Article pubs.acs.org/cm
Role of Solution Chemistry in Determining the Morphology and Photoconductivity of Electrodeposited Cuprous Oxide Films Anna Osherov, Changqiong Zhu, and Matthew J. Panzer* Department of Chemical & Biological Engineering, Tufts University, Medford, Massachusetts 02155, United States S Supporting Information *
ABSTRACT: Electrodeposition of single-phase, crystalline Cu2O thin films is demonstrated using acidic lactate/Cu2+ solutions. Relative speciation distributions of the various metal complex ions present under different growth conditions are calculated using reported equilibrium association constants and supported experimentally by UV−vis absorption spectroscopy. Cu2O films grown from acidic lactate solutions can exhibit a distinctive flowerlike, dendritic morphology in contrast to the faceted, dense films obtained using alkaline lactate solutions. Dependence of thin film morphology on the lactate/Cu2+ molar ratio and the applied potential is described. Hot point probe measurements have been used to determine the p-type conductivity of these Cu2O films. A short circuit current density of 23 μA/cm2 under 0.583 mW/cm2 monochromatic green light emitting diode (LED) illumination was measured for a Cu2O film grown at pH = 5.27, which was substantially larger than that of a Cu2O film deposited under basic conditions. KEYWORDS: cuprous oxide, electrodeposition, solution speciation, photoconductivity deposition, leading to various film morphologies and orientations depending on the pH.4,13−17 For instance, a transition from ⟨100⟩ oriented growth to ⟨111⟩ oriented growth was reported upon increasing the pH of the electrodeposition bath from 9 to 12.13 Interestingly, cuprous oxide films deposited from an acidic acetate solution showed a flowerlike, dendritic morphology.7,18 This flowerlike morphology was attributed to the mass transfer-limited growth of the polyhedron crystal apexes, while polyhedron formation was attributed to the preferential acetate adsorption on the {111} crystallographic planes.7,11 Furthermore, solution pH was found to be a main parameter of control over the film conductivity type; n-type conductivity attributed to oxygen deficiency was demonstrated in films deposited from slightly alkaline copper sulfate8 and slightly acidic copper acetate mediums.6−8,18 According to first-principles calculations, however, intrinsic ntype defects or defect complexes in Cu2O cannot be the source of the n-type behavior displayed by electrodeposited samples.19 Therefore, the question remains as to whether the n-type conductivity demonstrated in previous reports is related to the low pH deposition conditions, unusual film morphology, the measurement technique, or some combination thereof. Furthermore, although solution chemistry plays a major role in the film formation process, no correlation between solution speciation (i.e., relative concentrations of soluble complexes present) and electrodeposited Cu2O film characteristics (such
1. INTRODUCTION Abundance, environmental safety, and an estimated practical potential of ∼10% power conversion efficiency1 make cuprous oxide (Cu2O) an attractive alternative for widespread use in future solar cells. Natively, cuprous oxide is a p-type semiconductor with a direct energy band gap of approximately 2.2 eV. Intrinsic p-type conductivity is generally attributed to cation deficiency, and it was postulated that because of the polaronic nature of the defect states, copper vacancies should dominate under all growth conditions.2 Although this statement applies for the majority of cuprous oxide films,2−5 n-type conductivity has also been reported in electrodeposited films under certain deposition conditions.6−8 Electrodeposition offers a controllable and a cost-effective route to the fabrication of cuprous oxide films on conductive substrates. A variety of solution chemistries have been used for the electrodeposition of Cu2O over the years.3,5,9,10 Ligand chemistry is widely employed to maintain stable solutions and prevent bulk precipitation within the solution volume that may destroy homogeneity of the electrodeposited layer and consequently deteriorate physical properties. However, ligands were also shown to modify film growth, via preferential adsorption3,11,12 or reaction with the desired material.5 Lactate and acetate are the most commonly used ligands in cuprous oxide electrodeposition, and several attempts to elucidate the role of the ligand in the film formation process have been made. However, no comprehensive studies have yet been reported to thoroughly clarify this issue. Alkaline lactate solutions (in the pH range 9 to 12) have been the most conventional medium for Cu2O electro© 2013 American Chemical Society
