Combinative solution processing and Li doping approach to develop p-type NiO thin films with enchanced electrical properties

1 Introduction

Nickel oxide (NiO) is one of the few stable and transparent transition metal oxides, and it has a wide optical bandgap (Eg > 3.60 eV) and p-type electrical conductivity (Caruge et al., 2006; Napari et al., 2020). Due to these fascinating optical and electrical properties, numerous attempts have been made to integrate NiO_x thin films into various electronic and optoelectronic devices (Ohta et al., 2003a; Ohta et al., 2003b). Mesoscopic NiO_x thin films, for instance, have been widely employed as an electrochromic material for smart windows (Sato et al., 1993), an electroactive material for batteries or supercapacitors (Goodenough et al., 1958), and a photocathode for dye-sensitized photo-electrochemical cells (Zhang et al., 2016). In addition, owing to their p-type conductivity, nanocrystalline NiO_x thin films have recently gained enormous interest as electron barrier layers in light-emitting diodes and organic/inorganic photovoltaic devices (Irwin et al., 2008; Jeng et al., 2014). However, the development of NiO_x thin film has faced many challenges because, in the electronic structure of the p-type NiO_x semiconductor, the O 2p orbital is naturally localized in the valence band (VB), which makes the introduction of shallow acceptors challenging. Hence, undoped NiO_x thin film has been widely reported to be electrically resistive (10¹³ Ω cm) at room temperature (Barquinha et al., 2012; Krunks et al., 2014).

The electrical performance of NiO_x thin films can mainly be improved by inducing more Ni vacancies and forming interstitial oxygen within the bulk of NiO crystals. To achieve this, several groups have tried doping NiO_x with monovalent Li⁺ ions or co-doping with Mg²⁺ ions, assuming that each Li⁺ ion donates one hole to the top of the VB in order to keep charge neutrality (Sato et al., 1993; Krunks et al., 2014). In the literature, there is a wide discrepancy regarding the hole mobility and conduction mechanisms of NiO_x (Zhang et al., 2018). Guo et al. (Guo et al., 2013), for instance, reported a reduction in the resistivity of nanocrystalline Li:NiO_x thin film to ∼ 10³ Ωcm as a result of increasing the concentration of Li atoms in the precursor solution. (Chen et al., 2012), on the other hand, were able to achieve a lower resistivity of ∼ 0.02 Ωcm by doping NiO_x with copper (Cu). Doping was also reported to improve the crystallinity and the transparency of the NiO_x thin films (Chen et al., 2012). Alver et al. reported a decrease in the transparency of NiO_x thin film by introducing B³⁺ impurities into the NiO_x thin film (Alver et al., 2013) and an increase in transparency by doping with Li (CHEN et al., 2012).

For the deposition of NiO_x thin films, several physical (Sato et al., 1993; Ohta et al., 2003a; Pan et al., 2019) and chemical (Ohta et al., 2003b; Chen et al., 2021) (https://www.sciencedirect.com/science/article/pii/S0042207X14000657?via=ihub) methods have been employed, including chemical vapor deposition (CVD) (W Han et al., 2016), sputtering (Ohta et al., 2003a; Pan et al., 2019), atomic layer deposition (ALD) (Napari et al., 2020), sol-gel spin coating, chemical bath deposition (CBD) (Xia et al., 2008), electrodeposition, and ultrasonic spray pyrolysis (USP). The feasibility of all these deposition techniques has been proven for the synthesis of NiO, as well as for other functional oxides and a larger extent of the application range (Yu et al., 2022a; Yu et al., 2022b) (Yu et al., 2014). Among the various methods, USP has several advantages, such as low-cost, simplicity, offering the possibility of depositing uniform thin film over a large area, and ease of scaling for industrial applications (Oluwabi et al., 2020; Eensalu et al., 2022). One of the most important advantages of USP is a high degree of freedom in the choice of mixing the precursor solution (Wu and Yang, 2013; Krunks et al., 2014). In the literature, USP has been reported as a deposition method for charge transport layers (TiO₂ or NiO) in dye-sensitized solar cells (Chan et al., 2011; Eensalu et al., 2019). Moreover, USP-deposited TiO₂ has recently been employed as a heterojunction partnering layer in antimony chalcogenide-based solar cells (Eensalu et al., 2022; Koltsov et al., 2022). Compared with the n-type TiO₂ electron transport layer (ETL), USP-deposited p-type NiO films can be used as an electron barrier layer in similar chalcogenide-based single and tandem solar cells (Napari et al., 2020; Eensalu et al., 2022). In perovskite-chalcogenide tandem cells, for instance, NiO_x seems to be a good companion for organic hole transport layers (HTLs). For this application, expensive physical methods have been reported for the deposition of NiO_x, which are limited by poor coverage of the rough surface of the underlying chalcogenide absorber (Jošt et al., 2019). Our recent study on the development of ZnO nanorod/NiOx core-shell structures demonstrated that conformal uniform coverage of NiOx with a thickness of some nanometers could be deposited by USP on ZnO nanorods (Chen et al., 2022). Therefore, USP-processed NiO_x thin film could be an excellent approach to solve this problem. However, a low processing temperature ca 200°C is required in order to be compatible with perovskite and emerging chalcogenide thin film solar cells (Juneja et al., 2022) (Krautmann et al., 2023).

