Mo1âxWxSe2-Based Schottky Junction Photovoltaic Cells

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Mo1-xWxSe2-based Schottky junction photovoltaic cells Sum-Gyun Yi, Sung Hyun Kim, Sungjin Park, Donggun Oh, Hwan Young Choi, Nara Lee, Young Jai Choi, and Kyung-Hwa Yoo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11768 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 29, 2016

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ACS Applied Materials & Interfaces

Mo1-xWxSe2-based Schottky Junction Photovoltaic Cells Sum-Gyun Yi, Sung Hyun Kim, Sungjin Park, Donggun Oh, Hwan Young Choi, Nara Lee, and Young Jai Choi, Kyung-Hwa Yoo* Department of Physics, Yonsei University, 50 Yonsei-ro, Seoul, 03722, Republic of Korea

KEYWORDS Mo1-xWxSe2 alloys, photovoltaic cells, Schottky junction, graphene electrode, scanning photocurrent mapping

ABSTRACT

We developed Schottky junction photovoltaic cells based on multilayer Mo1-xWxSe2 with x = 0, 0.5, and 1. To generate built-in potentials, Pd and Al were used as the source and drain electrodes in a lateral structure, and Pd and graphene were used as the bottom and top electrodes in a vertical structure. These devices exhibited gate-tunable diode-like current rectification and photovoltaic responses. Mo0.5W0.5Se2 Schottky diodes with Pd and Al electrodes exhibited higher photovoltaic efficiency than MoSe2 and WSe2 devices with Pd and Al electrodes, likely because

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of the greater adjusted band alignment in Mo0.5W0.5Se2 devices. Furthermore, we showed that Mo0.5W0.5Se2 - based vertical Schottky diodes yield a power conversion efficiency of approximately 16% under 532 nm light and approximately 13% under a standard air mass 1.5 spectrum, demonstrating their remarkable potential for photovoltaic applications.

Introduction Recent research has explored the possible application of monolayer semiconducting transition metal dichalcogenides (TMDs), such as MoS2, MoSe2, WS2, and WSe2, in ultrathin, flexible, and nearly transparent electronic and optoelectronic devices.1-9 In particular, TMD-based photoresponse devices, including photodetectors and photovoltaic devices, have received much attention since semiconducting TMDs absorb light efficiently across a broad wavelength range.610

Typical photo-responsive devices, such as p-n diodes, are usually fabricated by chemical

doping; however, this will not work with TMDs because even low-energy doping damages the layered structures.11-12 Thus, electrostatic doping using pairs of split gates has been developed for p-n photodiodes.13-16 However, these electrostatically doped p-n junctions are ill-suited for practical applications because their lateral structures limit scale-up and require the operation of split-gate biases. Vertical p-n heterojunctions composed of MoS2, WSe2 and the other 2D materials have been also reported by several groups;17-26 however, they exhibit lower photovoltaic efficiency than electrostatically doped p-n junctions. More recent studies have explored monolayer Mo1-xWxSe2 alloys for their versatile and stable tunability of physical properties.27-28 This allows for performance optimization and a broader


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array of application options. Several reports showed the tuning of an energy bandgap of monolayer

Figure 1. Schematics of the Schottky diodes. (a) Mo1-xWxSe2-based lateral Schottky diode with Pd and Al as source and drain electrodes, respectively. (b) Mo1-xWxSe2-based vertical Schottky diode with Pd and graphene electrodes as bottom and top electrodes, respectively.

