NiO as Hole Transporting Layer for Inverted Perovskite Solar Cells: A Study of X‐Ray Photoelectron Spectroscopy

Hygroscopic and acidic nature of organic hole transport layers (HTLs) insisted to replace it with metal oxide semiconductors due to their favorable charge carrier transport with long chemical stability. Apart from large direct bandgap and high optical transmittance, ionization energy in the range of −5.0 to −5.4 eV leads to use NiO as HTL due to good energetic matching with lead halide perovskites. Analyzing X‐ray photoelectron spectroscopic (XPS) data of NiO, it is speculated that p‐type conductivity is related to the NiOOH or Ni2O3 states in the structure and the electrical conductivity can be modified by altering the concentration of nickel or oxygen vacancies. However, it is difficult to separate the contribution from nonlocal screening, surface effect and the presence of vacancy induced Ni3+ ion due to very strong satellite structure in the Ni 2p XPS spectrum of NiO. Thus, an effective approach to analyze the NiO XPS spectrum is presented and the way to correlate the presence of Ni3+ with the conductivity results which will help to avoid overestimation in finding the oxygen‐rich/deficient conditions in NiO.


Introduction
Nickel oxide (NiO) has a "rock salt" crystal structure with a facecentered cubic lattice that is very stable. [1]NiO is an intrinsic p-type metal oxide with a wide bandgap (>3.5 eV), high optical transmittance, and good thermal, and chemical stability. [2]Thus, it has been employed as a photocathode material in p-type/p-n DOI: 10.1002/admi.2023007510][11] To date, various deposition techniques, such as sol-gel, [12] electrochemical, [13,14] spray pyrolysis, [15,16] sputtering, [17] evaporation, [18] pulsed laser deposition (PLD), [19] and atomic layer deposition (ALD), [11] have been employed to deposit NiO HTLs, and the highest power conversion efficiency (PCE) of 22.13% was demonstrated, in which spin-coated NiO nanocrystalline films were treated with 3,6-difluoro-2,5,7, 7,8,8-hexacyanoquinodimethane (F 2 HCNQ) molecules. [20]e believe that a proper understanding of the character of holes in the valence band maxima is necessary to elucidate the transport properties of intrinsic and doped/deficient NiO.Initially, NiO was thought to be a Mott-Hubbard insulator in which Coulomb repulsion between 3d electrons created an insulating gap.Thereafter, Zaanen, Sawatzky, and Allen redefined NiO as an intermediate compound between Mott-Hubbard insulators and charge-transfer (CT) semiconductors. [21]][24] Overlap between Ni 3d and O 2p orbitals gives rise to a complex line shape in the experimental XPS spectrum of the Ni 2p photoemission peak, which has been widely discussed in the literature.[27] Peaks due to overlap between the frozen ground state (after removing a core electron) and the unscreened final states of c3d 8 and c3d 10 L 2 characters are found at 9.5 and 6 eV higher bind-ing energies (BE) from the main peak, respectively, and are assigned as satellite peaks.Although the assignment of the main and satellite peaks is well understood, the interpretation of the shoulder in the Ni 2p spectra of NiO is still under debate.The shoulder peak was assumed to be a result of the 3d 10 L 2 configuration or 2p 5 3d 9 multiplet splitting.In another explanation, the shoulder peak was attributed to the chemical shift of Ni 3+ species due to nickel vacancies, nickel hydroxides or nickel oxyhydroxides present at the NiO surface.However, the appearance of shoulder in a freshly cleaved sample and in the X-ray absorption spectroscopy (XAS) spectrum made this explanation doubtful. [28]urthermore, Van Veenendaal and Sawatzky introduced clusters comprising multiple transition-metal ions, facilitating interactions between the metal sites and ligands.As a result, a core hole can be screened by an electron originating from a neighboring NiO 6 unit, without exclusively relying on the ligands at the core-hole site.This screening mechanism, necessitating involvement from at least two sites, is termed nonlocal screening (Figure 