Electrochemically Synthesized Mesoscopic Nickel Oxide Films as

Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Electrochemically Synthesized Mesoscopic Nickel Oxide Films as Photocathodes for Dye-Sensitized Solar Cells Jin Soo Kang,†,‡,# Jin Kim,†,‡,# Jeong Soo Kim,§ Kyungju Nam,∥ Hyungyung Jo,∥ Yoon Jun Son,†,‡ Jiho Kang,†,‡ Juwon Jeong,†,‡ Heeman Choe,*,∥ Tae-Hyuk Kwon,*,§ and Yung-Eun Sung*,†,‡ †

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea § Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ∥ School of Materials Science and Engineering, Kookmin University, Seoul 02707, Republic of Korea

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‡

S Supporting Information *

ABSTRACT: Dye-sensitized solar cells (DSCs) have been considered as promising and reliable solar energy conversion devices that are suitable for various applications. In general, mesoscopic TiO2 films are often used as photoanodes in DSCs; however, significant effort has been put into the development of NiO photocathodes with the long-term goal of tandem DSCs for high-voltage output. In this study, we report the preparation of mesoscopic NiO films via electrochemical anodization of Ni foil and their application as photocathodes in p-type DSCs. The anodic NiO had a mesoporous structure composed of ∼10 nm nanoparticles, and to the best of our knowledge, this was the first demonstration of mesoscopic NiO synthesis by anodic oxidation. The performance of the anodic photocathodes was comparable to that of conventional NiO electrodes comprising 20 nm-sized particles, even though the preparation of the anodic oxide film was significantly easier. Additionally, charge transfer kinetics in the NiO films were compared with those in conventional photocathodes using electrochemical impedance analyses, and the feasibility of the anodic NiO photocathodes in tandem DSCs was also demonstrated. KEYWORDS: electrochemical anodization, nickel oxide, photocathodes, dye-sensitized solar cells, tandem solar cells

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or exhibited poor chemical stability,26 and thus, research on NiO electrode has continued until lately. There have been various approaches to enhance the performance of DSCs with NiO photocathodes. Significant advances were achieved by the development of high-performance p-type sensitizers,27−37 resulting in improved light harvesting properties and photocurrent generations. In addition, the developments of redox couples based on cobalt and iron complexes led to substantial increases in photovoltage,38,39 for their more favorable redox potential compared to conventional iodide electrolytes. There have been a number of studies on mesoscopic NiO films based on various deposition techniques, such as spraying and double layer screen-printing,25,40−44 and nanostructured NiO with morphological or compositional modifications have also been utilized in photocathodes.45−49 However, these approaches require additional procedures which complicate the overall photocathode preparation process.

INTRODUCTION

Because of the rapid increase in fossil fuel consumption despite the limited reserves, there have been numerous attempts to efficiently harness energy from incident sunlight using solar cells.1−4 In addition to Si and thin film photovoltaics, organic/ inorganic hybrid solar cells have received considerable attention because of their great potential in terms of performance and economic feasibility. In particular, dyesensitized solar cells (DSCs) are regarded as a promising device because of their favorable characteristics (e.g., transparency and colorful features) that facilitate their use in a wide range of applications.5−9 In general, DSCs are based on mesoscopic photoanodes employing colloidal TiO2 nanoparticles owing to their strong performance.7,8,10−14 On the other hand, there have been various attempts to utilize photocathodes in DSCs with the goal of tandem applications for high-voltage output, and a number of studies on NiO electrodes have been reported.15−23 Although the mobility of the charge carriers in NiO is more than 2 orders of magnitude lower compared to the case of TiO2,24−26 p-type oxide semiconductors other than NiO had inadequate band positions © XXXX American Chemical Society

Received: May 25, 2018 Accepted: July 18, 2018

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DOI: 10.1021/acsaem.8b00834 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 1. Schematic illustration of the preparation methods for an anodic NiO photocathode and digital photograph images of the electrode at each step.

