Correlation between Chemical and Electronic Properties of Solution

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On the Correlation between Chemical and Electronic Properties of Solution-Processed Nickel Oxide Florian Ullrich, Sabina Hillebrandt, Sebastian Hietzschold, Valentina Rohnacher, Tomasz Marszalek, Wolfgang Kowalsky, Robert Lovrincic, Sebastian Beck, Eric Mankel, and Annemarie Pucci ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00284 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 9, 2018

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ACS Applied Energy Materials

On the Correlation between Chemical and Electronic Properties of Solution-Processed Nickel Oxide Florian Ullrich,*,§,∞,‡ Sabina Hillebrandt,*,§,⊥,‡ Sebastian Hietzschold,§,∩ Valentina Rohnacher,§,⊥ Tomasz Marszalek,+ Wolfgang Kowalsky,⊥,§,∩ Robert Lovrincic,§,∩ Sebastian Beck,§,⊥ Eric Mankel*,§,∞ Annemarie Pucci,⊥,§,√ §

InnovationLab GmbH, Speyerer Str. 4, 69115 Heidelberg, Germany Materials Science Department, TU Darmstadt, Otto-Berndt-Str. 3, 64287 Darmstadt, Germany ⊥Kirchhoff-Institute for Physics, Heidelberg University, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany ∩ Institute for High-Frequency Technology, TU Braunschweig, Schleinitzstr. 22, 38106 Braunschweig, Germany √ Centre for Advanced Materials, Heidelberg University, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany + Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ∞

KEYWORDS: nickel oxide, oxygen plasma treatment, infrared spectroscopy, photoelectron spectroscopy, conductivity, chemical analysis, electronic analysis, structural order

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 spectroscopies 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 eV 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.

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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.

1. Introduction Interfaces in stacked organic or organic-inorganic hybrid devices crucially influence device performance.1 This is especially true for interfaces at the electrodes in organic photovoltaic cells (OPV), where energetic misalignment and missing charge selectivity can significantly reduce device efficiency. One solution to this problem is the implementation of functional interlayers. At the anode this layer ideally fulfills the following requirements: a sufficiently strong n- or p-type character, which is beneficial to the charge transport across the hole transport layer, a large work function (WF) to ensure energy level alignment with deep lying hole transport levels of typical donor materials, a small electron affinity (EA), desirable for efficient electron blocking behavior and transparency in the visible spectrum. Crystalline nickel oxide (NiO), which has been the subject of fundamental research for many years,2–9 fulfills all of these criteria. Additionally, it can be solution-processed, giving it potential to be used in low-cost, large-area manufacturing. Due to its functionality as a hole transport layer (HTL), NiO has been successfully implemented in thin film transistors,10 organic light-emitting diodes,11,12 OPVs13–21 and more recently in perovskite solar cells.22–30 When solution-processed, an annealing step is necessary to convert the precursor materials into NiO. The chemical composition of the NiO, and thus the film and device properties, is extremely sensitive to the annealing temperature (Ta) of the precursor material.10,11,20,31–33 In 2010 Berry et al. discovered that the application of an additional oxygen plasma (OP) treatment further enhances OPV performance.34 Subsequently, Ratcliff et al. were able to attribute this effect 2 ACS Paragon Plus Environment