1c). [27]In the calculation, they extended the size of the cluster to Ni 7 O 36 and revealed that the presence of the abovementioned shoulder in the Ni 2p XPS spectra is a result of a screening process by an electron that does not come from the oxygen orbital around the Ni atom with the core hole but from a neighboring NiO 6 unit (with a 2p3d character).Thus it is observed that a hole is transferred between the core-ionized ligand and nearest-neighbor cluster (cd n+1 L:d n → cd n+1 :d n L), and this nonlocal screening effect is responsible for the shoulder peak in the Ni 2p XPS spectrum.In simple word, "nonlocal screening" in transition metal (TM) compounds is basically subject to ligand-to-metal charge transfer and interatomic multiplet couplings and the idea of nonlocal screening was applied to clarify the origin of shoulder peak observed in the higher BE side of the main line.In their cluster model, Mossanek et al. considered NiO 6 octahedral cluster as the "bulk" contribution to the spectra, the NiO 5 pyramidal cluster as the "surface" contribution, and the connection between the NiO 6 :NiO 5 clusters allows for non-local fluctuations with surface contribution as shown in Figure 1d.In their calculation they find that the separation in energy between non-local screening and surface contribution is less than 0.2 eV; which is really hard to distinguish experimentally as the XPS resolution of around 0.1 eV. [29]1]

Ni2p Peak in NiO as HTL
Several groups have investigated to increase the p-type conductivity of NiO films as inorganic HTLs for inverted PSCs either by substitutional optimized replacement of Ni 2+ with similar ionic radius elements (Li, Mg, Ag, Co, Y, Cs, Cu, Zn) or by modifying the oxidation state (non-stoichiometry).To explain the mechanism of increased p-type conductivity in ionically doped/surfacetreated NiO films, researchers performed XPS analyses.Most researchers ascribed the main peak at ≈854.1 eV to the chemical state of Ni 2+ ions in the standard NiO 6 octahedral bonding configurations in the cubic rock-salt structure and the peak centered at ≈860.8 eV to the shake-up process.The other peaks were centered between 855.3 and 857.0 eV (in different reports) was assigned to the Ni 3+ ions due to the presence of Ni 2+ vacancies, which was further fitted with two peaks corresponding to NiOOH and Ni 2 O 3 .In some studies, another intersite peak of Ni(OH) 2 was also employed in the Ni 2p 3/2 XPS fitting.Most of the Ni 2p fittings in these reports are overestimated, as seen from the statistical fluctuations between the experimental and theoretical modeling values.Furthermore, the XPS spectrum of the O 1s core level splitting consists of two peaks centered at 529.2 eV for Ni-O octahedral bonding and 531.1 eV for Ni 3+ .Like the overestimation of Ni 2p peak analysis, the O 1s peak related to Ni 3+ was deconvoluted into two (NiOOH and Ni 2 O 3 ) peaks.[34][35][36][37][38][39][40] In contrast to the above, a few reports deconvoluted the Ni 2p 3/2 XPS spectrum into four peaks and assigned them to the cd 9 L state, a nonlocal screening process with a partial contribution from the surface states, the cd 10 L 2 state, and the cd 8 state (Row 44 in Table 1). [11,32]In addition to the cd 10 L 2 and cd 8 states, another peak of the shake-up transition was also employed in the broad satellite peak fitting.Here, the binding energy of the O 1s spectra is mainly resolved into three oxygen states, attributed to lattice oxygen, surface hydroxyl groups, and adsorbed water.The significant reduction of hydroxyl groups and water after annealing demonstrates that NiOOH and water are eliminated from the NiO surface when it is annealed at 300 °C.As the binding energies of the Ni 2p XPS peaks of NiOOH and the nonlocal screening overlap, commenting on the presence and estimating the amount of Ni 3+ (if present) is difficult.Thus, to find the exact amount (in percentage) of hydroxyl groups (mostly NiOOH) present in the NiO sample, one should carefully examine the O 1s spectra, as routinely performed in the XPS analysis of other oxide materials.

