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Letter

Properties of Plasmon-Induced Photoelectric Conversion on a TiO/NiO Junction With Au Nanoparticles 2

p-n

Keisuke Nakamura, Tomoya Oshikiri, Kosei Ueno, Yongming Wang, Yoshiomi Kamata, Yuki Kotake, and Hiroaki Misawa J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00291 • Publication Date (Web): 26 Feb 2016 Downloaded from http://pubs.acs.org on February 28, 2016

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The Journal of Physical Chemistry Letters

Properties

of

Plasmon-Induced

Photoelectric

Conversion on a TiO2/NiO p-n Junction with Au Nanoparticles Keisuke Nakamura,† Tomoya Oshikiri,† Kosei Ueno,† Yongming Wang,‡ Yoshiomi Kamata,† Yuki Kotake,† Hiroaki Misawa*,†,§ †

Research Institute for Electronic Science, Hokkaido University N21, W10, CRIS Bldg., Kita-ku,

Sapporo 001-0021, Japan ‡

Creative Research Institution, Hokkaido University, N21, W10, CRIS Bldg., Kita-ku, Sapporo

001-0021, Japan §

Department of Applied Chemistry & Institute of Molecular Science, National Chiao Tung

University, 1001 Ta Hsueh R., Hsinchu 30010, Taiwan

ACS Paragon Plus Environment

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ABSTRACT We have successfully fabricated all-solid-state plasmonic photoelectric conversion devices composed of titanium dioxide (TiO2)/nickel oxide (NiO) p-n junctions with gold nanoparticles (Au-NPs) as prototype devices for a plasmonic solar cell. The characteristics of the crystal structures and the photoelectric properties of the all-solid-state devices were demonstrated. We observed that the crystalline structure of the NiO thin film and the interfacial structure of TiO2/Au-NPs/NiO changed significantly during an annealing treatment. Furthermore, the photoelectric conversion devices exhibited plasmon-induced photocurrent generation in the visible-wavelength region. The photocurrent may result from plasmon-induced charge separation. The photoelectric conversion properties via plasmon-induced charge separation were strongly correlated with the morphology of the TiO2/Au-NPs/NiO interface. The long-term stability of the plasmonic photoelectric conversion device was found to be very high because a stable photocurrent was observed even after irradiation for 3 days.


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TOC GRAPHICS

KEYWORDS Localized Surface Plasmon, Photoelectric Conversion, Nanostructures, Titanium Dioxide, Nickel Oxide


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The widespread application of photoelectric conversion devices such as solar cells and photodiodes requires that they exhibit not only high efficiencies but also high durability in harsh environments. Dye-sensitized solar cells (DSSCs) that use nanocrystalline titanium dioxide (TiO2) particles incorporating organic sensitizers have been studied extensively since they were first reported by Grätzel et al.1 Although DSSCs exhibit a visible-light response and can be fabricated at low cost, one of the disadvantages of DSSCs is their poor long-term stability due to the degradation of their organic dye and electrolyte.2 Recently, the plasmon-enhanced photocurrent generation of TiO2 in the visible region has been gaining interest.3-5 Our group recently demonstrated plasmon-enhanced photocurrent generation and water oxidation in aqueous

supporting

electrolytes

using

gold-nanostructure-loaded

photoelectrodes, even in the near-infrared region.6,

7

TiO2

single-crystal

However, wet solar cells have serious

electrolyte stability issues that prevent the application of these devices. In addition, the photoelectric conversion process of wet solar cells includes many events such as the redox reactions of chemical species which undermine the precise understanding of the plasmoninduced charge separation and the transport of the electron-hole pair. All-solid-state solar cells with hole transport materials have been proposed to solve some of the problems encountered in wet solar cells. Recently, Miyasaka et al. reported a lead halide organometallic perovskite solar cell.8 Such perovskite solar cells have attracted significant attention because of their high power conversion efficiencies.9,