Mo1-xWxSe2 alloys, from 1.56 eV (x = 0) to 1.67 eV (x = 1), by varying x.27-28 However, few reports investigated specific applications using Mo1-xWxSe2 alloys. Here, we report on Mo1xWxSe2

- based Schottky diodes with source and drain electrodes made from materials having

different work functions φ (Figure 1). Different values of φ contribute to asymmetry in the Schottky barriers at the two contacts and induce a built-in potential, leading to a photovoltaic response upon illumination. Fontana et al.29 reported photovoltaic effects in MoS2-based Schottky diodes in lateral contact with Au and Pd electrodes. However, their photovoltaic efficiency was too low for feasible application in photovoltaic cells. As with energy bandgaps, the conduction band minimum (CBM), valence band maximum (VBM), and φ of Mo1-xWxSe2 alloys are expected to depend on x.27-28 Then, adjusting the band alignment may enhance the photovoltaic efficiency in Mo1-xWxSe2-based Schottky junctions. To test this idea, we fabricated Mo1-xWxSe2-based Schottky diodes in contact with Pd and Al


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electrodes with x = 0, 0.5, and 1 (Figure 1a), and we measured their photovoltaic efficiencies at various gate voltages (VG). Compared to MoSe2 or WSe2 devices, the Mo0.5W0.5Se2 devices exhibited a higher photovoltaic efficiency across the entire measurement range of VG. This enhanced efficiency was probably attributable to the greater adjusted band alignment in Mo0.5W0.5Se2 devices with Pd-Al compared to MoSe2 and WSe2 devices with Pd-Al. We also fabricated Mo0.5W0.5Se2-based vertical Schottky diodes with Pd and graphene electrodes for more practical applications (Figure 1b). These devices yielded a power conversion efficiency η of approximately 16% under 532 nm light and approximately 13% under a standard air mass (AM) 1.5 solar spectrum. These results suggest that Mo0.5W0.5Se2 – based vertical Schottky diodes are promising in photovoltaic applications since large-area graphene sheets are available and poly-crystalline Mo0.5W0.5Se2 layers probably work for constructing vertical Schottky junctions, allowing scale-up for applications.

EXPERIMENTAL SECTION Crystal growth: We grew Mo1-xWxSe2 single crystals (x = 0, 0.5, and 1) by utilizing the chemical vapor-transport method with a chemical agent of I2.30 Pure elements of Mo (99.99%), W (99.95%), and Se (99.999%) were placed inside an evacuated quartz tube according to the stoichiometric ratio. Table S1 summarizes the detailed condition for the synthesis of single crystals.

Device fabrication: We fabricated the devices, as shown in Supporting Information, Figure S1. To fabricate Mo1-xWx Se2 devices on hBN substrates (Supporting Information, Figure S1a), Mo1-xWxSe2 and hBN flakes were separately prepared on the SiO2/Si substrate by mechanical


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exfoliation. Then, the flakes were transferred using a pick-up method with a poly-propylene carbonate film (PPC, Sigma-Aldrich, CAS 25511-85-7).31 Briefly, 1-µm-thick PPC was spincoated onto separate glasses and gently peeled off the slide glass. The PPC films were put on a polydimethylsiloxane (PDMS) block mounted on another slide glass, followed by heating at 120 °C for 1 min to enhance adhesion of the PPC to PDMS. The Mo1-xWx Se2 and hBN flakes were picked up sequentially using the PPC film. Then, the stacked Mo1-xWx Se2 and hBN flakes were positioned onto an Au gate electrode patterned on the SiO2/Si substrate by a micromanipulator (Thorlabs, RB13M/M), and the PPC film was dissolved in chloroform. Finally, Cr (1 nm)/Pd (30 nm)/Au (30 nm) and Al (30 nm)/Au (30 nm) electrodes were made using electron-beam lithography and lift-off techniques. The thicknesses of the Mo1-xWx Se2 and hBN flakes, measured using an AFM, were approximately 3~5 nm and 40 nm, respectively (Supporting Information, Figure S2). To fabricate the vertical Mo0.5W0.5Se2 Schottky diodes (Supporting Information, Figure S1b), a graphene flake of mechanically exfoliated from natural kish-graphite (Graphene supermarket, SKU-NKG-300) or a chemical vapor deposition (CVD)-grown graphene was used as a top electrode. The graphene was picked up using the PPC film, followed by picking up the Mo0.5W0.5Se2 flake. This stacked graphene and Mo0.5W0.5Se2 flake were aligned with a Cr(1 nm)/Pd(30 nm) bottom electrode and an Au electrode connected to the graphene by the micromanipulator. Finally, the PPC film was dissolved in chloroform.

