Influence of ITO Electrode Surface Composition on the Growth and

Article pubs.acs.org/JPCC

Influence of ITO Electrode Surface Composition on the Growth and Optoelectronic Properties of Electrodeposited Cu2O Thin Films Anna Osherov, Changqiong Zhu, and Matthew J. Panzer* Department of Chemical & Biological Engineering, Tufts University, 4 Colby Street, Medford, Massachusetts 02155, United States ABSTRACT: Morphological inhomogeneity between electrodeposited cuprous oxide (Cu2O) thin films on nominally equivalent tin-doped indium oxide (ITO) electrodes under identical deposition conditions is reported. Dependence of the Cu2O photocurrent generation ability on thin-film morphology is also described. Films exhibiting compact, polyhedral crystallites produce short circuit photocurrents more than 10 times greater than those consisting of sparse, dendritic crystallites. A correlation between the ITO surface stoichiometry and the resulting Cu2O film morphology is established using X-ray photoelectron spectroscopy for identification of the ITO electrode surface chemical states. Variation in the Sn/In atomic ratio up to 33% is measured at the surfaces of ITO substrates from the same supplier lot, highlighting the importance of thorough electrode characterization prior to electrodeposition to obtain repeatable Cu2O performance for energy conversion applications.

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INTRODUCTION Electrodeposition is a promising technique for the fabrication of metal oxide thin films for many applications, including solar energy conversion devices.1−4 It offers precise control of the driving force for the reactions involved in deposition, enabling structural and compositional control over the deposited films.5 Current distribution through the working electrode is one of the most important features in the electrochemical process that influences the homogeneity of the deposit.6 However, while the mechanisms of electrodeposition on metallic substrates are well studied and established,7−9 a theory describing the electrodeposition of semiconducting thin films (metal oxides in particular) on multicomponent, semiconducting working electrodes (such as Sn-doped indium oxide, ITO) has not been well developed. This is particularly relevant, given that several groups have recently reported on the electrodeposition of metal oxide films (such as Cu2O) for photoconversion applications on transparent ITO working electrodes.3,10−12 The potential distribution at the semiconducting electrode/solution interface is more complicated than that at the metal electrode/solution interface because the applied potential is partitioned between the space charge layer of the semiconductor and the Helmholtz layer in solution.13 Furthermore, the possibility of electronic structure heterogeneity due to local variations in stoichiometry at the surface of an ITO working electrode could contribute to a nonuniform potential distribution at the ITO/solution interface. Nonetheless, electronic structure homogeneity of the working electrode is a common assumption of electrodeposition, and depositions employing a constant potential or a constant current for the fabrication of thin films on ITO electrodes are widely reported. ITO coatings find many industrial applications as a transparent conductive material, and ITO-coated glass substrates have become widely commercially available. The © 2013 American Chemical Society

processing conditions used to deposit ITO are of particular importance because they can significantly affect properties such as microstructure, transparency, and conductivity.14−18 It has also been shown that film thickness can have an influence on the texture, morphology and consequent optoelectronic properties of ITO films.19,20 For instance, both the doping efficiency of SnO2 and film crystallinity were found to be affected by film thickness.21 Furthermore, one of the most important outcomes determined by the fabrication process is the distribution of surface chemical states. Various oxygen chemical states have been detected on ITO surfaces, and differences in oxygen surface states were attributed to varying degrees of crystallinity,22 segregation of the Sn dopant to the substrate surface,19,20,23 and adsorption of water.24,25 In this work, semiconducting Cu2O thin films are electrodeposited on nominally equivalent ITO electrodes under identical conditions using a common growth solution, resulting in striking variations in Cu2O film morphology and photocurrent generation ability. As will be shown below, these observations can be attributed to local variations of the electrode surface stoichiometry due to chemical heterogeneity between − as well as within − individual ITO-coated glass substrates from a single supplier lot.