Nickel chloride (Krunks et al., 2014), nitrate (Joseph et al., 2008; Tenent et al., 2010; Juybari et al., 2011), acetate (Desai et al., 2006), and acetylacetonate (Chan et al., 2011) are common precursors used to deposit NiO_x thin film via the spray pyrolysis method. The suitability of these precursors has been reported by different research groups. In the case of chloride and acetate salts, in our previous report (Krunks et al., 2014), we comparatively studied the structure and morphological changes in spray-deposited p-type NiO_x thin film from aqueous and alcoholic solutions of nickel chloride and nickel acetate. We reported that the difference in the thermal decomposition of the chloride or acetate precursor salts was crucial in the formation of sprayed NiO_x thin film and led to changes in the film’s microstructures. Similarly, Kamal et al. (Kamal et al., 2004) reported a porous NiO_x thin film deposited from chloride precursor salts by spray, and a traceable amount of chlorine residue. In the case of the nitrate precursor route, the stability of the precursor was still uncertain during the decomposition process. On the other hand, Gomaa et al. (Gomaa et al., 2017) showed that the optical transparency of NiO_x thin film could be enhanced when deposited from nitrate precursor.

The deposition of nickel oxide (NiO_x) thin film from an acetylacetonate route has been an avoided precursor source for many solution-based techniques owing to its poor solubility in alcohol solvents. In addition, acetylacetonate is a coordinating ligand with two bidentate organic moieties that enhances the chelating complex that dissolves poorly at room temperature in alcohol (Williams et al., 2001). This has discouraged the use of acetylacetonate precursor by solution-based deposition methods such as spin coating. The great uniqueness of this work was to show that ultrasonic spray pyrolysis (USP) could be used to deposit a uniform, transparent, NiO_x thin film with fine grain structure without any drawback in terms of performance when compared with spin coating, which is a common technique used to deposit NiO_x from solutions. We provide for the first time a systematic comparative study of the development of NiOx by USP, using acetonitrile and methanol as solvents in the precursor. Furthermore, this solution processing is combined with a Li doping approach to develop p-type NiO thin films with enhanced electrical properties. We aim to optimize the deposition of NiO_x thin film using these solvents, and to describe the effect of Li alloying in the solution of NiO_x on the structural and electrical properties of NiO_x thin film.

2 Experimental procedure

2.1 Precursor solution preparation

The NiO_x precursor solution was synthesised by dissolving nickel acetylacetonate (Ni(C₅H₇O₂)₂); Sigma Aldrich, 98%) in methanol (Me-OH, CH₃OH; Honeywell, 99.8%) and acetonitrile (ACN, C₂H₃N; Honeywell, 99.9%) at room temperature to yield a solution whose Ni²⁺ ion concentration was equivalent to 2 mmol of solute in 100 mL. The solution was stirred for ∼ 30 min to ensure homogenous dissolution of solute into the solvent, which was completely transparent with a green colouration. All the reagents used were HPLC-graded, therefore there was no reason to further purify them.

Lithium nitrate (LiNO₃) [99%; Alfa Aesar] salt was employed as the Li dopant source. The Li:NiO_x solution was synthesised by mixing a quantitative amount LiNO₃ salt with Ni(C₅H₇O₂)₂ salt in methanol, and the [Li]/[Ni] mol ratio in solution was varied from 0 to 60 mol%.

2.2 Thin film deposition and post deposition treatment

Before the thin film deposition process, all the borosilicate glass and quartz substrates were ultrasonically cleaned at 60°C in acetone, ethanol, and distilled water for 10 min in respective order, and later air-dried. To optimize the NiO_x thin film deposition, the films were deposited via the ultrasonic spray pyrolysis (USP) method at different deposition conditions, such as temperature (T_s) being varied from T_s of 300–450°C. The process and the technique involved with the USP set-up has been previously reported in (Oluwabi et al., 2020), but in this work, the film deposition process took about 30 min with 24 spraying cycles for each temperature group. For the Li:NiO_x thin films, similar deposition conditions were employed, in that the value of T_s was fixed at 400°C. After the film deposition, all the as-prepared Li:NiO_x thin films (0%–60% Li) were then treated in air at 500 and 600°C on a hot plate; and later in evacuated seal ampoules in a preheated furnace at 500°C for 30 min. For convenience, the Li:NiO_x samples were labelled “y-Li:NiO_x”, where “y” was an integral number (0, 10, 20, 40, 60) representing the Li amount in the precursor solution.