Thin Films/Nanostructured Films
Sol-gel-driven NiO thin film preparation is the most conventional route to use for planar PSCs due to its simple process and good reliability.In this process, nickel salt and a stabilizer are dissolved in an organic solvent to form a precursor sol, which is then deposited on the substrate by using spin-coating or spray pyrolysis methods.Thereafter, a high-temperature (≥300 °C) annealing process is often conducted to remove the organic components and crystallize the as-deposited films.In addition to thin films, a sol-gel-processed NiO nanoporous layer was also used as the HTL in inverted PSCs.[35][36][37] The existence of both Ni 2 O 3 and NiOOH has stimulated researchers to term thin/nanoporous films NiO x rather than NiO.][39] On a contradictory note, Lee et al. treated the surface of a sol-gel-driven NiO thin film using a tetramethylammonium hydroxide (TMAH) solution and found that the ratio of Ni 3+ to Ni 2+ decreased from 1.91 to 0.47 after TMAH treatment due to reduced surface defects of the NiO thin film, which boosted the PCE of planar heterojunction PSCs from 12.30% to 17.03% (Row 5 in Table 1). [36]][42] On the other hand, Dalavi et al. claimed the deconvoluted peak corresponds to the presence of Ni 2+ cations, and not of Ni 3+ ; which led them to confirm sol-gel deposited NiO thin film is composed of pure stoichiometric NiO   1). [43]][46][47][48][49][50][51] The Ni 3+ /Ni 2+ ratio extracted from the Ni 2p XPS spectrum has been observed to decrease with increasing sputtering power and Ar pressure when the NiO film is prepared via.radio-frequency (rf) magnetron sputtering (Row 7, 42 in Table 1). [44,50,51]Furthermore, researchers also observed that the Ni 3+ /Ni 2+ ratio in the Ni 2p XPS spectra of porous-NiO and bilayer-NiO is higher than that of compact-NiO, indicating the superior hole conductivity of porous-NiO and bilayer-NiO films (Row 16 in Table 1). [52,53]

UV-Ozone/O 2 Plasma Treatment
In order to increase the conductivity of NiO layers and improve the band alignment with the perovskite, several groups have adopted UV-ozone (UVO) [54][55][56][57][58][59] and oxygen plasma (OP) [60,61] treatments on NiO films.Comparing the XPS spectra of surface-treated films with that of the pristine sample, an oxygen-rich stoichiometry is reported to create nickel vacancies, and the Ni 3+ state increases with surface treatment.A few reports have demonstrated that UVO/OP treatment creates Ni 2 O 3 , NiOOH, and Ni(OH) 2 on the NiO surface, which helps to increase the work function and hopping transport barrier for holes. [56,57,59,60]owever, most of the reports deconvoluted the shoulder into two peaks, assigned them to NiOOH and Ni(OH) 2 and denied the formation of Ni 2 O 3 (Row 43, 47 in Table 1). [54,55,58,61]Interestingly, the highest value for the Ni 3+ /Ni 2+ ratio was obtained for the OP-treated films, followed by UVO-treated films, which was determined by observing the reduction in the Ni 2+ peak width with UVO/OP treatment, and superior NiO properties resulted in the suppression of interfacial defects and recombination, which boosted the PSC performance. [57]In addition to UVO treatment, many researchers have annealed samples to adjust the NiO stoichiometry, but the O/Ni ratio only increases from 1.010 to 1.016 as the annealing temperature increases from 350 to 600 °C (Row 8 in Table 1). [62,63]Although Zhao et al. showed that the Ni 3+ /Ni 2+ ratio in NiO films depends on the annealing temperature, and the values were 2.87, 3.02, 2.44, and 2.38 for air, O 2 , N 2 , and Ar, respectively, [64] which was supported by Guo et al. [65] 1). [66]Furthermore, Zheng et al. claimed the presence of Ni 2 O 3 when NiO is annealed above 250 °C as enough driving force is required for the dehydration/dehydroxylation reaction to convert Ni(OH) 2 /NiOOH to higher-order oxidation states as in Ni 2 O 3 . [67]In contrast to the above observations, several groups reported an increase in the Ni 2+ peak intensity and a decrease in the Ni 3+ peak intensity with increasing