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FULL PAPER Precursors for p-type Nickel Oxide: Atmospheric Pressure MOCVD of Nickel Oxide thin films with high work functions. Samuel. D. Cosham, [a] Stephen P. Richards, [a] Troy Manning, [b] Michael S. Hill, * [a] Andrew L. Johnson* [a] and Kieran C. Molloy [a] Dedication ((optional)) Abstract: A series of unsymmetrical nickel - diketonate derivates have been synthesised and structurally characterised for application as atmospheric pressure metalorganic chemical vapour deposition (AP-MOCVD) precursors for nickel oxide. (TMEDA)Ni(MeC(O)CHC(O)OEt) 2 was selected and used to deposit NiO films of varying thickness onto commercial indium tin oxide-coated glass (ITO); the work function of the ITO is raised as a consequence. Introduction The development of transparent p-type semiconducting oxides is truly enabling technology, [1] opening the floodgates to a multitude of applications such as coatings for transparent electronics, thin film transistor displays, battery electrodes [2] , selective gas sensors [3] , inorganic thin film photovoltaic [4] and thermoelectric devices [5] , memristors [6] and electrochromic smart windows devices. [7] Of the known, and established, p-type semiconductor oxides nickel oxide, NiO, which is an intrinsic nonstoichiometric (O/Ni atomic ratio >1), wide bandgap (3.6-4 eV) p-type semiconductor material [8] , is possibly the most widely studied with a number of routes to thin films including both chemical and physical methods. While the most popular methods of thin film deposition appear to be sol-gel, [9] magnetron sputtering, [10] evaporation [11] and laser ablation deposition, [12] and electrospray deposition, [13] a number of single source precursors for application in chemical vapor deposition (CVD) [14] and atomic layer deposition (ALD) [15] have also been developed (Scheme 1). In very thin films of NiO have sufficient optical transparency to allow access of light to the photoactive layers, and as such have found application as a photocatode material in p-type dye-sensitized solar cells, in p-n tandem dye sensitised solar cells and as hole collector in BHJ solar cells. [16] More recently NiO has also found application in perovskite heterojunction solar cells in which the NiO layer acts as an electron blocking layer by virtue of its higher conduction band compared to [CH 3 NH 3 ][PbI 3 ]. [17] . As an electron blocking layer (EBL) in BHJ solar cells, NiO, prevents unwanted leakage from the BHJ to the anode, and in this regard is superior to more conventional organic blocking layers such as a PEDOT/PSS mixture (PEDOT: poly-3,4- ethylenedioxythiophene; PSS: poly- styrenesulphonate) whose deposition from an acidic solution attacks the ITO surface [18] and generates inconsistent film morphologies. [19] Scheme 1. Examples of CVD and ALD precursors used in the deposition of NiO. Finally, NiO, with a work function of ca. 5.0 - 5.4 eV, [20] has been shown to increases the work function of ITO anode anodes (ca. 4.75 eV), [20a] thereby enhancing the efficiency of photogenerated hole extraction from the BHJ. Marks et al have reported that thin (5 – 80 nm) films of pulsed-laser deposited NiO, when incorporated into a typical BHJ device involving P3HT / PCBM as the active layer (P3HT: poly-3-hexylthiophene; PCBM: 6,6-phenyl- C 61 -butyric acid methyl ester), affords a cell [a] S. D. Cosham, S. P. Richards, M. S. Hill, * A. L. Johnson* and K. C. Molloy Department of Chemistry, University of Bath, Bath, BA2 7AY (UK) E-mail: [email protected] [b] T. D. Manning Department of Chemistry University of Liverpool Liverpool L69 7ZF, (UK) Supporting information for this article is given via a link at the end of the document.

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Page 1: ((Title)) · Web viewtandem dye sensitised solar cells and as hole collector in BHJ solar cells.[16] More recently NiO has also found application in perovskite heterojunction solar

FULL PAPER

Precursors for p-type Nickel Oxide: Atmospheric Pressure MOCVD of Nickel Oxide thin films with high work functions.Samuel. D. Cosham,[a] Stephen P. Richards,[a] Troy Manning,[b] Michael S. Hill, * [a] Andrew L. Johnson*

[a] and Kieran C. Molloy [a]

Dedication ((optional))

Abstract: A series of unsymmetrical nickel -diketonate derivates have been synthesised and structurally characterised for application as atmospheric pressure metalorganic chemical vapour deposition (AP-MOCVD) precursors for nickel oxide. (TMEDA)Ni(MeC(O)CHC(O)OEt)2 was selected and used to deposit NiO films of varying thickness onto commercial indium tin oxide-coated glass (ITO); the work function of the ITO is raised as a consequence.

Introduction

The development of transparent p-type semiconducting oxides is truly enabling technology,[1] opening the floodgates to a multitude of applications such as coatings for transparent electronics, thin film transistor displays, battery electrodes[2], selective gas sensors[3], inorganic thin film photovoltaic[4] and thermoelectric devices[5], memristors[6] and electrochromic smart windows devices.[7] Of the known, and established, p-type semiconductor oxides nickel oxide, NiO, which is an intrinsic nonstoichiometric (O/Ni atomic ratio >1), wide bandgap (3.6-4 eV) p-type semiconductor material[8], is possibly the most widely studied with a number of routes to thin films including both chemical and physical methods. While the most popular methods of thin film deposition appear to be sol-gel,[9] magnetron sputtering,[10] evaporation[11] and laser ablation deposition,[12] and electrospray deposition,[13] a number of single source precursors for application in chemical vapor deposition (CVD)[14] and atomic layer deposition (ALD)[15] have also been developed (Scheme 1).