2.3 Thin film characterization

The structural analysis of the deposited NiO_x thin film was assessed from X-ray diffraction (XRD) measurement using the Rigaku Ultima IV diffractometer. The instrument was equipped with a silicon detector and a Cu K_α irradiation source, and could be operated at 40 kV and 40 mA. The surface morphology was measured through the scanning electron microscope (ZEISS HR Ultra 55 with Bruker EDS system ESPRIT 1.8), and the accelerated voltage was 4.0 kV. The optical measurement was obtained from the total transmittance and total reflectance using the Jasco-V670 spectrophotometer, which was fitted with an integrating sphere. The measurements were in the range of 250–1,600 nm and the acquired data was used to calculate the optical bandgap of the film. The surface composition of the NiO_x films was determined by X-ray photoelectron spectroscopy (XPS) in the CISSY-setup at Helmholtz-Zentrum Berlin, details of which can be found elsewhere (Lauermann and Steigert, 2016). All detailed XPS spectra were recorded with an Mg Kα X-ray source with an energy of 1,253.6 eV and a pass energy of 20 eV. The raw XPS spectra of the various NiO_x thin films were first calibrated to the C-C component of the C 1s emission at a binding energy of 285 eV and then fitted with a Voigt function using the Peak Fit v4.12 software. The electrical properties, such as resistivity, carrier concentration, and mobility, were acquired via the four-point probe method at room temperature using MMR’s Variable Temperature Hall System supplied with Hall Van der Pauw Controller H-50. The electrode material used for the Van der Pauw and Hall measurements was indium.

3 Results and discussion

3.1 Impact of USP precursor solvent and deposition temperature on the properties of NiO_X thin film

The first step in the development of NiO_x thin films by USP was set towards investigation of the impact of the solvent and substrate temperature on the structural, optical, and morphological properties of the deposited NiO_x films.

Figure 1A reveals the X-ray diffraction (XRD) patterns for as-deposited NiO_x thin films deposited from methanol-based precursor on quartz substrate at different substrate temperatures (Ts). The film deposited at Ts = 300°C was amorphous, while those deposited at T_s ≥ 350°C were polycrystalline with characteristic peaks at 37.38, 43.41, 63.08, and 75.72°, being indicative of the (111), (200), (220), and (311) planes of the NiO cubic structure (PDF 01-081-8124) (International Centre for Diffraction Data (ICDD), 2008). Figure 1B reveals the XRD pattern of as-deposited NiO_x thin film from acetonitrile-based precursor on the same quartz substrate at different T_s. In a similar manner, the NiO_x film deposited at T_s = 300°C was amorphous, while the film deposited at 350 and 400°C were both crystalline, with the X-ray reflection peak at the (200) plane being the strongest and only peak on the diffractogram. However, the reflection peak along the (111) plane appeared diminished on the diffractogram for NiO_x thin film deposited at T_s = 450°C. The ratio of intensities of the (111) and (200) diffraction peaks [I₍₁₁₁₎/I₍₂₀₀₎] increased from 0.47 (at T_s = 400°C) to 0.67 (at T_s = 450°C) in NiO_x deposited from methanol-based precursor, while the intensities of the (111) and (200) diffraction peaks [I₍₁₁₁₎/I₍₂₀₀₎] was 0.13 (at T_s = 400°C) for NiO_x deposited from acetonitrile-based precursor. A comparison with the intensity of XRD reference lines of NiO_x powder [I₍₁₁₁₎/I₍₂₀₀₎ = 0.6] (PDF 01-081-8124) implied that the NiO_x thin films deposited from both methanol- and acetonitrile-based precursors exhibited preferential growth of the crystallites along the (200) plane. The full width at half maximum (FWHM) of the diffraction peak at the (200) plane decreased with the increase in T_s, which suggested the growth of the crystallites. The mean crystallite sizes calculated from Paul Scherrer’s equation for the NiO_x thin film at different temperatures for both solvents are presented in Table 1. As seen from the result, the crystallite size increased from 18 (T_s = 350°C) to 28 nm (T_s = 450°C) in the NiO_x deposited from methanol-based precursor, while the crystallite size increased from 14 (T_s = 350°C) to 41 nm (T_s = 450°C) in the NiO_x deposited from acetonitrile-based precursor.

FIGURE 1

FIGURE 1. X-ray diffraction patterns of NiO_x thin films obtained from (A) methanol, and (B) acetonitrile as solvent. The deposition temperatures for NiO_x are 300, 350, 400, and 450°C, respectively.

TABLE 1

TABLE 1. Influence of deposition temperature and precursor solvent on the crystals structure NiO_x thin film, obtained from XRD analysis.

The scanning electron microscope (SEM) images presented in Figure 2 revealed no difference in morphology of the NiO_x thin films deposited from methanol- and acetonitrile-based precursors. As depicted in Figure 2A, the NiO_x thin film deposited from methanol-based precursor was uniform with fine regular grains which had flat edges, while the films deposited from acetonitrile-based precursor had fine regular grains with rounded edges. However, the mechanisms by which the NiO_x grains changed with solvent has not been fully studied at this stage of the work, but it is a known fact that spray from alcohol-based precursors result in much smoother films due to the smaller solution-droplet size (Otto et al., 2011; Krunks et al., 2014).