Scanning

Photocurrent

Mapping:

Confocal

Raman

and

photocurrent

mapping

measurements were performed using a confocal Raman microscope (Ntegra Spectra, NT-MDT, Russia) and current preamplifier (Model 1211, DL Instruments, USA). Control over the


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instruments and data acquisition was obtained via the confocal Raman microscope’s built-in software.32-33

RESULTS AND DISCUSSION We grew Mo1-xWxSe2 single crystals, with x = 0, 0.5, and 1, using a chemical-vapor transport method with a chemical agent of I2.30 In the Mo1-xWxSe2 structure, each Se-Mo/W-Se layer consists of three planes, with one hexagonal plane comprising Mo/W ions placed between two other hexagonal planes with Se ions (Figures 2a, b). The bonding of ions within the Se-Mo/W-Se

Figure 2. Characterization of Mo1-xWxSe2 crystals and multilayer flakes. (a) Top and (b) side views of Mo1-xWxSe2 crystal structure. Pink and blue spheres represent Mo/W and Se ions,


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respectively. (c) The XRD pattern obtained from MoSe2, Mo0.5W0.5Se2, and WSe2 crystals. (d) Raman and (e) photoluminescence spectra with 532 nm excitation obtained from multilayer MoSe2, Mo0.5W0.5Se2, and WSe2 flakes. layer is strongly covalent. However, the bonding between the layers arises from the weak van der Waals force. The crystallographic structures were determined by Rietveld refinement for X-ray diffraction patterns (Figure 2c). As the W doping level x increased, the lattice constant along the c-axis became slightly elongated due to the size difference between the W and Mo ions, as summarized in Table S2. To confirm the chemical composition of the Mo0.5W0.5Se2 alloy, we produced elemental mapping images of Mo, W, and Se using energy dispersive X-ray spectroscopy (EDS) within a scanning transmission electron microscope (STEM) (Supporting Information, Figure S3). Uniform distributions were observed through the crystal, and we estimated the value of x to be approximately 0.5. We then mechanically exfoliated multilayer Mo1-xWxSe2 flakes from single crystals and characterized them by their Raman spectrum using 532 nm laser excitation (Figure 2d). As x increased, the out-of-plane Ag mode shifted from 243 cm-1 to 252 cm-1, which agrees well with previously reported results.34-37 Figure 2e shows photoluminescence (PL) spectra obtained from multilayer MoSe2, Mo0.5W0.5Se2, and WSe2 flakes. We observed two maxima at 1.35 and 1.53 eV in the MoSe2 flake and at 1.46 and 1.64 eV in the WSe2 flake. This is consistent with the results reported for trilayer MoSe2 and WSe2.36-37 However, the PL spectrum of multilayer Mo0.5W0.5Se2 showed a maximum centered at 1.46 eV with side maxima of 1.25 and 1.53 eV. We noted that the main PL peak of multilayer Mo0.5W0.5Se2 appeared at almost the same position as the PL peak of multilayer WSe2, and the positions of the side maxima were nearly the same as the peak positions of multilayer MoSe2. These results indicate that the indirect energy bandgaps


