Dual Role of Sb-Incorporated Buffer Layers for High Efficiency

Jul 17, 2017 - Dual Role of Sb-Incorporated Buffer Layers for High Efficiency Cuprous Oxide Photocathodic Performance: Remarkably Enhanced Crystallini...
0 downloads 0 Views 2MB Size
Download PDF
Research Article pubs.acs.org/journal/ascecg

Dual Role of Sb-Incorporated Buffer Layers for High Efficiency Cuprous Oxide Photocathodic Performance: Remarkably Enhanced Crystallinity and Effective Hole Transport Seung Ki Baek,† Joo Sung Kim,† Young Been Kim,† Jae Hong Yoon,‡,§ Han-Bo-Ram Lee,‡,⊥ and Hyung Koun Cho*,† †

School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do 16419, Republic of Korea ‡ Innovation Center for Chemical Engineering, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon 22012, Republic of Korea § School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-Gu, Seoul 03722, Republic of Korea ⊥ Department of Materials Science and Engineering, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon 22012, Republic of Korea S Supporting Information *

ABSTRACT: The high performance of electrodeposited cuprous oxide (Cu2O)-based photoelectrochemical (PEC) cells has been limited due to low electrical conductivity hindering effective carrier transport to electrodes and chemically unstable properties in aqueous environments, despite their several advantages such as suitable band gap, band position, and cost-effective and environmentally friend elements. To improve the fundamental photoelectrochemical properties of photocathode Cu2O layers, particularly their photocurrent and stability, we present a simple approach using a double-layer photocathode, where the doublelayer structure consists of Sb-incorporated Cu2O (Cu2O:Sb) and undoped Cu2O. The Cu2O:Sb double layer enhanced the preferred crystal growth along the [111] direction, as well as the crystallinity of the Cu2O. This microstructural change resulted in high electrical conductivity owing to high hole mobility and the suppression of instability related to surface facets. Consequently, the introduction of Cu2O:Sb led to the simultaneous roles of seed crystal and effective hole transport, and our double-layer photoelectrodes have shown good photocurrent without any metal photocatalysts and relatively better photostability without the help of protection layers. KEYWORDS: Cuprous oxide, Photoelectrochemical, Double-layer, Electrodeposition, Hole transport

■

INTRODUCTION

Cuprous oxide (Cu2O), natively a p-type oxide, is a suitable photocathode for hydrogen evolution because of its adequate direct band gap of 2 eV, which can absorb a significant part of the solar spectrum, and favorable energy band position, with the conduction band (CB) lying at −0.7 V of the hydrogen evolution potential. Thus, the electrons excited into the CB of Cu2O are energetic enough to be transferred to water for hydrogen evolution.2 In addition, this attractive Cu2O material is abundant, inexpensive, and an environmentally friend element, and it can be synthesized by low-cost methods such as solution-based and electrochemical processes.3,4 Nevertheless, Cu2O photoelectrodes have critical limitations in terms of high PEC performance. First, Cu2O films are

The demand for steady and dependable power sources is very high in the field of sustainable energy because of the limited amount of fossil fuels reserves. Among several sustainable alternatives, solar energy may be the most efficient solution because it constitutes the largest renewable energy source. However, because solar energy is only available during the day, an efficient energy storage solution is necessary for the storage of solar energy under various circumstances. So far, the only practical way to store such large amounts of energy has been to use a chemical energy carrier like a fuel. In various solar energy to power conversion systems, the photoelectrochemical (PEC) splitting of water into hydrogen and oxygen by the direct use of solar energy is an ideal process.1 It is a renewable method of hydrogen production integrated with solar energy absorption and water electrolysis using a single photoelectrode. © 2017 American Chemical Society

