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. 2022 Aug 31;2(6):527–538. doi: 10.1021/acsnanoscienceau.2c00027

High-Performance Indium–Tin Oxide (ITO) Electrode Enabled by a Counteranion-Free Metal–Polymer Complex

Gyeongbae Park 1, Dongbeom Kim 1, Geonwoo Kim 1, Unyong Jeong 1,*
PMCID: PMC10125366  PMID: 37101853

Abstract

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Although multicomponent inorganic thin films (metal-oxides, -carbides, -nitrides, and -chalcogenides) have been synthesized by polymer-assisted deposition (PAD), synthesis of high-performance transparent conducting oxides (TCOs) has been rarely reported. TCO requires (i) removal of impurities, (ii) high-density oxide film, (iii) homogeneity in crystal structures and film morphology, and (iv) controllable elemental doping. This study performs a systematic investigation on preparation of stable multicomponent metal–polymer complex solutions by removing the counteranions in the solution. This study also proposes accurate acid–base titration for each metal species in order to minimize the amount of PEI, thus maximizing the density of the film. As a representative TCO, Sn-doped In2O3 (ITO) films have been achieved. The ITO film has an excellent sheet resistance (24.5 Ω/sq) at 93% optical transparency, with a figure of merit of 2.1 × 10–2 Ω–1, which is comparable to the best.

Keywords: metal−polymer complex, counter anion-free, Sn-doped In2O3 electrode, ITO, acid−base titration, high-density oxide film

Introduction

Polymer-assisted deposition (PAD) has been first introduced in 2004 to synthesize high-quality inorganic thin films.1 A solution of metal–polymer complex is coated on a substrate, and the metal is oxidized to form a crystalline metal-oxide thin film during the thermal treatment. The polymer is used for coatability of the metal precursors, and it is decomposed and removed during the thermal treatment. Stable complexation of metal cations has been readily achieved by chelating with small molecules and effectively prevented the formation of metal hydroxides. Ethylenediaminetetraacetic acid (EDTA) has been extensively used for the metal complex anion in the form of [M-EDTA](4–n)–, where n is valence of the metal ion.2−7 Polyethyleneimine (PEI) is commonly used to bind to the metal complex anions.8,9 PEI can be protonated to give positive charges and make the solution basic, [−CH2CH2–NH−]n + H2O Inline graphic [−CH2CH2–NH2+−]n + OH–; hence, the electrostatic attraction between [M-EDTA](4–n)– and the amines results in homogeneous metal–polymer complexes.10−12 The advantages of the PAD approach, compared to the sol–gel process, are the precise control of the film thickness, easy achievement of homogeneous doping concentration in large area, and the use of the conventional coating technologies.13,14 A variety of inorganic thin films have been synthesized by the PAD process, including metal oxides,15−17 metal carbides,18−20 metal nitrides,21−24 and metal chalcogenides13,25 for the potential uses in magnetics,15,26,27 optoelectronics,14,28 and superconductors.21,24

Transparent flexible conductors have been produced successfully through metal nanowires,29−31 monolayer graphene,32,33 and metal grids,34,35 composite of metal and amorphous carbon films,36,37 and printed metal lines.38 With elastic substrates, those electrodes could also be used as transparent stretchable electrodes.29,31,32,37 Despite such remarkable advances in the transparent electrodes, transparent conducting oxides (TCOs) are still the primary interest in the industrial fabrication in rigid and flexible devices.39−43 This is partly because TCO is an intrinsically conductive material rather than relying on the percolation between the conductive components, and also because the appropriate hardness with a high degree of bendability. The processing compatibility to the contemporary lithography-based production lines is additional reason for the continuous use of TCO.

Even though the PAD process can readily produce TCO, only few literature using the PAD process have been reported so far.44−47 Vishwanath et al. synthesized Mo-, Ti-, and W-doped In2O3 thin films by PAD with varying dopant concentrations for the use as transparent conducting oxide electrodes.44,45,47 The Mo-doped (1.5 at. %) In2O3 with a thickness of 130 nm and the Ti-doped (3 at. %) In2O3 with a thickness of 270 nm exhibited 279 and 110 Ω/sq, respectively. Even though harsh H2 plasma treatment increased oxygen vacancy and activated Mo dopant,46 the sheet resistance of 1.5 at. % Mo-doped In2O3 remained at 165 Ω/sq, which is inferior to that processed by the conventional vapor deposition techniques. PAD-processed W-doped (3 at. %) In2O3 thin films with the thickness of 230 nm showed 38 Ω/sq and optical transmittance over 85% after harsh reduction under H2/Ar at high-temperature annealing (700 °C). Su et al. prepared Al-doped (9 at. %) ZnO thin films by PAD, exhibiting 7600 Ω/sq for a thickness of 150 nm.48

