Magnetron sputtering fabrication and photoelectric properties of WSe2 film solar cell device

Accepted Manuscript Full Length Article Magnetron sputtering fabrication and photoelectric properties of WSe2 film solar cell device Xu Mao, Jianpeng Zou, Hongchao Li, Zhengqi Song, Siru He PII: DOI: Reference:

S0169-4332(18)30612-3 https://doi.org/10.1016/j.apsusc.2018.02.249 APSUSC 38707

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

21 January 2018 28 January 2018 25 February 2018

Please cite this article as: X. Mao, J. Zou, H. Li, Z. Song, S. He, Magnetron sputtering fabrication and photoelectric properties of WSe2 film solar cell device, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc. 2018.02.249

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Magnetron sputtering fabrication and photoelectric properties of WSe2 film solar cell device Xu Mao, Jianpeng Zou*, Hongchao Li, Zhengqi Song, Siru He State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China *Corresponding Author. E-mail: [email protected] (J.P.Zou) . ABSTRACT: Tungsten diselenide (WSe2) films with different growing orientations exhibit diverse photoelectric properties. The WSe2 film with C-axis⊥substrate texture has been prepared and applied to thin-film solar cells. W nanofilms with a thickness of 270 nm were deposited onto the Mo bottom electrode and then heat-treated in selenium vapor to synthesize WSe2 films with a thickness of 1 µm. ZnO films were deposited onto WSe2 films to form a P-N junction and ITO films were deposited subsequently as the conductive layer. X-ray diffractometry, scanning electron microscopy and UV-VIS-NIR spectro-analysis instrument were employed to analyze the phase composition, optical absorptivity and micromorphology of WSe2 films and the WSe2 solar cell device. WSe2 films exhibit excellent photoelectric performance with an optical absorption coefficient greater than 105 cm-1 across the visible spectrum. The calculated direct and indirect band gap of the WSe2 films is 1.48 eV and 1.25 eV, respectively. With the application of standard glass/Mo/WSe2/ZnO/ITO/Ag device structure, the open-circuit voltage of the battery device is 82 mV. The short-circuit current density is 2.98mA/cm2 and the filling factor is 0.32. The photoelectric conversion efficiency of the WSe2 film solar cell device is 0.79%. Key words: tungsten diselenide, magnetron sputtering, solar cell device, layered structure, fabrication

1.

Introduction

Compared with traditional silicon solar cells, thin-film solar cells have attracted great attention recently for their excellent features, such as low cost, relatively simple production process and high material utilization ratio [1]. However, the representative thin-film solar cells such as Cu(In,Ga)Se2 and CdTe solar cells are either with complex composition or certain toxicity. So it is urgent and challenging to find an inexpensive and environmental-friendly absorption layer material with ordinary ingredients. WSe2 is a kind of transition metal dichalcogenides (TMDCs) with graphite-like layered microstructure [2]. W and Se only form one stable compound (WSe2) without any other stoichiometries [3]. So at room temperature, the crystalline structure of WSe2 is highly stable, which makes the components of WSe2 simple and controllable [4]. The band gap for WSe2 lies between 1.16 and 1.54 eV, which is similar to the indirect band gap of visible light absorption [5]. WSe2 films usually have two different kinds of growth texture (C-axis ∥substrate and C-axis⊥substrate) [6,7]. WSe2 with C-axis∥substrate texture can be employed in hydrogen evolution reaction, which has been studied in our former research [8], and the other can be used as the absorption layer of solar cells. The surface of WSe2 crystal with C-axis⊥substrate texture is composed of selenium atomic layers without dangling bonds, which provides the potential for a non-surface-state heterojunction with highly mismatched lattice [9]. So WSe2 is perfect to be used as the optical absorption layer of solar cells for its simple components and excellent photoelectric performance. The existing synthetic methods of WSe2 films include soft selenization [10], pulsed-laser deposition [11], chemical vapor deposition [12], electro-deposition [13], selenium-oxygen ion exchange [14], etc. A kind of environmental-friendly method should be favored for further development. Also, the application of WSe2 films in thin-film solar cells is still a great challenge. Herein we present a WSe2 solar cell device on the basis of the preparation methods of WSe2 films reported before in our research group [15,16]. WSe2 films with C-axis⊥substrate texture have been successfully synthesized and applied to thin-film solar cells.