Received: October 11, 2012 Revised: December 14, 2012 Published: February 17, 2013 692
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determined from linear sweep voltammetry data (10 mV/s) using a custom MATLAB routine (see Supporting Information). The magnitude of the potential difference between the constant applied potential for film growth and the measured onset potential for each solution is denoted as Δϕ. Cathodic current densities are expressed as negative quantities in this work. 2.4. Characterization. Solution absorption spectra were collected in a quartz cuvette using an Evolution 300 UV−vis spectrometer (Thermo-Scientific). A reference spectrum from the cuvette containing DI water was subtracted from all copper-containing solution spectra. X-ray diffraction (XRD) patterns were recorded using a Bruker D8 with GADDS apparatus (Cu Kα radiation) operating at 40 kV and 40 mA. Scans were collected over a 2θ range of 15°−80°. Scanning electron microscopy (SEM) was employed for topographical characterization using a Zeiss FESEM Ultra Plus without coating the samples. The secondary electron signal was used to obtain topography images. Acceleration voltages ranged from 1.8 to 3.5 kV. Hot point probe measurements were conducted using two retractable gold pins (approximately 1 cm apart) connected to a Keithley 2602A sourcemeter. One of the gold pins was heated using a soldering iron, and the polarity of the resulting current between the pins was observed. Schottky junction solar cell structures were formed by depositing an EGaIn liquid metal droplet of controlled contact area using a Teflon ring (I.D. = 3.3 mm). Photoresponse was characterized under monochromatic green light emitting diode (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.
as orientation, microstructure, morphology, and consequent physical properties) has been established. To elucidate the influences of pH and ligand selection on the deposition process and consequent metal oxide conductivity type, systems with wide pH stability windows should be investigated. While acetate does not form a stable copper complex in alkaline media, copper-lactate complexes can be stable over a wide pH range. Two complexes are believed to dominate within acidic copper/lactate systems, CuLac+ and CuLac2.20,21 It has also been suggested that the chelation process is affected by the hydrogen ion concentration.22 In the high pH regime commonly used to electrodeposit Cu2O, the existence of alkaline copper-lactate complexes has been proposed, but not experimentally confirmed.23 To the best of our knowledge, electrodeposition of cuprous oxide from an acidic lactate medium has not been extensively investigated. The goals of this work are 2-fold: (1) to describe the tunable, flowerlike Cu2O thin film morphology obtained via cathodic electrodeposition from acidic copper/lactate solutions and compare the photoconductive behavior of these films with those grown from traditional basic media, and (2) to highlight the important connection between metal complex ion speciation in the growth solution and the resulting semiconductor film microstructure, as well as the optoelectronic properties of the metal oxide film.
