Fabrication of Highly Transparent and Conductive Indium–Tin Oxide

Article pubs.acs.org/Langmuir

Fabrication of Highly Transparent and Conductive Indium−Tin Oxide Thin Films with a High Figure of Merit via Solution Processing Zhangxian Chen,† Wanchao Li,† Ran Li,† Yunfeng Zhang,†,‡ Guoqin Xu,*,† and Hansong Cheng*,†,‡ †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 Sustainable Energy Laboratory, China University of Geosciences Wuhan, 388 Lumo Road, Wuhan 430074, China

‡

S Supporting Information *

ABSTRACT: Deposition technology of transparent conducting oxide (TCO) thin films is critical for high performance of optoelectronic devices. Solution-based fabrication methods can result in substantial cost reduction and enable broad applicability of the TCO thin films. Here we report a simple and highly effective solution process to fabricate indium−tin oxide (ITO) thin films with high uniformity, reproducibility, and scalability. The ITO films are highly transparent (90.2%) and conductive (ρ = 7.2 × 10−4 Ω·cm) with the highest figure of merit (1.19 × 10−2 Ω−1) among all the solution-processed ITO films reported to date. The high transparency and figure of merit, low sheet resistance (30 Ω/sq), and roughness (1.14 nm) are comparable with the benchmark properties of dc sputtering and can meet the requirements for most practical applications.

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× 10−3 Ω·cm) and high transparency (93%). The sheet resistance of 356 Ω/sq is the lowest of all the reported ITO nanoparticulate films and essentially meets the requirement of touch screens.12,14 In general, long chain surfactants or polymers are required to prevent agglomeration of the nanoparticles. The decomposition of these organics during thermal annealing and packing of the nanoparticulate building blocks on a substrate gives rise to high porosity of the TCO films, which limits the film resistivity still at 1−2 orders of magnitude higher than the sputtering benchmark.12,13,15 In addition, the weak control of nanoparticle size, morphology, and dispersibility in large-scale synthesis can exert a strong influence on the film roughness. A competitive alternative is based on a sol−gel process, which has been shown to produce TCO films with better quality, lower porosity, and lower resistivity.16 It was reported that the resistivity of a sol−gel-processed ITO film can even be at the level of 10−4 Ω·cm,17−21 close to the standard value commonly achieved by sputtering. Only a reasonably thin ITO film at this level of resistivity may have a low sheet resistance and, at the same time, a high transparency. To quantitatively evaluate the performance of a transparent conductive film with different thickness, resistivity, and transparency, Haacke proposed a revised figure of merit (FOM) defined by22

INTRODUCTION Transparent conducting oxide (TCO) film is one of the essential components in various state-of-the-art optoelectronic devices, including liquid crystal displays (LCD),1,2 organic solar cells,3 touch screens,4 and organic light emitting devices (OLED).5,6 Their high conductivity and transparency have been conventionally achieved by physical vapor deposition (PVD) techniques with sputtering on a commercial scale.7 PVD offers TCO films with dense structures and benchmark properties [resistivity, ρ = (1−2) × 10−4 Ω·cm; transparency, T > 85%]. Unfortunately, the requirement of high vacuum and sophisticated equipment, indispensably associated with the PVD processes, makes the fabrication of TCO films very expensive, which severely limits the applicability of the technology for broad industrial utilities. The possibility of replacing the PVD process with simple and solution-based methods to achieve a similar quality of TCO films represents a great challenge for materials chemistry and surface science and has been a subject of intense research activities in recent years.7,8 The solution deposition methods require no vacuum or sophisticated equipment and thus enable thin film fabrication with larger scalability and lower processing costs.8−10 To date, two solution processes, one based on TCO nanoparticles and another on sol−gels, for film deposition have been proposed. For a nanoparticle-based process, dispersions of the nanoparticles remain to be a delicate choice. One can tune the sizes, morphologies, compositions, and dispersibilities of TCO nanoparticles in a nonpolar or polar solvent through careful control of synthesis conditions.11−13 Recently, Lee et al. assembled an indium−tin oxide (ITO) thin film from monodisperse nanoparticles with relatively low resistivity (5.2 © 2013 American Chemical Society