FIGURE 2

FIGURE 2. Top-down view of SEM images of the deposited NiO_x thin film obtained from (A) methanol, and (B) acetonitrile as precursor solvent at T_s = 400°C.

The optical transmittance spectra of NiO_x thin film deposited from methanol- and acetonitrile-based precursor is presented in Figure 3A. Both films demonstrated a high optical transmittance along the visible light region and an absorption band edge at around 400 nm, indicating that they could be employed in transparent window applications. Independent of precursor solvent, above 75% transparency was recorded in the deposited NiO_x thin films, which surpassed all other reports on NiO_x thin film produced using this method (Krunks et al., 2014) (Reguig et al., 2007). The optical bandgap (E_g) values of these thin films were estimated using both the reflectance and transmittance data. According to the transmission spectra, the E_g of NiO_x thin films was extrapolated from the Tauc plot (Pankove, 1971). The method was based on the following relation:

$\left(\mathit{\alpha }\mathit{h}\mathit{\nu }\right)=\mathit{A}{\left(\mathit{h}\mathit{\nu }-{\mathit{E}}_{\mathit{g}}\right)}^{\mathit{n}}(1)$

where α is the absorption coefficient, hv is the photon energy, A is a constant known as the critical absorption, E_g is the optical energy bandgap (eV), and n is the power coefficient for the respective energy transition. The value of E_g for the deposited NiO_x thin films (from methanol- or acetonitrile-based precursor) at different T_s (between 300 and 450°C) were obtained by plotting ${\left(\alpha h\upsilon \right)}^2$ against photon energy (hʋ), and the obtained report is summarized in Figure 3B. The plot of ${\left(\alpha h\upsilon \right)}^2$ vs. hʋ for the samples deposited at 400°C is presented in the supporting information (Supplementary Figure S1). The E_g value of NiO_x thin film deposited from methanol-based precursor (3.78 eV; T_s = 400°C) was higher than the value obtained in NiO_x thin films deposited from acetonitrile-based precursor (3.59 eV; T_s = 400°C), which was evident in the transmittance spectra shifting to lower wavelengths (Figure 3A). In addition, irrespective of the solvent used, the bandgap value decreased with increase in T_s. The decrease in E_g with T_s may be related to structural changes and/or the presence of residuals originating from the precursor. The E_g values at higher T_s were in good correlation with the ones in the literature on NiO_x thin film produced using the chemical spray pyrolysis method (Wu and Yang, 2013) (Reguig et al., 2007).

FIGURE 3

FIGURE 3. (A) Optical transmittance spectra of NiO_x films deposited at T_s = 400°C from methanol- and acetonitrile-based precursors, and (B) bandgap variation with deposition temperature for NiO_x films deposited at T_s = 400C from methanol- or acetonitrile-based precursors.

To sum-up this part regarding the impact of solvent and growth temperature on the properties of spray-deposited NiO_x thin film, it can be inferred that, by changing the solvent, the grain orientation and the NiO_x thin film transparency can be tuned. In addition, increasing the growth temperature of NiO_x thin film promoted the phase transition from amorphous (in T_s = 300°C) to crystalline (in T_s ≥ 400°C), and these changes were evident in the film’s optical bandgap energy.

3.2 Influence of lithium doping on the morphological, structural, and chemical properties of NiO_x thin film

After demonstrating that a uniform and transparent NiO_x thin film could be deposited on quartz substrate from a methanol-based precursor by USP, a similar deposition condition could be applied using borosilicate glass, which is suitable for post-deposition annealing (600°C). The condition for depositing Li:NiO_x thin film by spray was optimized at T_s = 400°C, and the effect of doping with Li was studied using LiNO₃ as the source of Li and Ni (acac)_2, due to the fact that relatively uniform and crack-free NiO films were obtained from this precursor. Li:NiO_x samples were labelled “y-Li:NiO_x” where “y” was an integral number (0, 10, 20, 40, 60) representing the mol% of Li in the spray solution.

Figure 4 shows the surface morphologies of the Li:NiO_x thin films with and without 60 mol% of Li in the precursor. Independent of the amount of Li in the solution, the surface morphology of the films was composed of uniform granular grains, and it is important to note that the film was continuous and compact. The film thickness, which was obtained from SEM cross section (image not shown) revealed that the film thickness reduced from 80 nm (0-Li:NiO_x) to 55 nm (60-Li:NiO_x) with the increase in the amount of Li in the precursor spray solution. The morphology of NiO thin film was similar to that reported by Chan et al. (Chan et al., 2011).

FIGURE 4

FIGURE 4. SEM images for (A) 0-Li:NiO_x and (B) 60-Li:NiO_x thin films deposited at 400°C using methanol-based precursor on borosilicate glass substrate.

