Effect of RF Power on the Properties of Sputtered-Cus Thin Films For

energies

Article Eﬀect of RF Power on the Properties of Sputtered-CuS Thin Films for Photovoltaic Applications

Donghyeok Shin 1, SangWoon Lee 1, Dong Ryeol Kim 2,3, Joo Hyung Park 2, Yangdo Kim 1, Woo-Jin Choi 4 , Chang Sik Son 5, Young Guk Son 1,* and Donghyun Hwang 5,*

1 School of Materials Science and Engineering, Pusan National University, Busan 46241, Korea; [email protected] (D.S.); [email protected] (S.L.); [email protected] (Y.K.) 2 Photovoltaics Laboratory, Korea Institute of Energy Research, Daejeon 34129, Korea; [email protected] (D.R.K.); [email protected] (J.H.P.) 3 School of Materials Science and Engineering, Kyungpook National University, Daegu 41566, Korea 4 Energy Convergence Technology Center, Silla University, Busan 46958, Korea; [email protected] 5 Division of Materials Science and Engineering, Silla University, Busan 46958, Korea; [email protected] * Correspondence: [email protected] (Y.G.S.); [email protected] (D.H.); Tel.: +82-10-4553-0034 (Y.G.S.); +82-10-3156-4055 (D.H.) Received: 6 January 2020; Accepted: 3 February 2020; Published: 5 February 2020

Abstract: Copper sulfide (CuS) thin films were deposited on a glass substrate at room temperature using the radio-frequency (RF) magnetron-sputtering method at RF powers in the range of 40–100 W, and the structural and optical properties of the CuS thin film were investigated. The CuS thin films fabricated at varying deposition powers all exhibited hexagonal crystalline structures and preferred growth orientation of the (110) plane. Raman spectra revealed a primary sharp and intense 1 1 peak at the 474 cm− frequency, and a relatively wide peak was found at 265 cm− frequency. In the CuS thin film deposited at an RF power of 40 W, relatively small dense particles with small void spacing formed a smooth thin-film surface. As the power increased, it was observed that grain size and grain-boundary spacing increased in order. The binding energy peaks of Cu 2p3/2 and Cu 2p1/2 were observed at 932.1 and 952.0 eV, respectively. Regardless of deposition power, the difference in the Cu2+ state binding energies for all the CuS thin films was equivalent at 19.9 eV. We 2 observed the binding energy peaks of S 2p3/2 and S 2p1/2 corresponding to the S − state at 162.2 and 163.2 eV, respectively. The transmittance and band-gap energy in the visible spectral range showed decreasing trends as deposition power increased. For the CuS/tin sulfide (SnS) absorber-layer-based solar cell (glass/Mo/absorber(CuS/SnS)/cadmium sulfide (CdS)/intrinsic zinc oxide (i-ZnO)/indium tin oxide (ITO)/aluminum (Al)) with a stacked structure of SnS thin films on top of the CuS layer deposited at 100 W RF power, an open-circuit voltage (Voc) of 115 mA, short circuit current density 2 (Jsc) of 9.81 mA/cm , fill factor (FF) of 35%, and highest power conversion efficiency (PCE) of 0.39% were recorded.

Keywords: covellite; CuS thin ﬁlm; CuS/SnS absorber; RF magnetron sputtering; solar cell

1. Introduction Copper sulﬁde (CuS) is a transition metal chalcogenide in the IB-VIA group. The material is a typical p-type semiconductor where holes are the major carrier, Cu 4s and S 3p hybridized states become the conduction-band minimum, and the Cu 3d and S 3p antibonding states become the valence-band maximum [1,2]. Most copper sulﬁde phases that show semiconductor behavior are reported to exist in four types, Cu2 xS (digenite), Cu1.96S (djurleite), Cu1.75S (anilite), and CuS − (covellite), which are crystallographically and stoichiometrically stable at room temperature [1,3].