High-performance TCO requires several conditions: (i) removal of impurities, (ii) high-density oxide film, (iii) homogeneity in crystal structures and film morphology, and (iv) controllable elemental doping.49 The impurities in the PAD process are caused by the carbon residue from the polymer and also by the elemental doping from counteranions bound to the cationic sites of the polymer. Although thermal treatment to completely remove the carbon traces has been well developed, the contamination by the counteranions has not been investigated. Since the counteranions compete with the [M-EDTA](4–n)– anions to take up the cationic sites in the polymer chains (see the scheme in Figure 1a), the unnecessary counteranions (Cl– or NO3–) coming from the metal precursors should be removed in the metal–polymer complex solution. Although Amicon ultrafiltration has been used to separate such counteranions,20−22 complete separation was infeasible.50,51 Branched PEI consists of primary, secondary, and tertiary amines in a ratio of about 1:2:1. The site binding model considering the interactions between ionized sites explained that all primary amines and half of the secondary amines are protonated at high pH (8 < pH < 10), most of the secondary amines are protonated at pH = 4, and the tertiary amines are rarely protonated.36 Therefore, the degree of protonation at pH = 3, 5, and 8 are approximately 0.8, 0.6, and 0.4, respectively (see the scheme in Figure 1b, black).36−38 The fraction of the [M-EDTA](4–n)– anions varies with pH and metal ion species.52−55 The metal–EDTA complex is not formed at low pH due to the limited solubility of H4EDTA. The metal ion and EDTA create a complex in a neutral form of H(4–n)[M-EDTA], which is not useful for interaction with protonated PEI. The [M-EDTA](4–n)– anions are formed as pH increases further (Figure 1b, red). However, high-density oxide thin films can be achieved by using minimum amount of PEI, maximizing the volumetric concentration of the metal elements in the printed film. Addition of an equivalent amount of PEI (CPEI = Ceq) to take up all the [M-EDTA](4–n)– in the solution is the optimal condition (see the scheme in Figure 1c), ensuring the polymer use of minimum amount but enough for metal elements. When CPEI < Ceq at the low PEI concentration, the complex solution is in an acidic condition and a large fraction of metal–EDTA exists in the neutral form of H(4–n)[M-EDTA]. The unbound neutral complex is phase-separated during the coating process, resulting in an inhomogeneous oxide film after thermal treatment. When CPEI > Ceq, the excess amount of PEI generates pores in the resultant oxide film during the thermal treatment, hence decreasing the material density. When multiple elements are involved in the oxide film, the abovementioned conditions should be met for both the metal elements; therefore, precise titrations for controlled elemental fractions are required.

Figure 1.

Figure 1

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(a) Scheme of the interaction between [M-EDTA] and PEI in the solution without (left) and with (right) counteranions. (b) Schematic plot of degree of protonation of PEI (black) and fraction of [M-EDTA] (red) versus solution pH. (c) Schematic drawing of the precursor solution, describing the effect of relative PEI concentration (CPEI) versus the equivalent concentration (Ceq) on the spin-coated precursor film. When CPEI < Ceq, the neutral [M-EDTA] phase separates. When CPEI > Ceq, excess PEI is used, decreasing the density of metal oxide. Green, red, and pink circles, and black lines represent [M-EDTA], counteranion, proton, and PEI, respectively.

Herein, we conducted a systematic study to prepare the metal–polymer complex solution in which no counteranions exist and two metal elements are included stably for controllable doping concentration. In addition, we propose the accurate acid–base titration for each metal species to optimize the PEI concentration for homogeneous high-density oxide film formation. As an example of metal-oxide film, we synthesized Sn-doped In2O3 (ITO) films. The ITO film has a low sheet resistance (24.5 Ω/sq) at 93% optical transparency, with a figure of merit (FOM) of 2.1 × 10–2 Ω–1, which is comparable to the best industrial standard and the best among the polycrystalline ITO thin films obtained through the solution-based processes.

Experimental Section

Materials

The chemicals used in this study were indium nitrate hydrate (In(NO3)3·xH2O, 99.99%, Sigma-Aldrich), tin chloride dihydrate (SnCl2·2H2O, ≥98%, Sigma-Aldrich), ethylenediaminetetraacetic acid (H4EDTA, ≥99%, Sigma-Aldrich), polyethylenimine (PEI, branched, Mw = ∼25,000, Sigma-Aldrich), and ammonia solution (25.0–30.0%, Samchun). Deionized water (DI water) was obtained using an 18.2 MΩ (VIVAGEN EXL3) system.

Preparation of HIn(H2O)EDTA

HIn(H2O)EDTA was synthesized by reacting In(NO3)3·xH2O and (NH4)2H2EDTA. The (NH4)2H2EDTA aqueous solution was prepared by dissolving H4EDTA (2.9 g, 10 mmol) in ammonia solution (2 M, 10 mL), followed by filtration to remove residual H4EDTA. The (NH4)2H2EDTA aqueous solution (1 M, 10 mL) was added to In(NO3)3·xH2O (1 M, 10 mL) and heated to 80 °C. After 60 min of reaction, the solution was cooled to room temperature and vacuum-filtered. The resulting white crystal of HIn(H2O)EDTA was dried in vacuum.

Preparation of Sn2EDTA·2H2O

Sn2EDTA·2H2O was prepared as a white crystal by reacting SnCl2·2H2O and H4EDTA. 4.50 g of solid SnCl2·2H2O (10 mmol) and 1.46 g of H4EDTA (5 mmol) were dissolved together in 100 mL of DI water with vigorous stirring. A white precipitate of Sn2EDTA·2H2O was immediately formed, and the reaction was completed in 30 min. The white crystal was filtered and washed with DI water and then dried in vacuum.