Figure 5 shows the XRD patterns for Li:NiO_x thin films with different doping concentrations of lithium in the precursor solution and the effects of annealing in air and ampoules for 1 h. As seen in Figure 5A, all the as-deposited Li:NiO_x thin films were polycrystalline with crystallographic reflection peaks belonging to the cubic NiO phase, and no other phases were suspected. We observed that the introduction of Li into the precursor solution caused an increase in the intensity of the diffraction peak along the (111) plane, and a decrease in the intensity of the (200) diffraction peak. Furthermore, we observed that the addition of Li dopant into the spray solution caused the diffraction peak at the (200) plane to shift from a lower angle of diffraction (2θ = 43.46°) to a higher angle of diffraction (2θ = 43.51°) with the increase in the amount of Li in the solution from 0% to 60%, respectively. The mean crystallite size was calculated to be ca 18.0 nm in the 0-Li:NiO_x, ca 6 nm in the 10-Li:NiO_x, and later increased to ca 18 nm in the 20-Li:NiO_x as-deposited films.

FIGURE 5

FIGURE 5. X-ray diffraction patterns for (A) (0–60)-Li:NiO_x thin films deposited at 400°C from methanol-based precursor, (B) after annealing at 600°C for 1 h in air, and (C) (0–60)-Li:NiO_x thin films after annealing in evacuated ampules at 500°C for 1 h. The solid black diamonds represent the crystallographic reflection peaks of cubic NiO, while the solid red clubs represent that of metallic Ni.

On the other hand, Figure 5B shows the XRD pattern of (0–60)-Li:NiO_x thin film after annealing in air at 600°C for 1 h. As depicted on the diffractogram, the (0–60)-Li:NiO_x films showed a well-defined peak orientation at a similar position to what was previously observed in the as-deposited Li:NiO_x. The calculated crystallite sizes and the lattice parameter for both the undoped NiO_x and (10–60)-Li:NiO_x thin films after annealing at 600°C is presented in Table 2. According to the presented data, it was observed that after annealing at 600°C, the mean crystallite was reduced from 21 nm (in 0-LiNiO_x film) to 9 nm (in 10-Li:NiO_x film) and later increased to 13 nm (in 40-Li:NiO_x film). The XRD pattern did not show any evident changes between the as-deposited and 600°C annealed Li:NiO_x thin films, hence the understanding that the doping mechanism at this stage may be very difficult. Furthermore, no Li phases either as Li₂O or Li-NiO_x alloy was detected in the film. Presumably, because Li was trapped at a defect within the lattice during deposition. In order to understand the Li dopant incorporation, ampoule annealing treatment was done.

TABLE 2

TABLE 2. Changes in the mean crystallite sizes (D) and lattice constant (a) of Li:NiO thin films as a function of Li amount in solution for as-deposited films and after different post-deposition treatment conditions.

Figure 5C represents the XRD result obtained from the (0–60)-Li:NiO_x thin film ampoules annealed at 500°C. The diffraction peaks located at 2θ, with values of 37.45, 43.47, 63.11, and 75.69° could be readily indexed as the (111), (200), (220), and (311) crystal planes of the bulk p-NiO respectively, and corresponded to the cubic structure of NiO_x, which was in accordance with that of the PDF card No: 01-081-8124. The relative intensity of the peak at the (200) plane decreased, while that of the peak at the (111) plane increased as the amount of Li in the precursor solution was increased. In addition, there occurred two additional peaks at 2θ with values of 44.52° and 51.96° that corresponded to the metallic nickel. The intensity of these peaks increased with the increase in the Li concentration in the precursor solution. It is important to say that the ampoule annealing led to the formation of a mixture of phases and the explanation for this observation is given in the Supplementary Note S1.

To summarize the XRD study, many have reported that Li-doped NiO_x favoured crystallization along the (200) plane. This, however, is very subjective, depending on the nature of nickel and lithium sources employed during deposition. For instance, Krunks et al. (Krunks et al., 2014) reported on the growth of Li:NiO_x thin film along the (200) plane by spray pyrolysis with LiCl as the dopant source. In our case, the as-deposited Li:NiO_x thin film grew along the (111) plane with a reduction in the lattice parameters, which could be associated with Li incorporation within the lattice of the Li:NiO_x solid solution formed. Krunks et al. (Krunks et al., 2014) and Tenent et al. (Tenent et al., 2010) also reported no observable deviation between the XRD diffractograms of the lithium-doped and undoped NiO_x thin film deposited by spray pyrolysis.

However, as observed in our results, the shift in the 43.47° peak towards a higher angle of diffraction, accompanied by the reduction in the lattice parameters with the increase in the amount of Li in the spray solution, could be explained according to the phenomena which is explained by Bragg’s law (Li et al., 2005) (Lu et al., 2012). This means that, during doping, Li⁺ impurities have the tendency to substitute Ni²⁺ and thus reduce the bond length or occupy an interstitial site, resulting in an increase in bond length (Wu and Yang, 2013) (Moulki et al., 2012). A shift in the X-ray diffraction peaks toward a higher 2θ value with the increasing amount of Li in the solution has been reported for solution-processed NiO_x thin films (Moulki et al., 2012; Wu and Yang, 2013; Krunks et al., 2014) and for Ni_1−xLi_xO powders (Li et al., 2005; Lu et al., 2012). Maulki et al. (Moulki et al., 2012) explained that when Li⁺ (0.76 Å) substituted Ni²⁺ (0.69 Å) in the NiO_x film lattice, the formation of a defective Ni³⁺ (0.56 Å) was induced at an appreciable amount proportional to the amount of the Li incorporated.