2.

Experimental

Device Fabrication. W films were DC-sputtered (direct-current) on molybdenum/glass substrates (20 mm×15 mm×4 mm) in an Ar (99.999%) atmosphere. The substrates were thoroughly cleaned employing a standard procedure, which included 15 min ultrasonic cleaning with the substrates immersed into acetone and ethanol respectively. The substrates were dried with Nitrogen (99.999%) prior to insertion into the sputtering system (JGP-450, Sky Technology Development). The deposition chamber was pumped down to a background pressure of 5.0 × 10-4 Pa and then washed with Ar (99.999%) for several times in order to reduce the oxygen content in the chamber. The substrates were fixed onto a rotating holder to ensure the homogeneity of the films. The distance between the substrate and W target (ZhongNuo Advanced Material Technology Co.,Ltd) was 153 mm. The W target was pre-sputtered for 5 min in order to remove the impurities on the surface. The specific deposition parameters for W films prepared on Mo/glass substrates are displayed in Table 1. After that, the W films were exposed to selenium vapor to form WSe2 films in a sliding tube furnace (Hefei Ke Jing Materials Technology Co., Ltd). The tube furnace was pumped down to a background pressure of 0.7 Pa and filled with N2 (99.999%). This process was repeated for three times in order to remove the oxygen in the tube completely. 0.7 grams of selenium powder (bought from Aladdin) was put on one side of the tube and the heating system was on the other side. The furnace was pushed to the direction of W films and selenium powder when the temperature rose up to 600 ℃ and was pushed away after 10 min of reaction. ZnO films were deposited on WSe2 films by radio-frequency (RF) magnetron sputtering in Ar (99.999%) atmosphere. Subsequently, ITO (indium tin oxide) films were DC-sputtered on the ZnO films. The experimental operation of sputtering ZnO films and ITO films was similar to that of W films. The distance between the substrates and the ZnO and ITO targets (ZhongNuo Advanced Material Technology Co., Ltd) were 75 mm and 153mm, respectively. ZnO and ITO films were both deposited after breaking the vacuum because the sputtering targets in the chamber had to be changed. WSe2, ZnO and ITO are all stable and the problem of oxidation does not exist. The specific deposition parameters for ZnO and ITO films are shown in Table 1. Finally, the surface of the device was evenly

divided into grids (3 × 3 mm) by sharp needles. Fine sandpapers were used to polish the edge of the device to expose the Mo bottom electrode. Ag conductive paste was spread onto the surface of ITO films and Mo films before the device was dried for 3 h for further testing. Fig. 1a indicates the preparation process of WSe2 solar cell device and the general structure of the device is shown in Fig. 1b.

Figure. 1. (a) Preparation process of WSe2 solar cell device. (b) A general structure of WSe2 solar cell device. Table 1 Specific deposition parameters for W, ZnO and ITO films Sputtering

Sputtering

Sputtering

Substrate

power(W)

pressure(Pa)

time(min)

temperature(℃)

W

120

0.2

25

25

ZnO

110

1.4

13

25

ITO

60

0.5

25

25

Target

Characterization techniques. X-ray diffraction was applied to analyze the phase composition and orientation of WSe2 films using a Rigaku D/max2550VB diffractometer with CuK radiation at U=40kV and I=250 mA. Scanning electron microscopy (FEI Nova Nano SEM 230) was used to characterize the surface and cross-section morphology of the films. The transmittance of WSe2 films was obtained by UV-VIS-NIR spectrophotometer (Hitachi U- 4100). The electrical properties of WSe2 film solar cell device were tested by two-probe current-voltage measurement (Newport 91160) under calibrated standard AM1.5 illumination (1000W/m2) at room temperature (25 ℃). 3.