In very thin films of NiO have sufficient optical transparency to allow access of light to the photoactive layers, and as such have found application as a photocatode material in p-type dye-sensitized solar cells, in p-n tandem dye sensitised solar cells and as hole collector in BHJ solar cells. [16] More recently NiO has also found application in perovskite heterojunction solar cells in which the NiO layer acts as an electron blocking layer by virtue

of its higher conduction band compared to [CH3NH3][PbI3].[17]. As an electron blocking layer (EBL) in BHJ solar cells, NiO, prevents unwanted leakage from the BHJ to the anode, and in this regard is superior to more conventional organic blocking layers such as a PEDOT/PSS mixture (PEDOT: poly-3,4-ethylenedioxythiophene; PSS: poly-styrenesulphonate) whose deposition from an acidic solution attacks the ITO surface[18] and generates inconsistent film morphologies.[19]

Scheme 1. Examples of CVD and ALD precursors used in the deposition of NiO.

Finally, NiO, with a work function of ca. 5.0 - 5.4 eV,[20] has been shown to increases the work function of ITO anode anodes (ca. 4.75 eV),[20a] thereby enhancing the efficiency of photogenerated hole extraction from the BHJ. Marks et al have reported that thin (5 – 80 nm) films of pulsed-laser deposited NiO, when incorporated into a typical BHJ device involving P3HT / PCBM as the active layer (P3HT: poly-3-hexylthiophene; PCBM: 6,6-phenyl-C61-butyric acid methyl ester), affords a cell power conversion efficiency as high as 5.2%, and both an enhanced fill factor (69%) and open-circuit voltage (Voc; 638 mV) relative to ITO : P3HT / PCBM control device;[21] the enhanced effect on hole injection in an organic light-emitting diode (OLED) from addition of an ultrathin film of NiO on ITO has also been reported.[22]

For the cost-effective engineering of large-scale photovoltaic devices, deposition of NiO by laser-assisted sputtering,[4, 10f] or by other methods such as vacuum evaporation,[11d] sputtering[10b, 10c] or ALD,[15d-g] is less attractive than by chemical vapour deposition (CVD), which is capable of depositing films of good uniformity and composition and which is currently employed to coat large areas of float glass in an on-line process. However, precursors for the CVD of NiO remain rather limited. The majority of those reported [Ni(dmamb)2 (Hdmamb = 1-dimethylamino-2-methyl-2-butanol),[23] Ni(dpm)2 (Hdpm = dipivaloylmethane),[24]

Ni(tta)2.TMEDA (Htta = 2-thenoyltrifluoroacetone; TMEDA = tetramethylethylenediamine),[14a, 25] Ni(thd)2 (thd = 2,2,6,6-tetramethyl-3,5-heptanedione),[26] Ni(C5H5)2,

[27] Ni(bic)2 (Hbic =

[a] S. D. Cosham, S. P. Richards, M. S. Hill, * A. L. Johnson* and K. C. Molloy

Department of Chemistry, University of Bath,Bath, BA2 7AY (UK)E-mail: [email protected]

[b] T. D. ManningDepartment of ChemistryUniversity of LiverpoolLiverpool L69 7ZF, (UK)

Supporting information for this article is given via a link at the end of the document.

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FULL PAPER(MeCO)(MeOCO)C=C(NH2)Me),[28]

Ni(pda)(hfac)2, Ni(pda)(thd)2

(pda = 1,3-diaminopropane, Hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione) [29] have all been employed at low pressures due to their limited volatility. Additionally, Ni(acac)2 (Hacac = 2,4-pentanedione) has been used at both low-[30] and atmospheric-pressures[31] as a precursor in conjunction with O2, and as a gas-transported solid single-source precursor (SSP).[14h] NiO thin films have also been fabricated by spray pyrolysis methods using nickel nitrate,[32] nickel chloride,[32b, 33] nickel acetate[34] and nickel acetylacetanoate.[35]

The nickel -diketonates studied to date as CVD precursors suffer from several limitations. Firstly, they often have high melting points and consequently low volatility, leading to incomplete volatilisation and unstable growth kinetics. From a molecular perspective, these characteristics can be associated with the tendency for nominally monomeric species to aggregate into trimers,[36] or to coordinate solvent molecules such as water which, through H-bonding, generate associated lattice structures.[37] Fluorinated -diketonates e.g. Ni(hfac)2 (Hhfac = 2,2,2,6,6,6-hexafluoro-2,4-pentanedione), which have been used in Ni0 deposition,[38] have the drawback of potential fluorine incorporation into the final film.

In this paper we report (i) the synthesis and characterization of a number of unsymmetrical Ni(II) -diketonates (Scheme 2) whose coordination sphere is saturated by the inclusion of a coordinated bidentate Lewis base (I), (ii) the evaluation of a selected example as an APCVD precursor for the deposition of NiO, (iii) the deposition of NiO onto ITO surfaces and a study of the influence of the deposited film on the ITO work function.