According to Deren et al. (Jarzebski, 1973) the conditions for Li atoms to occupy an interstitial position in the Ni-Li-O system are very slight, preferably when the Li amount is approx. 0.2 at.%, which is much lower than our experimental parameters. The annealing process caused an improvement the crystallite growth of Li:NiO_x thin film. Although, the isothermal annealing process led to the formation of unexpectedly mixed (NiO_x + Ni) phases in the deposited Li:NiO_x thin film, it is also worth mentioning that the ampoule annealing process was applied herein for the first time. Sealed evacuated ampoules are a notable technology for homogeneous doping in CdTe (Spalatu et al., 2017) and SnS (Spalatu et al., 2019) chalcogenide photovoltaic materials. Such annealing methods allow the understanding of the mechanism of dopant incorporation into the lattices of thin film material. Based on our results, we claim that during the deposition process Li was incorporated into the NiO_x thin film lattice and most probably accumulated at the grain boundaries. Subsequent ampoule annealing at 500°C generated conditions for mass transport through the gas phase, promoting crystallite growth and grain sintering in the NiO_x film (as seen on Table 2). The entire process was accompanied by the incorporation and redistribution of Li dopant into the lattice, and it was dictated by the partial pressure of the gas components above the solid NiO phase (the mechanism of the ampoule annealing is described in the Supplementary Note S1).

To ascertain the chemical composition and investigate the changes in the core-level state of the deposited (0, 60)-Li:NiO_x thin films, XPS spectra were recorded for each Li 1s, Ni 2p, and O 1s regions. As shown in Figure 6A, the XPS spectra of the Li 1s core level for both as-deposited (0, 60)-Li:NiO_x showed no observable peak that corresponded to Li BE, while the (60)-Li:NiO_x thin film exhibited a peak at 52.04 ± 0.2 eV BE. Keeping in mind that the relative (to carbon) photoionization cross section of Li was only 0.06 compared to the one of Ni 2p^3/2 (13.9) (Trzhaskovskaya et al., 2001), it was no surprise to miss Li 1s signals on the surface of (60)-Li:NiO_x. The interesting observation was its detection after isothermal treatment (in ampoules at 500°C) at 52.04 ± 0.2 eV BE, a value attributed in literature to the BE of Li interstitials (Li_ii) and correlated to the valence state of incomplete oxidation (Lu et al., 2006). One can interpret this by assuming that during the annealing in isothermal conditions, the Li atoms were strongly diffused towards the Li:NiO_x surface and detected by XPS.

FIGURE 6

FIGURE 6. X-ray photoelectron core-level spectra for both as-deposited (Ts = 400°C) and ampoule-annealed (T_amp = 500°C) films: (A) Li 1s region for (0, 60)-Li:NiO_x, (B) Ni 2p region for the 0-Li:NiO_x, and (C) Ni 2p region for the 60-Li:NiO_x thin films.

The XPS spectra of Ni 2p core level from the surface of NiO_x films (Figure 6B) represented a sum of two envelopes of 5-6 peaks for Ni²⁺ and Ni³⁺, and at least one more envelope for nickel hydroxide (Grosvenor et al., 2006). For easy comparison, we have presented here the non-deconvoluted spectra for 0-Li:NiO_x and 60-Li:NiO_x thin films. As shown in Figures 6B, C, the XPS spectra of Ni 2p peak clearly demonstrated that the NiO (Ni²⁺ BE at 854.2 ± 0.1 eV) and Ni₂O₃ (Ni³⁺ BE at 855.9 ± 0.2 eV) phases coexisted in the 0-Li:NiO_x and 60-Li:NiO_x films. On comparing the spectra before and after annealing, we observed a shift in the low BE emission in the 0-Li:NiO_x films, which was absent in the 60-Li:NiO_x films after annealing (Figure 6C). In addition, there was no observable difference between the Ni 2p spectra from undoped and Li-doped NiO_x thin films. This indicated that the addition of Li in the precursor solution induced no chemical change in the as-deposited NiO_x thin film surface. An illustration of such an observation is presented in the supporting information (Supplementary Figure S2).

In our view, the XPS profile in Figures 6B, C is not asymmetric and there seem to be emissions from other Ni 2p components that are present on the surface of deposited Li:NiO_x thin film, thus contributing to the broadness of the peak. To check this assumption, we deconvoluted Ni 2p spectra for the ampoule-annealed NiO_x thin films (Supplementary Figure S3) by three main peaks at BE of 853.04 ± 0.2 eV, 854.22 ± 0.2 eV, and 856.06 ± 0.2 eV, and other satellite and shake-up peaks were positioned at 860.44 ± 0.2 eV, and 862.55 ± 0.2 eV (Klein and Hercules, 1986). The O 1s core-level spectral for as-deposited Li:NiOx thin film at different amount of Li in the spray precursor solution are given in the supporting information (Supplementary Figure S4). Based on literature records, the peak at low BE 853.04 ± 0.2 eV could be attributed to Ni-atoms in the zero state (Ni⁰), the peak at 860.44 ± 0.2 eV—to its satellite peak, whereas the peaks at 854.29 ± 0.2 eV and 856.04 ± 0.2 eV BE were attributed to Ni²⁺, and Ni³⁺, respectively (Lei et al., 2014) (Soo Kim and Lee, 2012). This result supports the current observation of metallic (Ni⁰) by the XRD analysis after the ampoule-annealing process, and the explanation is given in the Supplementary Material.