color of as-anodized foils indicate that there are large number of defects in the anodic oxide films, which could be amorphous NiO at grain boundaries or Ni with low oxidation states.61 In addition, as-prepared anodic oxide films may contain hydroxides and oxyhydroxides. Their color turned to yellowgreen after the thermal annealing, which matches the color of NiO. Figure 2a displays the XRD patterns of Ni foil anodized for 2 h, before and after heat treatment. For the as-anodized Ni foil, only the signals from the metallic Ni substrate (JCPDS 040850) were observable, indicating that anodic Ni oxide film was amorphous. On the other hand, the diffraction peaks of NiO (JCPDS 47-1049) were apparent after thermal annealing, clearly indicating the formation of crystalline NiO. The XRD patterns of anodized Ni prepared via 1 to 4 h of anodic oxidation are displayed in Figure S2. The morphological characteristics of the anodic Ni oxides were then characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 2b presents a top-view SEM image of the NiO film, in which nanoparticle-based mesoporous structure is clearly observable. The detailed morphology of the anodic NiO was characterized by TEM analysis (Figure 2c), with the sizes of the NiO nanoparticles falling within the range of around 5−15 nm. In addition, a lattice spacing of 0.24 nm, which matches the dspacing of NiO (111) plane, was observed in the high resolution TEM image. The polycrystalline nature of the anodic oxides was also confirmed by selected area electron diffraction (SAED) patterns (Figure 2d). The thickness of the films in relation to anodization time was characterized by obtaining cross-sectional SEM images with the assistance of focused ion beam (FIB) milling; these images are displayed in Figures 3a−d. The thickness of the NiO layer increased as the anodization time was elongated from 1 to 4 h, with a growth rate for the oxide film, as calculated from the linear fit (Figure 3e), of approximately 593 nm/h. For the application of anodic Ni oxide films in DSCs, P1 dye (DN-FP01, Dyenamo) was used as a sensitizer. Digital photograph images of dye-sensitized anodic NiO films are displayed in Figure S1c. Highly transparent platinized FTO films were prepared as counter electrodes, and the cell was assembled using 25 μm-thick thermoplastic sealants (Surlyn,

In this study, we prepared nanostructured NiO photocathodes via the electrochemical anodization of Ni foil. Although the anodic oxidation of Ni has been demonstrated under various conditions,50−60 which have been verified as effective processes for a diverse range of energy conversion and storage applications,53−59 the anodic NiO materials in previous research are not suitable for use in DSCs due to their insufficient surface area for a heavy loading of dyes. In the present study, mesoscopic NiO films composed of 10 nm particles were prepared by anodic oxidation and were employed as photocathodes of DSCs; this is the first time this approach has been attempted according to the best of our knowledge. The anodic photocathode in p-type DSCs exhibited a performance which was comparable to that obtained from conventional NiO electrodes comprising 20 nm particles; though the thickness of the anodic NiO film was less than half that of conventional electrode in optimized conditions, dye-loading was doubled in the anodic photocathode. Our approach is expected to be favorable for practical applications, because electrode fabrication is much simpler for anodic NiO films. The anodic NiO photocathode can be directly synthesized by electrochemical anodization and thermal annealing, whereas the process for conventional photoelectrodes requires multiple steps, including precursor preparation, casting, and the transition into NiO nanocrystals. A comparative analysis on conventional and anodic NiO photocathodes was carried out throughout this study, and the feasibility of the anodic NiO in tandem DSCs was also demonstrated.

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RESULTS AND DISCUSSION

As illustrated in Figure 1, electrochemical anodization of Ni foil was performed in a two-electrode system; 50 V was applied at 25 °C, and an ethylene glycol electrolyte containing 0.25 wt % of NH4F and 2 vol % of H2O was used. As-anodized Ni foils were cleaned in ethanol to remove the remaining electrolyte on the surface and were then heat-treated in air at 450 °C for 4 h to enhance their crystallinity and interconnections. Figure S1a and b show the digital photograph images of anodic Ni oxide before and after the heat treatment, respectively. The dark B