Preparation of [In-EDTA]-PEI Precursor Solution (0.75 M)

422 mg (1 mmol) of HIn(H2O)EDTA was dispersed in 0.93 mL of DI Water. When PEI (0.4 mL, 5 M) was added to the solution, HIn(H2O)EDTA began to dissolve and made a clear solution of [In-EDTA]-PEI.

Preparation of [Sn-EDTA]-PEI Precursor Solution (0.75 M)

280 mg (0.5 mmol) of Sn2EDTA·2H2O and 146 mg (0.5 mmol) of additional H4EDTA were dispersed in 0.53 mL of DI water. The additional H4EDTA was introduced to match the stoichiometry of Sn and EDTA because EDTA was deficient for the formation of [Sn-EDTA]-PEI at the condition of pH = 5. The solution was heated to 60 °C. PEI (0.8 mL, 5 M) was slowly added to the heated solution under magnetic stirring. The solution became optically clear, indicating the formation of [Sn-EDTA]-PEI.

Titration of the Metal–Polymer Precursors

211 mg (0.5 mmol) of HIn(H2O)EDTA was dispersed in 5 mL of DI water. A mixture of 140 mg (0.25 mmol) of Sn2EDTA·2H2O and 73 mg (0.25 mmol) of H4EDTA was separately dispersed in 5 mL of DI water. 40 μL of 1 M aqueous base solution of NaOH, NH4OH, and PEI was added in each solution. The solution became transparent after the base addition. The acid–base reaction was maintained for 5 min for complete reaction, and then, the pH was measured.

Formation of Sn-Doped In2O3 Thin Films by PAD

Alkaline earth boroaluminosilicate glass substrate (Corning 1737) was cleaned sequentially using acetone, isopropanol, and deionized water. The precursor solution was spin-coated onto the glass substrate at 3000 rpm for 30 s, followed by thermal annealing at 600 °C for 1 h in the air. One procedure of the coating and thermal annealing led to an ITO film of 80 nm. The procedure was repeated to tune the thickness of the ITO film. Three cycles were applied to achieve the desired thickness (240 nm). The oxide film was further annealed at 200 °C for 3 h in the environment of a H2/Ar mixture gas (v/v = 5/95). The ramping rate was 5 °C/min.

Characterization

pH was measured using a pH meter (Orion 3-Star, Thermo Scientific). Oxidative decomposition of complex precursors was measured by thermogravimetric analysis (TGA, TA instrument, Q50). Surface topology and roughness of the samples were investigated with atomic force microscopy (AFM, BRUKER Nanoscope V). X-ray diffraction (XRD, RIGAKU D/MAX-2500/PC) was used to analyze the crystallographic structures. Thickness and elemental distribution of the samples were analyzed with a field emission scanning electron microscope (FE-SEM, JSM 7800F PRIME, JEOL.LTD). The sample was mounted at the center of a Hall measurement system (HMS-3000, Ecopia) with a 0.54 T magnet. The electrical signals from Hall measurement were collected using a Keithley 4200-SCS and a probe station (MS-TECH, Korea). The transmittance of the samples was measured by a UV–Vis Spectrophotometer (S-3100; SCINCO Co. LTD.) Chemical composition and the electronic state of each sample were estimated using X-ray photoemission spectroscopy (XPS, VG SCIENTIFIC ESCALAB 250).

Results and Discussion

Preparation of Counteranion-Free EDTA Precursor Solutions

Counteranion-free EDTA complexes of In and Sn were prepared in the form of HIn(H2O)EDTA56 and Sn2EDTA·2H2O57 on the basis of the following reactions.

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To obtain HIn(H2O)EDTA, an (NH4)2H2EDTA aqueous solution was prepared by dissolving EDTA in ammonia solution and the precipitate was filtered to remove residual EDTA. The (NH4)2H2EDTA aqueous solution was added to an aqueous solution of In(NO3)3·xH2O. After heating at 80 °C for 1 h, the solution was cooled to room temperature and the HIn(H2O)EDTA was vacuum-filtered in a white crystal (Figure 2a). To obtain Sn2EDTA·2H2O, SnCl2·2H2O and H4EDTA were dissolved together in DI water and the white crystal precipitate of Sn2EDTA·2H2O was filtered and washed with DI water (Figure 2b).

Figure 2.

Figure 2

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(a,b) Photographs showing the crystal precipitates of (a) HIn(H2O)EDTA and (b) Sn2EDTA·2H2O on square weighing dish (40 mm × 40 mm). (c) FT-IR spectra of H4EDTA, HIn(H2O)EDTA, and Sn2EDTA·2H2O.