3.3 Electrical properties of Li:NiO_x thin film

The electrical properties of the spray-deposited (0–60)-Li:NiO_x thin film were measured using the van der Pauw method at room temperature, and the results are presented in Table 3. According to the electrical results of the as-deposited (0–60)-Li:NiO_x thin films as a function of Li concentration in the precursor solution, all the deposited Li:NiO_x films exhibited p-type electrical conductivity as determined by hot probe technique and verified by the sign of the Hall coefficient. The resistivity of 0-Li:NiO_x thin film was 2.2 × 10⁴ Ωcm; increasing the Li amount in the precursor solution yielded a resistivity value of 3.6 × 10² Ω-cm for the 60-Li:NiO_x films. This was because the amount of Li atoms available to replace the sites of Ni atoms increased, and the carrier (hole) concentration increased from 3.0 × 10¹³ to 5.0 × 10¹⁵ cm⁻³. The carrier mobility of the Li:NiO_x thin films decreased from 6.0 cm²/(Vs.) to 1.5 cm²/(Vs.) as the amount of Li in the precursor solution was increased from 0 to 60 mol%. In Li:NiO_x thin films, the Li atom, which was the dopant material, can act as scattering centres; as the amount of Li was increased, the carriers were trapped within the film matrix, thus resulting in a decrease in carrier mobility. Based on the electrical properties of the as-deposited samples (0-60)-Li:NiO_x, it could be inferred that the sprayed-deposited Li:NiO_x thin film may not be sufficiently conductive to yield higher carrier concentrations (ρ ≥ 10^–4 Ω cm), indicating no significant density of electrically active defects, i.e., Ni vacancies (V_Ni). Several authors have reported a lower resistive NiO thin film in the order of 300 Ωcm by spray pyrolysis and other solution-based methods when NiNO₃ was used as Ni source (Moulki et al., 2012; Wu and Yang, 2013; Krunks et al., 2014).

TABLE 3

TABLE 3. Carrier concentration, mobility, and resistivity of the as-deposited at 400°C and 600°C annealed Li:NiO_x thin films as a function of Li amount in the precursor solution.

To further improve the electrical properties of the deposited Li:NiO_x thin films, post-deposition thermal annealing in air at 600°C was carried out. Sequential measurements were made to extract reliable carrier concentration (p) and mobility (μ) data during the Hall effect measurement. The result obtained from the (60)-Li:NiO_x thin film after annealing in air at 600°C showed a decrease in resistivity (∼22 Ωcm), a decrease in mobility (∼0.1 cm²/Vs.), and a two orders of magnitude increase in carrier concentration (∼2.0 × 10¹⁸ cm⁻³) relative to the as-deposited Li:NiO_x films. To explain the impact of Li doping concentration on the enhanced carrier density in Li:NiO_x films, the changes in the structural properties of the films should be also considered. Although grain size and grain boundaries have a major impact on the electrical parameters, no evident changes in the morphology of the Li:NiO_x films were observed with the increase of Li dopant concentration. Moreover, according to XRD analysis, for both as-deposited and 600°C air-annealed Li:NiO_x thin films, the crystallite size decreased with the increase of Li dopant concentration from 10 to 60 mol% (Table 2). Thus, these results indicated that the resulting hole concentration in Li:NiO_x was greatly impacted by the concentration of intrinsic point defects rather than by the changes in the crystallite size (and consequently grain size) of the films. Considering the vacancy-type mechanism as a predominant disorder in the NiO films, the defect formation within the NiO_x lattice with incorporation of Li led to the generation of holes. With the addition of Li, more Li⁺ ions substituted Ni²⁺ sites in the original crystal structure and created more holes into the NiOx lattice. The detailed mechanism of the formation of Schottky defects (considering the equilibrium constant in the NiO_x lattice) is described in the Supplementary Note S1. Although the Li dopant enhanced the hole density in Li:NiO_x films, the decrease in the mobility (Table 3) was an indication that the Li dopant may imply scattering phenomena via ionized impurity. On the other hand, there was a strong hindrance in the deposited Li:NiO_x films, which was consequently due to the presence of grain boundaries, electrically active defects, and/or impurities playing an influential role in determining the transport properties of polycrystalline films (Zhang et al., 2018).