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DuPont). I3−/I− redox electrolyte with a previously reported composition for p-type DSCs was used.25,28 The photovoltaic performance of the DSCs was evaluated under standard AM 1.5G illumination with the light intensity of 100 mW/cm2, and the measurements were carried out under back-illumination conditions (i.e., light is coming from the counter electrode side) because the Ni substrate beneath the NiO film was completely opaque. Figure 4 shows the photocurrent density−voltage (J−V) results of the DSCs employing anodic Ni oxides (see the

Figure 4. J−V curves of DSCs employing P1-sensitized anodic NiO photocathodes prepared by electrochemical anodization of Ni for 1, 2, 3, and 4 h.

summary of detailed photovoltaic parameters in Table S1). In addition to this, we also confirmed the importance of electrochemical anodization on the performance of DSCs by measuring the J−V characteristics of the cell using thermally oxidized Ni foil as photocathode, which manifested an extremely poor performance (Table S2 and Figure S3). From the J−V curves of DSCs employing the anodic NiO photocathodes prepared by 1 to 4 h of anodization, it is notable that the power conversion efficiency (η) increased when the anodization time for the photocathodes increased from 1 to 2 h, while a gradual drop was observed thereafter. This trend followed the tendency in the short-circuit current density (Jsc) values. In general, Jsc is governed by light

Figure 2. (a) XRD patterns of anodized Ni foil before and after the 450 °C heat treatment in air. The columns at the bottom show the reference peak positions and intensities for Ni (black bars, JCPDS 040850) and NiO (red bars, JCPDS 47-1049). (b) Top-view SEM image of an anodic NiO film. (c) TEM images of anodic NiO and (d) the corresponding SAED pattern. All of the results in this figure were obtained using 2 h-anodized electrodes.

Figure 3. (a−d) Cross-sectional SEM images of anodic NiO prepared by 1−4 h of anodization: (a) 1, (b) 2, (c) 3, and (d) 4 h. (e) Timedependent growth rate of anodic NiO films. C


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operational behaviors of the photocathodes, the charge transfer kinetics were investigated using electrochemical impedance spectroscopy (EIS). Figure 5a presents the Nyquist diagram of

harvesting and charge collection properties of photoelectrodes. Thicker NiO films allow a greater amount of dye molecules to be loaded for light harvesting, but the increased travel distance of the charge carriers leads to a significant rise in interfacial charge recombination. It is widely known that NiO suffers from intrinsically poor carrier mobility among oxide semiconductors,24−26 and thus the influence of charge collection on overall cell performance is more significant in NiO compared to other oxides used in DSCs (e.g., TiO2, ZnO, and SnO2).7,62−64 It should also be noted that the dependence of charge collection on film thickness is more significant under back illumination, because the upper region (that is apart from the Ni foil substrate) of the photoelectrode absorbs the largest portion of the incident photons, while this part has the greatest distance from the current collector. Anodic NiO film prepared via 2 h of anodization seems to have an optimum thickness with regard to the balance between light harvesting and charge collection. Observed changes in open-circuit voltage (Voc) was also noteworthy. The Voc increased rapidly as the anodization time increased from 1 to 3 h, and this could be attributed to the superior passivation of the metallic Ni substrate when the NiO film is thicker. However, the DSC with the photocathode anodized for 4 h exhibited a slight drop in Voc, possibly because of the increased interfacial charge recombination originated from the longer travel distance required for the holes to reach the current collector. These interpretations regarding the trend of Voc were confirmed by the similar trend in the fill factors (FFs), which represent various types of resistance as well as charge recombination characteristics. The performance of p-type DSCs with NiO photocathodes that had been anodized for 2 h was then compared with that of conventional NiO electrodes prepared using nanoparticles with the size of around 20 nm (see Figure S4 for the XRD pattern). The SEM images displayed in Figure S5a and b show the morphology of the conventional NiO electrodes. Their thickness was around 2.5 μm according to the cross-sectional SEM image (Figure S5c) obtained after FIB milling. Figure S6 presents the J−V curves of the p-type DSC comprising conventional NiO-based photoelectrode. For the evaluation of the conventional cells, measurements were taken under both front- and back-illumination conditions (see Table S3 for more details on the J−V characteristics). The conventional NiO photocathodes demonstrated superior performance compared to that of the anodic NiO electrodes when the cells were frontilluminated; however, DSCs employing conventional and anodic NiO photocathodes manifested comparable performances under back-illumination, though η was slightly lower in the case of anodized photocathode. In addition, we could perceive from the similar Voc values that the metallic Ni substrate beneath the anodic NiO did not induce severe charge recombination, probably because of the formation of NiO blocking layer during the thermal annealing process. Given that NiO photocathodes are generally designed for tandem cells, where back-illumination is unavoidable for either photocathode or photoanode, this result is promising because NiO electrodes are able to be fabricated more conveniently without a notable drop in performance. Although the conversion efficiency was comparable between DSCs with conventional and anodic NiO, there were significant differences in particle size and film thickness, both of which are known to have a critical effect on the dynamics of charge carriers. To further understand the characteristic