Energies 2020, 13, 688; doi:10.3390/en13030688 www.mdpi.com/journal/energies Energies 2020, 13, 688 2 of 12

From these, covellite CuS has a peculiar crystallographic structure that allows the reaction and intercalation of alkali metal ions between the S–Cu–S layers [4]. CuS also exhibits outstanding metallic conductivity, as well as physical and chemical properties, which make the material suitable for various electronics applications, such as cathode materials for lithium rechargeable batteries [5], gas sensors [6], photocatalysis [7], and solar cells [2]. CuS thin films are generally grown by methods like spray pyrolysis [8], the hydrothermal method [9], chemical-vapor deposition [10], atomic-layer deposition [11], and sputtering [12]. Among these, radio-frequency (RF) magnetron sputtering, which is one of the physical vapor-deposition methods, employs simple equipment and can form a strong adhesive force between the as-sputtered thin films and the substrate. It is also low-cost and appropriate for large-scale industrial productions, features that increase its potential value in various industrial applications [13,14]. Additionally, the sputtering method allows for the convenient control of various process parameters, such as substrate temperature, deposition time, deposition power, working pressure, and distance between target and substrate. Using the method, it is also relatively easy to produce thin films with consistent properties and control the elemental composition. However, despite these aspects, only a few studies have been reported on the growth of pure covellite CuS thin films using RF magnetron sputtering, and research on the application of covellite CuS thin films as the absorber layer in photovoltaic devices is scarce. In this study, the RF magnetron-sputtering technique was used to successfully deposit covellite CuS thin films on a glass substrate, and the structural and optical properties of the CuS thin films were thoroughly investigated according to the process parameter of deposition-power variation (40–100 W). To evaluate the solar-cell application, CuS thin films with the same thickness were deposited under different power conditions on Mo-doped glass substrates at room temperature. Then, tin sulfide (SnS) thin films were directly stacked on top of the CuS thin film to produce an absorber layer with a CuS/SnS structure. Finally, we evaluated the electrical properties of the CuS/SnS absorber-based solar cell fabricated by stacking the buffer layer, window layer, and electrode on top of the CuS/SnS absorber.

2. Materials and Methods A CuS target (TASCO, Seoul, Korea) of 50 mm (1.97 inch) diameter, 4 mm (0.16 inch) thickness, and 99.999% (5N) purity was installed on a target fixture located in the upper part of the chamber, so that the distance between target and substrate was 100 mm. Soda-lime glass was cut into 25 25 mm × squares and used for the substrate, and fine particles and other contaminants produced by the cutting process were removed using an N2 gas gun. Residual organic matter on the specimen surface was removed through ultrasonic cleaning for 5 min using deionized (DI) water and 99% purity acetone, ethyl alcohol, and isopropyl alcohol, in the listed order. A UV ozone cleaner was also used for the removal of solvent remaining on the specimen surface, and the cleaning was finished after 20 min of exposure. The initial pressure of the vacuum chamber was set to below 5.0 10 6 Torr (0.67 mPa) × − using a rotary pump and turbo molecular pump (TMP), and working pressure was maintained at 5.0 10 3 Torr (0.67 Pa) using a 20 sccm unit inflow of argon (Ar) gas via a mass-flow controller × − (MFC). Prior to the deposition of the CuS thin film, presputtering was carried out for 20 min at room temperature (RT) at 60 W RF power to remove target surface contaminants. The process parameter, RF power, was modified from 40 to 100 W to conduct CuS thin-film deposition for 40 min. For the solar-cell absorber-layer application, CuS thin films of 500 nm thickness were deposited at room temperature on a molybdenum-coated soda-lime glass substrate. In order to grow CuS thin films of equal thickness, the main process parameter, the RF power, was adjusted from 40 to 100 W, and deposition time was varied at a working pressure condition of 5.0 10 3 Torr. Additionally, the RF × − power, substrate temperature, and working pressure were set to 60 W, 300 C, and 1.5 10 2 Torr ◦ × − (2.0 Pa), respectively, to stack 500 nm thickness SnS thin films on top of the grown CuS thin film. Through this process, the heterojunction absorber layer of a CuS/SnS structure with equal thicknesses was produced. The fabricated absorber layers (CuS/SnS) were immersed in a bath containing Energies 2020, 13, 688 3 of 12

