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Organic Electronic Devices

Photovoltaic field-effect-transistors using a MoS and organic rubrene van der Waals hybrid 2

Cheol-Joon Park, Hyeon Jung Park, Jae Yoon Lee, Jeongyong Kim, Chul-Ho Lee, and Jinsoo Joo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11559 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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ACS Applied Materials & Interfaces 1

Photovoltaic field-effect-transistors using a MoS2 and organic rubrene van der Waals hybrid

Cheol-Joon Park,† Hyeon Jung Park,† Jae Yoon Lee,‡ Jeongyong Kim,§ Chul-Ho Lee,‡ and Jinsoo Joo*,† †

Department of Physics, Korea University, Seoul 02841, Republic of Korea

‡

KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul

02841, Republic of Korea §

Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of

Korea

KEYWORDS: rubrene, molybdenum disulfide, ambipolar, photovoltaic, transistor

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ABSTRACT A several-layer n-type MoS2 was partially hybridized with an organic crystalline p-type rubrene nanosheet through van der Waals interactions to fabricate a two-dimensional lateraltype n-p hetero-junction optoelectronic device. The field-effect-transistors (FET) using lateral-type MoS2/rubrene hybrids exhibited both gate-tunable diode and anti-ambipolar transistor characteristics. The FET devices show the coexistence of n-type, p-type states, and off-states controlled by the gate bias. From the photocurrent mapping experiments, the gatebias-dependent photovoltaic effect was observed from the hetero-junction regions of the MoS2/rubrene FETs. Furthermore, the photovoltaic FETs were successfully operated by light irradiation without applying source-drain bias and controlled using gate bias. These devices represent new solar-energy-driven two-dimensional multifunctional electronic devices.

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1. INTRODUCTION Transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) have been intensively studied owing to their two-dimensional (2D) semiconductor properties1–9 and applications to nanoscale optoelectronic devices.10–15 Mono-layer (1L) TMDC systems have a direct band gap in the visible range, whereas multilayer TMDC systems have an indirect band gap owing to inter-layer interactions. For example, a 1L MoS2 structure has a direct band gap at 1.84 eV in the visible range, whereas bulk MoS2 exhibits an indirect band gap at 1.29 eV in the near infrared region.16,17 The intraand inter-layers of 2-D TMDC systems feature a covalent bond and van der Waals interactions, respectively, resulting in anisotropic optical and electrical properties depending on the number of layers. TMDC systems have been applied to field-effect transistors (FETs), 1–9

light-emitting diodes,10–13 solar cells,14,15 valleytronics,18,19 and chemical/bio-sensors.20–23

Conventional semiconducting n-p junctions induce depletion regions and built-in potential, resulting in the rectification effect. Recently, an n-p hetero-junction using a 1L n-type MoS2 and p-type WSe2 through van der Waals interactions has been reported by Lee et al. for controlling nanoscale optical and electrical properties and for fabricating multifunctional optoelectronic devices.8 A significant variation of photoluminescence and charge transport properties has been observed in the n-p hetero-junction region of the MoS2/WSe2 hybrid.8,24,25 Tuning the inter-layer tunneling recombination of major carriers across the van der Waals interface by gating, which differs from the conventional bulk n-p junction, has been reported. Moreover, the photovoltaic and gate-tunable diode characteristics of MoS2/WSe2 n-p heterojunction FETs have been observed.7–9 A new type of hybrid structure using MoS2 with a ptype organic semiconductor, such as pentacene, has also been studied.26–29 In this case, a MoS2/pentacene n-p hetero-junction combined with van der Waals interactions, which results 3