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of multilayer MoSe2 and WSe2 were superposed in multilayer Mo0.5W0.5Se2, unlike in monolayer Mo1-xWxSe2, where the PL peak position was reported to shift to a larger energy with increasing x.23 Prior to fabricating an Mo0.5W0.5Se2 Schottky diode with asymmetric contacts, we fabricated an Mo0.5W0.5Se2 field-effect transistor (FET) with symmetric contacts on a hexagonal boron nitride (hBN) substrate, where Pd was used for both the source and drain electrodes (Pd-Pd) (inset of Figure 3a). The thickness of the Mo0.5W0.5Se2 layer, measured using an atomic force microscope (AFM), was approximately 3 nm. This corresponded to three to four layers (Supporting Information, Figure S2). The FET device exhibited a linear source-drain currentvoltage (ISD - VSD) curve, indicating the formation of good contacts between the Mo0.5W0.5Se2 and Pd (Figure 3a). Figure 3b shows the ISD - VG transfer curves measured at various values of VSD. An n-type semiconducting behavior was observed, although ISD slightly increased with decreasing VG at VG < -5 V (Figure S4a). From the ISD - VG transfer curve, the field-effect electron

mobility

(µ)


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Figure 3. Characterization of Mo1-xWxSe2 FET devices with Pd and Pd electrodes. (a) ISD - VSD curves measured for a Mo0.5W0.5Se2 FET device with symmetric Pd electrodes on an hBN substrate in dark (black) and under illumination (λ = 532 nm, Pin = 239 mW/cm2, red). The inset shows a schematic diagram of the Mo0.5W0.5Se2 FET device. ISD - VG transfer curves measured at various values of VSD in dark for (b) Mo0.5W0.5Se2, (c) MoSe2 and (d) WSe2 FET devices. The thickness is about 4.2, 3.5, and 5 nm for Mo0.5W0.5Se2, MoSe2 and WSe2, respectively. was estimated to be about 11. 4 cm2/V⋅s using the equation µ = (dI/dVG)×L/(WCiV),31 where L and W are the channel length and width, respectively, and Ci = ε/d ≈ 5.1×10-8 C/V⋅cm2 (d = 60 nm is the thickness of the hBN and ε = 3.1×10-13 F/cm is the dielectric constant for hBN). For comparison, we also fabricated MoSe2 and WSe2 FETs with Pd-Pd on the hBN substrates. The


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MoSe2 and WSe2 FET devices with Pd-Pd showed n-type and p-type semiconducting behaviors, respectively (Figures 3c and 3d), as also reported by others.38-41 The MoSe2 device yielded µ of approximately 0.2 cm2/V⋅s, smaller than µ of the Mo0.5W0.5Se2 device. On the other hand, for the WSe2 device the field effect hole mobility was estimated to be approximately 21.3 cm2/V⋅s.

Figure 4. Optoelectrical properties of Mo1-xWxSe2 – based lateral Schottky diodes with Pd and Al electrodes. (a) Optical image of Mo0.5W0.5Se2 devices with Pd and Al electrodes on an hBN substrate. (b) ISD - VSD curves measured in dark for Mo0.5W0.5Se2 (black), MoSe2 (red), and WSe2 (blue) devices with Pd and Al electrodes. The inset shows ISD - VSD curves measured in dark on a semi-logarithmic scale. ISD - VG transfer curves measured at various values of VSD in dark for (c) Mo0.5W0.5Se2, (d) MoSe2, and (e) WSe2 devices with Pd-Al electrodes. (f) ISD - VSD curves measured for Mo0.5W0.5Se2 (black), MoSe2 (red), and WSe2 (blue) with Pd and Al electrodes under illumination (λ = 532 nm, Pin = 239 mW/cm2). The thickness is about 4, 4, and 5 nm for Mo0.5W0.5Se2, MoSe2 and WSe2, respectively.