postdeposition annealing temperature (Row 18 in Table 1). [68,69]n this context, we would like to emphasize that Ni 2 O 3 has a trigonal hexagonal crystal structure, while -NiOOH and Ni(OH) 2 have the same rhombohedral structure, and UVO/OP processes cannot provide sufficient energy to induce such crystallographic reorganization.Additionally, dehydration/dehydroxylation to form a higher order oxide requires temperature above 250°C, and the UVO/OP surface treatment did not reach such a high temperature.Thus, UVO/OP treatment might only introduce more dipolar species, such as NiOOH, into the film surface and change the surface dipole, which in turn shifts the energy level at the inorganic-organic interface to facilitate hole injection.Using angle-resolved XPS measurements between 30 and 60°, Islam et al. showed that Ni vacancies in UVOtreated NiO films are not a surface effect; they are also distributed throughout the film (Row 54 in Table 1). [70]Furthermore, during NiO film deposition through PLD or sputtering, the Ni 3+ /Ni 2+ ratio was observed to increase with increasing oxygen pressure from 10 to 200 mTorr, and the ratio was maintained with a further increase in oxygen pressure (200-900 mTorr), which indicates that the defect concentrations of the films fabricated at oxygen partial pressures over 200 mTorr are very similar to each other (Row 29 in Table 1). [17,19,71]As a cumulative result of this, almost all the studies reported a PCE enhancement of 2-3% when these UVO/OP-treated NiO films were used as HTLs in inverted PSCs.However, the 2-3% PCE enhancement in PSCs falls in the error bar region and depends on environmental factors, such as temperature, humidity, and even the skill of the researcher.Thus, to confirm that the concentration of hydroxyl species is higher at the NiO surface, the authors should perform angle-resolved XPS measurement on both pristine and UVO/OP-treated NiO films with the same photoelectron take-off angles, as the intensity of the shoulder peak has a nonlocal screening effect with a significant contribution from the surface states.

Elemental Doping
Elemental doping is an effective strategy to increase the p-type conductivity of NiO films by reducing the trap states and modifying the work function.Thus, dopants such as transition metals (i.e., Cu, Ag, Co, Zn, etc.) and alkali/alkaline earth metals (i.e., Li, Mg, Sr, etc.) can introduce lattice distortion/defects into the NiO lattices, affecting the carrier mobility and optical transmittance.Copper (Cu) is the most frequently used doping element in NiO, as Cu and Ni have similar atomic radii (mismatch of ≈2%), electronegativities (difference of ≈2%), and crystal structures (face-centered cubic).Substitution of Ni 2+ by monovalent copper (Cu 1+ ) is an effective approach to form an acceptor-like defect of a Ni vacancy (V 2− Ni ), which increases the density of holes (h + ).Similar to the previous discussion, many researchers ascribed the shoulder peak in Ni 2p to NiOOH and Ni 2 O 3 and claimed that the concentration ratio for NiOOH, Ni 2 O 3 , and NiO according to the Ni 2p 3/2 spectra increased from 0.70:0.95:1 to 0.86:1.58:1with 2% Cu doping (Row 29, 30 in Table 1). [72,73]Interestingly, Chen et al. showed that there is no significant change in the Ni 2p peaks of Cu-doped NiO films and proposed that the commonly accepted mechanism of increased conductivity due to increased Ni vacancy concentration is not applicable in their measurement (Row 53 in Table 1). [74]They showed that Cu doping forms a shallower acceptor level (only ≈0.7 eV above the Fermi level) than the nickel vacancies (≈1.3-2.0 eV above the Fermi level) of bulk NiO, which increases the conductivity and work function (5.25 eV).A similar lack of significant changes in both the Ni 2p and O 1s peaks, with the clear presence of the dopant, were observed for Li-doped, [75] Ag-doped, [76] Co-doped, [77,78] Vdoped, [79] Sr-doped, [80] and boron nitride-incorporated NiO. [81]n contrast, a slightly higher (compared to Ni 2+ ) Ni 3+ XPS peak area