a solution synthesized with 0.2 M thiourea (CH4N2S, TCI, Tokyo, Japan), 0.003 M cadmium (Cd) sulphate hydrate (CdSO4, Sigma-Aldrich, St. Louis, MO, USA), and 5.95 M ammonium solution (NH4OH, Junsei, Kyoto, Japan). Then, the chemical-bath-deposition (CBD) method was used to synthesize a cadmium sulfide (CdS) buffer layer while maintaining the solution temperature at 80 ◦C for 12 min. Intrinsic zinc oxide (i-ZnO) and indium tin oxide (ITO) thin films, corresponding to the window layers, were stacked at 50 and 200 nm using RF magnetron-sputtering and DC sputtering methods, respectively. Finally, a 1 um thick aluminum (Al) upper electrode was deposited using the thermal-evaporation method, completing the fabricated structure of the CuS/SnS absorber-based solar cell (glass/Mo/absorber(CuS/SnS)/CdS/i-ZnO/ITO/Al), with an active area of 0.4 cm2. The structural properties of the CuS thin films were determined using an X-ray diffractometer (XRD, Rigaku, Tokyo, Japan, Ultima IV) employing CuKα (λ = 1.541 Å) radiation in the 2 θ range 1 from 20◦ to 80◦. Raman spectra in the range from 200 to 700 cm− were collected by dispersive Raman microscopy (WItec, Ulm, Germany, Alpha-300R) with an excitation wavelength of 532 nm. The thickness, surface morphology, and chemical composition of the films were observed by field emission scanning electron microscope (FESEM, TESCAN, Brno, Czechia, MIRA 3) combined with energy dispersive X-ray spectroscopy (EDS). The valance states of the constitutive elements were measured by X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Waltham, MA, USA, K-Alpha+) at a binding energy ranging from 0 to 1000 eV. The optical transmittance and band gap were studied using an ultraviolet-visible-near-infrared spectrophotometer (UV-Vis-NIR, Jasco, Hachioji, Japan, V-570) in the wavelength range of 400–2000 nm. The performance of the CuS/SnS absorber-based solar cells was investigated using a solar simulator (McScience, Yeongtong-gu, Korea, Xe55) at AM 1.5G illumination.

3. Results and Discussion

3.1. CuS Thin-Film Structural Properties The XRD patterns of the CuS thin films are shown in Figure1. The positions of the di ffraction lines matched the location of standard JCPDS card number 00-001-1281 for the crystalline covellite CuS hexagonal phase. The diffraction peaks corresponding to the (101) and (102) planes were observed at 27.3◦ and 29.4◦, respectively, on the basis of 2 θ, while diffraction peaks corresponding to the (103) and (116) planes were observed at 31.8◦ and 59.2◦, respectively. For all CuS thin films grown using different power conditions, the preferred growth orientation for the (110) plane was observed, and the intensity of the primary diffraction peak increased as the power was increased from 40 to 100 W. The full width at half maximum (FWHM) of the diffraction peak for the (110) plane was 0.58 for 40 W, 0.58 for 60 W, 0.48 for 80 W, and 0.38 at 100 W, showing a decreasing trend. The grain size calculated by substitution into the Scherrer formula [15] revealed that grain size increased from 15.7 nm at 40 W to 23.9 nm at 100 W. Atomic vibration mode analysis can distinguish spatial variations and surface species in a sample composition, which is generally possible through Raman measurement [16]. Additional analysis of the structural properties of the CuS thin films was conducted by obtaining Raman spectra, and the results are shown in Figure2. All of the Raman spectra for the CuS thin films deposited at di fferent 1 RF powers showed two bands, with a primary sharp and intense peak at the frequency of 474 cm− , 1 and a relatively wide peak at the frequency of 265 cm− . The shark peak in the high-frequency region 2 is the stretching vibrational mode from the covalent S–S bonds of S − ions at 4e sites, and this signifies that the lattice atoms were arranged in a periodic array [6]. The broad Raman peak in the low-frequency region corresponded to the mode of the lattice vibration produced by the Cu–S bonds [17]. Theoretical analysis revealed that a total of 36 vibrational modes are possible for the CuS system, and among them, 14 Raman active modes (ГRaman = 2A1g + 4E2g + 2E1g) can be categorized into ГCu1 = A1g + E2g + E1g, ГCu2 = E2g, ГS1 = A1g + E2g + E1g, and ГS2 = E2g for copper (Cu) and sulfur (S) each [1,18]. The covalent S–S bonds only exist in the copper-deficient Cu S (0.6 x 1) phases, so the strong band observed 2-x ≤ ≤ Energies 2020, 13, 688 3 of 12