ΦH = T10/R s Received: August 29, 2013 Revised: October 8, 2013 Published: October 11, 2013 13836

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percentage is defined as Sn% = Sn/(Sn + In) in atomic ratio. The solutions were kept at 60 °C overnight. After incubation, the color of solutions became transparently brown, indicating the formation of metal−acetylacetonate complexes. The solutions were highly homogeneous and stable and can be stored for more than 6 months without any precipitates (Figure S1, Supporting Information). Thin Film Deposition. The ITO/glass films were prepared by spin coating of the solutions at 4000 rpm for 60 s in air for simple fabrication of the ITO films. Obviously, other scalable coating techniques, such as spray coating and printing, can be readily adopted with the sol−gel solutions for large-scale thin film fabrication. Prior to spin coating, the glass substrates (Asahi, 40 × 40 × 0.55 mm3) were sequentially washed under sonication with acetone, deionized water, and ethanol for 10 min each. The substrates were dried in an oven and then treated with a Novascan UV−ozone cleaner (PSDP-UV12T) for 15 min. After coating, the wet films were dried at 120 °C for 10 min and then annealed for 10 min at 400 °C in air. The procedure of coating, drying, and annealing was repeated five times to achieve the desired thickness. Annealing in H2/Ar gas mixture (v/v = 5/95) was conducted in a tube furnace at 300 °C for 3 h with a heating ramp of 5 °C/min. Characterization. X-ray diffraction (XRD) patterns were collected on a PANAlytical EMPYREAN X-ray diffractometer with Cu Kα source (λ = 1.5418 Å), operating on a θ/2θ configuration at 40 kV and 30 mA. Scan range was from 20° to 70° with the step of 0.02°. Jade 6.5 (Materials Data Inc.) was used to fit the XRD patterns and to calculate lattice constants. Images of scanning electron microscopy (SEM) and atomic force microscopy (AFM) were taken from a JEOL JSM-6701F field emission SEM and a Veeco Nanoscope IIIa tapping-mode AFM, respectively. Transmittance of the thin films was measured by a Shimadzu UV-2550 ultraviolet−visible (UV−vis) spectrometer. Fourier transform infrared (FTIR) spectra were collected on a Varian 3100 FTIR spectrometer (resolution 2 cm−1, 64 scans). Sheet resistance was recorded from a four-point probe station and a source meter (Keithley 2400C) by an average of five readings. X-ray photoelectron spectroscopy (XPS) was performed on a Kratos AXIS Ultra HAS spectrometer equipped with a monochromatic Al Kα X-ray source (hν = 1486 eV), operating at 15 kV and 5 mA. Peak fitting was performed with the software XPSPEAK 4.1 and a Shirley background subtraction.