was observed in lanthanum-doped, [82] rubidium-doped, [83] yttrium-doped, [84] silver-doped, [85] nitrogen-doped, [86] lithiumdoped, [87] sodium-doped, [88] NiO films, which was ascribed to an increase in the hole conductivity.Specifically, the Ni 3+ /Ni 2+ ratio is enhanced by ≈12.5%, 19.8%, 13.13%, 5.66%, 2.31%, 32.5%, 17.8%, and 18.7% in Cs-doped, [89] Zn-doped, [90,91] Cddoped, [92] Pb-doped, [93] Al-doped, [94,95] K-doped, [96] Fe-doped, [97] and Eu-doped NiO, [98] respectively, whereas it is suppressed by 14.3% in Co-doped NiO. [99]The Ni 3+ /Ni 2+ ratio was reported to increase in Li-Mg, [87] Li-Cu, [100][101][102] Li-Ag, [103] Zn-Ce, [104] Li-Co, [105] and Li-Co-Mg [106] codoped NiO layers compared to the pristine NiO layers, suggesting an increase in the hole conductivity.Therefore, when the Ni 3+ /Ni 2+ ratio is increased, the oxidation of Ni 2+ into Ni 3+ in NiO is also reported to increase, resulting in a high Ni 2+ deficiency and an oxygen-rich nature, which directly contribute to the p-type conductivity of NiO.In addition, Carley et al. have showed incorporation of the alkali metal into the NiO lattice suppressed both the nonlocal screening shoulder feature and the intrinsic XPS satellite structure (XPS peak ≈861 eV) and Ni 3+ species formed if the sample heated above 575K (Row 36 in Table 1). [107]This type of behavior is not observed in doped-NiO when they were used as HTL in inverted perovskite solar cells.Thus, to determine the influence of the doping concentration on increasing the conductivity or hole mobility in doped NiO films, a few reports have presented conductivity measurements [83,87,92,95,97,102] /Hall effect measurements, [80,84,97] which we believe is essential for this complex system.
In addition to the nickel deficiency in the NiO film, the interface defects between the perovskite layer and the NiO layer seriously affect the charge carrier transfer since charge extraction only occurs at the interfaces, which may be particularly subject to charge recombination.To reduce lattice mismatchinduced interface defects, KCl, [108] CsBr, [109] ammonium salt, [110] thioacetamide, [111] mandelic acid, [112] etc. were added as an interface layer between the perovskite and NiO.Similar to doping in the NiO film, the shoulder peak in the Ni 2p XPS spectrum was assigned to Ni 3+ (NiOOH), and the interface layer modification had no effect on the composition of the NiO film; however, the binding energy of modified NiO decreased compared to that of unmodified NiO. [109]Since the fitting of the Ni 2p peak is considered complex and the Ni 3+ concentration is likely overestimated from the XPS fitting, the authors should compare the normalized spectra and extract the Ni 3+ /Ni 2+ ratio from the O 1s XPS spectrum for a better understanding of the effect of doping in NiO films.
From the above discussion, it is clear that many ambiguities are present in the experiments and discussion regarding the Ni 2p XPS spectrum of pure/surface-treated/doped NiO thin films when they are used as HTLs in hybrid PSCs.The confusion mostly arises because the Ni 2p XPS peak of NiOOH/Ni 2 O 3 and the nonlocal screening effect with a contribution from surface states lie almost in the same binding energy region.To understand the hole transport mechanism, enhanced charge extraction at NiO/perovskite interfaces, and standardize the synthesis method and post-treatment for conductivity improvement, a proper explanation of the origin of shoulder peak is essential.In this context, our suggestion is to carefully record both Ni 2p (Ni 2p 3/2 and Ni 2p 1/2 ) and O 1s XPS spectra with a fixed take-off angle.By comparing the normalized data, one might be able to assign the origin of the shoulder peak in the Ni 2p XPS spectrum.In addition to XPS spectra, the authors should also present conductivity measurements and Hall effect measurements to show the influence of surface treatment or doping in increasing the conductivity or hole mobility.We believe that the overlap of the space charge regions in ultrathin NiO films