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from the absence of dangling bonds and native surface oxides of the organic solids, was fabricated and investigated. Beyond common optoelectronic devices, n-p hetero-junction devices consisting of nanoscale organic semiconducting crystals and TMDC with high mobility can exhibit distinct and extraordinary performance. The broad optical absorption bands of p-type organic semiconductors contribute to efficient solar energy harvesting, which optimizes the photogeneration effect of optoelectronic devices. In this paper, we propose and study the design of an organic crystalline p-type rubrene nanosheet (NS) with self-assembled and controlled physical dimensions as an excellent optical absorber30–33 to hybridize n-type TMDCs. The optical absorption spectrum of crystalline rubrene NS shows a sharp peak at 500 nm and a broad absorption band below 550 nm (Figure S1a in the Supporting Information), which can be optimized with few-layer TMDCs in terms of light harvesting. The FETs using rubrene crystals as the p-type active layer exhibit excellent hole carrier mobilities of 10–40 cm2·V–1s–1 owing to ordered π–π stacking. Therefore, crystalline rubrene NSs are promising p-type 2-D systems as a hetero-junction with n-type TMDCs.34–37 Until now, new functionalities of optoelectronic devices using nanoscale organic semiconductors and TMDC hybrids have not yet been systematically studied. In this study, we used a p-type organic crystalline rubrene NS with a broad optical absorption band to fabricate lateral-type MoS2/rubrene n-p hetero-junction FETs. We selected the crystalline rubrene NS (see Figure S1b) with a thickness of 100–400 nm having highly optical absorption characteristics (see Figure S1a). Few-layer MoS2 rather than 1L MoS2 was selected for the mobility balance of n-type and p-type materials in the hetero-junction.38,39 Also, few-layer MoS2 exhibited higher optical absorption than 1L MoS2 (see Figure S2). From the photocurrent mapping experiments, the gate-bias-dependent photovoltaic effect was 4


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observed from the hetero-junction region in the FETs. The characteristics of an antiambipolar transistor with hysteresis and the gate-bias-dependent photo-responsive current– voltage (I–V) characteristics were simultaneously observed in our FETs. This enables us to report, for the first time, gate-bias-controlled anti-ambipolar transistors that are driven by light irradiation without applying source–drain bias.

2. EXPERIMENTAL SECTION 2.1. Sample preparation and device fabrication. To prepare the TMDC/organic n-p hybrids, p-type organic crystalline rubrene NSs were self-assembled via a physical vapor transport (PVT) method using a home-made organic molecule vapor deposition instrument.30, 31

For PVT growth, rubrene powder purchased from Sigma–Aldrich Co. was ground for

better sublimation. An alumina boat containing rubrene powder was placed at the center of the home-made furnace tube (sublimation zone) at a temperature of approximately 310– 320 °C. The SiO2 substrate with scattered Au nanoparticles as seeds was placed at the edge of the heating zone. N2 gas continuously flowed into the furnace at a flow rate of 50–100 sccm during heating to transport vaporized rubrene molecules and to prevent oxidation. Subsequently, the rubrene molecules self-assembled on the SiO2 substrates in the form of well-crystallized NS (Figure S1b). A highly p-doped Si wafer (resistivity ≤ 5×10–3 Ω·m) was used as the gate electrode, and a SiO2 layer with a thickness of 300 nm was used as the insulating dielectric layer. Few-layer MoS2 flakes were mechanically exfoliated from bulk MoS2 onto the SiO2 substrate (Figure S3a). Electron-beam lithography was utilized to fabricate top-contact electrode patterns, and Al/Cr/Au ((50/10/50) nm) electrodes were deposited (Figure S3b). The Al and Au electrodes, 5



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matching the conduction band edge of MoS2 and the LUMO of rubrene, respectively, were deposited using an e-beam evaporator (ULVAC). A thin rubrene NS was transferred onto the surface of few-layer MoS2 using a micro-manipulator (Sutter Instrument, MP285) with a tungsten micro-tip (Figure S3c). Notably, the transferred rubrene NS was partially attached onto the surface of the MoS2 layer. To investigate thermal annealing effect, the MoS2/rubrene sample was then annealed in an ambient N2 environment at maximum temperatures of 45 °C and 50 °C for 2 h each and then cooled for 12 h. 2.2. Measurements. The electrical characteristics of the FETs under dark and light illumination conditions were measured in vacuum conditions (approximately 10–3 Torr) using a Keithley 237 source-measurement unit. The photocurrent mapping was performed using a digital multimeter (34401A, Keysight), source meter (GS210-F/MON, Yokogawa 2400, Keithley), current pre-amplifier (1211, DL Instrument), and mapping stage (SCAN 75X50 XY, NOST) under standard pressure. The incident laser for generating the photocurrent was a diode-pumped solid-state laser (λex =532 and 633 nm). A focused laser with an optical lens of magnification 100× and numerical aperture of 0.55 was used, and the focused laser spot size was approximately 1 µm.

