Electric and photovoltaic characteristics of a multi-layer ReS 2 / ReSe 2 heterostructure

Among the various unique properties of two-dimensional materials, the ability to form a van der Waals (vdW) heterojunction between them is very valuable, as it offers a superior interface quality without the lattice mismatch problem. In this work, a ReS2/ReSe2 vdW heterostructure was fabricated, and its electrical and photovoltaic behaviors were discovered. The heterojunction showed a gate-tunable diode property with the maximum rectification ratio of 3150. Under illumination, it exhibited a photovoltaic effect with an efficiency of ∼0.5%. This study outlines the potential of Re-based 2D semiconductors and their integration by forming a vdW heterojunction for use in optoelectronic devices.

Herein, we report on the electrical characteristics of a multi-layer ReS 2 /ReSe 2 heterojunction as a diode and its photovoltaic properties as a solar cell.This research is meaningful in that we studied the vdW heterostructure of Re-based TMDs, which have unique properties that differ from the widely studied Mo or W families, and still only little has been discovered.For a solar cell, the heterojunction enables the effective use of incident light by splitting the absorbing spectrum, while the direct bandgap of the multi-layer ReS 2 can enhance the efficiency of the solar cell.The multi-layer ReS 2 /ReSe 2 heterostructure showed clear rectifying and gate tunable properties as a diode and showed a photovoltaic effect.Although the efficiency as a photovoltaic device in current stage is low, this result suggests its potential to be utilized for a solar cell with further optimization.
In contrast to the common TMDs that have a hexagonal crystal structure, ReS 2 and ReSe 2 exist in a distorted 1T phase of triclinic symmetry.Figure 1(a) shows the top view of the atomic structures of ReS 2 and ReSe 2 along the c-axis.The zigzag Re chains forming a diamond shape along the b-axis form a stable semiconducting distorted 1T structure with an in-plane anisotropy. 30,32 highly p-doped Si wafer with 300 nm-thick dry oxidized SiO 2 on top was used as a substrate.By repeating conventional lithography, DC sputtering of metal, and performing the lift-off process twice, Ti (50 nm) and Ti/Pt (5/45 nm) electrodes were formed.By mechanically exfoliating the pristine bulk crystals onto the polydimethylsiloxane (PDMS) stamp, ReS 2 and ReSe 2 flakes were prepared for the heterostructure.Then, by utilizing the "dry transfer" method, 21 these flakes were transferred and precisely positioned onto the electrodes, maintaining an overlapping region between the flakes.After the fabrication of the ReS 2 /ReSe 2 heterojunction, the devices were annealed under 200 °C at N 2 atmosphere for 1 h.The completed bottom contact device structure is illustrated in Fig. 1  After fabricating a device, atomic force microscopy (AFM) was utilized to observe the topography image of the heterostructure in order to determine the thickness of each 2D material.Figure 1(d) shows the AFM image and its line profile along the solid blue line.From the height profile, the thicknesses of the ReSe 2 and ReS 2 flakes were confirmed to be 48 and 64 nm, respectively.Raman spectra of the heterostructure under 633 nm laser were observed at three different points: ReS 2 , ReSe 2 , and the overlapping region [Fig.1(e)].ReS 2 shows peaks at 149, 160, 210, and 316 cm 1 , which is consistent with the literature. 6,30,32In the case of ReSe 2 , characteristic vibration modes of 124 and 158 cm 1 were detected.For both ReS 2 and ReSe 2 , in-plane (E g ) and out-of-plane (A g ) vibration modes were observed to be in reasonable agreement with those given in previous reports. 30,31,36The Raman spectrum of the overlapped region was a superposition of ReS 2 and ReSe 2 , implying that these two materials show independent signals.This therefore reveals the clear formation of the ReS 2 /ReSe 2 heterostructure.
Prior to the electrical characterization of the ReS 2 /ReSe 2 vdW heterojunction, the electrical behavior of each 2D semiconductor was tested.Figures 2(a) and 2(b) show the output characteristics of the bottom gate ReS 2 and the ReSe 2 FETs, respectively.In this paper, all the electrical measurements were conducted using a parameter analyzer (Keithley 4200), with the samples placed inside a vacuum chamber.A gate voltage of from 20 V to 20 V was applied in a row through the highly doped silicon substrate for the measurement of the output characteristics.As the gate voltage increases toward a positive region, both samples exhibited increasing conductivity, demonstrating that they are n-type semiconductors.In addition, it has been confirmed that ReS 2 and ReSe 2 show a clear gating effect.
The basic electrical properties of the multi-layer ReS 2 /ReSe 2 heterojunction were then measured.Figure 2(c) shows the I-V characteristic of the device, while the electrode connected to the ReS 2 layer was grounded and a voltage sweep of from 1 V to 1 V was applied to another side, ReSe 2 flake.The heterojunction exhibited clear current rectifying behavior with a peak rectification ratio of ∼364.Under positive bias, the heterojunction shows abrupt increase of current flow, whereas the current under the reverse bias remains very low at around tens of pA level.This indicates that the vdW ReS 2 /ReSe 2 heterojunction operates as a diode.The mechanism demonstrating its rectifying In order to understand the mechanism of the diode-like rectification observed in the ReS 2 /ReSe 2 heterostructure, we examined its energy band diagram.Figure 3(a) shows the band structure of the isolated ReS 2 and ReSe 2 , based on the previously reported values (electron affinity and bandgap) calculated by first principles calculations. 