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Next, we fabricated Mo0.5W0.5Se2, MoSe2, and WSe2 devices contacted to Pd and Al electrodes (Pd-Al) (Figures 1a and 4a). Unlike the FET devices with Pd-Pd, the devices with Pd-Al exhibited diode-like rectified ISD-VSD curves (Figures 4b), indicating that they could be considered Schottky diodes because of the different values of φ. The on/off ratio was 103 -104 at VSD = ± 1 V (inset of Figure 4b). We estimated an ideality factor in the range 1.65 ~ 1.90 using the relationship ISD ≈ ISexp(qVSD/nkBT), where IS is a saturation current in dark, q is a carrier charge, n is the ideality factor, kB is the Boltzmann constant, and T is the temperature. These values are comparable to values in the range 1.2 ~ 2.3, reported for p-n heterojunctions consisting of MoS2 and WSe2.19-21 Figures 4c, 4d, and 4e show the ISD - VG transfer curves measured in dark at various values of VSD for Mo0.5W0.5Se2, MoSe2, and WSe2 devices with PdAl, respectively. As with the FET devices with Pd-Pd, Mo0.5W0.5Se2 and MoSe2 devices with PdAl exhibited n-type semiconducting behavior, and WSe2 devices with Pd-Al showed p-type semiconducting behavior. However, we found asymmetric conduction between the positive and negative values of VSD because of the asymmetric contacts, as in the rectified ISD - VSD curves. Most Schottky diodes exhibit a photovoltaic effect caused by a built-in potential called the Schottky barrier.42-44 To determine whether Mo0.5W0.5Se2, MoSe2, and WSe2 devices with Pd-Al also show the photovoltaic effect, we measured the ISD-VSD curves upon illumination using a 532 nm laser diode with power intensity Pin of 239 mW/cm2 (spot diameter ≈ 2.5 mm) (Figure 4f). We observed photovoltaic response in all devices with Pd-Al. The Mo0.5W0.5Se2 device with PdPd did not display the photovoltaic effect upon illumination, although the photocurrent was enhanced (Figure 3a). This confirmed that the photovoltaic effect observed in devices with Pd-Al can be attributed to asymmetric electrodes with different values of φ.


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Figure 5. Scanning photocurrent mapping measured at VSD = 0, -1, and +1 V for (a) Mo0.5W0.5Se2 and (b) WSe2 devices with Pd and Al electrodes. The thickness is about 4 and 5 nm for Mo0.5W0.5Se2 and WSe2, respectively.

To determine whether the Schottky junction caused the observed photovoltaic effects, we acquired photocurrent mapping images using a 532 nm laser with 4.3 nW power. For the Mo0.5W0.5Se2 device with Pd-Al and n-type semiconducting behavior, we detected |ISC| ≤ 2 nA near the Pd electrode, whereas ISC ≈ 0 was found near the Al electrode (Figure 5a). These results imply that electrons from the Pd were photoexcited over the barrier upon irradiation, creating electron-hole pairs in the Mo0.5W0.5Se2. The electron-hole pairs were separated by the built-in potential from the space charge at the contacts, leading to ISC. In contrast, the WSe2 device with Pd-Al, which displayed p-type semiconducting behavior, yielded |ISC| ≤ 0.2 nA near the Al electrode and ISC ≈ 0 near the Pd electrode (Figures 5b). Moreover, when VSD = 1 V was applied,


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we detected photocurrents near the Al electrode for the Mo0.5W0.5Se2 device with Pd-Al and near the Pd electrode of the WSe2 device with Pd-Al, indicating that the main charge carriers were electrons and holes in Mo0.5W0.5Se2 and WSe2, respectively. These results support the conclusion that the Schottky junction barrier caused the photovoltaic effect. To investigate whether the photovoltaic effect was gate-tunable, we measured the ISD - VSD curves at various values of VG under 532 nm light illumination with Pin ≈ 239 mW/cm2 for the Mo0.5W0.5Se2, MoSe2, and WSe2 devices with Pd-Al (Supporting Information, Figure S5). The ISD -VSD curves were dependent on VG. For the Mo0.5W0.5Se2 device with Pd-Al, we found the maximum values of short-circuit current ISC, open-circuit voltage VOC, and fill factor FF at VG=2, -4, and -6 V, respectively (Figure 6a). On the other hand, for the MoSe2 device with Pd-Al, |ISC| showed a peak at VG = -6 V (Figure 6b), whereas VOC and FF decreased with increasing VG in the range -10 V < VG < +10 V (Figure 6c). For the WSe2 device with Pd-Al, ISC and VOC were considerably smaller than they were for the Mo0.5W0.5Se2 and MoSe2 devices with Pd-Al (Figure 6b, Supporting Information Figure S5); thus, VOC and FF were not included in Figure 6c. Figure 6d depicts a photovoltaic efficiency calculated by ISC/L × VOC × FF/Pin at different values of VG, where L is the device length in contact with the Pd electrode. According to the photocurrent mapping images, ISC was generated at the Schottky junction with the Pd electrode (Figure 5a). Thus, we normalized ISC by L rather than by a device area. Compared to the MoSe2 device with Pd-Al, the Mo0.5W0.5Se2 device with Pd-Al exhibited a higher photovoltaic efficiency across the entire range of VG.