Then, the chemical‐bath‐deposition (CBD) method was used to synthesize a cadmium sulfide (CdS) buffer layer while maintaining the solution temperature at 80 °C for 12 min. Intrinsic zinc oxide (i‐ ZnO) and indium tin oxide (ITO) thin films, corresponding to the window layers, were stacked at 50 and 200 nm using RF magnetron‐sputtering and DC sputtering methods, respectively. Finally, a 1 um thick aluminum (Al) upper electrode was deposited using the thermal‐evaporation method, completing the fabricated structure of the CuS/SnS absorber‐based solar cell (glass/Mo/absorber(CuS/SnS)/CdS/i‐ZnO/ITO/Al), with an active area of 0.4 cm2. The structural properties of the CuS thin films were determined using an X‐ray diffractometer (XRD, Rigaku, Japan, Ultima IV) employing CuKα (λ = 1.541 Å) radiation in the 2 θ range from 20° to 80°. Raman spectra in the range from 200 to 700 cm−1 were collected by dispersive Raman microscopy (WItec, Germany, Alpha‐300R) with an excitation wavelength of 532 nm. The thickness, surface morphology, and chemical composition of the films were observed by field emission scanning electron microscope (FESEM, TESCAN, Czech, MIRA 3) combined with energy dispersive X‐ray spectroscopy (EDS). The valance states of the constitutive elements were measured by X‐ray photoelectron spectroscopy (XPS, Thermo Scientific, USA, K‐Alpha+) at a binding energy ranging from 0 to 1000 eV. The optical transmittance and band gap were studied using an ultraviolet‐visible‐ near‐infrared spectrophotometer (UV‐Vis‐NIR, Jasco, Japan, V‐570) in the wavelength range of 400– 2000 nm. The performance of the CuS/SnS absorber‐based solar cells was investigated using a solar simulator (McScience, Korea, Xe55) at AM 1.5G illumination.

3. Results and Discussion

3.1. CuS Thin‐Film Structural Properties The XRD patterns of the CuS thin films are shown in Figure 1. The positions of the diffraction lines matched the location of standard JCPDS card number 00‐001‐1281 for the crystalline covellite CuS hexagonal phase. The diffraction peaks corresponding to the (101) and (102) planes were observed at 27.3° and 29.4°, respectively, on the basis of 2 θ, while diffraction peaks corresponding to the (103) and (116) planes were observed at 31.8° and 59.2°, respectively. For all CuS thin films grown using different power conditions, the preferred growth orientation for the (110) plane was

Energiesobserved,2020 ,and13, 688 the intensity of the primary diffraction peak increased as the power was increased4 of 12 from 40 to 100 W. The full width at half maximum (FWHM) of the diffraction peak for the (110) plane was 0.58 for 40 W, 0.58 for 60 W, 0.48 for 80 W, and 0.38 at 100 W, showing a decreasing trend. The 1 atgrain 474 size cm− calculatedis a typical by peak substitution characteristic into the that Scherrer indicates formula the covellite [15] revealed CuS phase that grain [19]. Throughsize increased such Ramanfrom 15.7 analyses, nm at 40 it wasW to determined 23.9 nm at 100 that W. CuS thin ﬁlms with pure covellite CuS phase were fabricated.