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EXPERIMENTAL SECTION Cu2O electrodeposition was performed on clean ITO-coated glass substrates from a single lot supplied by Thin Film Devices. The ITO working electrode area was ∼1 cm2. Acidic (pH 5.27) growth baths contained 0.02 M copper(II) sulfate pentahydrate Received: September 13, 2013 Revised: November 8, 2013 Published: November 8, 2013 24937

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Figure 1. (a) Current density versus film growth time for Cu2O films electrodeposited on nominally equivalent ITO-coated glass substrates using a copper(II) sulfate/sodium lactate solution at pH 5.27, [lactate]/[Cu2+] = 5, and a constant applied potential of −0.034 V versus Ag/AgCl at 60 °C. Inset: zoomed-in view showing the initial nucleation step. (b,c) Representative top-view SEM images of electrodeposited Cu2O films. (b) Sample C. (c) Sample A.

Figure 2. (a) Top-view optical image of a Cu2O film electrodeposited on an ITO-coated glass substrate exhibiting two distinct morphologies obtained. This film refers to Sample B introduced in Figure 1. The smaller, faceted crystallites visible in the lower right portion of the image (denoted as region 1) coexist on the same ITO substrate with large, dendritic crystallites seen in the upper left (denoted as region 2). Although this image is centered on the boundary between the two regions, each distinct morphological region covered several square millimeters within the Cu2O film. Top-view SEM images highlight the Cu2O morphologies of (b) region 1 and (c) region 2. (d) Current−voltage characteristics of ITO/Cu2O/EGaIn structures created in region 1 and region 2 under monochromatic illumination (λmax = 520 nm, 0.583 mW/cm2). (e) Schematic illustration of the electrical characterization setup.

(Sigma-Aldrich, >98%) and 0.1 M sodium D-L lactate (SigmaAldrich, 60% w/w) as a chelating agent. All reagents were used as received. ITO films were cleaned by successive sonication in 2 vol % Micro-90 in deionized water, deionized water, acetone, and immersion in boiling isopropanol. Electrodepositions were performed for 30 min at 60 °C in a three-electrode, singlecompartment electrochemical cell using a constant applied potential of −0.034 V (vs Ag/AgCl). A high-surface-area carbon film served as the counter electrode. Scanning electron microscopy (SEM) was employed for topographical characterization using a Zeiss FESEM Ultra Plus. The secondary electron signal was used to obtain topography images. Acceleration voltages ranged from 1.8 to 3.5 kV.

X-ray photoelectron spectroscopy (XPS) was performed on ITO surfaces prior to Cu2O deposition using a Thermo Scientific K-Alpha XPS with a microfocused monochromatic Al X-ray source (excitation energy 1486.6 eV) at base pressure of 8 × 10−8 mbar. Measurements were obtained at several locations within each ITO electrode to assess the chemical homogeneity of each ITO film. XPS data were collected from a circular sampling area ∼400 μm in diameter at a photoelectron takeoff angle of 90°, and energy calibration was performed according to the position of the C1s line (284.8 eV). Work function values of the ITO substrates were determined using secondary electron cutoff spectra. During the work function measurements, a bias of −34 V was applied to the sample. 24938

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Figure 3. XPS spectra of (a,d) In3d; (b,e) Sn3d; and (c,f) O1s peaks for the ITO substrates of samples A and C. Intensities are normalized by the height of the In3d5/2 peak for each substrate.