3. RESULTS AND DISCUSSION 3.1. Electrical characteristics of MoS2/rubrene n-p hetero-junction field effect transistors under dark conditions. Figures 1a and 1b show a schematic illustration of the device structure and the real FET using the MoS2/rubrene hybrid, respectively. The detailed fabrication process of the FET can be found in Figure S3 of the Supporting Information. The number of MoS2 layers in the FET in Figure 1 was more than five, which was identified using 6


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Raman spectra (Figure S4a). The atomic force microscopy mapping image (Figures S4b and S4c) shows that the thickness of the rubrene NS was approximately 300 nm. Through the partial hybridization of the rubrene NS on the MoS2 layers with van der Waals interactions, our FET devices have a lateral-type n-p hetero-junction structure deposited with Au and Al electrodes.

Figure 1. (a) Schematic illustration of the lateral-type FET using a MoS2/rubrene n-p hetero-

junction demonstrating the photovoltaic effect. Insets: Schematic chemical structures of 1L 7



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MoS2 (left) and rubrene molecule (right). (b) Optical microscopy image of the MoS2/rubrene FET fabricated in this study. Figure 2a shows a schematic illustration of the energy band structure of the MoS2/rubrene hetero-junction with valance band/highest occupied molecular orbital (HOMO) and conduction band/lowest unoccupied molecular orbital (LUMO) levels. Figure 2b shows the output source–drain characteristic curves (ID–VD) of the MoS2/rubrene n-p hetero-junction FET as a three-dimensional (3-D) surface plot with gate bias (VG) varying from −30 V to 30 V. In the reverse bias regime (VD < 0 V), a Schottky barrier was formed, resulting in high resistance (i.e., a nearly insulating state). Gate bias dependence of the source–drain current (ID) was not observed in this reverse bias regime. In contrast, in the forward bias regime (VD ≥ 0 V), ID was a maximum of 3.8×10–8 A at VG = −9.5 V and sharply decreased as | | increased, as shown in Figure 2b. Figure 2c shows the transfer characteristic curve (ID–VG) of our FETs with VD = 30 V. Notably, Figures 2b and 2c illustrate the anti-ambipolar behavior and gate-controlled diode characteristics, which were simultaneously observed in our MoS2/rubrene FETs. The rectifying current behavior of the FETs was controlled using the gate bias. VG was sequentially swept from ① to ⑥ (i.e., from forward to backward bias). With the VG sweep from −30 V to −5 V, ID was turned on at VG = −20 V and it sharply increased (① regime), demonstrating n-type characteristics. Subsequently, ID rapidly decreased as | | decreased further, indicating p-type characteristics. ID in the regimes of high positive and negative VG values (③ and ⑥ regimes, respectively) drastically decreased and became saturated (i.e., there was almost no current flow), owing to either electrons or holes in the MoS2 or rubrene regions, respectively, as the major charge carriers became scarce. In the backward gate bias starting from VG = 30 V to VG = −4 V, the initial ID was 8


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approximately 10–11 A (③ regime), and thereafter ID rapidly increased up to approximately 3×10–8 A (④ regime), indicating p-type characteristics. However, from VG = −4 V to VG = −30 V, ID rapidly decreased (⑤ regime), demonstrating n-type characteristics, and subsequently drastically decreased and became saturated (⑥ regime), as shown in Figure 2c. These ID vs. VG characteristics indicate the anti-ambipolar behavior of our FETs. The ID onoff ratio controlled using VG was approximately 4×103. The carrier mobilities (µ) of the MoS2/rubrene FETs in the regimes ①, ②, ④, and ⑤ were estimated using the equation