10

However, the lead halide

perovskite films easily degrade into other chemical species in the presence of moisture.11 In addition, although the literature contains several reports on hole transport layers for all-solidstate plasmonic solar cells, improvements in the durability and photovoltaic properties of such cells are required.12, 13 The development of a durable inorganic hole transport material that can be


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used in photoelectric conversion devices with improved rectification characteristics is essential to overcoming the aforementioned problems and to enabling the fabrication of all-solid-state photoelectric conversion devices with high stabilities under harsh conditions such as high temperature, humidity and erosion. Nickel oxide (NiO) is transparent to visible light and is a chemically and electrically stable semiconductor oxide with p-type conductivity. The hole transport properties of NiO are derived from nickel vacancies and/or oxygen interstitials.14 Because of its p-type conducting properties and wide bandgap,15 NiO is a suitable hole transport layer for plasmonic solar cells. Various procedures, e.g., evaporation,16 sputtering17 and sol-gel processes,18 have been used to prepare high-quality NiO films. Previously, we reported that the interface between gold nanoparticles (Au-NPs) and TiO2 plays an important role in plasmon-induced photocurrent generation and water oxidation.19 For this reason, forming close and robust contact among TiO2, Au-NPs and NiO is critical for efficient plasmonic charge separation. Atomic layer deposition (ALD) is a procedure that enables both control of the thickness uniformity of a deposited film with submonolayer accuracy and isotropic deposition even on complex three-dimensional structures.20,

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These properties are due to the cyclic self-limiting mechanism of the ALD

process, which results in a constant thickness increase during each reaction cycle. The preparation of both the Au-NPs and NiO thin films and the device fabrication are described in detail in the Supporting Information. In this article, we describe the fabrication and characterization of all-solid-state plasmonic photoelectric conversion devices composed of TiO2/NiO p-n junctions with Au-NPs as prototype plasmonic solar cell devices. We describe the plasmon-induced photocurrent generation on TiO2/NiO junctions with Au-NPs. A p-type NiO thin film was prepared by ALD


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followed by an annealing step. X-ray diffraction (XRD) and high-resolution scanning transmission electron microscopy (STEM) were conducted to analyze the crystalline structure of the NiO thin film and the interfacial structure of the TiO2/Au-NPs/NiO with and without annealing. The changes in the photovoltaic properties due to annealing were also estimated. In particular, the importance of the structural properties of TiO2/Au-NPs/NiO junctions and the effect on their photovoltaic properties, such as the current and voltage derived from electron-hole transport and the plasmon-induced charge separation, will be discussed. In addition, we also discuss the stability of the all-solid-state plasmonic photoelectric conversion devices constructed with a TiO2/Au-NPs/NiO electrode. Figures 1(a) and (b) illustrate the structure and fabrication process of the all-solid-state plasmonic photoelectric conversion device. The Au-NPs were introduced between the TiO2 single crystal and the NiO film. The Au-NPs were fabricated according to the procedure described in a previous report.19 The average particle size and the standard deviation of the AuNPs were estimated from an SEM image to be 12 nm and 4.5 nm, respectively (Figure S1). The surface coverage of the TiO2 substrate by Au-NPs was also estimated to be 27.8%. The NiO film was deposited to a thickness of 200 nm using ALD with nickel acetylacetonate (Ni(acac)2) and ozone gas as the precursors,22 and the film was subsequently annealed at 500°C under atmospheric conditions to reorient the Ni and O atoms. A comparison of the devices with and without annealing is presented in the next section of this article. A counter electrode was prepared by sputtering Au to a thickness of 4 nm. The Au electrode was sufficiently thin to be capable of transmitting more than 90% of the visible light (Figure S2). The energy band diagram of the TiO2/Au-NPs/NiO is shown in Figure 1(c). The charge separation mechanism of the plasmonic photoelectric conversion device was hypothesized as