Received: June 12, 2017 Revised: July 3, 2017 Published: July 17, 2017 8213

DOI: 10.1021/acssuschemeng.7b01889 ACS Sustainable Chem. Eng. 2017, 5, 8213−8221

Research Article

ACS Sustainable Chemistry & Engineering

the best of our knowledge, no effective and systematic studies using buffer seed layers to obtain electrodeposited Cu2O films with high crystallinity and smooth morphology have been undertaken or reported. In addition, to mitigate the barrier height at the absorber/ electrode interface as a result of inefficient Schottky contact and to accelerate the charge transport of hole carriers to the electrode, the addition of a more electrically conductive hole transport layer (HTL) between the p-type absorber layer and the electrode can be considered as an intellectual approach, which was proven in various photovoltaic cells and organic light-emitting diodes.13,14 It is typically beneficial to have an HTL with a high hole mobility to facilitate high conductivity and an appropriate band position to enhance the hole collection efficiency. Until now, most studies on improving PEC cells have focused on the superjacent layer of the absorbers in an effort to reduce interfacial defects in various passivation layers.15,16 In some studies, an additional coating has been used to enhance the photovoltage by generating an electric field by the formation of p−n junctions.17 In contrast to the approach taken in these previous studies, our idea for improving the PEC characteristics of Cu2O photocathodes is to find an adequate underlying layer that provides a seed buffer layer for high crystallinity/conductivity while acting as an HTL at the same time. In the current study, it was confirmed that the incorporation of a small amount of Sb in the Cu2O deposition bath caused a significant enhancement in the [111] preferred orientation and induced a high carrier mobility.18 However, this considerable improvement in crystallinity decreased light scattering, resulting in high transmittance above 70% in the visible wavelength, which in turn adversely affected the generation of a photocurrent in photoactive devices and restricted the direct use of the highly conductive Cu2O:Sb film as an absorber layer. Thus, instead of a single-layer structure, we propose a double-layer structure consisting of an undoped Cu2O absorber layer with high absorbance and a highly conductive Cu2O:Sb layer to provide seed crystals. In particular, the introduction of a Cu2O:Sb underlayer is expected to offer several advantages for the improvement of Cu2O photocathode performance. First, it will reinforce the preferred orientation of the Cu2O absorber between the (100) and (111) facets by using a crystal seed layer. Second, the Cu2O:Sb film has higher conductivity and higher hole mobility than the Cu2O absorber, which will lead to effective hole charge transport to the electrode. Third, the Cu2O:Sb seed layer will produce favorable (111) crystal orientation, thus improving the PEC stability of Cu2O. Consequently, the use of double-layer Cu2O:Sb/Cu2O photoelectrodes is anticipated to induce profitable gains in the photocurrent and stability.