Figure 5. (a) Nyquist spectra of DSCs employing conventional and anodic NiO photocathodes. The equivalent circuit is displayed in the inset. (b) Absorbance of P1 dyes detached from the conventional and anodic NiO photocathode at 420 nm. The black dots and their linear fitted lines were experimentally obtained using P1 solutions of various concentrations.

p-type DSCs based on conventional and anodic NiO films obtained under dark condition with a forward bias of 50 mV. In both of the DSCs, two semicircles were clearly observable. It is generally well-known that the first semicircle in the higher frequency region is the signal from the interfacial charge transfer at the counter electrode/electrolyte interface, and the second semicircle in the lower frequency region represents the charge transfer at the photoelectrode/electrolyte interface.65 Since the counter electrodes used in the two types of DSCs were identical (i.e., platinized FTO), the difference in the first semicircle was negligibly small. On the other hand, a substantial gap was apparent in the second semicircle; the semicircle was significantly smaller for anodic NiO than for conventional NiO, indicating that interfacial charge recombination takes place more frequently in anodic oxide film. EIS results were then fitted to the equivalent circuit displayed in the inset of Figure 5a, and the detailed results can be found in Table S4. The series resistance (Rs), which reflects the ohmic resistance, was smaller in anodic NiO, and this was attributed to the higher conductivity of the metallic Ni foil compared to the FTO glass counterpart. Meanwhile, the charge transfer resistance (Rct) at the photoelectrode/electrolyte interface was around 1.5 times smaller in the anodic NiO photocathode; the Rct values of 57.53 and 32.33 Ω cm2 were, respectively, obtained from DSCs employing conventional and anodic photocathodes. This significant gap seems to be the cause of the slightly lower performance in anodic NiO, because D