2. EXPERIMENTAL DETAILS 2.1. Materials. Copper(II) sulfate pentahydrate (Sigma-Aldrich, >98.0%), sodium D-L lactate solution (Sigma-Aldrich, 60% w/w), sodium hydroxide (Sigma Aldrich, 50% in H2O), and sulfuric acid (Sigma Aldrich, 95−98%) were used without further purification. Tindoped indium oxide (ITO, resistivity 20 Ω/sq.) on glass was obtained from Thin Film Devices, Inc. Eutectic gallium−indium (EGaIn, SigmaAldrich, >99.99% trace metals basis) was employed as a soft top contact for electrical measurements. 2.2. pH Stability Measurements. An Extech PH220-C pH meter was used for measuring the pH of all solutions. To define the maximum pH stability limit of a given copper-lactate solution, the pH was increased gradually (intervals of 0.1 pH units) using sodium hydroxide solution and held at constant temperature (25 or 60 °C) by immersion in a water bath. A stable solution was defined as one with no visible precipitates after a period of 30 min. The presence or absence of precipitates was determined by shining a red laser beam (λmax = 653 nm) through the solution. Visible scattering of the laser beam was indicative of the presence of a precipitate. 2.3. Cu2O Electrodeposition. Films were electrodeposited from acidic (pH = 5.27) baths containing 0.02 M copper(II) sulfate pentahydrate and various concentrations of sodium lactate (NaLac) as a chelating agent to obtain Lac/Cu2+ molar ratios of 0.5, 1, 3, 5, 7.5, 12.5, and 15. Prior to deposition, the growth solution was preheated to 60 °C, and the temperature of the solution was maintained constant at 60 °C during the deposition by immersion in a water bath. Fine pH adjustments were made using sodium hydroxide or sulfuric acid to reach the desired pH at 60 °C. Depositions were performed for various periods of time (30−150 min.) to obtain complete coverage of the substrate, which depended on the solution characteristics and the magnitude of the applied cathodic potential. ITO substrates were thoroughly cleaned by successive sonication in 2 vol % Micro-90 in DI water, DI water, acetone, and finally immersion in boiling isopropanol. Cu2O electrodeposition was performed using a conventional threeelectrode, single compartment electrochemical cell. A clean ITO substrate as used as the working electrode (WE), a reticulated vitreous carbon structure with a high specific surface area was used as the counter electrode (CE), and Ag/AgCl in 4 M KCl solution served as the reference electrode (RE). All films were electrodeposited in potentiostatic mode using a Princeton Applied Research VersaSTAT 3 potentiostat. Onset potentials for cathodic reduction (Cu2+/Cu+) were
3. RESULTS AND DISCUSSION 3.1. Solution Characterization. To determine the pH stability range of precipitate-free copper-lactate solutions and establish the role of the lactate ligand in the electrodeposition process, solutions with various lactate/copper sulfate (Lac/ Cu2+) molar ratios were investigated. The values of the maximum pH at which Lac/Cu2+ solutions with various molar ratios maintain visual clarity with no visible precipitation at both 25 and 60 °C are shown in Figure 1. The data shown in Figure 1 indicate that increasing the Lac/ Cu2+ molar ratio increases the upper pH stability limit, allowing the solution to remain precipitate-free over a wider pH range. At 25 °C, two different regions can be clearly seen, with a transition point of about Lac/Cu2+ = 7.5, suggesting a
Figure 1. Upper limit of pH stability vs sodium lactate/copper(II) sulfate molar ratio at 25 °C (filled circles) and 60 °C (open circles); the copper sulfate concentration is 0.02 M. The data indicate the maximum pH value at each ratio that preserves a stable solution without visible precipitation (within 30 min). Dotted lines serve to guide the eye. 693
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Figure 2. (a) Calculated equilibrium distribution of copper species in solution as a function of lactate/copper ratio at pH = 5.27, 25 °C, 0.02 M CuSO4 (dotted lines indicates speciation at 60 °C calculated using data from ref 21). (b) Linear sweep voltammograms measured for copper-lactate solutions of various Lac/Cu2+ molar ratios at pH = 5.27. The potential was swept at a rate of 10 mV/s, from positive to negative potentials (vs Ag/ AgCl).
Figure 3. (a) UV−vis spectra of the copper(II) sulfate/sodium lactate solution with various Lac/Cu2+ molar ratios at pH = 4.5 and 25 °C. In all cases, 0.02 M copper sulfate was used; the spectrum of a solution containing no sodium lactate (curve labeled as Cu(H2O)62+) is also shown for comparison. (b) Calculated copper speciation diagram for Lac/Cu2+ = 0.5 vs pH at 25 °C (0.02 M CuSO4). (c) Calculated copper speciation diagram for Lac/Cu2+ = 15 vs pH at 25 °C (0.02 M CuSO4). Vertical dotted lines indicate the pH (4.5) of the solutions characterized in panel (a).