or movement of hole carriers through the crystal by hopping from one defect site to another is responsible for the enhanced hole transport ability in the hybrid perovskite system, which is described as follows.NiO has a highly stable ionic crystal structure, and due to the high bonding strength, NiO thicker than a few nanometers show intrinsically high insulating behavior.However, a high conductivity can be achieved if one can prepare ultrathin NiO films in which the film thickness is comparable to the characteristic length of the space charge region, i.e., the Debye length.When the film thickness approaches the Debye length, the space charge concentration at the center of the film no longer reverts to the bulk value, and the apparent work function, and hole concentration increase due to the overlap.Additionally, the formation of nickel vacancies (V Ni ) or the substitution of Ni 2+ due to surface treatment or the use of the p-doping process might change the intensity ratios of the Ni 2+ and Ni 3+ peaks in the Ni 2p XPS spectra.Unlike stoichiometric NiO, nonstoichiometric NiO with Ni vacancies releases two holes to maintain charge neutrality near the vacancy sites in the NiO lattice, and this change contributes a quasi-localized pair of holes to compensate for the formation of two Ni 3+ ions and results in greater p-type semiconducting behavior of the material.Compared to Ni 2+ , the higher valence Ni 3+ corresponds to a lack of one electron that produces a hole carrier in NiO.These hole carriers can move through the crystal by hopping from one Ni 3+ site to another-site or by moving in a narrow polaron band, indicative of enhanced hole transport ability. [113]

Results and Discussion
To fully understand the electronic structure of the NiO film and the presence of Ni 3+ species due to nickel vacancies, NiO films were deposited by O 3 and H 2 O 2 -derived atomic layer deposition (ALD) and sol-gel methods, respectively.To deposit sol-gel NiO film, we used Nickel(II) Nitrate Hexahydrate (Ni(NO 3 ) 2 • 6H 2 O, Sigma-Aldrich) as a solute and 2-methoxyethanol as a solvent, and detailed experimental procedure is discussed in the experimental section.We discussed details of ALD-grown NiO in the experimental section, following our group's previous report. [11,114,115]After that, the films underwent surface treatments, such as annealing at 300°C for 1 h, UV-O 3 treatment for 30 min, a combination of annealing at 300°C for 1 h and then UV-O 3 treatment for 30 min, and UV-O 3 treatment followed by annealing at 300°C for 1 h.XPS spectra of all the samples were recorded at a constant take-off (43.7 0 ) angle with reference to the adventitious C 1s peak at 284.8 eV, and the deconvolution of narrow scan Ni 2p and O 1s spectra of as-deposited O 3 -derived ALD-NiO are presented in Figure 2a,b, respectively.The backgrounds were subtracted using a Shirley-type background, and all peaks were fitted by a Gaussian/Lorentzian function (glmix).The Ni 2p spectrum consists of the spin-orbit split 2p 3/2 and 2p 1/2 components separated by 17.4 eV, which is in good agreement for all the samples.The Ni 2p 3/2 region (Figure 2a) comprises the main peak (Peak 1; c3d 9 L), a shoulder peak (Peak 2; nonlocal screening with a surface effect contribution), and satellites (Peak 3, c3d 8 , and Peak 4, c3d 10 L 2 ) at 853.5, 855.2, 860.6, and 864.7 eV, respectively.Similarly, the main peak (Peak 5), a shoulder (Peak 6), and satellites (Peak 7 and Peak 8) of Ni 2p 1/2 are found at binding energies of 870.9, 872.6, 877.6, and 880.7 eV.Furthermore, the binding energies of the O 1s spectra (Figure 2b) are mainly resolved into two oxygen states, which are ascribed to O-bonded Ni-O-Ni (Peak 1; 529.2 eV) and O-bonded Ni-OH (Peak 2; 530.9 eV).Details of peak position, FWHM, and area during the fitting of Ni2p and O1s peaks are tabulated in Table S1 and S2 (Supporting Information), respectively.Table 1 showed the representative XPS Ni 2p 3/2 and O1s 1/2 peak positions with binding energy of NiO prepared by many different methods.