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ACS Applied Energy Materials a smaller Rct indicates a more severe interfacial charge recombination. The lifetime of the charge carrier (τn) was additionally calculated by multiplying Rct by the chemical capacitance (Cμ).66,67 It was found that the τn of the anodic NiO-based DSCs was around a half that of the conventional cells; τn of charge carriers in conventional and anodic NiO electrodes were 377.8 and 225.2 ms, respectively. This implies that the holes within NiO are less likely to arrive at the current collector and participate in the solar-to-electric energy conversion. Despite this, anodic NiO film produced a performance comparable to conventional photocathodes, which may be due to the smaller particle size, thus enabling greater dye-loading for subsequent light harvesting. Figure 5b presents the absorbance of P1 dyes detached from the conventional and anodic photocathodes using a mixture solution of ethanol and toluene (in a volumetric ratio of 1:1) containing 0.1 M NaOH. The extinction coefficient (ε), calculated based on the Beer− Lambert equation using the absorbance spectra in Figure S7, was 49645 cm−1 M−1 at 420 nm wavelength, where the absorption peaks were observed. The dye-loading on the anodic photocathodes was almost two times larger than that on the conventional NiO electrodes; the loading levels were 8.9781 × 10−8 mol/cm2 (6.9062 × 10−4 mol/cm3) and 4.7960 × 10−8 mol/cm2 (1.9184 × 10−4 mol/cm3), respectively. Therefore, anodized Ni foil with the nanoparticle sizes of around 10 nm and film thickness of 1.3 μm is expected to have superior light harvesting and inferior charge transport properties when compared with conventional NiO electrode, which is a 3 μm-thick film composed of ∼20 nm nanoparticles. The trade-off between light harvesting and charge collection, which is governed by the size of the nanoparticles and the thickness of the film, seems to explain the comparable performance of DSCs employing anodic and conventional NiO photoelectrodes. Finally, anodic NiO photocathodes were incorporated into tandem DSCs employing mesoscopic TiO2 photoanodes prepared by casting and thermal sintering of colloidal TiO2 nanoparticle paste (DSL 18-NRT, Dyesol). SQ2 dye (Sensidizer SQ2, Solaronix) was used in the photoanode due to the mismatch between its absorption region and that of P1 dye; SQ2 dye has a narrow absorption window with a peak at around 650 nm.68−70 Figure 6 shows the J−V characteristics of an n-type DSC comprising a SQ2-sensitized TiO2 photoanode and a platinized FTO counter electrode, a p-type DSC employing a P1-sensitized NiO photocathode and a platinized FTO counter electrode), and a tandem DSC prepared by assembling the photoanode and the photocathode (for a summary of photovoltaic parameters, see Table S5). An I3−/I− electrolyte with an optimal composition71,72 for the photoanode was used in the tandem cell analysis, which led to an increase in Voc and Jsc in the p-type DSC, while η did not exhibit a notable change due to the significant drop in FF. It was clearly observable from the J−V curves that the use of an anodic NiO photocathode in tandem DSCs increased the overall photovoltage of the cell. In general, illumination from the photocathode-side is beneficial for the performance of tandem DSCs, because poor carrier mobility in NiO limits the overall photocurrent density. Light illumination from the TiO2 photoanode direction was inevitable in our case because of the presence of opaque Ni foil, and an increase in η is thereby expected if anodic NiO could be formed on transparent conducting oxides. We additionally prepared a tandem cell

Figure 6. J−V curves of n-type DSC employing SQ2-sensitized TiO2 photoanode, p-type DSC employing P1-sensitized anodic NiO photocathode, and tandem DSC comprising both of the photoelectrodes.

using an N719-sensitized TiO2 photoanode and could increase the photovoltage to a further extent, though current matching between the photoelectrodes was not successful (see Figure S8). These results clearly demonstrate the strong potential of anodic NiO in tandem DSCs designed for high-voltage output, such as in solar-to-hydrogen conversion via water photoelectrolysis with a suitable photoanode.

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CONCLUSIONS In this study, mesoscopic NiO films were prepared via electrochemical anodization, a facile and robust procedure, and they were employed in p-type DSCs as photocathodes. These anodic oxide films were composed of ∼10 nm-sized NiO particles, and their performance was comparable to that of conventional NiO photoelectrodes with nanoparticles of around 20 nm in size. These results are promising, given that electrochemical anodization is significantly simpler than the fabrication method required for conventional photocathodes, which generally consists of multiple steps. The feasibility of the use of anodic NiO electrode in tandem DSCs was also tested, and their suitability for high-voltage output cells was clearly demonstrated. To the best of our knowledge, this is the first anodic NiO film synthesized with mesoscopic architecture. There thus remains room for further advances in this proposed design, and the performance would increase further by using state-of-the-art sensitizers and redox couples, including quantum dots and various metal−organic complexes. Because this method can be applied to any type of metallic Ni substrate, it would be also possible to prepare anodic NiO photocathodes for flexible DSCs by using sufficiently thin Ni foil. Given that this study is one of the few that focuses on the development of new NiO materials for p-type DSC applications, it is strongly anticipated that our approach will provide significant insights into the future design of photocathodes, not only for mesoscopic solar cells but also for various types of photoelectrochemical cells, such as those for water splitting and artificial photosynthesis.