(130 ± 10 nm) were also accompanied by a small degree of variability in the measured ITO sheet resistance (21 ± 2 Ω/□), which fell within the specifications outlined by the supplier. It is also essential to note that topographical characterization (via AFM) coupled to bulk structural characterization (via XRD) did not reveal any apparent significant differences between the ITO films. The current density versus film growth time data for three Cu2O films deposited under identical conditions from the same growth solution (Figure 1a) exhibit largely similar shapes, with an increasing current at early times, followed by a continuously decreasing current after the first few minutes. Decreasing deposition current with increasing deposition time indicates the relatively high resistivity of the growing film. The shapes of the current density versus deposition time curves in the first few seconds reflect the instantaneous nature of the nucleation step (see inset in Figure 1a), and the overall shape of the current− time transient is typical for a 3D nucleation-and-growth mechanism.26 In addition, secondary nucleation is observed for Samples B and C, as indicated by the current fluctuations apparent in the deposition curves after approximately 15−20 s.5 Among a number of Cu2O films deposited using the identical conditions described here, three distinct morphological manifestations were observed: (i) branched, dendritic crystal-

Schottky junction photovoltaic structures were formed by depositing a eutectic gallium−indium (EGaIn) liquid metal droplet of controlled contact area using a Teflon ring (I.D. = 3.3 mm) on top of Cu2 O films. Photoresponse was characterized under monochromatic green LED illumination (Lamina Titan light engine, λmax = 520 nm, 0.583 mW/cm2). Current−voltage data were collected using a Keithley 2602A sourcemeter under full computer control.

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RESULTS AND DISCUSSION It has been observed that in some cases cuprous oxide (Cu2O) thin films electrodeposited under identical conditions on nominally equivalent ITO-coated glass substrates exhibit markedly different morphologies (Figure 1). This is an important phenomenon to address because it has been previously demonstrated that variation in the Cu2O film microstructure can correspond to substantially different optoelectronic properties.4 Because the growth conditions were kept strictly consistent for all films in the present study, it is hypothesized that these morphological variations are due to unexpected inhomogeneity among the ITO-coated glass substrates. From profilometry measurements, a small amount of variability in the ITO film thickness between substrates was observed (data not shown). These minor thickness variations 24939

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lites throughout the film (Figure 1b); (ii) faceted, densely spaced crystallites throughout (Figure 1c); and (iii) distinct regions of both dendritic and faceted crystallites within the same film (see Figure 2). These three archetypes are exemplified by Samples C, A, and B, respectively. While it was initially thought that the dendritic versus faceted Cu2O morphologies corresponded to ITO-coated substrates of slightly different thickness or sheet resistance, the observation of both morphologies present within the same film does not support this theory. Therefore, it was hypothesized that the differences in Cu2O morphology could be attributed to localized spatial variation in the conductivity of the ITO substrate. Even though current density is usually assumed to be uniform across the ITO electrode, any real surface can exhibit locally heterogeneous properties. Furthermore, electrochemical deposition begins first on the high energy sites, dictating nucleation and subsequent film growth. 6 The present experimental results show that for some ITO substrates a mode of sparse Cu2O nucleation and lateral growth is dominant, as indicated by a lower current density during the nucleation stage (see Figure 1a, inset) and reflected by the branched film morphology (Figure 1b), while for other ITO substrates, a greater density of nucleation sites is favorable, corresponding to a higher current density during the nucleation stage and resulting in a more compact, dense Cu2O morphology (Figure 1c). For certain ITO substrates, local regions exhibiting different conductivities and thus leading to the coexistence of both Cu2O morphologies also exist (Figure 2a). The two different Cu2O morphologies obtained correspond to clear differences in metal oxide film optoelectronic performance. This is exemplified by the current−voltage characteristics measured under illumination shown in Figure 2d, wherein the photocurrents generated by two distinct Cu2O morphological regions present within the same film (Sample B) were probed separately. Two-electrode current−voltage measurements were performed under monochromatic green LED illumination through the glass/ITO substrate (Figure 2e). The ITO/Cu2O/EGaIn structures displayed rectifying behavior and photovoltaic characteristics under illumination in both morphological regions.4 The denser, faceted Cu2O morphology present in region 1 (Figure 2b) produced a larger photovoltaic response, exhibiting a short circuit current density of 13 μA/ cm 2 and an open circuit voltage of 0.56 V under monochromatic green LED illumination (λmax = 520 nm, 0.583 mW/cm2). The short-circuit photocurrent of the dendritic Cu2O morphology found in region 2 (Figure 2c) was approximately one order of magnitude lower in comparison (0.97 μA/cm2) with an open circuit voltage of 0.45 V. Notably, this same relative discrepancy in the photocurrent generation ability of Cu2O regions 1 and 2 of Sample B was also observed during the current−voltage characterization of ITO/Cu2O/ EGaIn structures on Samples A and C, respectively. It should also be noted that while the photovoltaic performance of the Schottky junctions formed between Cu2O and EGaIn here are not as high as those of more optimized p-n heterojunction structures, such as the ones formed between Cu2O and ZnO27−30 or Ga2O3,31 the clear differences in photocurrent generation ability for Cu 2 O films exhibiting different morphologies observed here have broad implications for solution-deposited Cu2O-based optoelectronic devices. To support the hypothesis that different Cu2O morphologies obtained under identical growth conditions could be caused by