where Rs and T represent the sheet resistance and transmittance, respectively. A higher value of FOM indicates better performance of a film, which requires a low sheet resistance and high transparency. However, to achieve a high transmittance, the TCO film must be sufficiently thin, which makes the sheet resistance higher. Therefore, the two properties compromise each other. As a consequence, the FOM value cannot be unlimitedly high. With ΦH as the basic measurement of film quality, nanoparticulate TCO films usually exhibit a low FOM on the order of 10−4 Ω−1. To the best of our knowledge, most sol−gelprocessed ITO films exhibit the same order or modestly higher value of FOM as nanoparticulate ITO films due to the high sheet resistance and/or low transparency.9,19,21,23 One exceptionally high FOM of 1.17 × 10−2 Ω−1 was acquired by Seki,20 which is comparable to that of the polycrystalline ITO film prepared by sputtering.24 The minimum sheet resistance achieved is 7 Ω/sq, which can meet the high-end applications. Unfortunately, the transparency around 78% is much lower than the basic requirement for transparent conductors (T > 85%). In addition, fabrication of the ITO film requires 30 coatings, which makes the process highly time-consuming and inefficient. The toxicity, high cost, and inaccessibility of the metal alkoxide precursors further compromise the advantage of the sol−gel process. Most sol−gel processes of ITO film deposition employ acetylacetone as the ligand to stabilize the In3+ cations by forming chelation compounds to lower the hydrolysis rate of In3+. The dopant from tin salts (SnCl2, SnCl4, etc.) is dissolved in alcohol (ethanol, methanol, 2-propanol, etc.) to form an alcoholic solution. Prior to film coating, the two solutions are simply mixed.17,25−27 Several previous studies showed that the hydrolysis of tin(II) and tin(IV) readily occurs in ethanol to form SnO2 nanocrystals (1−3 nm).28−30 The preformed SnO2 nanocrystal may act as a nucleation center to grow In2O3.31 Nonuniform growth can lead to aggregation of nanoparticles and formation of islands,32 which affect the film microstructures, uniformity, and conductivity. In this paper, we report a simple but effective sol−gel method to fabricate highly uniform, conductive, and transparent ITO thin films. In a conventional sol−gel process, solutions of indium and tin salts were first made separately and then mixed together upon film deposition. This procedure may give rise to inconsistent quality and performance of the ITO films due to the poor uniformity. In the present study, the indium and tin salts are both dissolved together in acetylacetone to stabilize Sn4+ cations. As a consequence, highly uniform, transparent, and conductive ITO films were obtained. The film resistivity (7.2 × 10−4 Ω·cm), sheet resistance (30 Ω), transparency (90.2%), and surface roughness (1.14 nm) all surpass or become comparable to the best performance of recently developed nanoparticle-based solution processes and, more importantly, to the benchmark performance of dc sputtering. The film displays the best combination of conductivity and transparency, showing the highest figure of merit (1.19 × 10−2 Ω−1) among all the reported solutionprocessed ITO films.

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RESULTS AND DISCUSSION A typical XRD pattern of the ITO film (Sn% = 10%) after annealing in H2/Ar is shown in Figure 1A. All the diffraction peaks can be indexed to the body-centered cubic bixbyite In2O3 structure. The standard pattern (PDF no. 06-0416) is given for comparison. The increased background toward low angle is attributed to the amorphous glass substrate. The pattern shows three major diffraction peaks of (222), (440), and (400) Miller planes, similar to the pattern of randomly oriented powder. After peak fitting, the integrated peak area ratio of (222) to (440), S(222)/S(400), was found to be 5.09, much larger than the random orientation ratio of 2.86. The ratio of S(440) to S(400) remains close to the standard (1.15 vs 1.17). Therefore, the film shows a weak (222) texture, which has the lowest surface energy among the three low-index surfaces γ(100) > γ(110) > γ(222).33 Figure 1B displays the dependence of (222) peak positions versus the dopant percentage. As the dopant percentage increases from 0% to 10%, the (222) peak position slightly shifts toward a lower angle. The variation of lattice constants of the ITO films vs dopant percentage was obtained after peak fitting of the (222) diffraction plane. The results are presented in Figure 1C. Without doping with Sn, the lattice constant of the In2O3 film (a = 10.117 Å) is close to that of the standard In2O3 powder (a = 10.118 Å). As the dopant percentage increases, the lattice constant increases significantly. The variation of the lattice constant contradicts the Vegard’s law, which predicts a linear decrease of alloy lattice as the

EXPERIMENTAL SECTION

All chemicals were used without any purification. Indium nitrate hydrate (99.9%, Aldrich) and tin(IV) chloride hydrate (99.9%, Alfa) were dissolved together in acetylacetone with [In3+] = 1 mol/L and different dopant percentages from 0% to 15%. Here the dopant 13837

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Figure 2. SEM images (A−C) and AFM image (D) of ITO film (Sn% = 10%) after annealing in H2/Ar. (Part C shows the film crosssection.)

Figure 1. (A) XRD pattern of the ITO film after annealing in H2/Ar (Sn% = 10%); Dependence of (B) the XRD (222) peak positions, (C) the lattice constants, and (D) the film resistivities versus the dopant percentages: 0%, 2%, 5%, 8%, 10%, 12%, and 15%. (Blue lines are the corresponding fitted curves. The ITO films without annealing in H2/ Ar are used for all the measurements except part A.).