Comparing the ratios of the curve areas of the main (Peaks 1 and 5) and shoulder (Peaks 2 and 6) peaks for both Ni 2p 3/2 and Ni 2p 1/2, we found that the values are not identical in the asdeposited and post-treated (annealing, UV-O 3 , combination of UVO and annealing) ALD-NiO films.Extracted curve area ratio of peaks 1 and 2 for the as-deposited, annealed, UVO-treated, and UVO-treated followed by annealing at 300°C for 1 h ALD-NiO films are ≈1:2.47,1:2.40, 1:2.49, and 1:2.44, respectively (Table S1, Supporting Information).This observation demonstrates the presence of more dipolar NiOOH species on the film surface, which could change the surface dipole and hence cause energy level alignment for hole transport over the interface. [116]Furthermore, the corresponding curve area ratios of Peak 2 and Peak 1 in the O 1s spectra are 0.21, 0.24, 0.19, 0.21, and 0.20, respectively.This analysis demonstrates that 20-24% NiOOH is present in NiO depending on the surface treatment.Reduction or enhancement of the curve area ratio of both Ni 2p and O 1s under annealing and UVO treatment is an indication of oxygen-rich/deficient conditions in NiO and thus the presence of nickel vacancies only at the surfaces.
For a better understanding, we also prepared NiO thin films deposited via.As we know the metallic Ni peak is primarily attributed to a sequence of unfilled one-electron levels within the conduction band.These levels can accept electrons that have undergone shake-up type processes following ejection of the initial core electron.In contrast to the discrete features typically associated with shake-up peaks, what we observe is a tail on the higher binding energy side of the main peak.tail accounts for the peak's asymmetry in XPS spectrum. [117]Further, the deconvoluted Ni   4d).To determine how the increased conductivity relates to absolute charge carrier mobility (μ) in the NiO, we have fabricated sandwich-type "hole-only" devices with top and bottom gold electrodes and results are depicted in Figure S1 (Support-ing Information).We know if charge injection is Ohmic then the current flowing through the device will be limited by the spacecharge buildup within the film, with μ being proportional to the current density (J).The current-voltage (J-V2) characteristics can be described by the SCLC formalism: J (V) = 9 8  0 V 2 d 3 ; where  is the dielectric constant of the material ( ≈11.9 for NiO),  0 is free space permittivity (8.854187817 × 10 −12 F m −1 ) and d is the film thickness.Thus, if we solely focused on the conductivity and mobility of NiO films then solgel method is better; however, to increase the device reliability and stability by compact and pin-hole free characteristics, we chose ALD(O 3 )-NiO over other methods.

Conclusion
In conclusion, we accepted that due to the very strong satellite structure in the Ni 2p photoelectron spectrum of NiO, separating all the contributions to obtain a better understanding is difficult.In particular, the satellite at 1.5 eV above the lowest binding energy line has been interpreted (as nonlocal screening, a contribution from the surface, the presence of vacancy induced Ni 3+ ions) by several authors.However, due to the overlap in the binding energies of the nonlocal screening and vacancy-induced Ni 3+ ion (NiOOH) peaks in the Ni 2p XPS spectrum of NiO, most of the literature overestimated the oxygen-rich/deficient conditions in NiO (used as an HTL in inverted PSCs) and thus the presence of nickel vacancies in the sample.In order to avoid this, both the Ni 2p (Ni 2p 3/2 and Ni 2p 1/2 ) and O 1s XPS spectra should be carefully recorded with a fixed take-off angle (and/or depending on take-off angle), and the percentage of vacancy induced Ni 3+ ions (NiOOH) present in the sample should be extracted from the O 1s XPS spectrum.In addition to XPS spectra, the authors should also present some direct experimental evidence such as conductivity or Hall effect measurements to show the influence of surface treatment/doping in increasing the conductivity or hole mobility in NiO thin films used as HTLs for inverted PSCs.

Figure 1 .
Figure 1.a) Ni 2p 3/2 core-level XPS spectrum of NiO.CT assignment of the main peak (≈854.6 eV in NiO; c3d 9 L, where c stands for a hole in the Ni 2p core level), shoulder peak (≈855.1 eV; combination of nonlocal screening with a contribution from the surface effect and Ni 3+ ) and a broad satellite (centered at ≈861 eV in NiO; c3d 8 and c3d 10 L 2 ).b) Schematic energy level diagram of NiO before and after XPS measurement.Schematic representation: c) non-local screening; d) bulk versus surface cluster in NiO.