interested in varying the Lac/Cu2+ molar ratio over a wide range to study the potential effects of copper complex speciation on film formation and photoconductivity. Therefore, growth solutions were prepared at pH = 5.27, as this value allowed the deposition of continuous films at 60 °C and the ability to tune the Lac/Cu2+ molar ratio over a wide range, based on the measured stability data. Our working hypothesis states that identification of the predominant species present in the copper-lactate solution at various deposition conditions will facilitate a deeper understanding of the Cu2O film formation process, and as a result, better control of film properties. Equilibrium concentrations of all species present in the copper-lactate aqueous system under acidic pH conditions (pH = 5.27) were calculated using the Hydra-Medusa multitasking software.24 Although most studies suggest the existence of two primary copper lactate complexes (CuLac+ and CuLac2), the formation thermochemistry of the different complexes is still unclear, as several contradictory trends have been suggested.20−22 For this work, the stability constants for the copper-lactate complexes were adopted from reference 21:
transformation to a more thermodynamically stable chelated form of copper. As discussed below, the formation of a basic copper dimer complex may explain this observed behavior. Interestingly, the upper limit of the pH stability vs Lac/Cu2+ molar ratio for solutions at 60 °C shows a nearly linear trend over all studied Lac/Cu2+ molar ratios, suggesting the formation of a single dominant chelated form of Cu2+ (CuLac2) over a wide range of lactate concentration in the solution (Lac/Cu2+ ≥ 2). While the small drop in pH stability for Lac/Cu2+ molar ratios smaller than 7.5 upon increasing temperature from 25 to 60 °C can be explained mainly by water dissociation and the decreasing solubility of copper hydroxide with increasing temperature, the major drop in the pH stability at 60 °C for Lac/Cu2+ molar ratios higher than 7.5 indicates the exothermic nature of the particular complex formed at high Lac/Cu2+ molar ratios and 25 °C under basic conditions. Stability investigations such as these are important for Cu2O electrodeposition because precipitate-free growth conditions should be maintained to minimize homogeneous nucleation in solution and subsequent particle incorporation into the growing semiconductor film. In this work, we were particularly 694
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the absorption edge position and a strong increase in the absorption peak maximum near 800 nm. Since the relative shift in the absorption edge is related to the concentrations of the different complexes, once an appreciable amount of CuLac2 is present in the solution (i.e., for Lac/Cu2+ ≥ 1 at this pH) no further shift in the UV absorption edge is observed. Although the UV−vis spectra currently do not provide us with a quantitative determination of solution speciation (work ongoing), the qualitative picture they can provide regarding the relative amounts of expected complexes (e.g., CuLac+ vs CuLac2) makes this technique inherently valuable for systems where equilibrium complex stability constant values are not available (e.g., at different temperatures or ionic strengths). Similar phenomena are observed upon varying the pH for a fixed molar ratio of Lac/Cu2+ = 15. Figure 4 shows the variation
log K = 2.66
Cu 2 + + 2Lac− = CuLac 2 log K = 4.28
All other species association constants were taken from the software database (critically reviewed values).24 According to the copper-containing species distribution diagram shown in Figure 2a, two copper lactate complexes are dominant at pH = 5.27. It can be seen that CuLac+ is the dominating species up to a Lac/Cu2+ molar ratio of about 1.9. Upon further increase of the Lac/Cu2+ molar ratio, CuLac2 is predicted to be the predominant copper complex. The relationship between ligand concentration and the Cu2+/Cu+ reduction onset potential can be demonstrated experimentally by linear sweep voltammetry (LSV) in solutions containing different Lac/Cu2+ ratios, as demonstrated in Figure 2b. A gradual shift to more negative onset potential values with increasing Lac/Cu 2+ molar ratio up to 7.5 is clearly demonstrated, while further increases of the lactate concentration result in a stagnation of this trend. Furthermore, two different regions are apparent in the cathodic LSV curve: a gradual increase in the cathodic current at low applied potentials followed by a steeper rise in the cathodic current at more negative potentials may be clearly seen in all curves (Figure 2b). The solution speciation diagram shown in Figure 2a may partially explain the trends seen in the LSV data depicted in Figure 2b. Reduction of Cu2+ from the positively charged species CuLac+, present in an appreciable amount for Lac/Cu2+ molar ratios below ∼3, requires the shedding of only one lactate anion at the working electrode surface, and thus lower applied Δϕ values are required to facilitate Cu2O deposition. On the other hand, the neutral CuLac2 complex prevalent at higher Lac/Cu2+ molar ratios contains two chelating anions. The overlap in the LSV curves for Lac/Cu2+ ratios of 7.5 and 15 can be explained by a similarity in the relative amounts of CuLac+ and CuLac2 species present at these conditions (see Figure 2a). 3.2. UV−vis Spectroscopy. Changes in aqueous metal complex ion speciation can be monitored using UV−vis absorption spectroscopy. Figure 3 shows the variation in the absorption of solutions containing a constant concentration of CuSO4 (0.02 M) and various Lac/Cu2+ molar ratios. The shape of the UV−vis absorbance spectrum is indicative of the dominant aqueous metal complex present at a fixed pH (here, pH = 4.5 to obtain a stable solution with a Lac/Cu2+ ratio as low as 0.5). It is well-established that copper(II) ions form pale blue [Cu(H2O)n]2+ complexes in aqueous solutions.25 The addition of the lactate ligand results in a substitution reaction whereby lactate anions replace some of the water molecules to form more stable chelated copper complexes. As a result of copper-lactate complex formation, there is an increase in the copper d orbital energy level splitting, expressed as a shift of the ∼800 nm peak in UV−vis spectra to smaller wavelengths (Figure 3a). In addition, ligand−metal charge transfer results in stronger absorption of copper complexes in the UV region of the spectrum, as seen by a gradual shift toward higher wavelengths of the ∼350 nm absorption edge. The correlation between the measured absorption spectra (Figure 3a) and the calculated speciation diagrams for Lac/Cu2+ = 0.5 and Lac/Cu2+ = 15 shown in Figures 3b and 3c, respectively, implies a gradual transition from prevalent CuLac+ to CuLac2 with increasing Lac/Cu2+ molar ratio. This transition is expressed by a shift of
Figure 4. UV−vis spectra of a copper(II) sulfate/sodium lactate solution (Lac/Cu2+ = 15) at 25 °C and various pH values. The spectrum of a 0.02 M copper sulfate solution (0 M lactate) is also shown for comparison (as-prepared pH = 4.6 at 25 °C).