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Figure 6. Gate-tunable photovoltaic responses of Mo1-xWxSe2 – based lateral Schottky diodes with Pd and Al electrodes. (a) ISD - VSD curves of the Mo0.5W0.5Se2 device with Pd and Al electrodes measured at VG = -2, -4, and -8 V under illumination (λ = 532 nm, Pin = 239 mW/cm2). (b) ISC versus VG for Mo0.5W0.5Se2 (black), MoSe2 (red) and WSe2 (blue) devices with Pd and Al electrodes. (c) VOC versus VG for Mo0.5W0.5Se2 (black) and MoSe2 (red) devices with Pd and Al electrodes. The inset shows the plot of fill factor versus VG for Mo0.5W0.5Se2 (black) and MoSe2 (red) devices with Pd and Al electrodes. (d) Photovoltaic efficiency versus VG for Mo0.5W0.5Se2 (black) and MoSe2 (red) devices with Pd and Al electrodes. The thickness is about 4, 4, and 5 nm for Mo0.5W0.5Se2, MoSe2 and WSe2, respectively.


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Figure 7. ISC/L measured at VSD=0 V as a function of VG under 532 nm light illumination with different values of Pin for (a) Mo0.5W0.5Se2 and (b) MoSe2 devices with Pd and Al electrodes. L is the device length in contact with the Pd electrode. (c) Maximum values of |ISC|/L versus Pin for Mo0.5W0.5Se2 (solid symbols) and MoSe2 devices (open symbols) with Pd and Al electrodes. (d) Maximum values of |ISC|/L versus Pin measured under illumination with different values of λ for Mo0.5W0.5Se2 devices with Pd and Al electrodes.

To further study the VG dependence of the photovoltaic effect, we measured ISC at VSD=0 V as a function of VG for the Mo0.5W0.5Se2 and MoSe2 devices with Pd-Al, with the devices exposed to 532 nm light and different values of Pin (Figures 7a and 7b). As expected from the ISD-VSD curves


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measured at different values of VG (Figures 6a and S4), |ISC|/L showed a peak and it increased with increasing Pin. Figure 7c shows plots of the maximum values of |ISC|/L (|ISC|max/L) as a function of Pin. Three different Mo0.5W0.5Se2 devices with Al-Pd showed similar power dependences, implying that it was reasonable, for the sake of comparison, to normalize ISC by L rather than by a channel area. As Pin increased, the |ISC|max/L of the Mo0.5W0.5Se2 devices with Al-Pd increased more rapidly than that of the MoSe2 device with Al-Pd. This supports the assertion that the photovoltaic efficiency of the Mo0.5W0.5Se2 device with Al-Pd was higher than that of the MoSe2 device with Al-Pd. In addition, |ISC|max/L was measured under illumination with different values of wavelength λ for Mo0.5W0.5Se2 devices with Al-Pd (Figure 7d). Compared to light with λ = 405, 532, and 635 nm, light with λ=808 nm resulted in smaller values of |ISC|max/L. These findings suggested that the Schottky barrier height might be higher than 1.5 eV. In other words, few electrons were probably photoexcited over the barrier by λ = 808 nm light irradiation, so that |ISC|max/L was smaller upon λ = 808 nm light irradiation than upon λ < 808 nm light irradiation. Based on the above results, we propose energy band diagrams, as shown in Figure 8. According to a theoretical calculation,45-46 the CBM and VBM of MoSe2 are lower than those of WSe2. This band alignment explains the n-type and p-type semiconducting conduction of MoSe2 and WSe2 devices, respectively, and the formation of a Schottky barrier at an interface in contact with the Pd electrode for MoSe2 and with the Al electrode for WSe2. For Mo0.5W0.5Se2 devices with Pd-Al, n-type semiconducting behavior was observed (Figure 4c) and large leakage currents were found in dark when VSD < 0 V and VG > 0 V (Supporting Information, Figure S6). These results imply that the CBM and VBM of Mo0.5W0.5Se2 may be higher than those of MoSe2 and lower than those of WSe2. Furthermore, the φ of Mo0.5W0.5Se2 may be higher than the φ of