Results and Discussion

Synthesis and Structural Studies

A series of Ni(II) derivatives of unsymmetrical -diketonate ligands, stabilized by the presence of a bidentate nitrogen-donor Lewis base, have been synthesized (Scheme 2). Also prepared by the same method was a bis-pyridine derivative Ni[MeC(O)CHC(O)Et]2.2py (7). Yields are in the range 44 – 96% and all the compounds are air-stable solids (1, 2, 5 - 7) or oils (3, 4) which are soluble in common organic solvents (e.g. CH2Cl2, Et2O, hexanes).

All the solids are relatively low melting (83 – 115 oC) with the exception of 6 (191-4 oC) and 7 (148-164 oC). For 6, this is plausibly due to a series of weak hydrogen bonds between the oxygen atoms of the fused ring [Fig. 4, O(3) and O(6)] and hydrogen atoms of a neighbouring TMEDA fragment on one side [O(3)-H(13B) 2.791; O(6)-H(14B) 2.668 Å] and ring -diketonate oxygens [Fig. 4, O(2) and O(5)] which form a bifurcated

hydrogen bond with a neighbouring CH2 group on the other [O(2)-H(25B) 2.514; O(5)-H(25B) 2.565 Å] leading to weak lattice association. In general, the data would suggest that the unsymmetrical nature of the ligands leads to poor lattice packing and hence low melting points.

R1 R2 D1 H OMe TMEDA2 H OEt TMEDA3 H OEt PMDETA4 H NEt2 TMEDA5 Et Me TMEDA6 (CH2)2O TMEDA7 H OEt 2Py

Scheme 2. Scheme showing the synthesis of complexes 1-7.

The structures of 1, 2, 5 – 7 are shown in Figure 1, relevant geometric data are given in the figure caption (see supporting information for crystallographic table, S1). Compounds 1, 2, 5 and 6 all adopt a cis-, cis-, cis-configuration of ligands about the metal. The Ni-O and Ni-N distances are unexceptional and are consistent with previous reports on such type of complex.[25] The -diketonate ligands are most symmetrically bound in 2 and, particularly, 5, and in a more anisobidentate manner in the structures of lower symmetry (1, 6). In contrast, the pyridine-coordinated complex 7 has the donors disposed in a trans manner, with the Ni-N bonds the shortest across the series of five compounds.

TGA data for selected compounds are shown in Fig. 2. Compounds 1, 2 and 5 all show evidence of volatility with residues of ca. 10% mass at 400 oC, compared to ca. 18% for a NiO or 14% for Ni residues. 4 has a residue close to that of NiO (16.5%; theoretical 15.3%), while 3 (20.3 vs 15.3%) and 6 (22.9 vs. 17.4%) show incomplete decomposition by this temperature, the mass remaining exceeding that of NiO.

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FULL PAPER

Figure 1. Diagram showing the molecular structures of complexes 1(A), 2(B), 5(C), 6(D) & 7(E) and the labeling scheme used. All thermal ellipsoids are at the 50% probability level, Hydrogen atoms have been omitted of clarity: A) Shows only one of two independent, but essentially similar, molecules in the asymmetric unit 1. Selected geometric data: Ni(1)-O(1) 2.0277(16), Ni(1)-O(2) 2.0565(17), Ni(1)-O(4) 2.0123(16), Ni(1)-O(5) 2.0559(16), Ni(1)-N(1) 2.1332(19), Ni(1)-N(2) 2.1554(19) Å; O(1)-Ni(1)-O(4) 175.59(7), O(2)-Ni(1)-N(1) 173.97(7), O(5)-Ni(1)-N(2) 178.04(7) o. B) Shows the molecular structure of 2; Selected geometric data: Ni(1)-O(1) 2.0264(12), Ni(1)-O(2) 2.0473(13), Ni(1)-N(1) 2.1497(15) Å; O(1)-Ni(1)-O(1A) 174.16(7), O(2)-Ni(1)-N(1) 175.17(6), O(2A)-Ni(1)-N(1A) 1 175.16(6) o. Symmetry transformations used to generate equivalent atoms: 1/4-y,1/4-x,1/4-z; C) Shows the asymmetric unit of 5; Selected geometric data: Ni(1)-O(1) 2.0183(7), Ni(1)-O(2) 2.0112(8), Ni(1)-N(1) 2.1609(9) Å; O(1)-Ni(1)-O(1A) 173.58(4), O(2)-Ni(1)-N(1) 174.84(3), O(2A)-Ni(1)-N(1A) 174.84(3) o. Symmetry transformations used to generate equivalent atoms: 1-x,y,1/2-z; D) Shows the asymmetric unit of 6; Selected geometric data: Ni(1)-O(1) 2.0230(15), Ni(1)-O(2) 2.0678(16), Ni(1)-O(4) 2.0325(16), Ni(1)-O(5) 2.0528(16), Ni(1)-N(1) 2.1572(19), Ni(1)-N(2) 2.1745(19) Å; O(1)-Ni(1)-O(5) 178.50(6), O(2)-Ni(1)-N(1) 177.31(7), O(4)-Ni(1)-N(2) 175.54(7) o ; E) Shows the asymmetric unit of 7; Selected geometric data: Ni(1)-O(1) 2.0186(12), Ni(1)-O(2) 2.0531(12), Ni(1)-N(1) 2.0994(16) Å. Symmetry transformations used to generate equivalent atoms: 1-x,1-y,1-z.