in the absorption spectra of solutions containing constant concentrations of CuSO4 (0.02 M) and sodium lactate (0.3 M) as the pH is varied between 2.43 and 12.8. A blue shift in the λmax ∼750−800 nm region with increasing pH is accompanied by increasing absorption intensity up to pH ∼7.4. This behavior can be attributed to the formation of a greater population of CuLac2 as more lactate anions become available for binding copper (the pKa of lactic acid is ∼3.8 at 25 °C). However, upon further increase of the pH there is an inversion of this trend, and further pH increases lead to both a reduction in intensity and continued blue shifting of this absorption peak. These observations clearly suggest the formation of an additional complex, which we tentatively designate as a basic copperlactate dimer complex, supported by the decrease in absorption peak intensity.26,27 Importantly, the UV−vis spectra of the aqueous copper lactate system with an excess of lactate present indicate a clear difference in speciation between acidic and basic pH conditions, which may be expected to lead to variations in film morphology and/or electronic properties. To test this idea, we performed photoconductivity measurements on Cu2O films electrodeposited from both acidic and basic lactate media (vide infra). In addition, the endothermic nature of the formation of the dominant complex present under acidic conditions (pH = 5.27), CuLac2, was confirmed by temperature-resolved UV−vis spectroscopy (Figure 5). Differences in the absorption peak wavelength position and an opposite trend in the peak position shift direction with increasing temperature shown in Figures 5a and 5b support the idea of distinctly different speciation under acidic and basic conditions in the Lac/Cu2+ = 15 solution. Comparison with Figure 4 provides further insight into the 695
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Figure 5. UV−vis spectra of 0.02 M copper(II) sulfate/sodium lactate solutions (Lac/Cu2+ = 15) at 25 °C, 45 °C, and 60 °C. (a) pH = 5.27 (at 25 °C), (b) pH = 10 (at 25 °C). A pH decrease of ∼0.8−1.5 pH units is observed when both solutions are heated to 60 °C.