Results and discussion 3.1. Photoelectric performance of WSe2 films According to our previous studies [15,16], WSe2 films with C-axis∥substrate or C-axis⊥substrate texture is determined by W film precursors’ morphology. Furthermore, W film precursors with a dense or porous morphology are up to the sputtering parameters. In order to investigate the photoelectric performance of WSe2 films conveniently, W films with a dense morphology were deposited on sodium calcium glass adopting the specific sputtering power (120 W), sputtering pressure (0.2 Pa) and sputtering time (5 min). Then the W films were heat-treated in selenium atmosphere at 600 ℃ for 30 min to synthesize WSe2 films with C-axis⊥substrate texture. The thermal-probe tests show that WSe2 films prepared are p-type semiconductors. Fig. 2(a) shows the XRD patterns of WSe2 films synthetized by W films and selenium vapor, which indicate that the films exhibited the 2H-WSe2 hexagonal structure. According to the standard powder diffraction file card (No.38-1388), the three primary diffraction peaks correspond to the (002), (006), and (008) crystal face of WSe2 , which means the WSe2 films have a preferential growth texture with C-axis⊥substrate. The results show good agreement with Qinglei Ma’s work [5]. The microstructure and the stress state of W films have a great influence on the orientation of WSe2 films during the reaction. W films tend to have a dense microstructure under low sputtering pressure (0.2 Pa), which is a crucial factor for the growth of WSe2 with C-axis⊥substrate texture [2]. As shown in Fig. 2(c), the nanocrystalline grown horizontally can be distinctly observed in the SEM images of cross-sectional WSe2 films, which shows good agreements with the XRD results. The results of X-ray diffraction indicate that WSe2 films prepared by forementioned process are suitable for the absorption layer of thin-film solar cells. The transmittance (T %) of WSe2 films in the 300 ~ 2400 nm wavelength range was measured by UV-VIS-NIR spectrophotometer. The optical absorption coefficient can be obtained by the following formula (1), in which d is the thickness of the measured film, R is the reflectivity and a is the absorption coefficient of the films. ܽ = lnሾ(100 − ܴ)/ܶሿ /݀ (1) (aℎ‫)ݒ‬௡ = ‫(ܣ‬ℎ‫ ݒ‬− ‫ܧ‬௚ ) (2) Fig. 2(b) depicts the relationship between the optical absorption coefficient (a) and the photon energy (hv) of WSe2 films when ignoring the reflectivity of the films. From the graph, we can see that the optical absorption coefficient of WSe2 films is greater than 105 cm-1 in the range of visible photon energy (1.61 eV ~ 3.10 eV), which means that WSe2 films with a thickness of only 100 nm can

absorb more than 90% of the incident light without considering the reflection and refraction.

Figure. 1. (a) XRD patterns of WSe2 films. (b)The relationship between the optical absorption coefficient and hv. (c)SEM image of cross-sectional WSe2 films. Figure. 3 illustrates the forbidden band width of WSe2 films. According to the formula (2), the band gap of WSe2 can be calculated in the graph taking hv as a horizontal coordinate and (ahv)n as a longitudinal coordinate. After drawing the tangent line at the straight part of the relation curve between (ahv)2 and photon energy (hv), the intersection point between the tangent line and the X-axis is the direct band gap of WSe2 (1.48 eV). When the same processing is done between (ahv)1/2 and photon energy (hv), the width of the indirect band gap of WSe2 is 1.25 eV. Therefore, the WSe2 films prepared by the above method are applicable for the absorption layer of thin film solar cells owing to the high optical absorption coefficient and appropriate band gap width.

Figure. 3. Energy gap of WSe2 films selenized at 600 ℃. (a)The relationship between (ahv)2 and hv; (b) The relationship between (ahv1/2 ) and hv. 3.2. Process control of WSe 2 films prepared on Mo/glass substrates When the Mo/glass substrates with W films were exposed to selenium vapor at 600 ℃, selenium atoms tended to diffuse into Mo films with the formation of unfavorable MoSe2 films. On one hand, high temperature and long reaction time ensured the thorough selenization of W films, but the Mo bottom electrode was selenized partly at the same time. On the other hand, low temperature and short reaction time protected the Mo bottom electrode without turning W into WSe2 completely. So it is of vital importance to figure out the appropriate temperature and reaction time of the selenization process. In consideration of the deformation of soda-lime glass at

650 ℃, the temperature of the reaction is determined at 600 ℃.