Energies 2020, 13, 688 4 of 12 60W 100W 40W 80W Atomic vibration mode analysis can distinguish(110) spatial variations Ref. 00-001-1281 and surface species in a (103) (101) (102)

sample composition, which is generally possible through (116) Raman measurement [16]. Additional analysis of the structural properties of the CuS thin films was conducted by obtaining Raman spectra, and the results are shown in Figure 2. All of the Raman spectra for the CuS thin films deposited at different RF powers showed two bands, with a primary sharp and intense peak at the frequency of 474 cm−1, and a relatively wide peak at the frequency of 265 cm−1. The shark peak in the high‐ frequency region is the stretching vibrational mode from the covalent S–S bonds of S2− ions at 4e sites, and this signifies that the lattice atoms were arranged in a periodic array [6]. The broad Raman peak in the low‐frequency region corresponded to the mode of the lattice vibration produced by the Cu–S bonds [17]. Theoretical Intensity (arb.unit.) analysis revealed that a total of 36 vibrational modes are possible for the CuS system, and among them, 14 Raman active modes (ГRaman = 2A1g + 4E2g + 2E1g) can be categorized into ГCu1 = A1g + E2g + E1g, ГCu2 = E2g, ГS1 = A1g + E2g + E1g, and ГS2 = E2g for copper (Cu) and sulfur (S) each 20 30 40 50 60 70 80 [1,18]. The covalent S–S bonds only exist in the copper‐deficient Cu2‐xS (0.6 ≤ x ≤ 1) phases, so the strong band observed at 474 cm−1 is a typical peak2 (Degree) characteristic that indicates the covellite CuS phase [19]. Through such Raman analyses, it was determined that CuS thin films with pure covellite CuS Figure 1. XX-ray‐ray diffraction diffraction (XRD) (XRD) patterns patterns of of copper copper sulfide sulfide (CuS) (CuS) thin thin films films grown at different different phase were fabricated. sputtering powers.

474 40W 60W 80W 100W

265 Intensitiy(arb.unit)

200 300 400 500 600 700 Raman Shift (cm-1)

FigureFigure 2. 2. RamanRaman spectra spectra of of CuS CuS thin thin films films grown grown at at different different sputtering powers. 3.2. CuS Thin-Film Morphological Properties 3.2. CuS Thin‐Film Morphological Properties FESEM images of the CuS thin films grown on glass substrates at four different sputtering powers FESEM images of the CuS thin films grown on glass substrates at four different sputtering (40–100 W) are presented in Figure3. In the case of the CuS thin film deposited with the RF power powers (40–100 W) are presented in Figure 3. In the case of the CuS thin film deposited with the RF condition of 40 W, relatively small dense particles with small void spacing composed a smooth and power condition of 40 W, relatively small dense particles with small void spacing composed a smooth uniform surface, as shown in Figure3. As the deposition power was increased to 100 W, the grain size and uniform surface, as shown in Figure 3. As the deposition power was increased to 100 W, the grain gradually increased, and the grain boundaries also increased. This result was in good agreement with size gradually increased, and the grain boundaries also increased. This result was in good agreement XRD analysis results that showed that the FWHM value decreased and crystallite size improved as with XRD analysis results that showed that the FWHM value decreased and crystallite size improved the deposition power increased. The cross-sectional image of the CuS thin film showed that thickness as the deposition power increased. The cross‐sectional image of the CuS thin film showed that varied for different deposition powers. Thickness was around 250 nm for 40 W, 310 nm for 60 W, thickness varied for different deposition powers. Thickness was around 250 nm for 40 W, 310 nm for 460 nm for 80 W, and 580 nm for 100 W. It was also observed that CuS thin-film growth occurred in 60 W, 460 nm for 80 W, and 580 nm for 100 W. It was also observed that CuS thin‐film growth long and flat columnar structures vertically about the glass substrate. occurred in long and flat columnar structures vertically about the glass substrate.