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mainly to an increase of the bandgap caused by a decreasing EA.17 They identified the formation of nickel oxyhydroxide (NiOOH) as the origin of this decreasing EA. According to their interpretation NiOOH induces a dipole on the surface and thus increases the work function of the film. However, in a later study by Steirer et al.35 it was observed that films annealed at different temperatures, and therefore presumably with different work functions,31 exhibit similar WFs after the OP treatment. This annealing temperature dependent increase in the WF cannot be explained by the commonly accepted model by Ratcliff et al., where OP treatment simply causes a certain additional surface dipole. This example shows that the effects of OP treatment of NiO are still not fully understood. Herein, in an attempt to address this ambiguity, we investigate the chemical and electronic processes which occur as a result of the OP treatment of solution-processed NiO (sNiO). The chemical and electronic properties of sNiO, annealed at different temperatures, were examined before and after OP treatment. We varied Ta from 275 °C, where the conversion is sufficiently complete for optoelectronic applications,11,20 to 400 °C, where the efficiency of OPVs exhibits a maximum.20 The samples were studied with infrared (IR) spectroscopy for the analysis of the bulk properties of NiO. Information on the work function, band bending, surface dipole and chemical components of the surface region were obtained via X-ray photoelectron spectroscopy (XPS) and angle-resolved XPS (ARXPS). By correlating the results of these two complementary methods, IR and XPS, a consistent description of the effects of OP treatment was obtained and further supported by lateral surface conductivity measurements. We find that the OP treatment converts non-conductive NiO and β-Ni(OH)2 into conductive β-NiOOH, thus creating a thin, conductive surface layer. This layer is both chemically and electronically similar for all annealing temperatures, despite the samples having different compositions and work functions prior to the treatment. 3 ACS Paragon Plus Environment


With this spectroscopic study, we provide information which contribute to a deeper understanding of the correlations between the fabrication parameters, chemical composition and electronic properties of solution-processed NiO.

2. Results and Discussion The NiO films analyzed in this work were spin-casted from a nickel acetate tetrahydrate precursor solution on silicon substrates. After deposition, the films were thermally annealed in ambient atmosphere for 45 min at three different temperatures (275 °C, 325 °C and 400 °C). In a further step, these as-deposited (AD) samples were OP treated to examine the changes induced by this surface treatment.

2.1. IR Spectroscopic Characterization The transmittance measurements were taken in the far infrared (FIR) (250 – 700 cm-1) and mid infrared (MIR) (700 – 4000 cm-1) spectral range. The FIR measurements taken under various polarization conditions probe optical phonons in the reststrahl band of the material. This optical phonon response strongly depends on the precise stoichiometry and on the crystalline quality (i.e. grain sizes and the grain packing) of the sample. The MIR range gives insight into molecular vibrations from which conclusions about the chemical composition of possible species between NiO grains can be drawn.


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Figure 1. (a) FIR relative transmission spectra of sNiO films annealed at 275 °C (blue), 325 °C (green) and 400 °C (red) measured under 10° and 60° (dashed spectra) angle of incidence (AOI). The dashed vertical lines indicate the TO and surface mode (400 cm-1 and 547 cm-1, respectively) of the sNiO thin film. The black curve shows the fit of the dielectric function of sNiO-400°C with Gervais oscillators (see SI for details). (b) The MIR spectra of as deposited (AD, dark colored curves) and oxygen plasma treated (OP, bright colored curves) sNiO show characteristic bands of α-Ni(OH)2 between 1300 cm-1 and 1750 cm-1 as well as in the range between 3000 and 3600 cm-1 (shaded area), the dashed line at 3685 cm-1 indicates that β-Ni(OH)2 is present in the films.46 At 1286 cm-1 an absorption band assigned to β-NiOOH is marked with another dashed line. (c) Scheme of conversion processes between Ni(OH)2 and NiOOH.46 (d) Relative intensity changes in the MIR range of sNiO films upon OP treatment are depicted for the three different Ta.

Figure 1a shows the FIR spectra of three sNiO samples annealed at the different temperatures, each measured under two angles of incidence (AOI): 10° (which is referred to as normal incidence) and 60° (with an electric field component of the incoming IR light perpendicular to the layer). If the intensity of a vibrational signal of non-spherical molecules changes when the AOI is changed, this indicates a preferred orientation of the transition dipole moment of the vibrational mode.36–39 In a layer of an optically isotropic material like NiO IR irradiation under 5 ACS Paragon Plus Environment