The spectra shown in Figure 6 indicate the presence of single-phase crystalline Cu2O films; all nonsubstrate related reflections observed correspond to the cuprite structure (JCPDS No. 78-2076). The absence of additional diffraction peaks indicates that no other crystalline phases, such as metallic copper, copper(II) oxide, or copper carbonates exist with any detectable concentration within the layers. The XRD spectra also indicate an intensification of preferred ⟨111⟩ orientation with increasing Lac/Cu2+ molar ratio. This phenomenon may be attributed to a higher out of plane growth rate along the ⟨111⟩ direction. However, an increase in Cu2O grain size for a constant X-ray sampling area (as shown in Figure 7) may also contribute to this observation. Secondary electron SEM images (Figure 7) show topography evolution as a function of Lac/Cu2+ molar ratio and Δϕ for Cu2O films grown at pH = 5.27. It can be seen that a flowerlike morphology is dominant at low Δϕ (152 mV) for all studied Lac/Cu2+ molar ratios. This morphology is radically different from the typically observed microstructures of films deposited from basic copper/lactate solutions4,16,17,28 (see Supporting Information, Figure S1). The SEM images indicate that increasing the Lac/Cu2+ molar ratio leads to an increase in the domain sizes, similar to the case of depositions from acetate solutions.7 The flowerlike morphology reveals that the nucleation of Cu2O predominantly occurs on the ITO substrate, and that lateral growth is preferable over increasing thickness. Previously, it has also been suggested that increasing domain size results in higher photocurrents for n-type Cu2O films deposited from acidic acetate medium.7 Increasing the applied potential difference relative to the onset of Cu2+ reduction produces drastic changes in film morphology. There is a clear transition from branched, flowerlike domain structures to more dense and truncated polyhedral grains with increasing Δϕ. These changes observed in thin film morphology can be partially explained by a greater initial crystallite nucleation density at larger overpotential values.6 In addition, the speciation of copper complexes in the deposition solution shown in Figure 2a should be considered. Decomposition of CuLac+ (a more prevalent species at low Lac/Cu2+ molar ratios) requires smaller values of Δϕ. Therefore, for depositions from low Lac/Cu2+ molar ratio solutions, dense nucleation is observed across a wide range of Δϕ values (Figures 7a, 7e, 7i). On the other hand, for solutions dominated by the CuLac2 complex at large Lac/Cu2+ molar
thermodynamics of the dominant complex formation under these two conditions. It is essential to note that an increase in the solution temperature from 25 to 60 °C is accompanied by a decrease in the solution pH value by about 0.8 pH unit for a starting pH value of 5.27 and a decrease of about 1.5 pH units for a starting pH value of 10. Although the expected result of decreasing pH (at fixed temperature) from a starting point of around pH = 5.3 is a decrease in the absorption peak and a redshift of the peak wavelength position (Figure 4), in fact, an increase in the absorption peak located at ∼775 nm is observed upon decreasing the pH by heating the solution, as seen in Figure 5a. Therefore, we attribute the increasing absorption peak to the endothermic nature of the dominant complex formed under these conditions (CuLac2).21 For the basic solution (starting pH = 10 at 25 °C), the increasing peak absorption intensity and red-shift of the peak wavelength position with increasing temperature (decreasing pH) seen in Figure 5b mirror the observed behavior upon changing pH alone (Figure 4). As such, the thermochemistry of forming the dominant complex present under highly basic conditions cannot be determined by these measurements alone. 3.3. Film Characterization. XRD scans of Cu2O films deposited from an acidic copper/lactate medium (pH = 5.27) with various Lac/Cu2+ molar ratios at Δϕ = 0.152 V are shown in Figure 6.
Figure 6. XRD spectra of Cu2O films deposited from CuSO4 solution with various Lac/Cu2+ molar ratios at pH = 5.27, 60 °C, and Δϕ = 0.152 V. Spectra have been vertically offset for clarity. The dotted vertical lines indicate positions of the ITO substrate diffraction peaks. 696
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Figure 7. Top view SEM images of Cu2O films electrodeposited from copper(II) sulfate/sodium lactate solutions at pH = 5.27 for various Lac/Cu2+ molar ratios and Δϕ values.
Figure 8. (a) Current−voltage characteristics of Cu2O films electrodeposited from acidic (pH = 5.27, Lac/Cu2+ = 3, Δϕ = 172 mV) and basic (pH = 8.5, Lac/Cu2+ = 15, Δϕ = 180 mV) growth solutions (0.02 M CuSO4) in the dark, and illuminated by a green LED (λmax = 520 nm, 0.583 mW/ cm2). (b) Schematic cross-section of the electrical testing setup (not to scale). (c) Approximate energy band diagram of the ITO/Cu2O/EGaIn structure under short circuit conditions.