Figure. 4. XRD patterns of WSe2 films prepared with different reaction times Fig. 4 shows the XRD patterns of WSe2 films prepared with different reaction times. The Mo/glass substrates with W films were characterized by XRD before the reaction. As shown in Fig. 4, the diffraction peaks at the position of 2θ=40° and 2θ=40.48° in the XRD patterns correspond to the crystal face of W (210) and Mo (110) respectively. The Mo films we bought show a preferred growing orientation with low resistivity, which is a crucial factor for the bottom electrode in thin film solar cells [17]. After selenization for 5 min, a weak diffraction peak appears at the position of 2θ=13.44°, which corresponds to the crystal face of WSe2 (002). Selenium vapor diffused into W films and reacted with the W films to form WSe2 films. The low sputtering pressure (0.2 Pa) contributes to the dense microstructure of W films and allows WSe2 to grow parallel to the substrates [2], which explains the appearance of (002) diffraction peak of WSe2. The strong diffraction peak of W films indicates that the time of 5 min is insufficient and the selenization is incomplete. When the reaction time was extended to 7 min, the (002) diffraction peak of WSe2 gets stronger with the distinct reduction of the diffraction peak of W films spontaneously. Se atoms continued to diffuse into the rest W films and more WSe2 films were formed. But the existing peak at 2θ=40° indicates that the W films were not fully involved in the reaction and the reaction time needed to be increased. Further enhancement of (002) diffraction peak of WSe2 is observed after the selenization for 10 min. In addition, the diffraction peak of W films disappears and the diffraction peak of Mo films exists, which means the time of 10 min is adequate for the selenization. When the reaction time increased from 5 min to 10 min, the increasing diffraction peak of WSe2 and the decreasing peak of W verified the reaction between W and Se and the formation of WSe2. But the diffraction peak of Mo decreases simultaneously, which could possibly be caused by the reaction between Mo and Se or the shielding effect of the WSe2 film on the X-ray. The specific reason still needs to be studied. When the reaction time increased to 12 min, the (002) diffraction peak of WSe2 gets much stronger with the appearance of faint (100), (006) and (110) diffraction peaks. The diffraction peak of Mo weakens and Se atoms may have diffused into the Mo layer to form MoSe2. To sum up, the suitable selenization time is preliminarily judged to be 10 min by XRD. The SEM images of cross-sectional WSe2 films prepared at different reaction times are listed in Fig. 5. As shown in Fig. 5a, the interface can be obviously observed between the threaded columnar crystals of Mo films and the vertical columnar crystal of W films. On the top surface, WSe2 films of about 85 nm were synthesized after the reaction for 5 min, which correspond to the emerging diffraction peak of WSe2 (002) in Fig. 4. The thickness of W films which are not involved in the reaction is 207 nm, indicating that 5 min is insufficient to complete the selenization process. This also explains the strong diffraction peak of W films in Fig.4. As shown in Fig. 5b, WSe2 films of about 220 nm were obtained when the reaction time increased to 7 min, and W films with a thickness of 190 nm did not participate in the reaction. The Mo layer keeps the original threaded columnar crystal with a thickness of 900 nm. When the W films were selenized at 600 ℃ for 10 min, WSe2 films with a thickness of 1 µm were prepared with the disappearance of W films (Fig. 5c). Also, the thickness of Mo films maintains 900 nm. The WSe2 and Mo films show distinctly different cross-sectional morphology and the interface between the two films is quite clear. The chemical reaction between W and Se to generate WSe2 is a progress of volume expansion. The thickness of the produced WSe2 film is about 3.84 times the thickness of the origin W film, which has been confirmed by Yu [7]. WSe2 films of about 1 µm appear in Fig. 5d, similar to that in Fig. 5c. However, the thickness of Mo films decreases to 850 nm because some Mo films involved in the selenization generated unfavorable MoSe2 (Fig. 5d). Compared to Fig. 5c, three layers with distinctly different microstructures are show in Fig. 5d and the interfaces between each layers are quiet obvious. This