oblique incidence measures not only the transverse optical (TO) phonon frequency but also the thin-film phonon polariton. For a perfect planar layer this polariton has the frequency of the longitudinal optical (LO) phonon frequency (Berreman effect40). Its mode shifts to lower frequencies with film roughness and therefore is a measure of the film quality.41 Under normal incidence, AD-sNiO-400°C shows a strong absorption peak at 400 cm-1. When the sample is tilted to 60° AOI, an additional mode at around 560 cm-1 is observed. A model of the dielectric function of AD-sNiO-400°C, as described in the SI, yields resonance frequencies of 398 cm-1 and 547 cm-1. While the first mode at lower wavenumbers is in very good agreement with literature values for the TO mode of NiO, the second mode deviates significantly from those for the LO mode of NiO at 580 cm-1 where an ideally flat NiO layer should show the IR signal.42,43 The mode appears at lower frequencies (547 cm-1), which can be explained by the granular morphology and sNiO grain sizes in the range of 10 nm43,44 (see SI for further details and Figure S1 for AFM measurements). Thus, we conclude that both modes in the FIR indicate the presence of stoichiometric NiO. This finding is supported by x-ray diffraction (XRD) patterns of sNiO films (see Figure S5). When Ta is reduced these peaks become broader, indicating a less ordered structure. Possible overlapping contributions from Ni(OH)2, that also exhibit modes in this spectral range,45,46 might slightly modify the FIR spectra. The presence of Ni(OH)2 is indicative of incomplete conversion of the precursor materials,47 and can be further classified as α- and β-Ni(OH)2. The α-phase contains water in its structure and is less ordered than the β-phase.46,48 A sketch of both phases can be found in Figure S2. Since these two phases show different characteristic absorption modes in the MIR range, it is possible to distinguish between them. The relative amount of both species is dependent on the annealing temperature, as observed in the IR transmission spectra in Figure 1b. α-Ni(OH)2 can be identified by a broad absorption band between 3200 cm-1and 3700 cm-1 and several vibrational modes 6 ACS Paragon Plus Environment

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between 1300 cm-1 and 1750 cm-1.45,46,49 These modes exhibit a clear temperature dependency: the higher Ta the smaller their intensities, most probably because of the reduction of the αNi(OH)2 concentration in the layers (we note that all films exhibit the similar thicknesses, see Figure S4). This is a reasonable result since it is known that α-Ni(OH)2 has the tendency to dehydrate especially under thermal treatment,50,51 and confirms that the degree of precursor transformation is dependent on Ta. The absorption band at 3680 cm-1 (marked with a grey dashed line) indicates the presence of βNi(OH)2.46 Unfortunately, because of its weak intensity and its superposition with the α-Ni(OH)2 absorption mode, a possible temperature dependency of this mode cannot be extracted. Furthermore, according to our measurements, neither α- nor β-Ni(OH)2 vibrational modes show an angular dependent IR absorption, which means that there is no preferred overall orientation of these molecules (see Figure S3). The absence of a preferred orientation rather indicates molecular species between the NiO crystallites. Additionally, the spectra of each AD sample after OP treatment are displayed in Figure 1b. As it is known that this procedure only affects the topmost region of sNiO samples,52 the respective changes in IR spectra can be ascribed to changes of the composition close to the surface. The most prominent spectral changes occur in the fingerprint region at 1286 cm-1 and in the Ni(OH)2 vibrational modes between 3000 and 3700 cm-1. Detailed insight into the spectral changes caused by the OP treatment can be obtained by analyzing the relative changes between the spectra of AD and OP samples shown in Figure 1d. Positive peaks indicate a reduction of the vibration relative to the respective AD sample, and vice versa. All three annealing temperatures show a positive feature at 3685 cm-1 induced by a decreasing amount of β-Ni(OH)2. At the same time, the absorption band at 1286 cm-1 changes: for 275 °C the shape of the spectra indicates a small energetic shift of the vibrational mode, but no increase. In the case of the samples annealed at 7 ACS Paragon Plus Environment