local regions of varying conductivity due to stoichiometric variations within and among the surfaces of the ITO substrates, we performed XPS characterization of the ITO substrates. XPS spectra were recorded at multiple locations on the three ITO substrates used to grow Samples A−C discussed above, prior to Cu2O electrodeposition. Two distinct surface stoichiometries (primarily differing in Sn/In atomic ratio) were uniformly observed across ITO substrates A and C (see Figure 3), while regions exhibiting these two different spectral signatures were both present at different locations within ITO substrate B. Previously, it has been established that Sn in ITO films has a tendency to preferentially segregate to the surface.32,33 A larger amount of Sn in certain regions of the surface is expected to influence the electrical properties of the ITO electrode due to the n-type behavior of substitutional Sn dopants in In2O3, resulting in a localized conductivity increase. Analyses of the In3d, Sn3d, and O1s spectral envelopes for the ITO substrates of samples A and C are presented in Figure 3. The XPS peaks are deconvoluted to show the various chemical states present at the surface. Binding energies of the In3d, Sn3d, and O1s peaks are referenced to the C1s peak at 284.8 eV. The In3d3/2 and the In3d5/2 doublets are visible in Figures 3a,d. Both spectral envelopes are resolved into two major peaks, which can be attributed to the Sn-screened and Sn-unscreened In3+ in In2O3.34 These peaks are positioned at 445.5 and 444.5 eV for In3d5/2, respectively. According to the data shown in Figure 3, the In3+(Sn-screened)/In3+(Snunscreened) atomic ratio for ITO substrate A is 0.33 compared with 0.22 for ITO substrate C. The greater In3+(Sn-screened)/ In3+(Sn-unscreened) ratio for ITO substrate A is consistent with a larger amount of Sn present at the ITO surface, which would increase the local carrier concentration and thus the electronic conductivity. Indeed, comparison of the Innormalized intensities of Sn at the surfaces of ITO substrates A and C (Figures 3b,e) clearly confirms the larger relative amount of Sn on the surface of ITO substrate A. It is therefore posited that the larger Sn/In ratio at the surface of ITO substrate A compared with that of ITO substrate C ([Sn/In]A/ [Sn/In]C = 1.33) is responsible for an increased conductivity at the ITO surface that results in a higher Cu2O nucleation density at fixed deposition potential, resulting in a more dense Cu2O film morphology. The Sn3d3/2 and Sn3d5/2 doublet spectra displayed in Figure 3b,e can also be deconvoluted into two components for each Sn3d photoemission peak. For the Sn3d5/2 spectra, the peak positioned at 486.5 eV can be attributed to the Sn4+ bonding state in SnO2, while the peak located at 487.6 eV was previously assigned to Sn substituting In or Sn−OH-like species.33,35,36 Under ambient conditions, SnO2 is highly thermodynamically stable; therefore, Sn4+ is the predominant state observed at the surface of the ITO films.37 The calculated atomic ratios of substitutional Sn or Sn−OH-like species/Sn4+ using these spectra are 0.30 for ITO substrate A and 0.25 for ITO substrate C, which are nearly equivalent. The XPS O1s data can be resolved into three components. The lowest energy peak positioned at 530.1 eV (Figures 3c,f) may represent the O−In bonding in In2O3;35,38 however, it has also been suggested that this peak might be associated with O2− ions in the tetrahedral interstices of the ITO structure.35 The peak positioned at 531.4 eV likely corresponds to the Sn4+− O2− bond37 or Sn−OH-like bond,36 and the small peak positioned at 532.6 eV that is more pronounced for ITO substrate A (Figure 3c) was previously shown to correspond to 24940