images of the films (Supporting Information, Figure S2). As seen in Figure 8a, the ITO/Cu2O/EGaIn (eutectic gallium− indium) structures created (Figure 8b) displayed rectifying behavior and photovoltaic characteristics under illumination. The large mismatch between the EGaIn workfunction (4.2 eV below vacuum29) and the valence band level of Cu2O (∼5.2 eV below vacuum1,30) produced a Schottky junction at this interface and enabled the observed diode-like response. As shown previously by McShane and Choi, the ITO/Cu2O junction exhibits a nearly Ohmic response;28 using the minimum ITO work function specified by the manufacturer (4.8 eV) and the Cu2O Fermi and valence band maximum energy levels determined by Yoo et al. using photoelectron spectroscopy,31 an approximate energy band diagram for the ITO/Cu2O/EGaIn structure under short circuit conditions is shown in Figure 8c. Interestingly, the Cu2O film grown from acidic (pH = 5.27) lactate media with Lac/Cu2+ = 3 and Δϕ = 172 mV produced the largest photovoltaic response of all films measured, exhibiting a short circuit current density of 23 μA/cm2, an open circuit voltage of 0.48 V, and a fill factor of 20.52% under green LED (λmax = 520 nm, 0.583 mW/cm2) illumination. The short circuit photocurrents of flowerlike Cu2O films with larger apparent grains (such as those shown in Figure 7c, 7d, 7h) were at least 1 order of magnitude lower in comparison. It should
ratios, very large and flowerlike domains are achievable if Δϕ is not too high (Figure 7d). 3.4. Electrical Characterization. While both p-type and ntype electrodeposited Cu2O films have been reported previously, the p-type conductivity of all electrodeposited Cu2O films produced in this work was confirmed using a hot point probe measurement. The positive sign of the current due to the Seebeck effect indicated that holes are the majority charge carriers. In contrast, a bare ITO substrate exhibited a negative current (n-type conductivity, as expected) in the same setup. To probe potential differences in photoconductivity between films grown from different solutions (varying pH, Lac/Cu2+ ratio, Δϕ), two-electrode current−voltage measurements were performed both in the dark and under monochromatic (green) LED illumination through the glass/ITO substrate. Current− voltage characteristics were highly repeatable for films subjected to multiple voltage sweeps. All Cu2 O films were of approximately equal thickness based on comparable values of integrated charge density versus growth time during electrodeposition. Although the film topography was typically quite rough, an approximate average film thickness of ∼750 nm was calculated (assuming 100% Coulombic efficiency) using the integrated charge density and the bulk density of Cu2O (6.0 g/ cm3); this value agreed well with typical SEM cross-sectional 697
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also be noted that the photoresponse of a Cu2O film deposited from basic lactate solution at pH = 8.5 was extremely low in comparison to the best acidic media-grown Cu2O film (Figure 8a). This large difference cannot be attributed to a significant variation in optical absorption between the films (Supporting Information, Figure S3). Collectively, these observations support the conclusion that differences in the dominant complex metal ion species present in solutions of varying pH and/or ligand/Cu2+ ratio (see Figures 1 and 4, and discussion thereof) can substantially influence the optoelectronic properties of the resulting Cu2O films.
Awardee of the Weizmann Institute of Science - National Postdoctoral Award Program for Advancing Women in Science.
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4. CONCLUSION Cuprous oxide thin films were successfully electrodeposited from acidic lactate solutions on ITO-coated glass substrates. Film morphology was shown to depend on both the ligand/ Cu2+ molar ratio as well as the applied potential, and a distinct flowerlike, dendritic microstructure was observed. Solution UV−vis absorption spectra supported equilibrium calculations of the relative speciation of copper complexes in the deposition medium and provided evidence of a basic copper-lactate dimer complex that forms under high pH conditions for large lactate/ Cu2+ ratios. A connection between the solution chemistry of the electrodeposition bath and the resulting Cu2O thin film material properties was evidenced by photocurrent measurements using Schottky junction photovoltaic structures. The ptype conductivity of Cu2O films electrodeposited from acidic lactate baths was determined using a solid-state hot point probe measurement, disproving the hypothesis that acidic conditions alone determine the material conductivity type. Further investigation of the effects of relative metal complex ion speciation for a variety of ligands on Cu2O electrodeposition is expected to provide important insights into the growth solution-film structure−property relationships that will lead the way to high performance Cu2O-based solar cells.
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ASSOCIATED CONTENT
* Supporting Information S
Description of onset potential determination, SEM image of a Cu2O film deposited from basic copper/lactate solution, crosssectional SEM image of a Cu2O film, and Cu2O film optical absorption data. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge Prof. Elena Rybak-Akimova at Tufts University for several insightful discussions. SEM images were obtained 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. This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under award no. DMR-08-19762. One of the authors (Anna Osherov) is an 698
dx.doi.org/10.1021/cm303287g | Chem. Mater. 2013, 25, 692−698