325 °C and 400 °C, however, the same absorption band strongly increases. These observations suggest that the β-Ni(OH)2 has been converted into another species. Since Ratcliff et al. has shown that the OP treatment of sNiO creates NiOOH17, and Bode et al. proved that the oxidation of β-Ni(OH)2 (e.g. induced by OP) results in the formation of β-NiOOH48 (see Figure 1c), we propose to assign the mode at 1286 cm-1 to an absorption band of β-NiOOH. The broad absorption peak of α-Ni(OH)2 is slightly increased in the case of NiO-275°C, but for 325 °C and 400 °C no significant changes are visible. With respect to the spectra of AD-sNiO (Figure 1b), it can be observed that while the mode of βNiOOH is the strongest for 275 °C, for 325 °C it is somewhat weaker and it completely disappears at 400 °C. Contrastingly, after the OP treatment, this mode is clearly visible in all three samples. Due to the superposition with vibrational modes of α-Ni(OH)2, especially in the case of 275 °C and 325 °C, the determination of the intensity and thus of an overall absolute amount of the β-NiOOH (and of Ni(OH)2) is not possible. However, the following important qualitative statement can be given: After OP treatment the amount of β-NiOOH is similar in all three films. In summary, we have demonstrated that a higher Ta leads to films with a higher content of stoichiometric NiO grains and less α-Ni(OH)2, reflecting a better conversion of the precursors. For the lowest Ta (i.e. 275 °C), the amount of α-Ni(OH)2 significantly increases upon OP treatment. For higher Ta and less α-Ni(OH)2, OP only marginally increases the α-Ni(OH)2 concentration. The amount of β-NiOOH present in AD-sNiO decreases with increasing Ta and for Ta = 400 °C it is below our detection limit. β-Ni(OH)2 is detected for all temperatures and upon OP treatment it is converted into β-NiOOH, resulting in similar amounts of β-NiOOH in the films independent of Ta. As such, for Ta ≥ 325°C and in the case of the OP treatment, it can be assumed that the NiO film surface is β-NiOOH terminated. 8 ACS Paragon Plus Environment

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2.2. Photoelectron Spectroscopic Characterization

Figure 2. (a) XP detail spectra of the O 1s region of sNiO, referenced in binding energy to the position of the main peak (529.6 eV17), for samples annealed at different temperatures (275 °C: blue, 325 °C: green and 400 °C: red). The lower three spectra refer to samples without any further processing step after annealing (AD). The higher Ta the more stoichiometric is the sNiO layer. The upper three curves belong to OP treated samples and exhibit this dependency as well. (b) Ni 2p3/2 spectra of AD- and OP-sNiO samples, corresponding to the O 1s spectra shown in (a), also referenced in binding energy to the position of the main peak (854.7 eV68). (c) Direct comparison of the O 1s spectra of sNiO-400°C before and after OP treatment. The inset magnifies the region with the biggest change and displays selected values for the BE of Ni(OH)2 from literature.11,17 (d) Fit of the OP-sNiO-400°C spectrum using five components, two of which are assigned to the two different oxygen species in NiOOH. Prior to the fits the data was corrected using a Shirley background.77

XPS measurements were used to further investigate the nature of the surface species of the sNiO. IR and XPS data interpretation should be compatible with each other under consideration of the limited information depth of XPS.