chemically unstable because of a self-photodegradation mechanism. Because the redox potential of Cu2O corresponding to self-reduction (Cu2O to Cu) or self-oxidation (Cu2O to CuO) by the photogenerated electrons or holes lies within its bandgap, photocorrosion is unavoidable under visible-light illumination.5 Thus, additional thin protective layers based on TiO2 have been used on top of the Cu2O film to prevent direct contact between the Cu2O and the electrolyte. However, highly resistive TiO2 requires high growth temperatures, resulting in an electrically resistive Cu2O film, and an appropriate film thickness, which reduces the photocurrent. In addition, the surface energy between Cu2O and TiO2 hinders uniform conformal coating of the additional TiO2 layer.6 Thus, to reduce the intrinsically detrimental effects of an additional protective coating, more robust Cu2O photocathodes are needed. Second, the electrodeposited Cu2O films generally exhibit low electrical conductivity and short minority-carrier diffusion lengths, which cause low photocurrents because of frequent nonradiative recombination.7 To enhance the PEC properties of photocathode Cu2O layers, particularly their photocurrent and stability, the preferential crystal orientation, which is strongly linked to the electrical conductivity and chemical stability of the Cu2O film, must be artificially controlled. In electrodeposited Cu2O synthesis, the preferred orientation is related to the solution pH because the concentration of hydroxyl ions (OH−) determines the primary crystal surfaces with the slowest growth rate. In previous reports, as the solution pH increased, the concentration of hydroxyl ions increased, and growth was favored along the crystallographic faces with the highest number of oxygen atoms per unit area, which corresponds to the {111} planes.8 As a result, the Cu2O crystals were preferentially oriented in the [111] direction, and the exposed crystal faces consisted of {200} crystallographic planes with three-sided pyramids. In addition, it was suggested that the high pH condition in p-type Cu2O synthesis induced an increase in the carrier concentration because of the presence of oxygen interstitials and enlarged grains.9 Furthermore, the exposed {200} crystal facets in the [111] preferentially oriented Cu2O layers were relatively more stable in chemical reduction reactions as compared to the {111} surface faces in [200]oriented Cu2O, which is attributed to differences in the respective surface energies theoretically expected by firstprinciples calculations.10 Therefore, it is proposed that highly stable and conductive Cu2O photocathodes can be produced from crystals with a [111]-preferred orientation. However, in the electrochemical deposition of Cu2O layers, the production of crystal films with a strongly preferred [111] orientation is fundamentally prohibited owing to the delay of complete film formation because of the low density of large nucleation crystals and irregular crystal coarsening between adjacent grains with different crystallographic orientations. Thus, up to now, most electrodeposited Cu2O layers have exhibited both [200]- and [111]-orientated crystals under optimized synthesis conditions. An alternative deposition design such as a two-step growth process utilizing an additional seed crystal layer is suggested herein as a method for expedient control of the preferred orientation. For example, for GaN and ZnO epitaxial films on sapphire substrates, low-temperature nucleation layers have been used prior to the deposition of high-temperature films to reduce the dislocation density generated from lattice mismatch.11,12 This method resulted in a breakthrough in obtaining high-quality smooth films. On the other hand, to

■

EXPERIMENTAL SECTION

ITO working electrodes with a sheet resistance of 10 Ω/sq and a thickness of 180 nm were fabricated on glass substrates. These ITO/ glass substrates were cut into 2 cm × 3 cm pieces and cleaned by sonication sequentially in acetone, ethanol, and distilled water for 10 min each. The Cu2O:Sb seed layers were electrodeposited from a solution of lactate-stabilized copper sulfate (CuSO4, Junsei, > 98%) buffered to pH 11 with a sodium sulfate (4 M NaOH) solution at 60 °C. For the Sb-incorporated Cu2O films, 3 mM antimony sulfate (Sigma-Aldrich, > 98%) was added to the chemical bath. The deposition conditions were optimized to provide enhanced electrical properties, and potentiostatic electrodeposition was carried out using a three-electrode system (Princeton Applied Research Versatate 4) with a Pt counter electrode and a Ag/AgCl (saturated NaCl) reference 8214

DOI: 10.1021/acssuschemeng.7b01889 ACS Sustainable Chem. Eng. 2017, 5, 8213−8221

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Chronoamperometry of (a) Cu2O on an ITO substrate and (b) Cu2O on Cu2O:Sb. The deposition condition of Cu2O was same (pH 11, −0.3 V, 60 °C).