Complexation of HIn(H2O)EDTA and Sn2EDTA·2H2O without containing other anions was confirmed by FT-IR analysis (Figure 2c). Table 1 compares the characteristic peaks of H4EDTA, H[In(H2O)EDTA, and Sn2EDTA·2H2O on the basis of previous reports (Table 1).39,58−61 The COO–H band in H4EDTA observed at 3015 and 1690 cm–1 disappeared in the HIn(H2O)EDTA and Sn2EDTA·2H2O because of deprotonation and coordination of H4EDTA to metal ions. The C–H stretching peak of alkane shifted from 2990 to 2955 cm–1 for HIn(H2O)EDTA and to 2935 cm–1 in Sn2EDTA·2H2O. The shift indicates that the COO– groups of EDTA were attached to the metal ions.60 The C=O stretching band from the carboxylate group (COO–) is located in 1600–1700 cm–1. The peak located at 1635 cm–1 in H4EDTA was downshifted and split to 1610 and 1580 cm–1 in HIn(H2O)EDTA and to 1590 and 1555 cm–1 in Sn2EDTA·2H2O. With this C=O stretching band shift, bonding nature can be estimated.59,60 The downshift indicates that metal ions were ionically bonded to the carboxylate groups in EDTA. The peak would upshift if the covalent bonding took place, as observed in the covalently bonded Co(III)–EDTA complex.62 Additionally, the C–N bands which were not observed in H4EDTA appeared after complexation (1095 cm–1 in HIn(H2O)EDTA and 1085 cm–1 in Sn2EDTA·2H2O). The FT-IR analysis confirmed that stable metal–EDTA complexes were prepared.

Table 1. Characteristic FT-IR Peaks of H4EDTA, HIn(H2O)EDTA, and Sn2EDTA·2H2O.

functional group H4EDTA HIn(H2O)EDTA   Sn2EDTA·2H2O  
COO–H 3015        
C–H 2990 2955   2935  
COO–H 1690        
COO– 1635 1610 1580 1590 1555
COO– 1390 1385   1400  
COO– 1315 1290   1300  
C–N   1095   1085  
COO– 965 915   930  
COO– 870 880   850  
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Potentiometric Titration for the Equivalent PEI Concentration (Ceq)

Proton dissociation of HIn(H2O)EDTA and Sn2EDTA·2H2O/H4EDTA is pH-dependent, so that titration is required to find the condition for equivalent amount of PEI for stable metal–polymer complexation. Photographs of aqueous solutions of HIn(H2O)EDTA and Sn2EDTA·2H2O/H4EDTA with varying amounts of PEI are shown in Figure 3a,b, respectively. The M-EDTA complexes were not soluble in water due to the limited solubility. The saturated solution exhibited pH = 1.2 for HIn(H2O)EDTA and pH = 1.68 for Sn2EDTA·2H2O/H4EDTA. During addition of PEI aqueous solution (5 M), the white powders were completely dissolved to become transparent solutions when pH was increased to 2.1 (NPEI/NIn = 1.25) for HIn(H2O)EDTA and 2.4 (NPEI/NSn = 2.5) for Sn2EDTA·2H2O/H4EDTA. The molar concentration of PEI was calculated based on the repeating unit of PEI with Mw = 43.04 g mol–1. It is notable that we introduced additional H4EDTA (146 mg, 0.5 mmol) in the dispersion of Sn2EDTA·2H2O (280 mg, 0.5 mmol). It was because white Sn(OH)2 precipitates were formed during the addition of PEI in Sn2EDTA·2H2O/H4EDTA and they were immediately complexed to form [Sn-EDTA]-PEI in the presence of the additional H4EDTA as following equations.

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Figure 3.

Figure 3

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(a,b) Photographs showing the aqueous solutions of (a) HIn(H2O)EDTA and (b) Sn2EDTA·2H2O/H4EDTA at various amounts of PEI; (a) NPEI/NIn = 0, 0.63, 1.25, 2, and 4, (b) NPEI/NSn = 0, 1.3, 2.5, 4, and 8 from left to right. (c,d) Potentiometric titration curves of (c) HIn(H2O)EDTA and (d) Sn2EDTA·2H2O/H4EDTA. Titration was performed with NaOH (black), NH4OH (red), and PEI (blue). Green lines represent the titration curves of (c) InCl3 and (d) SnCl2 with equimolar amount of H4EDTA by PEI.

To accelerate the reaction 4, reaction was carried out at an elevated temperature (60 °C).

Potentiometric titration was conducted by aqueous solutions (1 M) of NaOH, NH4OH, and PEI for HIn(H2O)EDTA (Figure 3c). There were two equivalence points in the HIn(H2O)EDTA titration curve when using a strong base like NaOH. The first equivalence point indicates the deprotonation to form [In(H2O)EDTA]−, and the second indicates deprotonation from the coordinating water to create [In(OH)EDTA]2–.63 There appeared only one equivalence point when using a weak base (NH4OH) because the proton of the coordinating water is not dissociated. Owing to the weak polyelectrolytic characteristic of PEI, there was no sharp increase in the titration curve. The equivalence pH for NaOH and NH4OH was obviously about 4, but it was difficult to pinpoint the exact equivalence point for PEI. The same solutions were used for titration of Sn2EDTA·2H2O (Figure 3d). The titration curves for the small molecules were sharp, showing an equivalence pH of about 7 for NaOH and about 5 for NH4OH. The titration curve in NaOH also showed only one equivalent point because the water molecules are not directly coordinated to Sn in Sn2EDTA·2H2O.64 Similar to HIn(H2O)EDTA, the titration curve with PEI showed the gradual increase of pH without an apparent equivalent point. When two metal complexes were mixed, the pH condition should be optimized considering the stability of the metal–polymer complex in the co-existing solution and least amount of PEI used. We set the equivalent point as pH = 5, at which NPEI/NIn = 2 and NPEI/NSn = 4. The pH condition allowed us to use only a small amount of PEI but accommodate an effective amount of [M-EDTA]. It is noteworthy that the degree of protonation in PEI reached over 0.8 at pH < 3, 0.6 at pH = 5, and 0.4 at pH = 8, which was sufficient to compensate the negative charges from [M-EDTA].65