37,38It shows the type II heterojunction, which is known to be advantageous for optoelectronic applications.Chiu et al. demonstrated that Anderson's model is valid in the vertical 2D heterojunction system due to its vdW interface which does not allow the charge transfer. 39According to Anderson's model, the conduction band offset (CBO) is the electron affinity difference, and the valence band offset (VBO) is the superposition of the CBO and bandgap difference.In our case, CBO and VBO are calculated to be 0.76 and 1.17 eV, respectively.Therefore, by adopting Anderson's model, the band alignment of our ReS 2 /ReSe 2 heterojunction under equilibrium state is shown in Fig. 3(b).When ReSe 2 is positively biased, the energy band diagram changes as shown in Fig. 3(c).In this case, the electron flow occurs easily without any barriers, indicating the forward bias condition of a diode.On the other hand, under negative bias, the band alignment forms a barrier in the conduction band as shown in Fig. 3(d).As the barrier height is greater than several kT at room temperature, the electron flow from ReSe 2 to ReS 2 is blocked, resulting in the low reverse current.
In addition to its electrical performance, the optoelectronic property of the ReS 2 /ReSe 2 heterojunction was examined.Figure 4(a) exhibits the J-V curves of the device under various light intensities.A clear photovoltaic effect under illumination can be observed.For the measurement, the light from a halogen lamp was directed onto the device through an optic fiber.As the incident light power increased, the J-V curve tends to shift toward the fourth quadrant, showing larger photovoltage and photocurrent.The J-V curve reached a short-circuit current density (J SC ) of 0.29 mA/cm 2 and open-circuit voltage (V OC ) of 0.175 V under the incident light power density of 4.25 W/m 2 .The power conversion efficiency (PCE) at this condition was calculated to be 0.48% with the fill factor (FF) of 0.37.Compared with the previously reported MoS 2 /ReSe 2 heterojunction, 22 which showed a PCE of 0.072% and FF of 0.34, our device exhibits much higher efficiency by replacing MoS 2 with ReS 2 .Such a result demonstrates the merit of ReS 2 for optoelectronic applications by having direct bandgap and high photoresponse in a multi-layer form.Figure 4(c) shows the variation of photovoltaic parameters with respect to the light intensity.These parameters were extracted from the J-V curves as shown in Fig. 4(a).Both the V OC and J SC values tend to increase in proportion to the light power.Pearson's r value of the V OC and J SC with respect to light power density was calculated to be 0.97 and 0.998, respectively, which indicates clear positive correlations.As a function of illumination intensity, FF tends to decrease with the calculated Pearson's r value of 0.95.The PCE showed a similar trend to that of the fill factor, with a negative correlation (Pearson's r = 0.994).The maximum PCE value of 0.55% was achieved at the lowest light intensity within the measured range.The power conversion efficiency follows the equation of PCE = P max /P inc = (V OC • J SC • FF/P inc ) × 100%, where P max is the maximum output power and P inc is the incident light power.In summary, both V OC and J SC increased as the intensity of the incident light increased, although FF decreased accordingly.When these factors with opposite trends were integrated as a PCE, they showed inverse proportion to the light intensity.
In addition, a time scale response of the ReS 2 /ReSe 2 heterostructure under monochromatic light pulse was tested.Through a monochromatic light source system, light of 400 nm (0.79 mW/cm 2 ) and 550 nm (1.31 mW/cm 2 ) were generated from the continuous light of an Hg/Xe lamp.By utilizing a digital timer and electronic shutter, the light was exactly controlled to switch on and off alternately for 3 s intervals.The real-time current of a ReS 2 /ReSe 2 heterojunction was recorded along with the exposure to the generated light pulse, without applying any voltage bias.The orange regions in Fig. 4(d) indicate the I SC values under illumination, while the other regions indicate the dark state.As the current response occurs immediately, the I SC -time curve appears to be very similar to the pulsed shape of incident light.The smaller photocurrent change under 400 nm light (orange line) compared to 550 nm light (blue line) is mainly caused by the difference in the incident light intensities.The response time of the device for the I SC variation was determined from the rising and falling edge of the second period.A similar rise and fall time of ∼0.4 s for both 400 and 550 nm wavelength lights was observed.From the photoresponse of the device under 550 nm light, the responsivity and external quantum efficiency (EQE) were also calculated.Responsivity, which is defined as a ratio of photo-generated current density to incident light power density, was found to be 21.07 mA/W, and the EQE was calculated to be 4.76%.
In conclusion, we fabricated a ReS 2 /ReSe 2 vdW heterostructure and examined its electrical and photovoltaic characteristics.ReS 2 and ReSe 2 formed a type II staggered-gap heterojunction and showed rectifying behavior similar to a diode.As the device is formed by thin 2D materials, the amount of current rectification was significantly modulated by the gate voltage.Under illumination, our ReS 2 /ReSe 2 heterojunction exhibited a photovoltaic effect with a PCE of ∼0.5%.The efficiency can be further enhanced by changing the configuration of the photovoltaic cell to a vertical structure, when the wafer-scale growth of ReS 2 and ReSe 2 is enabled.In addition, the swift change of I SC was shown under a repeated light pulse on and off switching.This study demonstrates the potential of Rebased TMD semiconductors and their integration by forming a vdW heterostructure for application to optoelectronic devices.
(b), the optical microscope image of which is shown in Fig.1(c).