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follows. An excited electron is transferred to the TiO2 conduction band immediately after the Au-NPs inter/intraband transition or a transition of a TiO2 surface-state electron induced by the plasmon-induced near field. This transfer leaves a hole trapped in a surface state of the TiO2. The trapped hole can be transferred to the NiO valence band. The difference between the valence band of NiO and the conduction band of rutile TiO2 was estimated to be 0.9 eV based on reported values.23 Figure 1(d) shows the extinction spectra for the Au-NPs on the TiO2 substrate with and without a deposited NiO thin film. After deposition of the NiO film, the localized surface plasmon resonance (LSPR) band was broadened and redshifted because of the change in the refractive index of the surrounding medium. The data with the NiO film clearly showed that the LSPR band peaked at 670 nm. The LSPR band did not undergo a spectral shift due to annealing of the NiO film (Figure S3). Figure 2 shows the XRD spectra of the NiO films with and without annealing. The XRD patterns clearly show only the diffractions attributed to the NiO (200), Au (111) and TiO2 (110) facets. Thus, crystalline NiO films were successfully deposited on the substrates via ALD and the NiO (200), Au (111) and TiO2 (110) crystal facets were parallel to each other. The (200) signal of NiO in the XRD spectrum became slightly narrower after annealing, as shown in Figure 2(a). The full-width at half-maximum (FWHM) of the NiO (200) peak decreased from 0.6° to 0.5° because of annealing, indicating that the grain size of the NiO crystal was increased by the annealing process. In addition, the XRD patterns about the χ-axis, which provides information about the crystal orientation, showed more obvious differences with and without annealing, as shown in Figure 2(b). The FWHM of the NiO (200) peak decreased from 5.3° to 1.6° because of annealing, indicating that the crystal orientation became highly aligned at high temperatures.


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Figure 3 shows cross-sectional STEM images of the NiO films. For the STEM measurements, a sample of a NiO film with annealing was prepared using the ion milling technique. Pillar-like structures composed of NiO were clearly observed, as shown in Figure 3(a) and Figure S4. The crystal orientation of each pillar was aligned in the same direction as demonstrated in Figure 2(b), suggesting that the pillars were obtained via epitaxial growth. In addition, the lattice structures of the Au and the NiO matched, as shown in Figure 3(b), indicating that the NiO and Au bounded at the atomic level. These results suggested that the NiO pillars grew epitaxially on the Au-NPs because the lattice parameters of Au and NiO were in good agreement, and Au is known to be a catalyst for NiO growth.24, 25 Figure 3(c) shows a cross-sectional STEM image of the interface between the NiO thin film and the TiO2 substrate after annealing. Some of the Au-NPs were embedded in the NiO pillars. The thickness of NiO between an embedded Au-NP and the TiO2 was typically approximately 2 nm to 7 nm. The pillar-like structures were also observed in the cross-sectional STEM image of the NiO film without annealing. The NiO layer was peeled off the TiO2 layer during the ion milling process prior to the STEM measurement because of the relatively weak adhesion between the NiO without annealing and the TiO2 (Figure S5). However, STEM images of the NiO films without annealing were obtained using the focused ion beam (FIB) technique, and the results indicated that close contact was achieved between the Au and the TiO2 substrate before annealing, as shown in Figures 3(d) and (e). These data suggest that the immersion of the Au-NPs occurred during the annealing step as a consequence of the lattice matching between the NiO and Au. The I-V characteristics of the all-solid-state photoelectric conversion device constructed with TiO2, Au-NPs and NiO without and with annealing are shown in Figures 4(a) and (b), respectively. The open-circuit voltage (Voc) increased significantly after annealing and reached