= / / , where L is the channel length, W is the channel width, and Ci is the capacitance of the gate-insulating layer. Table 1 lists the µ values of the FET in the different regimes ①, ②, ④, and ⑤, which were approximately 10–2 cm2·V–1s–1. These mobilities originated from charge transport through both n-type MoS2 and p-type rubrene, and they can be considered as an effective mobility controlled by gate bias rather than inherent characteristics of either n-type or p-type material. Our FET devices show the coexistence of n-type, p-type states, and off-states depending on the gate bias. Similar gatecontrolled anti-ambipolar FET characteristics were observed from the different batches of FETs (see Figure S5). The effective charge carrier mobility was used in previous systems and defined as eff = e h (e : electron mobility, h: hole mobility) to interpret the current– voltage characteristics.40 The ambipolar mobility, A = e h − e h / e − h , was previously used to understand the photoactive characteristics of various hole transport materials.41

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Figure 2. (a) Schematic energy band diagram of the MoS2/rubrene hybrid structure with Au

and Al electrodes. (b) 3-D surface plot of ID–VD–VG characteristic curves of the MoS2/rubrene FET. (c) Transfer characteristic curves (ID vs. VG) in forward (red arrows; ①, ②, ③) and backward (blue arrows; ④, ⑤, ⑥) VG directions. ①: n-type on-state regime, ②: p-type onstate regime, ③: p-type off-state regime (i.e., hole-poor state), ④: p-type on-state regime, ⑤: n-type on-state regime, ⑥: n-type off-state regime (i.e., electron-poor state). (d) Schematic energy band structure of the MoS2/rubrene FET after contact with Au and Al electrodes. The electrons and holes recombined at the interface of the MoS2 and rubrene hetero-junction according to the Shockley–Read–Hall (SRH) (denoted as ⓐ arrow) and Langevin (denoted as

ⓑ arrow) recombination processes.

Table 1. Charge carrier mobilities (µ) in four different regimes (①, ②, ④, and ⑤) obtained from the slopes of the transfer characteristic curves of the MoS2/rubrene FET in Figure 2c.

µ [cm2∙V–1s–1]

In ① regime (n-type)

In ② regime (p-type)

In ④ regime (p-type)

In ⑤ regime (n-type)

1.45×10–2

1.64×10–2

1.66×10–2

1.60×10–2

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Eventually, anti-ambipolar transistor characteristics with weak hysteresis were observed in our lateral-type MoS2/rubrene FETs. To improve the performance of the FET devices, lowering the resistances of the active layers and reducing trap states and/or defects at the interfaces should be considered. Inserting an insulating layer with a high dielectric constant, for example hexagonal-boron nitride (h-BN), between the layers has been intensively studied to reduce trap and defect states,3,42–44 in which the mobility of the FET using TMDCs with hBN was enhanced.45,46 Such defects combined with H2O and O2 molecules on the surface of TMDCs could be removed by thermal annealing, resulting in increasing the on/off ratio and charge mobility.47 A post heat treatment method for organic semiconducting materials to enhance molecular ordering and to reduce contact resistance will be described in Section 3.3.

3.2. Charge transport mechanism of MoS2/rubrene FET. The anti-ambipolar charge transport behavior of the n-p hetero-junction FETs can be controlled using the gate bias.7–9,27– 29

Figure 2d shows an energy band diagram that can explain the charge transport and

transistor characteristics of the FETs with the MoS2/rubrene n-p hetero-junction structure in terms of the trap-intermediated tunneling (denoted as ⓐ process) and the Coulomb interaction (denoted as ⓑ process) recombination mechanisms.8 The interface of the n-p hetero-junction fabricated with MoS2 and rubrene causes disturbance to the direct current flow in the lateral direction. Electrons from n-type MoS2 and holes from p-type rubrene at the interface of the n-p hetero-junction recombine according to the Shockley–Read–Hall (SRH) mechanism (ⓐ process in Figure 2d) through trap-intermediated tunneling and the Langevin mechanism (ⓑ process in Figure 2d) through electron–hole Coulomb interactions. Charge is transported through these recombination mechanisms, and the current was maximized when 12


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the charge densities and major carrier mobilities of electrons and holes were balanced as the gate bias was tuned. It is noted that the Langevin process through electron–hole Coulomb interactions can be considered the direct charge transport mechanism at the interface rather than the trap-intermediated indirect process. With a proper gate bias, either holes or electrons were the major charge carriers through the lateral n-p hetero-junction controlled using the gate bias, resulting in p-type or n-type characteristics, as shown in the ①, ②, ④, and ⑤ regimes of Figure 2c. Applying a relatively high positive or negative gate bias, either holes or electrons were nearly depleted, resulting in the absence of current flow (e.g., regimes ③ or

⑥).