Precursors 1, 2 and 5 decompose in a single step, though this must represent several synchronous processes. For 6 there is a clearly defined separation of decomposition steps, with the initial mass loss (ca. 35% by 260 oC) corresponding to elimination of TMEDA and, presumably from the fused ring, C2H4 (calculated %mass loss for TMEDA + C2H4 = 33.5%).

Both 3 and 4 also show a separation of decomposition processes, which in the latter case (40% mass loss by 200 oC) plausibly equates to loss of TMEDA and the ligand substituent NEt2 (calculated %mass loss = 38.7%). Surprisingly, the TGA of 7 shows a steady decrease in mass over a wide temperature range (50 – 450 oC; residual mass ca. 18%) and only reaches a residue commensurate with NiO (12.4%) at 600 oC. Figure 2. TGA profiles for complexes 1 - 6. Data points were collected every

second at a ramp rate of 5 oC min-1 in a flowing (50 mL min-1) N2 stream.

Thin Film DepositionBased on the TGA, compounds 2 and 5 appear the most promising precursors for atmospheric pressure CVD, of which 2 is the more cost-effective ligand. On this basis, thin films of NiO on ITO-coated glass were grown at atmospheric pressure from precursor 2, with a precursor temperature of 200 oC and a

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FULL PAPERsubstrate temperature of 400 oC. Deposition of NiO on ITO was carried out using a home-built reactor similar to that reported by others[31] under a nitrogen flow See experimental for details. Attempt to perform MOCVD under an oxidising atmosphere (air) resulted in negligible carryover of precursor in to the reaction zone, and significantly higher decomposition in the precursor boat. Under an inert atmosphere of N2, deposition runs of 30 (R1), 45 (R2), 60 (R3), 120 (R4) and 300s (R5) were performed. XPS analysis was carried out on the as deposited thin films R2, R3 and R5, and show surface concentration of nickel is low with relatively concentrations of high carbon. However, as can be seen in figure 3, Ar-etching of the samples reveals that carbon levels drop off rapidly and are localised to the Ni containing layer, presumably a result of inefficient decomposition of the MOCVD precursor.

Table 1. Atomic % of elements in Thin films deposited at for 45s (R2), 60s

(R3) and 300s (R5) pre Ar-etching. Values are expressed as atomic percentages.

Deposition Run

R2 R3 R5*

Element Atomic %

C 30.5% 56.1% 60.8%

O 44.3% 31.2% 25.8

Ni 5.9% 8.8% 5.8

In 17.4% 3.5% 0

Sn 1.8% 0.4% 0

* Si is also found to be present (7.6%)

Initially, the films appear dark in colour but become pale yellow and transparent on annealing in air at 550 oC.

Auger electron spectroscopy of the annealed thin films R1 show that the Ni 2p3/2, 1/2 binding energies (853.1, 870,3 eV) are consistent with those reported for Ni0 (852.7, 870.0 eV),[39] and lower than those expected for NiO (see below). EDX of the annealed films on ITO showed the presence of nickel, which can also be seen in the Auger depth profile at 0.5% energy resolution (Fig. 3A).

At higher resolution (Fig. 4B) both Ni2+ (associated with NiO) and Nio are resolved (the latter presumably being due to incomplete annealing of the metal initially deposited); Ni0, along with NiO and Ni2O3, has previously been observed in films grown from Cp2Ni / O2 precursors.[27b]

The Ni 2p3/2, 1/2 binding energies (as determined by XPS) are 855.2, 873.0 eV, somewhat higher than reported for NiO (853.0, 871.3 eV),[39] but ca. 2eV higher than those in the as-deposited film, consistent with oxidation of Ni0 to Ni2+ on annealing. The approximate composition of the final, post-annealing film was NiO0.89.

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Figure 3. XPS Depth profiles of the thin films R2, R3 and R5.

To obtain a film thick enough for analysis by XRD precursor, 2 was deposited over a 2 hour period onto a glass substrate and subsequently annealed at 550 oC. The resulting film (See supplementary information) had only two strong reflections in the 5o 2 60o at d = 2.403 (222), 2.084 (400) which can be indexed to cubic NiO (PDF-895881).[40]

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FULL PAPER

Figure 4. Auger depth profiles for the film produced after 30 s deposition (R1), at energy resolutions of (A) 0.5% and (B) 0.1%.

Figure 5. Images of films produced from precursor 2: (a) Auger image of R1, bar = 40nm; (b) SEM of R3 (bar = 500nm); (c, d) SEM of R5 (bar = 500nm).

Images of the films grown at 30, 60 and 300s deposition time (R1, R3, R5, respectively; Fig. 5) show the growth of small spherical particles on the ITO surface. Initially, these particles are of the order 10 nm diameter (Fig. 5a) and grow to ca 150 nm

(Fig.5c & d). The film thicknesses have been estimated from the SEM images, Auger and XPS depth profiling (Table 2)

Table 2. NiO film thickness estimates.