pattern based on the Scherrer equation (4.4 nm). The analysis of the AFM image gives a root-mean-square roughness, Rms, of 1.14 nm, comparable to that of the ITO film deposited by sputtering (1.1 nm).24 The surface properties of films, including work function and surface compositions are important for the applications of TCO films. A high work function of ITO films is always required to improve the hole-transport efficiency in organic electronics. The surface composition is extremely important, since the impurities can act as the surface trapping centers of free carriers to affect the electrical properties. The XPS survey scan on the ITO thin film indicates that no detectable elements are present on the film except carbon, oxygen, indium, and tin. No contamination from nitrogen and chlorine was detected during the fine scan (Figure S2, Supporting Information). Figure 3 displays the C1s core-level spectrum of the ITO film upon annealing in a H2/Ar mixed gas, fitted to a single composition located at the binding energy of 284.8 eV from the aliphatic airborne contamination, which can be removed through Ar+ bombardment.37 Fitting the O1s spectrum results in three

concentration of smaller size dopant increases.34 Nevertheless, a similar trend of lattice variation upon doping was also observed by several studies.35,36 Nadaud et al. demonstrated that an increase of tin content would simultaneously introduce interstitial oxygen atoms, which results in lattice expansion.35 Beyond 10%, lattice parameters remain nearly constant, indicating that the solubility of Sn in In2O3 is around 10%. The resistivity change of the ITO films with different dopant concentrations is shown in Figure 1D. Interestingly, with the increase of dopant concentration, the film resistivity exhibits an inversed trend compared with the lattice constant. The resistivity decreases rapidly as the tin concentration increases from 0% to 10%, indicating the effectiveness of the Sn4+ doping in the In2O3 matrix. Further increasing the dopant concentration does not lead to reduction of the electrical resistivity. The resistivity reaches the minimum value of 7.32 × 10−3 Ω·cm when Sn% = 10%. The slight increase of resistivity upon the dopant concentration exceeding 10% is attributed to the increase of concentration of the ionizable complex (Sṅ2Ö i) and the neutral complex (Sn2O4)x, which reduces the carrier mobility and doping efficiency.18,36 Therefore, 10% was chosen as the focal dopant percentage in the present study to investigate the morphology, roughness, surface composition, and optical property of the ITO thin films. Figure 2 shows the morphologies of an ITO film (Sn% = 10%) after annealing in hydrogen gas. Figure 2A,B indicates that the ITO film has a very smooth and uniform surface without any large aggregates, which is a highly desirable feature for applications in various electronic devices. The cross-section of the thin film displays a uniform thickness of roughly 240 nm after five layers of ITO coating on a glass substrate (Figure 2C). The AFM image (Figure 2D) further confirms the smoothness and uniformity of the ITO film. The particle size is around 5.2 nm based on the analysis of a 2 μm × 2 μm area of AFM image. The average size is close to the result obtained from the XRD