The balance of electron and hole mobilities of the n-p hetero-junction devices plays an important role in the device performance and the shape of the anti-ambipolar characteristic curves. For FETs with a lateral n-p hetero-junction structure, the peak position of VG in the transfer characteristic curve is determined by the carrier concentration and/or the relative channel length of n-type and p-type regions of the device.27 When the major carrier concentrations and the channel lengths of the n-type and p-type active layers are balanced, ID is maximized at VG = 0 V. In the case of our MoS2/rubrene FET, the negative shift of the VG peak indicates that the contribution of holes in the p-type material to the charge transport is dominant owing to the relatively low mobility and carrier concentration of the thin n-type MoS2 layer.

3.3. Thermal annealing effect of MoS2/rubrene FET. Hybridization of the MoS2 layer with organic rubrene devices to fabricate n-p heterojunction devices induced a high contact and traps/defects. To reduce the resistance and 13



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traps/defects and thereby improve device performance, the MoS2/rubrene FETs were annealed at various temperatures. Previous studies on pentacene thin-film transistors found an improvement in performance of the transistor by the annealing effect due to a reduction in the number of trap states and structural variation.48 Figures 3a, 3b, and 3c show the 3-D surface plots of ID–VD–VG characteristics of the MoS2/rubrene-based anti-ambipolar FET before and after the annealing process at 45 °C and 50 °C. The ID–VD–VG characteristics of the 3-D surface plots for the FET before and after the annealing were similar. However, the output values of ID clearly increased after the annealing process, as shown in Figure 3. The maximum ID of the FET following annealing at 50 °C increased compared with the FET before and after annealing at 45 °C. The ID peak was positively shifted after annealing,48,49 as shown in Figure 3d. The increase of ID after the annealing process was caused by a reduction of traps/defects as well as the high ordered rubrene molecules.

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Figure 3. 3-D surface plot of ID–VD–VG characteristic curves of (a) the pristine (without

annealing) MoS2/rubrene FET device, and the FET device annealed at (b) 45 °C, and (c) 50 °C. Inset: Optical microscopy image of the MoS2/rubrene FET device. (d) The transfer characteristic curves of the MoS2/rubrene FET without annealing and with annealing at 45 °C and 50 °C.

3.4. Gate-bias-dependent photovoltaic effect of MoS2/rubrene FET. The FETs using the MoS2/rubrene n-p hetero-junction studied herein can be applied to photo-responsive multifunctional electronic devices such as photovoltaic cells and photo-responsive memory devices. Previous devices using n-type and p-type lateral hybrid structures with van der Waals interactions (e.g., MoS2/WSe2 or MoS2/pentacene)8,27 exhibited a photovoltaic effect. Using our MoS2/rubrene FET shown in Figure 1, the gate-bias-dependent photovoltaic effect was observed, as shown in Figure 4. A focused laser of λex = 532 nm with 100 µW of power was incident on the MoS2/rubrene hybrid region. When VD = 2 V and VG = 25 V, the photoresponsivity, defined as Iphoto/Pin, was maximum at approximately 0.12 mA/W, where Iphoto is the photocurrent and Pin is the incident laser power. Figure 4a shows the photoresponsive ID vs. VD characteristic curves with various values of VG in the range of −1.0 V ≤ VD ≤ +2.0 V. Figure 4b shows magnified photoresponsive ID–VD curves in the positive VD range from 0.5 V to 2 V with various values of VG, where the photoresponsive ID linearly increased with the increase in VD. The measured ID–VD output characteristic curves in Figures 2b and 4b show diode-like behavior, and current saturation behavior due to pinch-off was not observed. This is mainly due to the additional contribution of dissociated charges from 16