Comparatively, the titration in the presence of the counteranions from the metal salts was considerably inefficient than the case in the absence of the counteranions. It is because PEI is required to dissociate one proton from HIn(H2O)EDTA and two protons from Sn2EDTA·2H2O/H4EDTA in the absence of the counteranions, whereas it should dissociate four protons from H4EDTA in the presence of the counteranions. The titration curves of InCl3 and SnCl2 with H4EDTA by PEI are shown as green lines in Figure 3c,d. At NPEI/NIn = 2, pH of the solution was still very low (pH = 1.0). The equivalent pH = 5 could be reached when NPEI/NIn = 6.5, which required three times larger amount of PEI than the case in the absence of the counteranions. A similar tendency was found for the SnCl2 titration. At NPEI/NSn = 4, pH of the solution was 2.1 and the equivalent pH was arrived when NPEI/NIn = 6.5. It was obvious that the counteranions competed with [M-EDTA] complexes and requested more PEI for accommodating the same amount of metal elements in the solution without those counteranions. The use of such unnecessarily large amount of PEI in the complex solution caused low-density metal-oxide thin films, which led to poor electrical performance (it will be discussed later).

Characterization of the [M-EDTA]-PEI Complexes

For FT-IR and TGA analyses, the [M-EDTA]-PEI powders were obtained separately for In and Sn by drying the [M-EDTA]-PEI solution at pH = 5 at room temperature under vacuum for 24 h. In Figure 4a, the FT-IR spectrum of pure PEI is compared with the [M-EDTA]-PEI complexes. The representative peaks of PEI are designated as follows: 3355 cm–1 (primary amine, N–H stretching), 3275 cm–1 (secondary amine, N–H stretching), 2920 cm–1 and 2800 cm–1 (C–H stretching), 1590 cm–1 (N–H bending), and 1460 cm–1 (CH2 bending). The N–H stretching bands (3355, 3275 cm–1) disappeared in both complexes. Broad strong absorption bands centered at 2940 cm–1 were developed which correspond to amine salt peaks, indicating strong electrostatic attraction in the [M-EDTA]-PEI complexes. The peaks from [M-EDTA] including most the prominent C=O stretching at 1580 cm–1 for [In-EDTA] and 1555 cm–1 for [Sn-EDTA] maintained the same in the [M-EDTA]-PEI complexes, which infers that PEI did not disturb the [M-EDTA] complexes. On the basis of the FT-IR results, the complexation of the [M-EDTA]-PEI was depicted like Figure 4b. Metal ions are ionically bound to EDTA to form [M-EDTA] complex and then protonated PEI by making amine salt in the form of [M-EDTA]-PEI without altering the [M-EDTA] bindings. This result confirmed the stable complexation between PEI, metal ions, and EDTA, ensuring atomically homogeneous doping.

Figure 4.

Figure 4

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(a) FT-IR spectra of PEI, [In-EDTA]-PEI, and [Sn-EDTA]-PEI complexes. (b) Schematic illustration of [In-EDTA]-PEI and [Sn-EDTA]-PEI complexes.

TGA analysis for the [M-EDTA]-PEI complexes is exhibited in Figure 5. Dehydration of [In-EDTA]-PEI and [Sn-EDTA]-PEI was completed at 200 °C. EDTA in [In-EDTA]-PEI was rapidly decomposed in the range of 300–350 °C and decomposition of PEI was completed at 420 °C. EDTA in [Sn-EDTA]-PEI was gradually decomposed in the range of 200–400 °C in a multiple-step process. It was reported that decomposition of [M-EDTA] varies depending on the metal species because carboxylic acids can be decomposed in a single step or multiple steps.66,67 Decomposition and oxidation were completed below 500 °C in [Sn-EDTA]-PEI. The weight fractions of the residual In2O3 and SnO2 were 26.3 and 24.5%, respectively. Those values are in excellent agreement with the predicted values of 27.3% for In2O3 and 25.9% for SnO2, respectively, calculated from the relative concentration of the metal species in the solution. It is notable that there was no further weight change even at above 600 °C, indicating that oxidation of Sn(II) to Sn(IV) occurred simultaneously during the thermal treatment. This result proves the previous report, in which [Sn-EDTA] complex underwent oxidation and decomposition took place at the same time, whereas Sn(II)-oxalate and Sn(II)–Na8–inositol–hexaphosphate were decomposed to Sn(II)O and then oxidized to Sn(IV)O2 over 500 °C.68

Figure 5.

Figure 5

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TGA analysis of [In-EDTA]-PEI and [Sn-EDTA]-PEI complexes.