Sample Run-Time (s) SEM (nm)[a] Auger (nm) [c],[d] XPS (nm) [b],[d]

ITO Substrate

- 135 - 143 110 - 154

R1 30 ~10 [e] 19 (15)

R2 45 ~12 5

R3 60 10 - 24 [f] 22 (24) 10

R4 120 33 (>55)

R5 300 139 - 167 45

[a] Thickness estimated from the size of selected particles seen on the film surface. [b] Depth until no Ni visible [c] Data relate to 0.5% energy resolution with data for 0.1% resolution in parentheses; based on an etch rate for SiO2. [d] Based on the etch rate of NiCrO [e] From Auger image [f] Typical film thickness ca. 10 nm with larger aggregates appearing on the surface consistent with some localised island growth

However, no well-defined ITO – NiO interface was visible from the Auger profile for films produced by shorter deposition runs is (see Fig. 5a for the case of R1); this poor interface definition was supported by spectroscopic ellipsometry studies (See supplementary information), again supporting the finding that the NiO is strongly diffused into the ITO. The thickness values listed differ because each technique estimates film thickness in a different way; SEM visually picks out peaks and troughs in a surface (large and small surface particles), while Auger and XPS can indicate the depth at which no further nickel is observed. As no available etch rate data was available for NiO, the depths are calculated on the basis of relative etch rates for either SiO2

(Auger) or NiCrO (XPS). Due to these differences the thickness data can be taken only as estimates of the NiO growth with time.

The film thickness (using SEM data) initially increases approximately linearly with time (Fig. 6). Over time, however, there is a visible change in the appearance of the precursor from blue to green and analysis of the crystalline material present at the end of a 2 h run shows that 2 has converted to the tri-nickel species Ni3[MeC(O)CHC(O)OEt)]6 (8), whose structure is shown in Fig. 7. There is an apparent decrease in deposition rate over extended periods of time which can thus be rationalized by the steady conversion of 2 to less volatile 8.

A)

B)

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FULL PAPER

Figure 6. Growth profile of thin films produced from precursor 2.

We assume that over a prolonged period complex 2 loses TMEDA and the resulting Ni[MeC(O)CHC(O)OEt]2 trimerizes in a similar fashion to other nickel -diketonates, typified by [Ni(acac)2]3.[41] The structure of 8, however, more closely resembles that of Ni3[MeC(O)CHC(O)OBut]6

[36] in which all the -diketonate ligands chelate the terminal metals and the overall complex can be formulated as [L3Ni]2[Ni2+], rather than [Ni(acac)2]3 in which all three metals are chelated by the ligands and a formula [NiL2]2[4-NiL2] is more appropriate. As expected from a (L3Ni)2Ni formulation, the terminal Ni-O bonds [Ni(1)-O(2) 2.0222(13) Å] are shorter than the chelate bonds involving the bridging oxygen [Ni(1)-O(1) 2.0360(12) Å], while bonds between oxygen and the central metal atom are longest [Ni(2)-O(1) 2.0764(12) Å].

Figure 7. The asymmetric unit of 8 showing the labeling scheme used; thermal ellipsoids are at the 50% probability level. Selected geometric data: Ni(1)-O(1) 2.0360(12), Ni(1)-O(2) 2.0222(13), Ni(2)-O(1) 2.0764(12) Å; O(1)-Ni(1)-O(2) 91.72(5), O(1)-Ni(1)-O(1a) 79.58(5), O(1)-Ni(1)-O(2a)170.76(5), O(1)-Ni(1)-O(2b) 95.90(5), O(2)-Ni(1)-O(2a) 92.26(5), O(1)-Ni(2)-O(1d) 102.26(5), O(1)-Ni(2)-O(1a) 77.74(5), O(1)-Ni(2)-O(1c) 180.00(8),. Symmetry transformations used to generate equivalent atoms:a -y,x-y,z; by-x,-x,z; c -x,-y,-z; d y,y-x,z; e x-y,x,z.

Work Function Results

Figure 8a shows light transmission spectra of bare ITO and ITO/NiO substrates. We observed reduced absorption in the UV - blue part of the spectrum and in the near-IR for the NiO coated samples in comparison with uncoated ITO. ITO/NiO substrates show higher transmittance than ITO over the range from 400 – 550 nm with a maximum value of ~90% and slightly lower transmittance beyond 550 nm. This suggests that ITO substrates

with APCVD-deposited NiO are suitable for organic photovoltaic (OPV) cells. We found no strong dependence of transmittance on deposition run time within 5-60 s deposition windows; however, thicker films do show more parasitic absorption (data not shown here).

The work function values, as measured by Kelvin Probe in ambient atmosphere, shows that addition of NiO generally raises the work function relative to the ITO control, but not to the values of work function typically cited for NiO (Figure 8b). The work function for thinner NiO film (5 s run time) is found to be 4.83 ± .09 eV which is significantly lower than the reported value. [21]

One reason for observing a lower work function value for our NiO films could be due to measurement in ambient atmosphere, which inevitably contaminates samples with various hydrous species of NiO such as Ni(OH)2, which may result in a vacuum level shift and a decrease in overall work function. [10b]

Furthermore we observed a substantial increase in resistivity with thickness of NiO coating (Figure 8b) which shows that thinner devices are desirable to minimize resistive losses in devices.

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Figure 8. Effect of deposition run time (thickness) of the film, from complex 2, on the optical characterisation of the ITO/NiO films. (a) UV-Vis transmission and (b) electrode work function and resistivity.