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photogeneration at the interface of the lateral n-p hetero-junction. Under forward bias, photogenerated and dissociated electrons from MoS2 and holes from rubrene were transported through a low barrier between the electrodes and active layers. Under reverse bias, due to Schottky barriers, the dissociated holes and electrons did not produce current and instead showed rectifying (i.e., diode-like) behavior. The photogenerated and dissociated charges at the n-p van der Waals interface were controlled by the gate bias. Previous studies using a TMDC/organic n-p hetero-junction also reported non-saturated diode-like behavior for the photo-responsive ID vs. VD.8,9,14,27–29

Figure 4. (a) Photovoltaic characteristic curves of the MoS2/rubrene FET with focused laser

(λex = 532 nm and 100 µW). (b) Magnification of ID vs. VD in the regime of VD ≥ 0.5 V with various gate biases. Magnification of the photovoltaic characteristic curves with various gate biases: (c) VG = 15, 20, 30, 40, and 50 V, and (d) VG = 20, 15, 10, 0, −10, −20, and −30 V. 17



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ID was maximum at VG = 30 V, that is, the anti-ambipolar ID peak was positively shifted, in contrast to the results of Figures 2b and 2c. Under light irradiation, the photogenerated electrons were excessively high, resulting in a positive shift of the anti-ambipolar ID peak. Figures 4c and 4d show the magnified photovoltaic ID–VD characteristic curves (−0.05 V ≤ VD ≤ +0.4 V) with two different VG regimes, which confirm the gate-bias-dependent photovoltaic effect. The open circuit voltages (Voc) were measured to be 0.3 V–0.4 V, and the short-circuit current (Isc) exhibited a maximum value of −1.67 nA at approximately VG = 15 V. Figures 4c and 4d show that, with varying VG, ID (= Isc) and the photovoltaic characteristics were changed owing to the anti-ambipolar behavior. From the photovoltaic I–V curves, the power conversion efficiency (PCE; η) was calculated using the equation = × / ×

× 100 %, where FF is the fill factor. For example, at VG = 25 V, the PCE under monochromatic conditions (λex = 532 nm) was estimated to be approximately 2.45 × 10–4 %. The relatively low η mainly originates from inefficient charge transport (i.e., scattering effect) along long current channels after the photoexcitation in the lateral FET structure and from the low Isc values owing to the thinner 2-D current channel. We observed that Isc varied with VG as shown in Figures 4c and 4d. The Fermi energy level (EF) was changed as VG varied, and the current densities varied at the interface of n-p heterostructure. Additionally, the modified Schottky barriers between the electrode and the n- or p-type active layers induced a change of the dissociation current (i.e., change of Isc). Figure 5a shows a photocurrent mapping image of a different batch of MoS2/rubrene FETs, which exhibit the photovoltaic effect from the hybrid region. The observed maximum photocurrent, which was stably observed during the experiments, was approximately 1.5 nA at the sharp edge of the hetero-junction owing to the collected charges at the edge region. The non-homogeneous photocurrent shown in the mapping image of Figure 5a was mainly caused 18


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by an irregular distribution of exciton dissociation by the mismatch of the n-p hetero-junction and trap/defect sites. The photovoltaic effect of the FET device was measured from −4 V to 4 V, as shown in Figure 5b and its inset. The maximum photoresponsivity of the FET was approximately 0.8 mA/W. Voc and Isc of the FET device with the photovoltaic effect were measured to be approximately 0.36 V and –0.37 nA, respectively. To confirm the photovoltaic effect, the photocurrent mapping and output characteristic curves for the other batch of MoS2/rubrene FETs was performed under monochromatic conditions using an excitation laser (λex= 532 nm, Pin= 10 µW). Notably, the photovoltaic characteristics were reproducible (see Figure S6). The photocurrent and photovoltaic efficiency of our FETs were relatively lower compared with those of bulk solar cells, because of thin 2-D active layers and long lateraltype current channels.50 However, previously reported photovoltaic characteristics using n-p TMDC structures showed similar behavior.14,51,52 Based on our results, the photovoltaic efficiency can be improved by using active layers with high mobility and efficient optical absorbance as well as reducing contact resistance and trap/defect sites.

