the leading edge of the valence band spectra with the background signal.

Anneal time / min VB – Ef / eV

0 0.59

3 0.94

10 0.97

20 0.92

With annealing, the valence band to Fermi level offset increased by > 300 meV, meaning it moved closer to mid-gap. However, an increase in intensity of sub-gap absorption was present for annealed samples, indicating the formation of defect states. Whilst the increase in Fermi level with annealing in a Ni- rich environment might suggest a reduction in density of nickel vacancies, the similar position of the Fermi level for all annealing times may be evidence of Fermi level pinning. Such pinning would be caused by a defect formed during the annealing process, supported by the increase in sub-gap signal above the band-edge. A similar shift in Fermi level was previously seen in vacuum annealing studies of NiOx, where the formation of oxygen

vacancies gave rise to Ni0 _{defects within the films which pinned the Fermi-level and caused an}

increase in signal above the valence band edge.296 _{Meanwhile, defect computations have predicted}

a low defect formation energy of ~1.2 eV for oxygen vacancies, with the defect level situated ~0.6 eV above the valence band edge.122 _{Thus, comparison with the literature indicates that Ni}0

defects formed on annealing may have caused Fermi level pinning in the samples.

This was further supported by analysing the Ni 3p3/2 core level spectrum. Figure 10.17a shows the Ni 2p3/2 spectra of the Ni|NiOx films before (0 min) and after annealing for 3, 10 and

20 minutes. With annealing, broadening at the onset of the signal at ~853 eV occurred. Fitting of the spectra revealed that as well as the Ni2+ _{and Ni}3+ _{peaks at binding energies 854 and 856 eV}

(Figure 10.17b), a third peak centred at 853 eV emerged for annealed samples (Figure 10.17c). This has previously been attributed to metallic Ni0 _{in the literature.}296 _{Note that the Ni}0 _{signal at}

853 eV was present in all annealed samples, and is presented as a percentage of the fitted Ni 3p3/2 spectrum in Figure 10.17d. The percentage of Ni0 _{was non-linear with annealing time, which may}

have been due to overshooting or fluctuations of the heat tape (which had temperature fluctuations of up to ± 50 ºC). Metallic nickel may have formed due to oxygen loss (the formation of oxygen vacancies) under the annealing conditions.296 _{Whilst the samples were annealed face down on Ni}

foil, the pressure at the sample surface would still be expected to be lower than atmospheric due to the vacuum conditions within the furnace (1 x 10-3 _{mbar). Thus, on oxygen vacancy formation,}

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reduction of Ni2+ _{to Ni}0 _{by the two resultant electrons in the lattice would be expected to maintain}

charge neutrality.

Figure 10.17 a) The XPS Ni 2p3/2 core level spectra of Ni|NiOx samples as deposited (blue) and

after annealing face-down on Ni foil under vacuum for 3 min (green), 10 min (orange) and 20 min (red), b) the fitted Ni 2p3/2 spectrum of Ni|NiOx as-deposited, c) the fitted Ni 2p3/2 spectrum of Ni|NiOx after annealing for 10 minutes under vacuum face-down on Ni foil and d) the

percentage of Ni0 _{contributing towards the Ni 2p}

3/2 core level spectra for as-deposited Ni|NiOx

samples and samples annealed under vacuum face-down on Ni foil for up to twenty minutes.

XPS data from these preliminary experiments showed that annealing Ni|NiOx films face down on

Ni foil at 700 ºC and 1 x 10-3 _{mbar for up to 20 minutes resulted in an increase in both Ni}3+ _and

Ni0 _{species within NiO}

x. Whilst the origin of Ni0 was speculated to come from charge

compensation on formation of oxygen vacancies within the lattice, the overall finding was that this set of experiments were unsuccessful (i.e. filling of nickel vacancies did not occur). In a successful experiment, filling of nickel vacancies by annealing NiOx in a nickel rich environment

would cause an increase in the Ni2+ _{content of the films. Whilst it would be interesting to further}

probe the reason behind the increase in both Ni3+ _{and Ni}0 _{content under these annealing}

conditions, it would be more useful in future to repeat the experiments (annealing NiOx in a Ni-

rich environment) in an inert atmosphere at 1 atm to eliminate (or reduce) the possibility of

a) b)

c)

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oxygen vacancy formation. This would require a furnace which can be fully evacuated to remove oxygen before filling with an inert gas such as argon; such equipment was not available when these experiments were carried out.