Treatment of both naked ITO as well as NiO on ITO substrates (with deposition times of 30s, 45s, 60s 120s and 300s) with an oxygen plasma at 100W for 3 mins results in a significant increase in the work function of the NiO films. Figure 9 shows a plot of work function of both the plasma treated (with plasma) and un-treated (without plasma) substrates. Plasma treatment of naked ITO substrates show a small increase in work function

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FULL PAPER(from 4.49 eV to 4.86 eV). In contrast, treatment of NiO coated ITO sees an average increase of 1.17 eV (from 4.37 to 5.54 eV), with a maximum increase of 1.36eV for the NiO coated ITO substrates with deposition times of 120 and 300s. While plasma treatment shows no observable changes in optical transparency, crystallinity (PXRD) or composition (EDX) of the substrates, whatever surface modification is affected by the plasma treatment, the effects appear to be long-lasting, as can be seen in Figure 10, which shows a plot of work function for both NiO-ITO deposited over 60s (60s-Ni-ITO) with and without plasma treatment alongside the naked ITO (again with and without plasma treatment).

Figure 9. Mean work function of ITO/NiO substrates with and without oxygen plasma treatment. Each substrate is treated with oxygen plasma at 100W for 3 minutes only and immediately measured work function with Scanning Kelvin Probe. Highly oriented pyrolytic graphite (HOPG), was used as a reference, which after cleavage had a work function of 4.48 eV.

Figure 10. Scanning of work function measurement across the small area of substrate with the progression of time (Mins) of ITO and 60s-NiO-ITO with and without plasma treatment. Each point in the plot is the average of 30 points. Here, 60s-NiO-ITO is chosen as a representative sample and compared with bare ITO substrate.

Conclusions

In summary, we present a series of unsymmetrical nickel -diketonate derivatives which we have investigated as potential NiO precursor using APCVD. Of the systems we have developed the ethylacetoacetate derivative, complex 2: By TGA complex 2 displays a single step synchronous decomposition process with an onset of decomposition >100 oC and displays some degree of volatilisation with a 10% mass residue at 400oC

(below that expected for NiO). Resultantly 2 has been taken forward and assessed as a precursor to NiO with deposition run times of 30s, 45s, 60s and 300s, a precursor temperature of 200 oC and a substrate temperature of 400 oC. Un annealed thin films appear dark drown in colour, changing to a transparent yellow on annealing in air at 550 °C for 20 mins. Auger spectroscopy clearly shows that initial deposition results in thin films with of a high proportion of nickel (Ni9O) which the small amount of oxygen (and carbon) is present at the surface of the film. Subsequent annealing reveals a partial conversion of the nickel to nickel oxide, resulting in an empirical composition of NiO0.89. As part of the study treatment of both naked ITO as well as NiO on ITO substrates (with deposition times of 30s, 45s, 60s 120s and 300s) with an oxygen plasma at 100W for 3 mins results in a significant and long lasting, increase in the work function of the NiO films. While the precise origin of this increased work function is unknown, analysis reveals there to no detectable change in surface chemistry. We are currently continuing to examine the potential of these precursors and developing new systems which do not suffer from the dissociation of ligands such as TMEDA, which will be described, along with our observations in future publications.

Experimental Section

General Experimental Details.

All reagents were obtained from commercial sources and used as received without purification; solvents were dried and degassed under an argon atmosphere over activated alumina columns using an Innovative Technology solvent purification system (SPS). Melting points were determined utilizing a Stuart SMP10 Melting Point Apparatus. Elemental analyses were performed by Mr Alan Carver (University of Bath Microanalysis Service) on an Exeter Analytical Inc. CE-440 Elemental Analyzer instrument. Magnetic susceptibilities of neat samples (powders and oils) were obtained using a Sherwood Scientific Magnetic Susceptibility Balance Mk 1. Solution phase magnetic susceptibilities were determined by recording the paramagnetic shift of the CH2Cl2

solution solvent resonance relative to an internal CH2Cl2 capillary via Evans’ NMR method with the aid of a Bruker Avance 400 MHz NMR Spectrometer.

Synthesis of Precursors

[(TMEDA)Ni(MeC(O)CHC(O)OEt)2] (2): A solution of ethyl acetoacetate (5.1 ml, 40 mmol) and N,N,N’,N’-tetramethylethylenediamine (TMEDA; 3.0 ml, 20 mmol) in dichloromethane (35 ml) was combined with an aqueous solution of nickel(II) acetate tetrahydrate (4.98 g, 20 mmol) and KOH (2.24 g, 40 mmol) in deionized water (30 ml), and the resulting biphasic mixture stirred vigorously for a period of 18 hours. The organic layer was separated and the aqueous layer extracted with dichloromethane (3 x 20 ml). The combined organic portions were sequentially washed with deionised water (2 x 20 ml), a saturated brine solution (1 x 20ml) and deionised water (3 x 20 ml). The resultant solution was dried over MgSO4 and evaporated to dryness using a rotary evaporator. The solid residue was dissolved into hot diethylether. On slow evaporation of the solvent a batch of blue prisms formed. The crude crystalline product was recrystallized from hexanes, isolated from the mother liquor by filtration and washed with ice cold diethylether to leave large blue blocks which dried in air. Yield 4.25 g, 49%. m.p. 83-84 ºC. Elemental analysis calcd (%) for C18H34N2O6Ni: C 49.91, H 7.91, N 6.47; found C 50.0, H 7.98, N 6.43. μeff powder (corrected) 2.92 (3.01). μeff

CH2Cl2 solution (corrected) 4.18 (4.25).