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Figure 5. (a) Photocurrent mapping image of the MoS2/rubrene FET device at VD = 0 V with

an excitation laser λex = 532 nm and power of 100 µW. The hetero-junction area is denoted as a black dotted box. The color scale bar on the right-hand side represents the photocurrent level. Inset: Optical microscopy image of the MoS2/rubrene FET device. (b) Photovoltaic characteristic curves of the MoS2/rubrene FET under monochromatic conditions (λex = 532 nm). Inset: Photovoltaic characteristic curves over the full range of measured bias (−4 V ≤ VD ≤ 4 V).

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3.5. Anti-ambipolar FETs operated by light irradiation (Photovoltaic FETs). In our MoS2/rubrene FET under light irradiation, a drastic Isc was observed without source–drain bias (i.e., VD = 0 V) (Figure 6a). The photovoltaic anti-ambipolar characteristics of our FET were modulated only using the gate bias. Thus, our transistors were operated using Isc driven by light irradiation, and the characteristics were controlled using the gate bias. These are the first such observations in the research field of TMDC/organic n-p hetero-junction FETs. The photovoltaic gate-bias-dependent FET characteristic curves were obtained with two incident irradiation conditions—λex = 532 nm (power = 100 µW; green curve) and λex= 633 nm (power = 200 µW; red curve)—as shown in Figure 6a. For reference, the black markers in Figure 6a represent the Isc vs. VG characteristics without light irradiation. For the condition VD = 0 V (without source–drain bias), the transistor was operated by light irradiation and the anti-ambipolar characteristics were controlled using the gate bias (green and red curves in Figure 6a). Light irradiation with a wavelength of 532 nm was easily absorbed by both MoS2 and the rubrene layer; however, that with a wavelength of 633 nm was difficult to be absorbed by the rubrene layer.34–37 Therefore, the Isc characteristic curves in Figure 6a are more sensitive to the excitation with a wavelength of 532 nm. With λex = 532 nm, Isc (green curve) exhibited a maximum of −1.25 nA at VG = +11.0 V. The photovoltaic Isc with both ntype and p-type characteristics was dependent on VG. For both light irradiation conditions, hysteresis of the photovoltaic gate-FET characteristics was observed, indicating the existence of trap states. For both excitations with wavelengths of 532 nm and 633 nm, the hysteresis effect was more severe for the backward gate sweep relative to that for the forward gate sweep. As the p-type rubrene layer was much thicker and larger as an active layer and absorbed a larger amount of light than the MoS2 layer, a hole current channel was easily formed. Therefore, in the backward gate sweep, the holes were dominant in creating the interfacial current channel and the charge concentrations were in a balanced state; 21



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subsequently, Isc rapidly increased (Figure 6a). Figure 6b shows the −Isc and PCE curves as a function of the gate bias for the MoS2/rubrene FET obtained from the results of Figure 4 and Figure 6a. The PCE characteristics were also tunable using the gate bias, and were similar to the −Isc curve. The mismatch of the VG values for the peaks of Isc and PCE in Figure 6b was due to the variation in Voc (see Figure 4).

Figure 6. (a) Isc vs. VG characteristic curves of the MoS2/rubrene FET under dark conditions

(black curve) and light irradiation conditions; λex = 532 nm (green curve) and λex = 633 nm (red curve). (b) Characteristics curves of −Isc (green curve, left side scale) and PCE (black curve, right side scale) as a function of gate bias for the MoS2/rubrene FET. The solid curves are eye-guided lines.