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FIG. 1.(a) A top-view schematic image of ReS 2 and ReSe 2 crystal structures.(b) Schematic of the ReS 2 /ReSe 2 heterostructure based device under illumination.(c) Optical microscope image of the heterostructure with electrodes.(d) AFM image of the heterostructure and its line profile showing the height of each 2D layer.(e) Raman spectra for multi-layer ReS 2 and ReSe 2 flakes and the overlapped region in the heterostructure under 633 nm laser.

FIG. 2 .
FIG. 2. Output characteristics of (a) ReS 2 and (b) ReS 2 FETs with the schematic image showing the device structure.(c) I-V plot of the ReS 2 /ReSe 2 heterojunction with the rectification ratio at V = ±0.995V. (d) I-V curves of the ReS 2 /ReSe 2 heterojunction modulated by varying gate voltage bias.

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FIG. 4. (a) J-V curves of the ReS 2 /ReSe 2 vdW heterostructure showing photovoltaic effect under various light intensities.The inset image shows the band diagram of the heterojunction under illumination.(b) Current and generated power from the device under white light (power density: 3.34 W/m 2 ) as a function of voltage with parameters calculated at that point.(c) Experimentally extracted photovoltaic parameters (V OC , J SC , FF, and PCE) with respect to the incident light power density.(d) Time domain short-circuit current plot obtained from the ReS 2 /ReSe 2 heterostructure, under periodic monochromatic light pulse of 400 nm and 550 nm wavelengths.
This research was supported by the MSIP (Ministry of Science, ICT and Future Planning), Korea, under the "ICT Consilience Creative Program" (No. IITP-2017-2017-0-01015) supervised by the IITP (Institute for Information & Communications Technology Promotion).