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however, variations in the ITO surface stoichiometry can have a substantial effect on the morphology and optoelectronic properties of the resulting metal oxide film. Cu2O morphological variations observed in subsequent depositions under identical conditions on nominally equivalent ITO electrodes from a single supplier lot were shown to be a result of variations in the surface composition, which are expected to result in local variations in electrical conductivity. Regional segregation of Sn to the ITO surface is likely responsible for the increased conductivity that leads to a higher nucleation density and a more compact Cu2O film morphology for some samples. This work highlights the importance of ITO electrode surface characterization as an essential preliminary stage in the Cu2O thin-film electrodeposition process to enhance repeatability for a particular set of growth conditions.

organic oxygen (contamination) or molecularly chemisorbed water.24,25,36 It is essential to note that although the positions of the deconvoluted O1s features are in good agreement with those reported by other groups, the assignments of these features are still a subject of some debate.22,24,25,33,38 XPS was also used to measure the work function (ΔΦ) of ITO substrates A and C (Figure 4). The work function values

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AUTHOR INFORMATION

Corresponding Author

*Tel: (617) 627-4633. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 4. XPS data of the high binding energy cutoff region for ITO substrates A and C used to determine the work function of each film. The vertical line indicates the incident photon energy of 1486.6 eV.

ACKNOWLEDGMENTS XPS and SEM were performed at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation under NSF award no. ECS0335765. CNS is part of Harvard University. A.O. is an Awardee of the Weizmann Institute of Science − National Postdoctoral Award Program for Advancing Women in Science.

were determined by the taking the difference between the photon energy (1486.6 eV) and the binding energy cutoff, and calibration to a standard gold sample was performed. Work function values of ΔΦ = 4.1 and 4.6 eV were obtained for ITO substrates A and C, respectively; these are consistent with the range of commonly accepted values for ITO.35,39 The positions of the valence band (data not shown) were similar for both samples. The difference in the work function values observed for ITO substrates A and C could arise due to several factors, such as surface crystallographic orientation, In/Sn stoichiometric variation between the films, variations in carrier concentration at the surface, or different surface defect densities.40−42 The lower work function value for ITO substrate A is consistent with the larger initial current (and nucleation density) seen for Sample A (Figure 1). Although determining the underlying cause of variable surface stoichiometry within ITO substrates from the same supplier lot was beyond the scope of this work, one possible explanation may be connected to the processing conditions used to deposit the ITO films. Several groups have investigated the influence of oxygen partial pressure during ITO deposition via ion beam sputtering.18,38 Their results indicated that an increase in oxygen partial pressure during deposition was accompanied by an increase in the Sn4+ component in the Sn3d XPS spectra.34 Although local variations in oxygen partial pressure within the deposition chamber are unlikely for a single ITO deposition run, this possibility cannot be entirely ruled out. Furthermore, it is entirely possible that the ITO substrates received as a single lot from the supplier were fabricated in multiple batches, during which the oxygen partial pressure may have varied more substantially. It should also be noted that any differences due to relative amounts of chemisorbed water (i.e., greater for ITO substrate A) can be neglected because Cu2O electrodeposition takes place in aqueous solution.

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REFERENCES

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CONCLUSIONS Homogeneity within the ITO working electrode is a common assumption in the electrodeposition of Cu2O thin films; 24941

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