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The formation of traps and defects at the interface has also been reported for the MoS2/pentacene and MoS2/WSe2 hybrid systems.8,27 Additionally, the charge transfer effect via the trap and/or defect states in the MoS2/rubrene hybrid was observed from steady-state and time-resolved PL experiments.53 The schematic illustrations in Figure 7 show the photoexcited charge transport process with applying gate bias in the MoS2/rubrene FET. Without a source–drain bias (VD = 0 V), trap-intermediated and/or Coulomb interaction recombination did not occur and the major carriers were not transported. However, under light irradiation, excitons were formed in both MoS2 and rubrene systems, and the dissociated electrons and holes drifted to the source and drain electrodes, respectively, that is, photocurrent was generated, as shown in Figure 7. Owing to the blocking of dissociated charges by the built-in potential at the hetero-junction, the photoexcited Isc flowed in a negative direction (see the left scheme of Figure 7). When the gate bias was applied to enable the formation of current channels on both sides, the accumulated electrons or holes at the hetero-junction were free to drift, resulting in additional generation of Isc. Irradiating light and the application of a proper gate bias to control the charges and current channel in the MoS2/rubrene hetero-junction enabled photogenerated anti-ambipolar charge transport. Further, a balance of electron and hole concentration and charge tunneling induced the optimal photocurrent, as shown in the middle scheme of Figure 7. Furthermore, when a relatively high negative or positive gate bias was applied, the electron and hole concentrations became unbalanced and the current channel was suppressed; subsequently, charge dissociation rarely occurred, resulting in nearly insulating states, as shown in the right scheme of Figure 7. The dissociated charges from the excitons and the trapped and/or accumulated charges at the interface contributed to the photovoltaic anti-ambipolar effect in our FETs.

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Figure 7. Schematic illustrations of the photocurrent generation processes under focused

laser irradiation at the hetero-junction without gate bias (left scheme), with moderate gate bias (middle scheme), and with high negative or positive gate bias (right scheme).

In this study, FET devices using a lateral-type MoS2/rubrene hybrid show multifunctional device performance such as a gate-tunable diode, anti-ambipolar FET, and photovoltaic cell. For the first time, our FETs using the MoS2/rubrene hybrid were driven by light irradiation without a source–drain bias and the anti-ambipolar transistor characteristics were controlled by the gate-bias.

4. CONCLUSION We fabricated FETs with an MoS2/rubrene van der Waals hybrid structure driven by light irradiation (i.e., the photovoltaic effect). In the case of our MoS2/rubrene FET devices, with variation in the gate bias, the net carrier concentration of electrons and holes changed and were controlled near the hetero-junction, and gate-tunable diode characteristics, including rectifying behavior and anti-ambipolar FET characteristics, were observed. From the transfer 24


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characteristic curves of the MoS2/rubrene FET, the coexistence of n-type and p-type characteristics in the off-state was observed. The ID on-off ratio of the device was approximately 4×103, and the n-type and p-type carrier mobilities of the MoS2/rubrene antiambipolar FET were estimated to be approximately 10–2 cm2·V–1s–1. From the photocurrent mapping, the photovoltaic effect was tuned using the gate bias. Furthermore, gate-dependent photocurrent without the use of source–drain bias (i.e., photovoltaic gate-field-controlled transistor characteristics) was first observed in our MoS2/rubrene FET. The dissociated charges from excitons and the trapped and accumulated charges at the interface contributed to the photovoltaic anti-ambipolar effect in the lateral-type n-p TMDC/organic transistors. Thus, the FETs fabricated in this study can be applied to energy-saving optoelectronic devices. ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge. UV/Vis absorption spectra of rubrene NS and solution, XRD patterns of rubrene NS, confocal optical absorption spectra of MoS2 layers, optical microscopy images of the fabrication process of MoS2/rubrene FETs, Raman spectra of MoS2, AFM profile and surface morphology of the MoS2/rubrene FETs, 3-D surface plot of ID–VD–VG characteristic curves of different batches of MoS2/rubrene FETs, and photocurrent mapping image and photovoltaic characteristic curves of a different batch of MoS2/rubrene FETs.

AUTHOR INFORMATION

Corresponding Author 25



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*E-mail (J. Joo): [email protected].

Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS This research was supported by National Research Foundation of Korea (NRF) grants funded by the Korean government (Nos. 2015R1A2A2A01003805 and 2018R1A2B2006369) and by the Center for Advanced Meta Materials (CAMM, as the Global Frontier Project No. 2014M3A6B3063710).

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SYNOPSIS TOC

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SYNOPSIS TOC

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Photovoltaic field-effect-transistors using a MoS2 ... - ACS Publications

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