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MoSe2, leading to a lower Schottky barrier in the Mo0.5W0.5Se2 device than in the MoSe2 device. Then, upon illumination,

Figure 8. Band diagrams of Mo0.5W0.5Se2, MoSe2, and WSe2 devices with Pd and Al electrodes.

more electron-hole pairs are generated in the Mo0.5W0.5Se2 device than in the MoSe2 device, resulting in a higher photovoltaic efficiency. These energy band diagrams are also consistent with the VG dependence of the photovoltaic response observed for Mo0.5W0.5Se2 and MoSe2 devices. When VG < 0 V, the bands shift upward, so the Schottky barrier at the Pd electrode is reduced and the photovoltaic efficiency is enhanced for the MoSe2 device. However, for the Mo0.5W0.5 Se2 device with Pd-Al, the φ of Mo0.5W0.5 Se2 is higher than the φ of MoSe2.Thus, VG < 0 V causes the Schottky barrier to decrease at the Pd


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electrode and increase at the Al electrode. As a result, when we measure the photovoltaic efficiency as a function of VG, we expect a peak, as shown in Figure 6d.

Figure 9. Optoelectrical properties of Mo0.5W0.5Se2 – based vertical Schottky diodes with Pd and graphene electrodes. (a) ISD - VSD curve measured in dark for a Mo0.5W0.5Se2 vertical Schottky diode with Pd and graphene electrodes. The inset shows an optical image of the Mo0.5W0.5Se2 vertical Schottky diode with Pd and graphene electrodes. (b) Scanning photocurrent mapping image obtained from the Mo0.5W0.5Se2 vertical Schottky diode with Pd and graphene electrodes. Red, yellow, and black dashed lines denote Mo0.5W0.5Se2 flake, Pd, and graphene electrodes, respectively. (c) ISD - VSD curve measured under illumination (λ = 532 nm, Pin = 239 mW/cm2)