H 2 O 2 -driven ALD and sol-gel method, in which more water was involved during the synthesis process than in the NiO film preparation by O 3 -driven ALD.Similar to the spectrum of the O 3 -driven ALD-NiO thin film the Ni 2p spectrum of NiO films synthesized via H 2 O 2 -driven ALD and sol-gel processes
2p 3/2 corresponds to the main peak observed at a binding energy of 853.3 (853.5)eV with shake-up satellites at 860.6 (860.4) and 865.8 (865.9)eV in H 2 O 2 -driven ALD-NiO film (Sol-gel NiO film).The peak at 855.3 (855.1)eV is ascribed to the superposition of nonlocal screening with a vacancy-induced Ni 3+ ion (NiOOH).The XPS spectrum of O 1s in H 2 O 2 -driven ALD-NiO film is depicted in Figure 3b; which mainly resolved into two oxygen states: O-bonded Ni-O-Ni (Peak 1; 529.2 eV) and O-bonded Ni-OH (Peak 2; 530.9 eV).Furthermore, Figure 3d represent the XPS spectrum of O 1s for NiO, with a peak centered at 529.2 eV, confirming the octahedral bonding of Ni-O.The peaks at 530.7 and 532.1 eV are ascribed to the presence of NiOOH and water.The curve area ratio of Peak 2 and Peak 1 in the Ni 2p 3/2 spectrum of the sol-gel-driven NiO film is 3.68, which is relatively high compared to that of the NiO film deposited Via.H 2 O 2 (3.28) and O 3 driven ALD method (3.11).This result indicates more dipolar NiOOH species on the solgel-NiO film surface, which was also confirmed by the curve area ratio of Peak 2 and Peak 1 in the O 1s spectrum (1.22).According to the XPS analysis, the O 3 -driven ALD process produces a more stoichiometric NiO x film than the sol-gel method, as the O/Ni ratio of sol-gel-NiO (1.05) is high compared to H 2 O 2 -(1.01) and O 3 -driven ALD-NiO (1.00).To calculate O/Ni ratio we first fitted the Ni 2p and O1s spectra carefully and estimated integrated intensity of main lattice peak of Ni and O (peak-1 in both cases).Following this, we divided these intensities by the photoionization cross-section corresponding to the Ni and O core levels, which are 0.2998 and 0.04005, respectively, at 1.487 keV.This step was crucial as the photoionization cross-section provides the probability of photoelectron emission and varies for each atomic core level at specific photon energies.Finally, we normalized these values to

Figure 3 .
Figure 3. XPS spectrum of a NiO film deposited: a,b) H 2 O 2 -derived ALD; c,d) sol-gel method.High-resolution (a,c) Ni 2p and (b,d) O 1s XPS acquisition for the as-deposited H 2 O 2 -derived ALD grown NiO and sol-gel-NiO film.The deconvolution of each peak is explained in the text.

Figure 4 .
Figure 4. Comparison of XPS spectrum of NiO films deposited via.O 3 -driven ALD, H 2 O 2 -driven ALD, and sol-gel method: a) Ni 2p; b) O 1s XPS spectra.c) Current-voltage (I-V) curves of as-deposited, annealed (≈300 °C), UV-O 3 -treated, first annealed and then UV-O 3 -treated; first UV-O 3 -treated and then annealed on ALD(O 3 )-grown NiO thin films.d) Current-voltage (I-V) curves of NiO films deposited via.O 3 -driven ALD, H 2 O 2 -driven ALD and sol-gel method.Extracted conductivity of each specimen are also marked in the figure.

Table 1 .
Combined literature survey and own work of Ni 2p peak assignments and their binding energy positions for various NiO surfaces.
Wang et al. also estimated the Ni 3+ /Ni 2+ ratio from the curve area of each peak and the values were 2.09, 2.22, 2.43, and 2.33 for hot-cast NiO films preheated at 25, 80, 120, and 150 °C, respectively (Row 17, 45 in Table