Morphology and Density of the In2O3 Thin Films Depending on PEI Concentration

Morphologies of the printed film and the ITO film were investigated with respect to the amount of PEI. On the basis of the equivalent point (pH = 5, NPEI/NIn = 2), we compared three cases with an optical microscope (OM); case (1) NPEI/NIn = 1.25 (pH = 2.1) for CPEI < Ceq (Figure 6a), case (2) NPEI/NIn = 2.0 (pH = 5) for CPEI = Ceq (Figure 6b), and case (3) NPEI/NIn = 4.0 (pH = 8) for CPEI > Ceq (Figure 6c). To compare the effect of counteranions, NPEI/NIn = 6.5 (pH = 5) with the counteranion (Cl–) was tested for CPEI = Ceq (Figure 6d). The condition of case (1) NPEI/NIn = 1.25 was chosen because it was the minimum PEI that could completely dissolve [In-EDTA]-PEI at room temperature. The solutions were spin-coated at 3000 rpm on a glass substrate. When NPEI/NIn = 1.25, the cracks with an average width of 300–600 nm were found in the spin-coated film (Figure 6a), which was caused by crystallization of the unbound H(4–n)[In-EDTA]. After thermal annealing, the cracks were widened up to 800–3000 nm in the metal oxide due to the volume shrinkage during thermal decomposition. When CPEI = Ceq or CPEI > Ceq, no crack was observed in both the spin-coated films and the In2O3 films (Figure 6b–d).

Figure 6.

Figure 6

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Structural characterization of In2O3 thin films with varying amounts of PEI. (a–d) OM images of the precursor films with different amounts of PEI after spin-coating and annealing (scale bar: 20 μm); NPEI/NIn = 1.25 (a), 2 (b), 4 (c), and 6.5 (d). (e) XRR spectra, and (f) density variation plot for NPEI/NIn = 2, 4, 6.5 obtained after thermal annealing.

Densities of the In2O3 thin films were obtained by fitting the X-ray reflectometry curves (Figure 6e). The film thickness (t) was fixed at t = 25 nm for all the specimens. The XRR result for the case 1 specimen (NPEI/NIn = 1.25, CPEI < Ceq) was omitted because its surface was very rough due to the cracks, so that the XRR measurement was not applicable. The critical angle (θc) for total external reflection decreased with the increasing amount of PEI. The density of the ITO films varied with PEI concentration; 5.4 g cm–3 for case 2 (NPEI/NIn = 2, CPEI = Ceq), 4.3 g/cm3 for case 3 (NPEI/NIn = 4, CPEI > Ceq), and 3.5 g/cm3 for case 4 (NPEI/NIn = 6.5 with Cl– counteranion). Figure 6f shows the correlation between the density and NPEI/NIn of the In2O3 films.

Control of Dopant (Sn) Concentration in the ITO Film

One of the advantages of the solution process is the easy homogeneous control of dopant concentration in large area. Figure 7a shows the correlation between the dopant concentration in the ITO film [CSn,film = NSn,film/(NSn,film + NIn,film)] and the elemental fraction of Sn in the solution phase [CSn,soln=NSn,soln./(NSn,soln. + NIn,soln.)]. The dotted line is a guidance to the slope of “1”. CSn,film, and CSn,soln had a linear relationship with an average slope of “1.08” in the range of CSn,soln = 0–15 at. %, which verifies the reliability of dopant concentration control by the PAD process. Since the slight mismatch (more concentration in CSn,film) is attributed to the segregation of Sn atoms at the surface,69,70 we used CSn,soln for the actual dopant fraction in the ITO films. The XPS spectra at CSn,soln = 0, 5, 15 at. % are exhibited in Figure 7b. Full XPS spectra for various CSn,soln. are shown in Figure S1. The peaks presenting In 3d5/2 (444.6 eV), In 3d3/2 (452.2 eV), Sn 3d5/2 (486.6 eV), and 3d3/2 (495.1 eV) indicate that the valence states of In and Sn are +3 and +4, respectively. It is notable that there was no Cl 2p peak (Figure S2), which further confirms the absence of Cl– and M–Cl bonds. The distribution of Sn was homogeneous in the entire area of the ITO film, as shown in In and Sn EDS mapping (Figure 7c).

Figure 7.

Figure 7

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(a) Correlation between the Sn concentration in the In2O3 thin film (CSn,film) and the Sn concentration in the precursor solution (CSn,soln.). The red line represents the linear regression, and the gray dashed line represents the linear line with the slope “1”. (b) XPS spectra of In 3d and Sn 3d for CSn,soln. = 0, 5, and 15 at. %. (c) SEM and EDS mapping of the 240 nm-thick ITO thin film with CSn,soln. = 5 at. %. (d) XRD spectra for the 240 nm-thick ITO thin films for CSn,soln. = 0, 5, and 15 at. %. (e) Changes in fwhm with respect to CSn,soln., obtained from the (222) peaks in (d). (f) AFM height image of the 240 nm-thick ITO thin films with CSn,soln. = 5 at. %.