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for a Mo0.5W0.5Se2 vertical Schottky diode with Pd and graphene electrodes. (d) ISD - VSD curve measured under the AM 1.5 spectrum for a Mo0.5W0.5Se2 vertical Schottky diode with Pd and graphene electrodes. The thickness of Mo0.5W0.5Se2 is about 10 nm. Finally, we fabricated Mo0.5W0.5Se2-based vertical Schottky diodes on an SiO2/Si substrate, where Pd and graphene were used as the bottom and top electrodes, respectively (Figure 1b and inset of Figure 9a). We used graphene as the top electrode instead of Al, because graphene is transparent and the φ of graphene (≈ 4.4 eV) is lower than the φ of Pd. As for Mo0.5W0.5Se2 lateral Schottky diodes with Pd-Al, the vertical Schottky diodes exhibited the rectified diode-like ISD-VSD curve in dark (Figure 9a). In addition, the photocurrent mapping image measured using a 532 nm laser with a power of 47 nW exhibited |ISC| < 17 nA only in a contact region overlapping the Pd and graphene electrodes, indicating that ISC was caused by the Schottky junction (Figure 9b). Figure 9c depicts the ISD - VSD curve measured under 532 nm light irradiation with Pin ≈ 239 mW/cm2. We estimated that JSC = ISC/Ajunction ≈ 0.73 A/cm2 (Ajunction ≈ 11.2 µm2), VOC ≈ 0.25 V, and FF ≈ 0.22. Accordingly, the power conversion efficiency (η) given by JSC × VOC × FF/Pin was calculated to be approximately 16%. In fact, we fabricated more than 10 Mo0.5W0.5Se2-based vertical Schottky diodes using mechanically exfoliated graphene or CVD-grown graphene. Most devices with mechanically exfoliated graphene yielded η ≈ 5 - 16%, although the smaller values of η were observed for the devices with CVD-grown graphene, probably due to better quality of mechanically exfoliated graphene compared to CVD-grown graphene. We also measured the ISD – VSD curves under an AM 1.5 spectrum (Figure 9d). JSC, VOC, and FF were estimated to be approximately 0.092 A/cm2, 0.44 V, and FF ≈0.32, resulting in η ≈ 13%. This value is comparable to the η ≈ 14% reported for electrically generated p-n junctions


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composed of approximately 10 atomic layers of MoSe2.15 However, it was smaller than the value of η estimated upon 532 nm light irradiation. Actually, other devices also showed the lower values of JSC and η under the AM 1.5 spectrum than under 532 nm light, although VOC was opposite. This difference was possibly attributed to the Schottky barrier of about 1.5 eV since few electrons could be photoexcited over the barrier by irradiation of λ > 820 nm (≈ 1.5 eV) light including in the AM 1.5 spectrum (Figure 7d).

Conclusion In summary, we fabricated Mo1-xWxSe2 - based lateral and vertical Schottky junctions at x = 0, 0.5, and 1. All devices exhibited gate-tunable diode-like current rectification and photovoltaic responses. Photocurrent mapping images of Mo1-xWxSe2 devices with Pd-Al revealed that a short-circuit current ISC was created from the Schottky junction in contact with the Pd electrode for the Mo0.5W0.5Se2 and MoSe2 devices (n-type). In contrast, ISC came from the Schottky junction in contact with the Al electrode for the WSe2 device (p-type). Mo0.5W0.5Se2 devices with Pd-Al yielded higher photovoltaic efficiency than MoSe2 or WSe2 devices with Pd-Al across the entire range of VG, probably because of the greater optimized band alignment for Mo0.5W0.5Se2 devices than for MoSe2 and WSe2 devices. In addition, we fabricated Mo0.5W0.5Se2 vertical Schottky diodes using Pd and graphene electrodes as bottom and top electrodes, respectively. Their power conversion efficiency η was estimated to be approximately 16% under 532 nm light and approximately 13% under the AM 1.5 spectrum. These results demonstrate the remarkable potential of Mo0.5W0.5Se2 – based vertical Schottky diodes for photovoltaic applications, because these diodes may be scaled-up more easily than other structures and their power conversion may be further improved by controlling VG and the number of Mo0.5W0.5Se2 or graphene layers.


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Supporting Information Additional figures and information. This material is available free of charge via the Internet at http://pubs.acs.org. Fabrication schematic; AFM; HRTEM; SAED; ISD-VG curves in logarithmic scale; leakage current; Tables of CVT condition and structural parameters (PDF).

AUTHOR INFORMATION Corresponding Author *Corresponding author e-mail: K-H Yoo, [email protected] Present Addresses Department of Physics, Yonsei University, Seoul, 120-749, Republic of Korea

ACKNOWLEDGMENT This work was financially supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (Grant Nos. 2016R1A2B3011980 and 2012R1A4A1029061).


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Mo1âxWxSe2-Based Schottky Junction Photovoltaic Cells

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