Figure 3. XPS narrow scans of C1s, O1s, In3d, and Sn3d core-level spectra. 13838

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prepared via sol−gel processes, nanoparticle dispersion, and dc magnetron sputtering, respectively. To the best of our knowledge, this is the highest FOM among the polycrystalline ITO films deposited via a solution process. It is important to point out that the effective chelation of tin(IV) ions by acetylacetone that slows down the hydrolysis rate of Sn4+ facilitates the control of the uniform growth of the ITO thin films. Previous investigations have shown that the growth of SnO2 nanoparticles (from 1 to 3 nm) occurs via the hydrolysis of Sn2+ in an ethanolic solution under ambient conditions without heat or hydrothermal treatment.28,29 The hydrolysis of Sn4+ is expected to occur more readily due to its higher charge/radius ratio, more in favor of nucleophilic reactions such as hydrolysis.41 The FTIR spectroscopy was used to investigate the hydrolysis of SnCl4 in ethanol. The details of FTIR experiment are provided in the Supporting Information. As shown in Figure 5A, there are several absorption bands below 1000 cm−1 assigned to the vibrations of SnO2 lattice. The strong absorption bands around 1533 and 1421 cm−1 are attributed to the overtones of the asymmetric Sn−O−Sn modes,42 as observed in the in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) during the annealing process of ITO hydroxide precursor from 80 to 360 °C.43 The intensities of these two peaks increase with the increment of annealing temperature above 240 °C, where the hydroxide precursor starts to form ITO nanocrystals. The corresponding assignments are listed in Table 2 on the basis of reported references.42−44 The two peaks at 1340 and 1023 cm−1 without assignment remain after drying the pellet under vacuum at 80 °C for 72 h, arising from the product upon hydrolysis, not the residual ethanol. The previous FTIR spectrum suggests that the hydrolysis of tin(IV) cations already occurs upon the dissolution of SnCl4 in ethanol. To examine the efficiency of the chelation of Sn4+ by acetylacetone after the hydrolysis, an equivalent volume of acetylacetone was added into the ethanolic solution of SnCl4 to mimic the process utilized in other references. The FTIR spectrum (Figure S5, Supporting Information) does not show a major difference compared with that of ethanolic SnCl4 solution, implying that the simple mixing process is ineffective to chelate the hydrolyzed product. In comparison, the chelation is more effective if SnCl4 is directly dissolved in acetylacetone followed by heating at 60 °C. The IR spectrum is shown in Figure 5B, which displays completely different absorption features. The absorption bands around 1700 and 1666 cm−1 are due to the stretching of CO in keto and enol forms of free acetylacetone. The peak at 1563 cm−1 clearly shows the chelation between Sn and acetylacetone.45 The corresponding assignments are listed in Table 2, based on the references.45−47 The controlled experiments we conducted indicate that the ITO films would have a relatively rough surface if without effective chelation of tin cations by acetylacetone (Figure S6, Supporting Information). The small SnO2 nanoparticles formed after hydrolysis of Sn4+ in ethanol may serve as the nucleation centers during the growth and crystallization of ITO nanocrystals. Therefore, nonuniform growth of ITO nanocrystals would result in rougher and more porous film microstructures,32 leading to a much higher electrical resistivity (∼4.4 × 10−2 Ω·cm). Therefore, by stabilizing both cations, our simplified sol−gel process can improve the film quality effectively, leading to the enhanced performance of the transparent conducting films.

components, located at 530.07, 531.29, and 532.39 eV, respectively. The first two are assigned to bulk oxide and surface oxygen-deficient oxide components. The third composition comes from the surface hydroxyl groups.38 The relative elemental ratio can be calculated from the formula (SA/ ASFA)/(SB/ASFB), where S and ASF refer to the integrated peak area and atomic sensitivity factor, respectively. The atomic sensitivity factors are 4.3 and 3.9 for Sn3d5/2 and In3d5/2 core level electrons, respectively. The ratio of Sn/(Sn + In) was calculated to be 10.9%, modestly higher than the nominal dopant concentration of 10%, which was attributed to the surface segregation of tin dopants.39 The ITO film with Sn% = 10% shows the lowest sheet resistance around 310 Ω/sq, still much higher than 50 Ω/sq required for most applications but slightly lower than the best nanoparticulate ITO film.12,14 To further reduce the resistivity, we undertook annealing of the thin film in a H2/Ar gas for 3 h at 300 °C. As a consequence, 1 order of magnitude improvement in the sheet resistance was achieved. The film shows an average sheet resistance of 30 Ω/sq, corresponding to a specific resistivity of 7.2 × 10−4 Ω·cm, both comparable to those prepared by dc sputtering.24 No significant difference in the phase and morphology was observed for the ITO films before and after annealing in H2/Ar, except the slightly increased mean crystalline size and improved surface smoothness after annealing (Figure S3 and Table S1, Supporting Information). The improvement of conductivity can be attributed to the increased concentration of fully ionizable oxygen vacancies under reducing conditions, 8,36 which enhances the carrier concentration. In addition, the increased Hall mobility is also responsible for the improved conductivity after annealing in a reducing atmosphere.20 The transmittance of the ITO film was measured by a UV− visible spectrometer. The ITO film is highly transparent in the visible region (Figure S4, Supporting Information). As shown in Figure 4, the film upon five-layer coating displays a

Figure 4. UV−vis spectrum of the ITO film (Sn% = 10%) after annealing in H2/Ar mixture (dashed line: transparency of the glass substrate).

transmittance higher than 82% in all the visible range from 400 to 700 nm. The spectrum of 0.55-mm-thick bare glass substrate is also shown as the reference. The transmittance of the ITO film is as high as 90.22% at the standard wavelength of 550 nm. On the basis of these parameters, we obtained the Haacke’s figure of merit of 1.191 × 10−2 Ω−1 for the ITO film.40 Table 1 compares the properties of the typical ITO films 13839

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Table 1. Properties of Typical ITO Films Prepared by Various Deposition Processes no.