■

electrode. The first Cu2O:Sb seed layer of thickness of ∼300 nm was deposited at −0.5 V with an electric charge of 0.3 C/cm2. The undoped Cu2O absorber layers were potentiostatically electrodeposited in a pH 11 lactate-stabilized copper sulfate chemical bath at −0.3 V, regardless of the single- or double-layer structure. The total charge density of a single layer was 3 C cm−2, whereas the charge densities in the double layer were 2.7 C cm−2 for the Cu2O layer and 0.3 C cm−2 for the Cu2O:Sb layer. The effect of the seed layer on the morphological properties was visualized by field-emission scanning electron microscopy (FE-SEM, JSM-6700F, 10 kV). The phase and crystallinity of the thin films were investigated by X-ray diffraction (XRD, Bruker AXS D8 Discover with a Cu Kα radiation source). The electrical properties of each structure were evaluated with an I−V measurement system (HP-4145B), and the optical transmittance spectra were measured using an ultraviolet− visible−near-infrared (UV−Vis−NIR) spectrophotometer (Varian Cary 5000). The PEC performance of the samples was investigated using a three-electrode system with front illumination (AM 1.5, 150 W xenon lamp) using the Princeton Applied Research Versatate 4 system. The reference electrode and counter electrode were Ag/AgCl (sat. NaCl) and Pt mesh, respectively. The electrolyte was a 1.0 M Na2SO4 solution buffered at pH 5 with 0.1 M K2HPO4. The scan rate for the linear sweep voltammetry was 10 mV/s in the cathodic direction. Photocurrent stability tests were carried out by measuring the photocurrent induced under chopped light illumination at a fixed electrode potential of 0 V versus a reversible hydrogen electrode (RHE). Electrochemical impedance spectroscopy measurements were performed in the dark using a potentiostat in a 1.0 M Na2SO4 solution (pH 6.5). The potential was swept in the stable range, as confirmed by a Pourbaix diagram, with an AC signal of 10 mV and a frequency range between 1 Hz and 10 kHz. The impedance data were fitted using ZSimWin electrochemical data software. A commercial ALD chamber with a 4-in. capable stage was used to deposit TiO2. The titanium tetraisopropoxide (TTIP) was contained in a stainless steel canister of which temperature was maintained at 50 °C to obtain an appropriate vapor pressure during the ALD process. The H2O counter reactant was contained in other canister, and its flow was controlled by a needle valve. N2 (99.999%) was used for purging gas of which flow was controlled by a mass-flow controller (MFC). Further information about the ALD chamber can be found in our previous report.19 One ALD step is composed of four steps: precursor exposure (ts), N2 purging (tp), reactant exposure (tr), and N2 purging. For better conformity and uniformity, an infiltration step was added after the precursor and reactant exposure step, and the deposition temperature was under 150 °C.

RESULTS AND DISCUSSION To investigate the effect of the Cu2O:Sb seed layer on the electrochemical behavior in the deposition process, the current transients of the Cu2O films on the ITO substrates and the Cu2O:Sb seed layers were compared, and the results are shown in Figure 1a and b, respectively. In general, this current density−time transient graph can be functionally separated into three sections: (i) the nucleation step, which is the negative increment of current density by adatom nucleation of Cu2O, (ii) the grain growth step, which is the subsequent decrease of the current density after the electroactive area is maximized, and (iii) the planar diffusion step, which is a nonzero steadystate region due to the formation of a continuous twodimensional film.20 These steps could be controlled using the electrodeposition parameters and the conductivity of the substrate. In the single-layer Cu2O deposition at −0.3 V and pH 11, the nucleation step occurred over a long period of 250 s. The planar diffusion step with saturation current is not typically detected because of the rough surface with a facet structure (Figure 1a). On the other hand, the Cu2O deposition on the Cu2O:Sb seed layer with high nucleation density and flat film morphology perfectly omits the nucleation step and quickly leads to a steady saturation current density after 30 s (Figure 1b). This result implies that the undoped Cu2O absorber layer follows the characteristics of Cu2O:Sb as a template, unlike the case with the single layer. As shown in Figure 1a, b, the final thickness of both samples was determined by the electric charges as −3 C, and the seed layer Cu2O:Sb of double layer structure was deposited with −0.3 C. The incorporation of Sb into the seed layer of double layer structure was confirmed as reported in our previous study.18 The effect of the growth behavior on the structural properties was confirmed by XRD, and the results are shown in Figure 2. The XRD peaks from the single Cu2O film were well indexed to Cu2O (JCPDS 65-3288) and assigned to the (110), (111), (200), and (220) planes at 29.6°, 36.5°, 42.4°, and 61.5° (Figure 2a). This result indicates the typical low (111)preferred orientation of the Cu2O film, despite the synthesis in a pH 11 solution. In contrast, the Cu2O:Sb/Cu2O double layer revealed strong (111) Cu2O diffraction at 36.5° and the absence of diffraction from the other crystal planes, even on a log scale (Figure 2b). This significant change in the XRD results was caused just from the use of the Cu2O:Sb seed layer, which is comparable to the results from the single Cu2O:Sb 8215