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approximately 0.73 V. This increase in the Voc was due to the firm adhesion between the TiO2 and NiO. This conclusion is based on the STEM experiments, which suggested that the adhesion of the interface increased during the annealing process. The STEM experiments also suggested that a wide depletion layer was formed as a consequence of the strong adhesion. In addition, the Voc derived from only LSPR charge separation also exhibited high values in both cases (0.40 and 0.66 V under visible light irradiation without and with annealing, respectively). Therefore, plasmon-induced electrons and holes are transported to the TiO2 conduction band and the NiO valence band, respectively. By contrast, the short-circuit current density (Jsc), fill factor (ff) and solar light conversion efficiency (η) decreased because of annealing, as summarized in Table 1. The change in the shape of the I-V curves indicated that the series resistance of the NiO was increased by annealing.26 In fact, the two-terminal resistivity of the NiO film fabricated on a nondoped TiO2 substrate increased from 0.9 kΩ/cm to more than 20 kΩ/cm by annealing. One reason for the increase in the series resistance is the increase in the size of the voids between the grain boundaries associated with the propagation of the NiO pillars, as shown in the SEM images (Figure S6). Figure 4(d) shows the action spectra of the incident-photon-to-current conversion efficiency (IPCE) with and without annealing. The shapes and peak wavelengths of the IPCE action spectra were in close agreement with those of the LSPR band in both cases. In addition, almost no photocurrent was observed under visible light irradiation in the case of the NiO/TiO2 junction without Au-NPs as shown in Figure 4(d) and Figure S7. These observations suggest that the charge separation via LSPR excitation actively contributes to the photoelectric conversion in the visible region rather than the efficient scattering of photons by plasmonic nanostructures. However, the I-t characteristics show that the photocurrent in the visible region decreased remarkably after annealing (Figure 4(c)). In addition, the peak photocurrent decreased with time


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and reached a constant value after a few seconds, indicating retardation of the charge-carrier transport.27, 28 These results and the STEM experiments suggest that electron injection from the Au-NPs into the single crystal TiO2 was prevented by the immersion of the Au-NPs into the NiO film during the annealing process. Therefore, tight contact at the interfaces between the TiO2, Au-NPs, and NiO is important for the electron and hole transport processes. To demonstrate the durability of the solid-state plasmonic photoelectric conversion, the device was irradiated under air mass (AM) 1.5 for 3 days (Figure 4(e)). The photocurrent was stable and exhibited almost no changes over 3 days, even though the device was not sealed. In addition, the I-t characteristics under visible-light irradiation after the long-term irradiation test were the same as the initial characteristics. Therefore, the TiO2 and NiO inorganic semiconductors and the Au-NPs are highly durable and can maintain their charge separation capacities for extended periods of time. In conclusion, we have successfully observed plasmon-induced photocurrent generation on all-solid-state plasmonic photoelectric conversion devices composed of TiO2 single-crystal substrates, Au-NPs and NiO thin films using self-organization and ALD. These processes are inexpensive, and the fabrication can be easily scaled up. In addition, XRD and STEM measurements revealed that the NiO thin film is composed of high-density and crystalline pillarlike structures grown epitaxially on the Au-NPs. The Voc of the photoelectric conversion device with annealing was higher than that of the device without annealing. In addition, the Jsc of the photoelectric conversion device without annealing was remarkably larger than that of the annealed device. The differences in the photovoltaic properties were strongly correlated with the changes in the morphologies of the TiO2, Au-NPs, and NiO interfaces. Furthermore, the photocurrent response exhibited good agreement with the LSPR band, and the Voc derived from