In a first step the different XP detail spectra of the O 1s region will be discussed. The spectra as well as all binding energy values from literature are referenced to the respective main peaks (529.6 eV17). In Figure 2a the O 1s spectra of AD and OP samples annealed at the three different temperatures are displayed. Independent of the surface treatment two clear trends with increasing Ta are observed, whether or not the samples were treated with OP. Firstly, the main peak, which is consistently assigned to the oxygen species of stoichiometric NiO (O2-),2,17,53–57 becomes more pronounced. This agrees with the previously discussed IR data, which indicates that the sNiO layers become more stoichiometric and proves that this statement is also valid for the surface region. Secondly, at higher annealing temperatures, we observe a reduction in the side peak at approximately 531.4 eV. In literature the presence of this peak has been attributed to the presence of various species, among which are different hydroxide species (α-, β-, oxy- and surface)2,10,11,17,27,31,53,54,57–61 and Ni2O3/defective oxygen.2,31,55,59 At higher binding energies (BE) around 533 eV interstitial and adsorbed water,11,17,55,58,59 and oxygen11,53,55 may contribute to the signal. Based on the IR results, and under the assumption that the chemical processes which occur during annealing are similar for both the bulk and the surface region, we can attribute the reduction of the side peak mainly to the presence of α-Ni(OH)2, with a smaller contribution from β-NiOOH. β-Ni(OH)2 , which is also present in small quantities, may be superposed. Due to the absence of high temperatures and the presence of moisture during the OP process the formation of Ni2O3 can be excluded.17 These trends are also reflected in the Ni 2p3/2 spectra of AD-sNiO films (Figure 2b). The intensity of the main feature, which originates from the Ni2+ species of stoichiometric NiO17,53,56,59,62–66, increases with increasing Ta (see also Figure S7). The same holds for the side peak around 856.5 eV, which is an inherent feature of stoichiometric NiO due to intersite charge transfer screening.67 At the same time, the valley between both lines at approximately 855.7 eV deepens 10 ACS Paragon Plus Environment

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slightly, which is consistent with our previous observations, as Ni(OH)2 is known to exhibit features in this region.66,68 This also explains the increase of integral intensity, since the Ni ion volume density of NiO is twice as high as the one of Ni(OH)2 while the inelastic mean free path is the same for both materials.68 For an annealing temperature of 400 °C, all these changes result in a spectrum which shows no significant differences to reference samples of crystalline stoichiometric NiO (see also Figure S9).59,60,66,69 Furthermore, Figure 2b displays the changes in the spectra due to the OP treatment: the intensities of both the main peak and the side peak as well as the one of the satellite decrease. As no additional damping adsorbate is observed (Figure S6), this indicates a reduction in the amount of NiO in the surface region, and the formation of an additional species. According to our IR measurements, where an OP induced increase of Ni(OH)2 is clearly seen only for the sNiO275°C sample, Ni(OH)2 cannot be that species for the sNiO-400°C. However, the reaction equation 2 NiO + H2O + O 2 NiOOH could explain how NiO can be oxidized to NiOOH. Atomic oxygen is supplied during the OP process and water is always present on our AD-sNiO samples at ambient conditions, and even after several minutes in vacuum or nitrogen atmosphere. Based on the standard enthalpies of formation of the species involved, the reaction is exothermic (reaction enthalpy ∆rH0 < 0). If the oxygen instead is ionized (O+ + e-), ∆Hr reaches even lower values. Also reactions of NiO with other oxygen species that are present in oxygen plasma (O-, O2, O2-, O2+, O3)70 have negative reaction enthalpies (see Table S1). Therefore, we believe that one or several of these reactions occur during the OP process. This would also explain the filling of the valley between the main and the side peak since – depending on the source and the sample fabrication – NiOOH shows binding energies in this range.59,68,71 The reaction would furthermore explain the integral intensity 11 ACS Paragon Plus Environment