The X-ray diffraction patterns (XRD) of the ITO thin films at CSn,soln = 0, 5, 15 at. % are shown in Figure 7d. Full XRD data for various CSn,soln are found in Figure S3a. The diffraction patterns observed at 2θ = 21.5, 30.58, 35.46, 51.04, and 60.67° correspond to (211), (222), (400), (440), and (622) of the In2O3 cubic structure (JCPDS 06-0416). There were no impurity peaks such as Sn or Sn-related oxides (e.g., SnO and SnO2), indicating that Sn was effectively doped in the In2O3 matrix. The dependence of full width at half maximum (fwhm) of the most prominent (222) peak on CSn,soln = 0, 5, and 15 at. % is exhibited in Figure 7e. Full fwhm data of (222) peak for various CSn,soln are shown in Figure S3b. The fwhm increased as CSn,soln. increased to 5 at. %, and it was saturated at above 5 at. %. The grain size of the ITO thin films calculated by the Scherrer equation decreased from 25.5 nm (CSn,soln = 0 at. %) to 18.6 nm (CSn,soln = 2 at. %), and 13.7 nm (CSn,soln ≥ 5 at. %). The films were atomically smooth. The RMS roughness measured by an atomic force microscope (AFM) for the ITO film with CSn,soln = 5 at. % was only 0.5 nm (Figure 7f). The RMS roughness was less than 0.53 nm without pinholes and cracks in all the ITO films of different CSn,soln (Figure S4). The measured grain size was 15.4 ± 2.1 nm, which is well-matched with the calculated grain size from XRD.

Electrical Performance of the ITO Film Depending on Dopant Concentration

It is well known that the electrical conductivity of ITO is highly dependent on the dopant concentration.41,71,72 The effects of Sn doping concentration on electrical properties [resistivity (ρ), Hall mobility (μ), and free carrier concentration (n)] of the ITO thin films with a fixed thickness (t = 240 nm) are compared in Figure 8a. The resistivity drastically decreased with increasing Sn concentration up to 5 at. % and then increased at higher concentrations, which shows the minimum value of 5.9 × 10–4 Ω cm at CSn,soln = 5 at. %. This result is consistent with the decrease in the sheet resistance from 187.7 Ω/sq (CSn,soln = 0 at. %) to 24.5 Ω/sq (CSn,soln = 5 at. %). The decrease in resistivity is attributed to the abrupt increase of carrier concentration, which has the maximum value of 2.29 × 1021 cm–3 at CSn,soln = 5 at. %. The carriers are generated from the ionizable complex (Inline graphic and oxygen vacancy (Inline graphic at low Sn concentrations (<5 at. %). The decrease of the carrier concentration when CSn,soln exceeds 5 at. % is attributed to the increased concentration of strongly bound neutral complex (Sn2O4)x. This neutral complex is not ionizable even after the H2 reduction process is applied at 200 °C73 and acts as carrier trap rather than electron donors. The XPS spectra of O 1s can provide the relative population of the oxygen vacancy in the oxygen states. Figure 8b shows the O 1s peaks at CSn,soln = 0, 5, and 15 at. %, with deconvolution into three components at 530.1 eV for lattice oxygen (M–O peak), 531.3 eV for oxygen vacancy (VO), and 532.6 eV for surface hydroxyl (M-OH peak).74−79 The O 1s XPS data for all CSn,soln are shown in Figure S5. The relative area of the VO peak increased from 13.3 to 22.8% as CSn,soln. increased from 0 to 5 at. %, and then, it dropped to 17–18% when CSn,soln > 5 at. % (Figure 8c). This tendency is the same as the carrier concentration. The result is consistent with the previous finding, in which the increased oxygen vacancies in ITO thin films induce large carrier concentration because one oxygen vacancy generates two free electrons.80−84

Figure 8.

Figure 8

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Electrical characterization of the 240 nm-thick ITO thin films. (a) CSn,soln.-dependent electrical resistivity, carrier concentration, and charge mobility measured by Hall measurement. (b) O 1s XPS spectra for CSn,soln. = 0, 5, and 15 at. %, showing M–O, oxygen vacancy (VO), and surface hydroxyl group peaks (M–OH). (c) Plot of the relative area of VO vs CSn. (d) Log–log plot between the carrier concentration and the charge mobility. The red line represents a linear fit for CSn,soln. = 0, 2, and 5 at. %.

Intriguingly, the Hall mobility monotonically decreased with increasing dopant concentration. It has been reported that the carrier mobility of ITO thin films is limited by the scatterings caused by ionized impurities, neutral impurities, and grain boundaries.85,86 Carrier mobility was reported to be proportional to n–α due to the ionized impurity scattering.87Figure 8d shows clear regression up to CSn,soln = 5 at. % with a slope of −0.369 (R2 = 0.99863) that matches well with the reported value (slope = −0.396),87 indicating the dominant scattering mechanism when CSn,soln ≤ 5 at. % is the ionized impurity scattering. The scattering mechanism at higher doping regime (CSn,soln > 5 at. %) is not clear; however, grain boundaries and the possible formation of neutral impurities are considered as the main scattering mechanisms to the mobility decrease.