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FULL PAPERAlso prepared by the same general method were:

[(TMEDA)Ni(MeC(O)CHC(O)OMe)2] (1): Using methyl acetoacetate in place of ethyl acetoacetate. Blue blocks, yield 73%. m.p. 113-115 ºC Elemental analysis calcd (%) for C16H30N2O6Ni: C 47.4, H 7.46, N 6.91; found: C 47.42, H 7.49, N 6.93. μeff powder (corrected) 3.42 (3.50). μeff

CH2Cl2 solution (corrected) 4.19 (4.25).

[(PMDETA)Ni(MeC(O)CHC(O)OEt)2] (3):

Using pentamethyldiethylenetriamine (PMDETA) in place of TMEDA. Blue viscous oil. Yield 51%. Elemental analysis calcd (%) for C21H41N3O6Ni: C 51.45, H 8.43, N 8.57; found: C 51.26, H 8.62, N 9.20. μeff oil (corrected) 2.76 (2.88). μeff CH2Cl2 solution (corrected) 3.98 (4.07).

[TMEDA)Ni(MeC(O)CHC(O)NEt2)2] (4): Using N,N-diethylacetoacetamide in place of ethyl acetoacetate. Dark green viscous oil, yield 96%. Elemental analysis calcd (%) for C22H44N4O4Ni: C 54.22, H 9.10, N 11.50; found: C 52.2, H 8.97, N 10.79. μeff oil (corrected) 2.58 (2.72). μeff CH2Cl2 solution (corrected) 3.77 (3.86).

[(TMEDA)Ni(MeC(O)CEtC(O)Me)2] (5): Using 3-ethyl-2,4-pentanedione in place of ethyl acetoacetate. Green blocks, yield 66%. m.p. 84-87 ºC. Elemental analysis calcd (%) for C20H38N2O4Ni: C 55.97, H 8.92, N 6.53; found: C 55.80, H 8.99, N 6.45. μeff powder (corrected) 4.41 (4.47). μeff

CH2Cl2 solution (corrected) 4.19 (4.26).

[(TMEDA)Ni(MeC(O)C(CH2CH2O)C(O))2] (6): Using α-acetyl-γ-butyrolactone in place of ethyl acetoacetate. Pale blue crystals, yield 44%. m.p. 191-194 ºC (decomp.). Elemental analysis calcd (%) for C18H30N2O6Ni: C 50.38, H 7.05, N 6.53; found: C 50.31, H 7.11, N 6.53. μeff powder (corrected) 4.85 (4.90). μeff CH2Cl2 solution (corrected) 4.26 (4.32).

[(C5H5N)2Ni(MeC(O)CHC(O)OEt)2] (7): A solution of ethyl acetoacetate (5.1 ml, 40 mmol) and pyridine (3.2 ml, 40 mmol) in dichloromethane (35 ml) was combined with an aqueous solution of nickel(II) acetate tetrahydrate (4.98 g, 20 mmol) and KOH (2.24 g, 40 mmol) in deionised water (30 ml), the flask covered to limit loss of pyridine and the resulting biphasic mixture stirred vigorously for a period of 18 hours. Large blocks formed on subsequent standing for a period of 48 hours. Dichloromethane (50 ml) was added and the crystals redissolved into the organic phase. The separated organic layer was allowed to evaporate slowly to yield a crop of blue prisms, which were isolated and dried in air. Yield 7.84 g, 83%. m.p. 148-164 ºC (decomp.). Elemental analysis calcd (%) for C22H28N2O6Ni: C 55.61, H 5.94, N 5.90; found: C 55.46, H 5.95, N 5.90. μeff powder (corrected) 3.06 (3.15). μeff CH2Cl2 solution (corrected) 4.12 (4.19).

[Ni3(MeC(O)CHC(O)OEt)6] (8): 8 was obtained from the precursor container at the end of a 120 min CVD deposition run. Elemental analysis calcd (%) for C36H54O18Ni3: C 45.47, H 5.72; found: C 44.49, H 5.59.

X-Ray Crystallography

Experimental details relating to the single-crystal X-ray crystallographic studies are summarized in Table S1 (see Supporting Information). For all structures, data were collected on a Nonius Kappa CCD diffractometer at 150(2) K using Mo-K radiation ( = 0.71073 Å). Structure solution was followed by full-matrix least squares refinement and was performed using the WinGX-1.70 suite of programmes.[42]

In complex 1: there are two independent molecules in the asymmetric unit; 2, 5: the asymmetric unit comprises one half of the molecule, the remainder generated by a two-fold axis running through the nickel and bisecting the C-C bond of the TMEDA ligand; 7: the asymmetric unit consists of one half of the molecule, the remainder generated by an

inversion centre at the metal. 8: The asymmetric unit consists of one -diketonate ligand, one nickel of occupancy 0.333 [Ni(1)] and one nickel of occupancy 0.167 [Ni(2)]. The remainder of the tri-metallic molecule is generated by a three-fold axis passing through Ni(1), Ni(2) and an inversion centre at Ni(2).

Thermogravimetric Analysis was performed on a Perkin Elmer TGA4000 analyser. Data points were collected every second at a ramp rate of 5 oC min-1 in a flowing (50 mL min-1) N2 stream.