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the LSPR charge separation exhibited a high value. These results indicate that the NiO thin film prepared by ALD functions as a hole transport layer in the plasmonic charge separation. Annealing the NiO film caused significant structural changes in the NiO layer and, consequently, changes in the charge separation properties of the plasmonic photoelectric conversion device. The enormous advantage of plasmonic Au nanostructures is that their resonant wavelengths can be tuned to cover a broad range of the solar spectrum by simply changing their shape and/or size. In addition, the all-solid-state plasmonic photoelectric conversion device is highly stable over time because the device does not include any organic components, such as molecular sensitizers or organic hole transport materials. The long-term stability of the plasmonic photoelectric conversion device was found to be very high because a stable photocurrent was observed even after irradiation for 3 days. Although the photocurrent and the energy conversion efficiency of this plasmonic photoelectric conversion device are still insufficient for practical use, these results indicate that fabricating a stable photoelectric conversion device with a tunable resonant wavelength is feasible. The application of plasmonic photoelectric conversion devices requires increasing the light-to-energy conversion efficiency. We expect that the efficiency will be improved by increasing of the area of the interfaces between the TiO2, Au-NPs and NiO and harvesting a proportion of the incident light flux by fabricating array-structured semiconductors with large surface area, such as nanotube or nanohole array structures. Understanding the plasmon-induced charge separation mechanism and optimizing light-harvesting plasmonic nanostructures are also important for increasing the efficiency of plasmonic photoelectric conversion.


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(a)

(b)

AM 1.5

Nb-TiO2 (0.5 mm) Au sputtering, 3 nm

Annealing at 800ºC, N2 atmosphere Au (4 nm, Anode) NiO (200 nm)

Au-NPs

Nb-TiO2 (0.5 mm)

In-Ga

NiO deposition by ALD, 200 nm

Annealing at 500ºC, atmospheric condition

Cu film (Cathode) (c)

(d)

E vs vac. (eV) -4.0 -4.5

0.25 0.20

CBTiO2 -4.4 eV -5.3 eV

-5.0

EF, Au -5.5

VBNiO

-6.0

VB

Extinction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.15 0.10 0.05 0.00 500

Nb-TiO2

Au

NiO

600

700

800

Wavelength (nm)

Figure 1. (a) A schematic of the all-solid-state plasmonic photoelectric conversion device. (b) The fabrication process for the all-solid-state plasmonic photoelectric conversion device. (c) Energy band positions of the all-solid-state plasmonic photoelectric conversion device. (d) Extinction spectra of the Au-NPs on Nb-TiO2. Black and blue: without and with a NiO thin film, respectively.


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(a)

(b)

38

40

42

TiO2 (110) 20

Intensity (a.u.)

36

NiO (200)

NiO (200)

Au (111)

Intensity (a.u.)

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44

TiO2 (220) 40

60

-20

-10

0

10

20

∆χ (degree)

2θ (degree)

Figure 2. XRD spectra of the NiO/Au-NPs/TiO2 substrate about the 2θ axis (a) and the χ axis (b). The black and red lines indicate without and with annealing, respectively. The inset shows a magnified view of the spectra about the 2θ axis.


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Figure 3. (a, b, and c) Cross-sectional high-angle annular dark field (HAADF)-STEM images of annealed NiO thin films on a Au-NPs/TiO2 substrate. The sample was prepared using the ion milling technique. (d and e) Cross-sectional HAADF-STEM image (d) and EDS image (e) of a NiO thin film on an Au-NPs/TiO2 substrate without annealing. The sample was prepared using the FIB technique


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(b)

2

Current density (µA/cm2)

(a) 0

-50

-100

(c) 120

0

100

1

-50

0 -1 -2

-100

-0.1 0.0 0.1 0.2

0.3 0.4 0.5 0.6

0.7 0.8

-3

0.4

0.6

0.8

80 60 40 20

10× ×

0 0

10

20

30

Voltage (V)

(e)

0.15

0.15

0.10

0.10 0.05 0.05 0.00

0.00

Extinction

0.20

IPCE(%)

0.2

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Voltage (V)

(d)

0.0

Cuurent density (µA/cm2)

Current density (µA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cuurent density (µA/cm2)

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40

50 0

10 20 30 40 50

Time (sec.) 140

120

on

100

80

60 450

500

550

600

650

700

750

800

850

0

10

20

30

40

50

60

70

Time (h)

Wavelength(nm)