reduction of the Ni 2p3/2 spectra upon OP treatment, as (similarly to Ni(OH)2) the Ni ion density of NiOOH is about a factor of two smaller than the one of NiO, while the inelastic mean free path and thus the information depth is the same.68 Moreover, the formation of NiOOH is confirmed by the shape of the multiplet satellite structure around 861.5 eV. Upon OP the rather sharp and narrow peaks of AD samples lose intensity and become quite flat. Additional structures on the left shoulder in the range of 869 eV to 861.5 eV strongly vanish such that this range becomes almost linear. These are precisely the differences between the spectra of the NiO and NiOOH reference samples examined by Grosvenor et al.66 and Payne et al.59 The satellite structure of Ni(OH)2, in contrast, is quite pronounced59,66 and differs strongly from the corresponding peaks of our OP samples. Also, the LMM Auger lines (Figure S11) are compatible with the generation of NiOOH and further suggest that we are dealing with β-NiOOH rather than γ-NiOOH. To summarize, we are able to confirm the IR results for the surface region: increasing Ta leads to films with a higher NiO content and upon OP treatment β-NiOOH is created. The XPS data suggests that also NiO and not only Ni(OH)2 reacts to NiOOH. However, due to the proximity of the Ni2+ and Ni3+ binding energies,72 more precise conclusions about the Ni(OH)2 and NiOOH content cannot be drawn only from the Ni 2p3/2 spectra. Therefore, in the following, the influence of OP treatment on the O 1s spectra will be discussed in more detail. For a better visualization, both spectra of sNiO-400°C from Figure 2a are depicted in Figure 2c. (The qualitative changes are similar for the other two Ta and can be seen in Figure S12.) Aside from the reduction of the main peak, which confirms the NiO decrease observed in the Ni 2p3/2 spectra, two regions show significant changes: around 530.8 eV and around 532.5 eV. For sNiO, the recent literature about OP treated sNiO uses four components for fitting the O 1s spectra.11,17 The authors of these publications state that the signal of Ni(OH)2 lies within the region around 12 ACS Paragon Plus Environment

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531 eV (light red vertical lines in inset of Figure 2c). By this interpretation, the XP spectra of this study would indicate a growth of Ni(OH)2 species upon OP treatment. However, this conclusion contradicts the IR results, which gives rise to a revision of the various contributions from the possible species NiO, Ni(OH)2, NiOOH, and H2O to the O 1s spectra. For this purpose, we used a five-component fit instead of a conventional four-component fit, giving us the ability to consider the two different oxygen species of NiOOH. Our fit is displayed in Figure 2d for OP-sNiO-400°C. For three of the fit components little doubt concerning the assignment exists (see Table S2 for BE values from literature): the main component located at 529.6 eV refers to O2-, the middle component located at 531.6 eV belongs to hydroxide species (α- and β-Ni(OH)2 and surface hydroxide) and the component located at the highest binding energy of 534.0 eV is caused by water. The two remaining peaks (530.9 eV and 532.4 eV) are located exactly in the two regions with the increased intensity after OP. Since for sNiO-400°C there is no increase in the IR signal other than in β-NiOOH, these two peaks must be attributed to this species. Therefore, it is reasonable to relate the O2- component to the peak at 530.9 eV, and the OH- component to the peak at higher binding energies around 532.4 eV. Our assignment is supported by the study of Payne et al.59 in which polycrystalline β-NiOOH powder was investigated by XPS. Their spectrum exhibits two main features with a distance of 1.5 eV, which is in agreement with the value we have found here. Further fits of AD and OP samples are given in Figure S8 and confirm the trends extracted by IR: for AD samples the β-NiOOH amount is the lowest for Ta = 400 °C, Ni(OH)2 decreases with increasing Ta while the NiO amount is growing, and, upon application of OP, β-NiOOH is formed again. Also O:Ni ratios, which were extracted from the O 1s and Ni 2p3/2 spectra, support this interpretation (see Table S3 for more information).



In summary, our XPS measurements and the revised fit model that considers five instead of four main contributions provide reasonable results for the sNiO composition in qualitative concordance with the IR findings and the literature on reference samples. Furthermore, our samples contained only trace amounts of carbon within the XPS information depth, meaning that the respective signals are not distinguishable from noise in the XP survey spectra (see Figure S6). Thus, we can exclude any significant contribution to the O 1s signal caused by oxygen species of carbon compounds like alcohol (–CO), carbonyl (-COH) or ester (COO-) groups.55 This is a necessary condition for a clean, reliable analysis and the validity of the performed fits. The absence of carbon species very likely contributes to the fact that the side peaks in our O 1s spectra are significantly smaller than in almost all other spectra of solutionprocessed NiO in literature. Angle-resolved XPS measurements give insight into the spatial distribution of atomic species with respect to the sample surface. Both OP- and AD-sNiO spectra show an angle dependence (Figure S10), indicating an inhomogeneity within the information depth of XPS of around 5 nm.60 As this is much more pronounced for OP samples, it can be concluded that the OP treatment impacts the surface only on the nm-range, which agrees with the findings in literature, e.g. by Widjonarko et al.52 In conclusion, there is strong evidence that the sNiO surface after OP treatment is β-NiOOH terminated.