It is worth mentioning that several authors have reported the resistivity values of the NiO_x and doped NiO_x films in the range of 10⁻¹ to 10⁻³ Ωcm after post-deposition annealing in the temperature range of 400–600°C (Tyagi et al., 2013; Menchini et al., 2013; Jlassi et al., 2014; Ivanova et al., 2022; Wu and Yang, 2013). The specific values largely depended on the employed deposition methods and the annealing ambient conditions (Jlassi et al., 2014; Pan et al., 2019; Ivanova et al., 2022). A common peculiarity for most of the previous studies is that the as-grown NiO_x films were always amorphous and/or exhibited very low crystallinity, independent of the method employed for the film synthesis. This implied that a second step of post-deposition annealing was required and usually the treatment should be performed at temperatures between 500 and 600°C. The results in this work show that by changing the solvent, the grain orientation and the NiO_x thin film transparency can be tuned and, importantly, by selection a proper solvent for the USP precursor, the temperature at which phases transition from amorphous to crystalline can be significantly lowered. These findings provide complementary knowledge to that already existing about wet-chemical methods, showing that Li:NiO films fabricated by ultrasonic spray, which is a large-area wet-chemical method, within the precursor system of Ni-acetylacetonate- LiNO₃- acetonitrile allowed the fabrication of crystalline NiOx films at 400°C. It was demonstrated that the incorporation of Li in NiOx decreased the mean crystallite size of the films by a factor of two compared to those undoped. Furthermore, p-type conductivity, resistivity, and carrier concentration could be tuned by controlling the amount of Li in the precursor solution. The 600°C annealing of NiOx films with 10 mol% Li in the precursor solution was identified as the optimal temperature to result in films with resistivity of 2.0 × 10² Ωcm and a carrier concentration of 2.2 × 10¹⁶ cm⁻³. Thus, ultrasonic spray pyrolysis is a reliable method, allowing an alternative wet-chemical method for the large-scale production of the p-type NiO films fabrication, and the suitability of these films for application due to the hole transport layers in inverted solar cells.

4 Conclusion

We have demonstrated the deposition of NiO_x and Li:NiO_x thin films by ultrasonic spray pyrolysis method with a precursor prepared by combinative mixing of nickel acetylacetonate salt in either methanol or acetonitrile solvents. The choice of solvent and growth temperature was found to influence the morphological, structural, optical, chemical, and electrical properties of the deposited NiO_x thin films. The deposited NiO_x thin films were uniform and well compact. The optical studies gave evidence that the bandgap of USP-deposited NiO_x thin film at 400°C could be tuned from 3.59 eV to 3.78 eV when changing the solvent of the precursor from acetonitrile to methanol. According to the XRD analysis, the deposited NiO_x films (from both methanol- and acetonitrile-based precursors) were polycrystalline with a mean crystallite size ranging from 18 to 30 nm (methanol based) and from 14 to 41 nm (acetonitrile-based) when the deposition temperature was varied from 350 to 450°C. To improve the electrical properties of spray deposited NiO_x, the doping of NiO_x films with monovalent Li⁺ was employed. The deposited Li:NiO_x thin films exhibited uniform morphology and a p-type conductivity. The incorporation of Li in NiO_x thin film was confirmed by both XRD and XPS analyses. Li⁺ doping in form of Li-NiO_x alloy solution reduced the electrical resistivity of the as-deposited thin film, from 10⁴ to 10² Ω cm, and increased the carrier concentration by two orders of magnitude as the lithium concentration in the precursor solution increased, from 0 to 60 mol%. An additional annealing step at 600°C in air resulted in a further three orders of magnitude enhancement in hole concentration. Based on these results, the mechanism of Li incorporation with the formation of point defects was proposed. This work provides new insight into the fundamental understanding of the doping mechanism in NiO_x, and brings technical advances in the fabrication of p-type conductivity in Li-doped NiO_x, a functional hole transport material for solar cells.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author contributions

AO: conceptualization, methodology, validation, formal analysis, investigation, writing—original draft, visualization. NS: conceptualization, investigation, formal analysis, writing—review and editing, resources. NM: XPS data acquisition, writing—review and editing. AK: validation of XPS data. AM: resources, validation. MK: conceptualization, resources, writing—review and editing, supervision, project administration, funding acquisition. IA: conceptualization, resources, writing—review and editing, supervision, project administration, funding acquisition.

Funding

All of the sources of funding for the work described in this publication are acknowledged below: Estonian Research Council project PRG627 “Antimony chalcogenide thin films for next-generation semi-transparent solar cells applicable in electricity producing windows.” Estonian Research Council project PSG689 “Bismuth Chalcogenide Thin-Film Disruptive Green Solar Technology for Next-Generation Photovoltaics.” Estonian Centre of Excellence project TK141 (TAR16016EK) “Advanced materials and high-technology devices for energy recuperation systems.” The European Union’s H2020 Programme under the ERA Chair project 5GSOLAR grant agreement No. 952509.

Acknowledgments

The authors also thank Valdek Mikli and Olga Volobujeva from the Laboratory of Optoelectronic Materials Physics at Tallinn University of Technology for recording the SEM images.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2023.1060420/full#supplementary-material

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