confirms the rationality of a reaction time of 10 min and the excessiveness of a reaction time of 12 min. The film expansion during the reaction between Mo and Se is similar to the selenization of W films, so the thickness of the MoSe2 film is about 200 nm. The formation of MoSe2 produces larger surface resistance and has an adverse effect on the carrier transmission between the Mo film and the WSe2 film [18-20]. When the reaction temperature is fixed, W films can be fully selenized while Mo films barely react with Se by reasonable control of the reaction time. In a word, the SEM analysis further demonstrates that a reaction time of 10 min can ensure the complete selenization of W films without involving the Mo bottom electrode.

Figure. 5. SEM images of cross-sectional WSe2 films prepared at different reaction times: (a)5 min; (b) 7 min; (c) 10 min; (d) 12 min. 3.3. Structure and performance of WSe2 film solar cell device

Figure. 6. (a) Cross-sectional SEM image of W/Mo film. (b) Cross-sectional SEM image of WSe2 /Mo film. (c) Top-view SEM image of WSe2 film (d) Cross-sectional SEM image of ITO/Zno/WSe2/Mo film.

Fig. 6 shows the representative SEM micrographs of the solar cell device during different preparation processes. The cross-sectional view of the W film deposited on the Mo/glass substrate is shown in Fig. 6a. The W film prepared by DC magnetron sputtering under low sputtering pressure presents a compact microstructure and the interface between the W film and Mo film can be obviously observed. The W nanocrystal film exhibits good adhesion on the Mo/glass substrate, which is verified by a simple tape test. The thickness of the W film and Mo film are 270 nm and 900 nm, correspondingly. After selenization for 10 min, WSe2 film with a thickness of 1 µm is obtained (Fig. 6b). The change of thickness illustrates the volume expansion of W film during the selenization in selenium vapor. And because of that, WSe2 films tend to fall off from the Mo/glass substrate when the stress in the film is not controlled effectively. The stress state of W film precursors as well as its effect on WSe2 layed texture is systematically studied in our previous study and the problem is perfectly solved [15]. Fig. 6c shows the top-view morphology of the WSe2 film, which indicates the surface of WSe2 layer is flat with many horizontal platelets of WSe2. WSe2 films with C-axis⊥substrate texture are composed of Se-W-Se layers held together by the Vander Waals bonding. The surface reconstruction of WSe2 /ZnO diodes is minimal and only the interface states associated with crystal defects are expected to be formed within the band gap. The closely-integrated layered structure of WSe2 can be clearly observed in Fig. 6d. After the deposition of ZnO films and ITO films, the cross-sectional view of the whole device can be seen in Fig. 6d. The thicknesses of ZnO and ITO films are 50 nm and 340 nm, respectively. Fig. 7 shows the J-V curves of WSe2 thin film solar cell device under calibrated standard AM1.5 illumination and in the dark. The open-circuit voltage (VOC) of the battery is 82 mV. The short-circuit current density (JSC) is 2.98mA/cm2 and the fill factor (FF) is 0.32. The J-V curve presents an obvious rectifying characteristic of P-N junction with a power conversion efficiency of 0.79%, showing good photoelectric conversion performance.

Figure. 7. The J-V curves of WSe2 thin film solar cell. (a) Under calibrated standard AM1.5 illumination. (b) In the dark 4.

Conclusion A WSe2 film solar cell device with layered structure (glass/Mo/WSe2/ZnO/ITO/Ag) has been successfully prepared. WSe2 films with C-axis⊥substrate texture present a compatible band gap and high visible-light-absorption coefficient, which are crucial factors for the absorption layer in thin film solar cells. The rectifying characteristic of the WSe2 /ZnO P-N junction has shown that WSe2 films with C-axis⊥substrate texture are suitable for photoelectric application. The simple preparation process, low cost as well as the environmental friendliness of WSe2 film solar cell device have made it a promising alternative to those thin film solar cells with high energy consumption and inevitable environmental pollution.

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