Figure 4. (a and b) I-V characteristics of an all-solid-state plasmonic photoelectric conversion device fabricated without (a) and with (b) annealing. The inset shows a magnified view of the IV characteristics. Black: dark current. Red and blue: under irradiation of AM1.5 without optical filters and with a long-pass filter (λ > 410 nm), respectively. (c) I-t characteristics of all-solidstate plasmonic photoelectric conversion devices without (left) and with annealing (right). Red and blue: under AM1.5 irradiation without an optical filter and with a long-pass filter (λ > 410 nm), respectively. (d) IPCE action spectra of the all-solid-state plasmonic photoelectric conversion device without (black) and with (red) annealing. The green square indicates a solidstate photoelectric conversion device without Au-NPs, without annealing. The solid line indicates the LSPR band. (e) Irradiation time dependence of the photocurrent value for the allsolid-state plasmonic photoelectric conversion device without annealing.


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Table 1. Comparison of the performance of the photoelectric conversion devices with and without annealing.

Voc [V]

Jsc [µA/cm2]

ff

η [%]

Without annealing

0.56

108

0.55

0.033

With annealing

0.73

44

0.33

0.011


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ASSOCIATED CONTENT Supporting Information. Experimental section: preparation of Au-NPs and NiO thin films and device fabrication; materials characterization; measurement of the photovoltaic properties; statistical analysis of the Au-NPs particle size distribution; the extinction spectra of the Au-NPs on TiO2; the absorption ratio of 4 nm Au film; Cross sectional STEM and EDS images of NiO thin films on Au-NPs/TiO2 with and without annealing; surface morphology of NiO thin films on Au-NPs/TiO2; I-V characteristics of photoelectric conversion devices composed of NiO/TiO2 junction without Au-NPs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We are grateful to Prof. Yasutaka Matsuo and Ms. Naomi Kawai at Hokkaido University for STEM measurements and fruitful discussions. This study was supported by funding from the Ministry of Education, Culture, Sports, Science, and Technology of Japan: KAKENHI Grant-inAid for Scientific Research (s) (no. 23225006), the Nanotechnology Platform (Hokkaido University), and Nano-Macro Materials, Devices and System Research Alliance of MEXT.


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REFERENCES (1) Oregan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar-Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737–740. (2) Hinsch, A.; Kroon, J. M.; Kern, R.; Uhlendorf, I.; Holzbock, J.; Meyer, A.; Ferber, J. LongTerm Stability of Dye-Sensitised Solar Cells. Prog. Photovoltaics. 2001, 9, 425–438. (3) Tian, Y.; Tatsuma, T. Mechanisms and Applications of Plasmon-Induced Charge Separation at TiO2 Films Loaded with Gold Nanoparticles. J. Am. Chem. Soc. 2005, 127, 7632–7637. (4) Ueno, K.; Misawa, H. Surface Plasmon-Enhanced Photochemical Reactions. J. Photochem. Photobiol. C Photochem. Rev. 2013, 15, 31–52. (5) Sheldon, M. T.; van de Groep, J.; Brown, A. M.; Polman, A.; Atwater, H. A. Plasmoelectric Potentials in Metal Nanostructures. Science 2014, 346, 828–831. (6) Nishijima, Y.; Ueno, K.; Yokota, Y.; Murakoshi, K.; Misawa, H. Plasmon-Assisted Photocurrent Generation from Visible to Near-Infrared Wavelength Using a Au-Nanorods/TiO2 Electrode. J. Phys. Chem. Lett. 2010, 1, 2031–2036. (7) Ueno, K.; Misawa, H. Plasmon-Enhanced Photocurrent Generation and Water Oxidation from Visible to Near-Infrared Wavelengths. NPG Asia Mater. 2013, 5, e61. (8) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050–6051. (9) Snaith, H. J. Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells. J. Phys. Chem. Lett. 2013, 4, 3623–3630. (10) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional Engineering of Perovskite Materials for High-Performance Solar Cells. Nature 2015, 517, 476– 480.


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