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EXPERIMENTAL SECTION

Fabrication of Anodic NiO Photocathodes. Ni foils (GoodFellow, 99% purity) with thickness of 250 μm were ultrasonically E


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ACS Applied Energy Materials Author Contributions

cleaned in acetone, ethanol, and deionized (DI) water for 10 min. Electrochemical anodization of the Ni foils was carried out at 25 °C by applying 50 V in a two-electrode system, using an ethylene glycol electrolyte containing 0.25 wt % NH4F and 2 vol % H2O. Pt mesh was used as a counter electrodes during anodization, with the distance of 3 cm between the Ni foil and Pt mesh. The anodized films were washed with ethanol, dried in an oven, and thermally treated at 450 °C. Preparation of p-type DSCs. The dye-sensitization of the anodic NiO films was carried out by immersing each film in an acetonitrile solution containing 0.1 mM of P1 sensitizer (DN-FP01, Dyenamo) at 30 °C for 48 h. Commercial NiO photocathodes comprising ∼20 nm nanoparticles (DN-S01, Dyenamo) were used for comparison. Pt counter electrodes for DSCs were prepared by spin-casting an isopropanol solution containing 10 mM of H2PtCl6 (Sigma-Aldrich) onto FTO glasses (TEC-8, Pilkington), followed by thermal annealing in air at 400 °C for 20 min. For the preparation of p-type DSCs, photocathodes and counter electrodes were assembled using 25 μmthick thermoplastic sealants, and the iodide redox electrolytes were injected into the cells through predrilled holes. Acetonitrile solution containing 1.0 M LiI (Sigma-Aldrich) and 0.1 M I2 (Sigma-Aldrich) was used as the electrolyte. Preparation of n-Type and Tandem DSCs. Mesoscopic TiO2 photoanodes were prepared by doctor-blading colloidal TiO2 nanoparticle paste (DSL 18-NRT, Dyesol) on FTO glass, followed by heat treatment in air at 500 °C for 30 min. These electrodes were then immersed in an acetonitrile solution containing 0.1 mM of SQ2 dye (Sensidizer SQ2, Solaronix) for 48 h at 30 °C. Pt counter electrodes for n-type DSCs were prepared by the same procedure used in the case of p-type DSCs, and the cell assemblies were also carried out identically. The electrolyte for the n-type and tandem DSCs was a mixture solution of acetonitrile and valeronitrile (at a volumetric ratio of 85:15) containing 0.6 M 1-butyl-3-methylimidazolium iodide, 30 mM I2, 0.1 M guanidinium thiocyanate, and 0.5 M 4-tert-butylpyridine. Characterizations and Measurements. XRD measurements were carried out using a Rigaku D-MAX2500-PC diffractometer equipped with a Cu−Kα radiation source. SEM images were obtained with a Carl Zeiss AURIGA, and TEM analysis was performed with a Jeol JEM-2100F. J−V curves were measured using a solar simulator (XIL model 05A50KS source measure units) and a potentiostat (1480 Multistat, Solartron). A Si reference cell certified by the National Institute of Advanced Industrial Science and Technology (AIST, Japan) was used to calibrate the solar simulator. An electrochemical workstation (Zennium, Zahner) was used for EIS measurements, which were carried out with sinusoidal perturbations of 10 mV.

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J.S.K. and J.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.-E.S. acknowledges that this work was supported by the Institute for Basic Science (IBS-R006-A2) in Republic of Korea. T.-H.K. acknowledges the financial support from the Technology Development Program to Solve Climate Change of the NRF funded by the Ministry of Science, ICT and Future Planning of Korea (grant numbers: 2016M1A2A2940910 and 2017M1A2A2087813). H.C. acknowledges the support from the International Research & Development Program of the NRF funded by the Ministry of Science and ICT of Korea (NRF-2017K1A3A1A30083363).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00834.

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REFERENCES

J−V results for control samples and tandem solar cells, fitted parameters for EIS spectra, and additional characterizations on conventional and anodic NiO photocathodes (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H. C.). *E-mail: [email protected] (T.-H. K.). *E-mail: [email protected] (Y.-E. S.). ORCID

Jin Soo Kang: 0000-0001-8894-2630 Jiho Kang: 0000-0002-0298-8943 Tae-Hyuk Kwon: 0000-0002-1633-6065 Yung-Eun Sung: 0000-0002-1563-8328 F


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