The thickness of the ITO film at CSn,soln = 5 at. % could be controlled by repeating the process of coating-and-thermal treatment. Figure 9a exhibits the thickness of the ITO films as a function of the number of repeated processes. The thickness was measured by cross-sectional FE-SEM (Figure S6). The surface of all the oxide films was smooth and uniform (Figure S4). By using a solution with a metal concentration of 0.75 M, the oxide film thickness linearly increased by 82 nm, which is beneficial for systematic control over required thickness. Even at the same oxide film thickness (82 nm) and CSn,soln = 5 at. %, the sheet resistance and film density were dependent on NPEI/Nmetal; 117.8 Ω/sq and 5.4 g/cm3 when NPEI/NIn = 2 (CPEI = Ceq), 173.0 Ω/sq and 4.3 g/cm3 when NPEI/NIn = 4.0 (CPEI > Ceq), and 216.3 Ω/sq and 3.5 g/cm3 when NPEI/NIn = 6.5 (CPEI = Ceq, with Cl– counteranion). The results indicate that a high density of thin films is essential for high-performance TCO. The ITO films with the thickness of 240 nm at different CSn,soln, measured by UV–vis spectrometer (Figure 9b), exhibited transmittance higher than 84% in all the visible ranges from 400 to 700 nm. It is notable that it is more transparent than the industrial, commercial ITO thin film (Figure 9c). The transmittance of the ITO film was 93.7% at the standard wavelength of 550 nm at CSn,soln = 5 at. %, and it had excellent resistivity (5.9 × 10–4 Ω cm) and sheet resistance (24.5 Ω/sq). Based on the measured electrical and optical parameters, we obtained Haacke’s figure of merit88 for the ITO films with different dopant concentrations (Figure 9d). The performance of the ITO films prepared by the sol–gel process, nanoparticle dispersion, and DC magnetron sputtering are compared in Table 2. The maximum figure of merit was obtained at CSn,soln = 5 at. % (Haacke’s FOM = 2.129 × 10–2 Ω–1 and Fraser and Cook’s FOM = 3.824 × 10–2 Ω–1). The FOM values are the highest among the polycrystalline ITO films deposited through the solution processes and comparable to the industrial standard.

Figure 9.

Figure 9

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(a) Thickness and sheet resistance change with repeating numbers. The red dashed line is linear fit line of thickness-repeating numbers. (b) UV–vis absorption spectra for varying CSn,soln.. (c) Digital camera images of bare glass (left), commercial ITO glass (middle), and ITO glass (t = 240 nm) prepared in this study (right). (d) Plot for figure of merit with varying CSn,soln..

Table 2. Performance Comparison of the ITO Films Prepared by Various Deposition Processes.

no. ρ (10–4 Ω cm) Rs (Ω/sq) t (nm) T (%@550 nm) FOM (10–2 Ω–1) process refs
1 5.8 64 90 87 0.386 sol–gel (89)
2 2.1 7.1 295 78 1.171 sol–gel (90)
3 7.2 30 241 90 1.191 sol–gel (39)
4 115 131.3 878 87 0.193 nanoparticles + Sol–gel (91)
5 26 133 195 88 0.21 nanoparticles (92)
6 30 309 137 96 0.215 nanoparticles (93)
7 18.9 14 1350 88 1.933 nanoparticles (94)
8 9.72 85 115 87 0.295 sol–gel (95)
9 2.2 22 100 92 1.974 sputtering (96)
10 2.4 14 171 90 2.5 sputtering (97)
11 5.9 23 240 93.7 2.129 PAD this work
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Conclusions

We successfully synthesized high-performance ITO thin films through counteranion-free metal–polymer complexes. The systematic studies on the multicomponent metal–polymer complex solution without counteranions enabled accurate and homogeneous doping and precise control over thickness. The effect of counteranion impurities was evaluated in terms of density of thin films, which has not been investigated. It was revealed that counteranions compete for cationic sites on PEI with [M-EDTA] anions and require excess PEI, resulting in reduced density of metal-oxide thin films. Additionally, precise potentiometric titration for each metal species to optimize the PEI concentration for high-density metal-oxide thin films was proposed. In case the added amount of PEI (C) is less than the equivalent amount (Ceq), the pH of precursor solution lies in highly acidic regime and the [M-EDTA] complex exists in the neutral form (H(4–n)[M-EDTA]), prohibiting electrostatic interaction with protonated PEI. While stable electrostatic interaction was established for C > Ceq, excess PEI results in porous films. At the optimized condition (C = Ceq), the ITO film (CSn,soln. = 5 at. %, t = 240 nm) has a low sheet resistance (24.5 Ω/sq) at 93% optical transparency, with a figure of merit of 2.1 × 10–2 Ω–1, which is comparable to the best industrial standard and the best among the polycrystalline ITO thin films obtained through the solution-based processes.

Acknowledgments

This work was supported by Samsung Electronics Co., Ltd (IO201214-08175-01).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnanoscienceau.2c00027.

  • XPS spectra, XRD patterns, AFM height images, and cross-sectional FE-SEM images (PDF)

Author Present Address

‡ Dongnam Division, Korea Institute of Industrial Technology, 42-7, Baegyang-daero 804beon-gil, Sasang-gu, Busan, 46938, Republic of Korea

Author Contributions

† G.P. and D.K. equally contributed to this work.

The authors declare no competing financial interest.

Supplementary Material

ng2c00027_si_001.pdf (627.9KB, pdf)

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