Film Deposition by Atmospheric Pressure CVD

Deposition of NiO on ITO was carried out using a home-built reactor similar to that reported by others.[31] In a typical experiment, dry N2 carrier gas was passed at a rate of approximately 5 dm3/min through a mineral oil bubbler (3-4 bubbles/sec) and into a quartz tube (o.d. 21 mm, i.d. 19 mm, length 220 mm) via a glass delivery tube fitted into a drilled natural rubber bung. The quartz tube was housed in a Carbolite MTF 10/25/130 tube furnace set to maintain a temperature of 200 ºC with the open end allowed to rest onto an Infrared Salamander (HTE 500 W/230 V) ceramic heater. The substrate was then introduced into the open end of the quartz tube and the ceramic heater set to maintain a temperature of 400 ºC. The apparatus was allowed to equilibrate under a flow of N2 gas. A small quartz sample boat was loaded evenly with approximately 0.10 g of the precursor under study. The carrier gas flow was halted temporarily in order to allow the positioning of the sample boat in the region of the quartz tube encompassed within the tube furnace. The carrier gas flow was re-initiated and the deposition commenced simultaneously upon the melting of the precursor within the quartz boat. Deposition runs of 30 (R1), 45 (R2), 60 (R3), 120 (R4) and 300s (R5) were carried out.

The quartz tube was disconnected from the carrier gas feed by removal of the rubber bung and the tube rapidly removed from the tube furnace to permit the retrieval of the quartz sample boat at the end of the deposition run. The ceramic heater was allowed to cool to ambient temperature and the coated substrate repositioned to the middle of the quartz tube. The coated substrate was then heated to 550 ºC in the tube furnace and annealed at this temperature for a period of 20 min. The tube furnace was switched off and the annealed substrate allowed to cool slowly to room temperature.

Optical and electronic characterisation of APCVD-deposited NiO thin films.

Auger electron spectrsocopy measurements were perfomred at JEOL Ltd, Japan, on a JAMP-9500F Auger electron spectrometer, utilizing a field emission electron gun, ~20 nm min. with a spot size for Auger analysis of 3-30 keV primary energy. Ellipsometry was peformed using a SE-850 UV / VIS / NIR Spectroscopic Ellipsometer, by SENTECH Instruments LtD, Berlin, Germany. Film thicknesses were measured by X-ray photoelectron spectrsocopy (XPS); measurements were perfomred on a Kratos Axis Ultra-DLD photoelectron spectrometer, utilizing monochromatic Al K radiation (photon energy 1486.6 eV). The instrument was pre-calibrated using pure gold and copper samples. Samples were sputtered for a pre-determined time over a 4 mm wide area using 4 kV argon ions using a minibeam I ion source. Spectra were collected at pass energies of 80 and 160 eV for high resolution and survey scans respectively, with the 100 m apperature in place to focus on the centre of the etch pit. X-Ray powder diffraction patterns were recorded on a Bruker D8 Powder Diffractometer. FE-SEM analysis of the films was undertaken on a Philips XL30 FEG SEM with Pheonix EDX capability. In the absence of a reference etch rate for NiO, reference etch rates for NiCrO (XPS) and SiO2 (Auger) respectively, were used to give an approximate film thickness and etching depths for the NiO thin film deposited.

The sheet resistivity of the samples were measured experimentally using a home-made standard “four point probe” system with a Keithley 2400

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FULL PAPERcurrent source and 2000 nanovoltmeter. A current is passed through the outer probes and induces a voltage in the inner voltage probes. Using the voltage and currents readings from the probe the typical sheet resistivity of the sample is calculated.

A vibrating Kelvin probe (Besocke Delta Phi GmbH, Jülich, Germany) technique was used to measure the work function at the surface of the NiO coated and uncoated ITO glass substrates. In order to estimate absolute work functions of the measured samples, freshly cleaved highly oriented pyrolitic graphite (HOPG) was used as a reference. The absolute values of work function were obtained assuming the HOPG work functions to have the value of 4.48 eV, independent of ambient atmosphere that has been reported previously.[43]

Acknowledgements

We thank the EPSRC for financial support (Energy Grand Challenge grant: EP/F056494/1, EP/F056710/1 and EP/F056648/2), K Tsutsumi and A Tanaka (JEOL) for recording the Auger spectra, and M Baines and L-M Deslandes (NSG Pilkington) for recording SEM, XPS spectra. The authors also thank Dr S. M.Tuladhar (Imperial College London) for assistance with workfunction measurements.

Keywords: Nickel Oxide • ITO • CVD • Precursor • p-type

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Page 11: ((Title)) · Web viewtandem dye sensitised solar cells and as hole collector in BHJ solar cells.[16] More recently NiO has also found application in perovskite heterojunction solar

FULL PAPER

FULL PAPER

We described here the synthesis, isolation and characterisation of a series of Nickel(II) β- diketonate precursors and their use in the atmospheric pressure chemical-vapour deposition (APCVD) of of very thin films of NiO on ITO at 400 °C. Auger spectroscopy reveals the as deposited films are rich in Ni metal which is partially transformed to NiO on annealing at 550 oC in air, and show a significant modification of the work function of the ITO.

*one or two words that highlight the emphasis of the paper or the field of the study

[a] S. D. Cosham, S. P. Richards, M. S. Hill, * A. L. Johnson*Molloy

Department of Chemistry, University of Bath,Bath, BA2 7AY (UK)E-mail: [email protected]

[b] T. D. ManningDepartment of ChemistryUniversity of LiverpoolLiverpool L69 7ZF, (UK)

Supporting information for this article is given via a link at the end of the document.