ρ (10−4 Ω·cm)

Rs (Ω)

t (nm)

T (%) at 550 nm

ΦH (10−2 Ω−1)

process

1 2

45.9 4.23 ∼55 3.2 5.8 2.5 2.1 5.7 0.52 2.2 7.2

379 29.4 ∼573 21 64 42 7.1 100 356 22 30

121 144 96 150 90 60 295 57 146 100 241

∼88

0.073

spin coating dip coating

9 17

∼96

0.116

∼87 ∼85 ∼78 ∼70 93 ∼92c 90.2

0.386 0.469 1.171 0.028 0.136 1.974 1.191

dip coating dip coating dip coating dip coating dip coating spin coating sputtering spin coating

18 19 21 20 23 12a 24b this work

3 4 5 6 7 8 9 10 a

ref

Prepared via ITO nanoparticle dispersion. bPrepared by DC magnetron sputtering. cThe transmittance without glass substrate.

to that of polycrystalline ITO film produced by dc magnetron sputtering. The high transparency, low sheet resistance (30 Ω/ sq), and low surface roughness (1.14 nm) can meet the requirements for most practical applications. Compared with the nanoparticle-based solution processes, the new sol−gel process is stable and simple but capable of delivering highly reproducible performance of thin films with the sheet resistance an order of magnitude lower and the FOM value an order of magnitude higher. While the cost of the new sol−gel process is substantially lower than that of dc sputtering, we show that the performance of the ITO thin films developed in the present study is comparable to the sputtering results. This makes the sol−gel process highly competitive for fabrications of TCO films at a large scale for broader optoelectronic applications.

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Figure 5. FTIR spectra of the dried sol−gel samples prepared by dissolving SnCl4 in (A) ethanol and (B) acetylacetone.

S Supporting Information *

Table 2. FTIR Peak Positions and Corresponding Assignments SnCl4 + ethanol frequency (cm−1)

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585 647 670 684 736 806 934 1023 1282 1340 1421 1533 1630 3160−3600

assignment ν(Sn−O) ν(Sn−O) ν(Sn−O) νas(Sn−O−Sn) δOH(Sn−OH) δOH(Sn−OH) 2νas(Sn−O−Sn) 2νas(Sn−O−Sn) δOH(H2O) νOH(H2O)

ASSOCIATED CONTENT

Information on the FTIR experiment, photos of sol−gel solution and ITO film, XPS survey scan, fine scan of Cl2p and N1s, XRD and AFM image of ITO film before annealing in hydrogen, FTIR spectra of ethanolic SnCl4 solution before and after addition of acetylacetone, and SEM images of ITO film without effective chelation of Sn4+. This material is available free of charge via the Internet at http://pubs.acs.org.

SnCl4 + acetylacetone frequency (cm−1)

assignment

956 1022 1070 1165 1357 1412 1563 1630 1666 1700 2921 2979

δ(CC)

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δCH δ(CH3) δ(CH3) ν(CC) δOH(H2O) ν(CO) ν(CO) ν(CH3) ν(CH)

AUTHOR INFORMATION

Corresponding Authors

*G.X.: e-mail, [email protected]. *H.C.: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support of a Start-up grant from NUS, a POC grant from National Research Foundation of Singapore, and the National Natural Science Foundation of China (No. 21233006).

CONCLUSIONS We have developed a simple and effective sol−gel process to prepare transparent conductive ITO films. The chelation of Sn4+ by acetylacetone molecules prior to its hydrolysis can be effectively achieved, which facilitates formation of more uniform and smooth film microstructures. As a consequence, the ITO thin film with Sn% = 10% displays a high transparency (90.2%) and low resistivity (7.2 × 10−4 Ω·cm) after annealing in a reducing gas. The highest figure of merit was achieved among all the solution-processed ITO films, even comparable

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