DOI: 10.1021/acssuschemeng.7b01889 ACS Sustainable Chem. Eng. 2017, 5, 8213−8221

Research Article

ACS Sustainable Chemistry & Engineering

Figure 2. XRD spectra of the Cu2O single layer and Cu2O:Sb/Cu2O double layer for comparison of the (111)- preferred orientation. The asterisk (∗) indicates the diffraction obtained from the ITO substrate.

layer with high transmittance. Consequently, it is verified that the two-step growth using the Cu2O:Sb seed layer produces a high-crystallinity absorber layer, thus facilitating low defect density and effective charge transport. The significantly improved crystallinity also significantly changed the microstructural morphology. Figure 3a and b are schematic diagrams showing the expected evolution of the crystal morphology of the Cu2O:Sb/Cu2O double layer and the Cu2O single layer in the initial (total charge ∼0.3 C) and final (∼3 C) states, respectively, as indicated in Figure 1. Figure 3c and d show the cross-sectional SEM images of the electrodeposited Cu2O:Sb/ Cu2O and Cu2O films, respectively. Despite the Cu2O:Sb thickness of just 300 nm, the Cu2O:Sb/Cu2O double layer exhibits a crystal shape that is morphologically similar to that of the Cu 2 O:Sb single layer with significantly improved crystallinity. These results imply that the thin homogeneous seed layer determines the structural and morphological characteristics of the whole Cu2O photocathode. To compare the electrical resistivity of the single and double Cu2O layers, the current density−voltage (J−V) measurements were carried out in the dark because Hall measurements were impossible because of the use of conductive ITO/glass substrates. Although there was a slight Schottky barrier between the Cu2O film and the indium metal, it is clear that the doublelayer Cu2O:Sb/Cu2O thin film was more conductive (Figure 4a). The current density at +3 V from the Cu2O single layer was 0.1 mA/cm2, but the Cu2O:Sb/Cu2O exhibited a current density of 1.37 mA/cm2, which is more than 10 times higher than that of the single layer. This result is quite surprising because the relatively thin Sb film incorporated as a buffer layer was inserted below the absorber Cu2O layer, and this was the only difference between the two films. In addition, although the J−V measurements were carried out for a vertical structure requiring charge transport via the undoped Cu2O film, a current enhancement above 1 order of magnitude was observed. Figure 4b shows the UV−vis absorbance spectra from the single and double Cu2O layers. The electrodeposited

Figure 3. Schematic diagrams showing the growth evolution of (a) the Cu2O:Sb/Cu2O double layer and (b) the Cu2O single layer in the initial and final states. Cross-sectional SEM images of the (c) double layer and (d) single layer.

Figure 4. (a) I−V characteristics of Cu2O and Cu2O:Sb/Cu2O layers. (b) Optical transmission spectra of Cu2O:Sb, Cu2O, and Cu2O:Sb/ Cu2O films. The inset shows optical microscopy images of each samples.