In the following, the impact of the chemical changes of sNiO upon annealing at different temperatures and the OP treatment on electronic properties will be discussed. Figure 3a gives an overview of the measured work functions and valence band maxima of the differently prepared films. The work function of AD samples ranges from 4.4 eV to 4.9 eV and decreases with 14 ACS Paragon Plus Environment

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Figure 3. (a) Work functions (filled symbols) and valence band maxima (open symbols) of AD- and OP-sNiO for the three annealing temperatures. While the work function for AD-sNiO decreases with increasing annealing temperature, it saturates at a value of about 5.5 to 5.6 eV for OP samples. The purple and light-blue bars illustrate the changes in surface band bending (Δ) and surface dipole (Δ) upon OP treatment. The data is averaged over several measurements, exact values are given in Table S4. Representative spectra and additional information are given in Figure S13. (b) Schematic band diagrams of average AD and OP samples annealed at 275 °C and 400 °C, respectively, illustrating the values depicted in (a). The proposed OP-phase is shaded in grey.

increasing Ta. This correlates with the concentration of hydroxide species (Ni(OH)2 and NiOOH). Thus, we conclude that the hydroxide species increase the WF while a higher content of stoichiometric NiO leads to lower WF values. Upon OP treatment, however, the WF increases markedly. While Shim et al.73 have previously attributed this increase in work function to the removal of carbon contamination, this is unlikely to be the case for our samples as XPS data shows the carbon contamination to be minimal. Instead, the strong increase of β-NiOOH species upon OP treatment appears to be the main factor. This conclusion is further supported by the fact that the effect is more pronounced for higher annealing temperatures, where also the increase of β-NiOOH is larger. Interestingly, we measure a common work function of about 5.5 eV to 5.6 eV for all OP samples, independent of the initial work function (Figure 3b). These values agree with the results of Steirer et al. In this work, they investigated the influence of OP treatment duration on the work function of sNiO-250°C and observed a saturation in the range of 5.6 eV to 5.7 eV.15



The details of the observed work function changes (Δ) can be divided into two contributions: a change in the surface band bending (Δ) (which is equal to a change in the position of the Fermi level at the surface w.r.t. the band edges) and a change of the surface dipole Δ. This is expressed by the following formula: Δ = Δ + Δ Here, Δ is tracked via the valence band maxima (as leading edge, see Figure S13 for spectra and further explanation). The residual work function change is attributed to a change of the surface dipole. As indicated in Figure 3a, Δ is in the range of 150 meV - 300 meV, so the valence band shifts closer to the Fermi level. Δ starts with 0.35 eV in the case of sNiO-275°C and increases to almost 1 eV in the case of sNiO-400°C. A possible explanation for the saturation of the valence band maximum and the work function upon OP treatment is the formation of the β-NiOOH layer. Since in this surface layer the valence band is significantly closer to the Fermi level than in the bulk, the charge carrier density is assumed to be higher. Therefore, a space charge region in the NiOOH-phase might be thinner than the overall layer thickness, leading to similar work functions for all OP samples. Resultantly, one would expect that the surface dipole for all three OP samples should be similar and the different changes of the surface dipole lead to the conclusion that AD samples with lower annealing temperatures already had larger surface dipoles prior to OP treatment, which are removed and reestablished during OP treatment. The size of the surface dipoles very likely can be ascribed to the higher β-NiOOH content for AD samples with lower Ta. These results and considerations are summarized in schematic band diagrams in Figure 3b, for which two assumptions were made: flat band conditions for AD samples and no surface dipole for ADsNiO-400°C. This picture is further backed by Hoppe et al. who have deduced a very similar potential structure at the interphase between NiO and NiOOH in a different context.68 16 ACS Paragon Plus Environment