8216

DOI: 10.1021/acssuschemeng.7b01889 ACS Sustainable Chem. Eng. 2017, 5, 8213−8221

Research Article

ACS Sustainable Chemistry & Engineering Cu2O films had high absorption coefficients, leading to high absorbance below its band gap of 2.1 eV. However, a significant increase in the transmittance in the Cu2O:Sb films was observed at wavelengths higher than 520 nm (∼70%) because of the decrease in light scattering owing to the high crystallinity (Figure S1). In contrast, the low transmittance of the undoped Cu2O may be attributed to the low crystallinity of the lesspreferred orientation and the rough surface, resulting in a considerably lower photocurrent than expected. In addition, the greater conductivity is expected to induce highly effective hole charge transport behavior and result in a high photocurrent value if the absorber layers have relatively good crystallinity and high absorbance. Thus, to apply the Cu2O double layer with enhanced conductivity to photoelectrochemical cells, its semitransparent property (single Cu2O:Sb) should be nonpreferred for photoinduced charge generation. It is noticeable that the double-layer structure with a thick undoped Cu2O absorber resulted in a considerably higher absorbance in comparison to the Cu2O:Sb single layer (Figure 4b). In particular, the surface contrast was not murky, unlike the single Cu2O, but shiny red owing to its high crystallinity, well-ordered grain boundaries, and small surface facets as shown in the inset of Figure 4b. It is thus feasible to apply the double layer as the photoabsorber layer. Therefore, we can suggest that the combination of the thin Cu2O:Sb buffer layer as the seed crystal layer and the thick undoped Cu2O layer leads to enhanced crystallinity and conductivity and moderate optical characteristics. To investigate the effects of the Cu2O:Sb buffer layer on the behavior of the photoinduced charge carriers in the Cu2O absorber layer, PEC measurements were performed. When the Cu2O layer was exposed to light illumination with energy higher than 2.1 eV, photoinduced electron−hole pairs were generated. Then, under a negative bias (vs Ag/AgCl), electrons flowed from the counter electrode to the Cu2O working electrode, generating a cathodic current. These results indicate that the Cu2O single and Cu2O:Sb/Cu2O double layers have ptype characteristics (Figure 5). As the potential sweeps to the

increment of the applied potential due to the enhancement of the charge transfer to the electrolyte. For a good photocathode driving water reduction, the photocurrents at positive potentials of the reversible hydrogen evolution should be as high as possible. In the case of a semiconductor photocathode without an additional coating and catalysts, the typical photocurrent in the half-cells, coated on the transparent conducting substrate, show values of ∼1 mA/cm2 under 1-sun irradiation.22 From our single- and double-layer photocathodes without an additional coating (photocatalyst, surface passivation, and n-type coating), the Cu2O:Sb/Cu2O double layer reached a cathodic photocurrent of approximately −1.7 mA cm−2 at zero potential, whereas the Cu2O single layer reached a cathodic photocurrent of −1.0 mA cm−2 at 0 V vs RHE. Thus, it can be concluded that the double-layer structure absorber increases the photocurrent, despite its higher transmittance. For the high photocurrent density, additional important factors except for the absorption property should be considered, which include charge transport and carrier collection efficiency. Basically, the overall photocurrent density of the photocathode can be determined from the following three contributions: (i) the light harvesting efficiency corresponding to the light absorption and photogenerated electrons, (ii) the charge transport efficiency related to the charge separation in the depletion region in the electrolyte and electrode interface, and (iii) the carrier collection efficiency, which is the fraction of surface electrons that reduces water via the suppression of recombined carriers. Since no photocatalyst was used to improve the carrier collection efficiency, it can be considered that the high performance of the double-layer structure originated from the high charge transport efficiency. We measured the incident photon-to-current efficiency (IPCE) under monochromatic illumination to survey the changes of the spectral response from 350 to 650 nm (Figure 6). Because these samples are susceptible to photoelectrochem-

Figure 6. IPCE spectra for Cu2O and Cu2O:Sb/Cu2O films biased at +0.4 V vs RHE. Both current efficiencies were normalized by the maximum value of the Cu2O:Sb/Cu2O film. Figure 5. Linear sweep voltammetry of the Cu2O-based photocathodes with and without a Cu2O:Sb seed layer under chopped AM 1.5 light illumination.

ical corrosion owing to the lack of protection layers, the IPCE responses were measured at a safe potential of +0.3 V (vs RHE) and normalized by the highest value. The first allowed transition of Cu2O occurred at approximately 2.5 eV, which resulted in a sharp increase in absorption at ∼500 nm. Thus, the IPCE spectra should be analyzed with two sections. At lower photon energies (>500 nm), the conversion efficiency from these samples was poor because of the short minority carrier diffusion length. In contrast, the response in the shortwavelength region (