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2.3. Electrical Transport NiOOH is known to have a much higher conductivity than Ni(OH)2 and NiO.46,74–76 Therefore, according to our model, the resistivity of the surface should be reduced after OP application and reach similar values for all OP samples. A first evidence for this hypothesis could be obtained from our previous work, where OPV devices with OP treated sNiO HTLs exhibited lower series resistances than such with AD films.21 In the present work, lateral I-V measurements were performed. The extracted resistances for channels with a length of 70 µm are shown in Figure 4. For AD samples, sNiO-275°C has the lowest value, the values of films annealed at 325 °C and 400 °C are approximately one order of magnitude larger. Upon OP treatment there is a slight decrease in the resistance of the film annealed at 275 °C, and a drastic reduction of about one order of magnitude for films annealed at 325 °C and 400 °C. The final absolute values are very similar for all three temperatures (see Table S5 for details). As OP treatment was performed after the gold contact deposition, we assume no significant changes of the contact resistance. Therefore, the change in overall resistance is attributed to a change in channel resistance. These observations agree with the previous findings of this study: in the case of ADsNiO-275°C, NiOOH is already present in the Figure 4. Total resistance of sNiO films before and after

films (leading to a low resistance) and is only OP treatment for all three annealing temperatures extracted slightly increased upon OP treatment. AD-

from I-V measurements with a channel length of 70 µm. Exact values are given in Table S5.

sNiO-400°C, in contrast, is free of NiOOH and has a high resistance. Upon OP treatment a large 17 ACS Paragon Plus Environment


amount of NiOOH is produced and the resistance becomes low. Finally, we show that OP treatment results in a similar amount of β-NiOOH for all Ta, which is also reflected in similar resistances. We note that the reduction of channel resistance may also be related to the change of surface band bending und thus an increase in charge carrier density. These results support our suggested model, that for all three annealing temperatures similar conductive surface layers, mainly being composed of β-NiOOH, are formed on polycrystalline NiO upon OP treatment.

3. Conclusion In summary, we analyzed the impact of annealing temperature and oxygen plasma treatment on the chemical and electronic properties of solution-processed NiO films. We found that the work function of these films decreases from 4.9 eV to 4.4 eV with increasing annealing temperature, a phenomenon which we attribute to a decrease in the concentration of hydroxide species in the bulk and on the surface. This decrease is accompanied by an increase in the amount of stoichiometric NiO both in the bulk and at the surface. Based on our observation we identify the formation of a β-NiOOH surface layer on the sNiO bulk upon OP treatment. The combination of IR and XPS results enabled us to unveil a contradiction related to the previously used four-component fit of the sNiO O 1s spectrum and shows strong evidence that NiOOH has to be taken into account with two components instead of only one. Both experimental techniques consistently demonstrated that more β-NiOOH is created upon OP treatment for higher annealing temperatures, explaining the larger increase of the work function. After the treatment all samples exhibit similar overall amounts of β-NiOOH and similar electronic properties at the surface such as work functions of 5.5 eV to 5.6 eV. These findings led us to the conclusion that the topmost layers of OP samples are chemically similar, despite the different chemical compositions of the as-deposited samples. In terms of device application this 18 ACS Paragon Plus Environment

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implies that OP films exhibit a low sensitivity for chemical inhomogeneities at the samples’ surfaces prior to OP treatment, which may be beneficial for the reproducibility of OP-sNiO based devices. In general, this work allows the detailed understanding of chemical and electronic processes of solution-processed NiO and thus supports the interpretation of both spectroscopic data of NiO containing samples and NiO based device characteristics.

4. Experimental Section Materials: Nickel(II) acetate tetrahydrate (99.998% trace metal basis) and monoethanolamine (MEA) (

Correlation between Chemical and Electronic Properties of Solution

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