Energies 2020, 13, 688 5 of 12 Energies 2020, 13, 688 5 of 12

40W 60W

80W 100W

FigureFigure 3. 3. SurfaceSurface and and cross cross-sectional‐sectional images images of of CuS CuS thin thin films ﬁlms grown grown at different diﬀerent sputtering powers.

3.3. CuS CuS Thin Thin-Film‐Film Compositional Compositional Properties Properties The atomic percentages of the CuS thin film film according to deposition power, obtained in EDS analyses, are shown in Table 11.. TheThe SS/Cu/Cu ratio of the CuS thin film film deposited with the RF power of 40 W was 0.98, which was similar to the stoichiometric composition. As As deposition deposition power power increased, increased, the S/Cu S/Cu ratio was 0.96 for for 60 60 W, W, 0.95 for 80 W, and 0.94 for 100 W. The SS/Cu/Cu ratio decreased with increasing depositiondeposition power power compared compared with with a CuSa CuS thin thin film film grown grown with with relatively relatively less less S than S than Cu. ThisCu. Thisshowed showed that thethat S elementthe S element was more was dependentmore dependent on deposition on deposition power comparedpower compared to the Cu to element the Cu elementwhen fabricating when fabricating CuS thin CuS films thin using films the using RF magnetron-sputtering the RF magnetron‐sputtering method. method.

Table 1.1. Estimated full width at half maximum (FWHM) value, crystalline size, film film thickness, and chemical composition of CuS thin filmsfilms grown at didifferentfferent sputteringsputtering powers.powers.

EDSEDS (Atomic (Atomic %) %) SputteringSputtering Power Power (W) FWHMFWHM Value Value (Degree) CrystalliteCrystallite Size Size (nm) FilmFilm Thickness Thickness (nm) (W) (Degree) (nm) (nm) SS CuCu 40 0.58 15.7 250 49.6 50.6 4060 0.580.58 15.715.7 250310 49.649.1 50.650.9 6080 0.580.48 15.718.9 310460 49.148.9 50.951.1 10080 0.480.38 18.923.9 460580 48.948.4 51.151.6 100 0.38 23.9 580 48.4 51.6 XPS measurement was carried out to precisely analyze the purity, composition, and valence statesXPS of the measurement CuS thin film. was carriedFigure out4 shows to precisely the XPS analyze results the for purity, a specimen composition, with surface and valence impurities states removedof the CuS by thin etching film. for Figure 1 min.4 shows Strong the peaks XPS at results 932.1 for and a 952.0 specimen eV were with observed, surface impurities in good agreement removed withby etching the literature for 1 min. data Strong regarding peaks the at 932.1Cu 2p and3/2 and 952.0 Cu eV 2p were1/2 binding observed, energies, in good respectively. agreement withThis corresponds to the Cu2+ state in the CuS structure [20]. Theoretically, the difference in binding the literature data regarding the Cu 2p3/2 and Cu 2p1/2 binding energies, respectively. This corresponds energiesto the Cu for2+ state these in two the CuSstates structure is 20 eV [20 [9].]. Theoretically, For CuS thin the films diff erencedeposited in binding with varying energies sputtering for these powers,two states the is 20difference eV [9]. For in the CuS binding thin films energies deposited of Cu with 2p varying3/2 and sputteringCu 2p1/2 was powers, found the to dibeff erence19.9 eV, in regardless of the deposition power, which is almost equivalent to the theoretical value. Figure 4b the binding energies of Cu 2p3/2 and Cu 2p1/2 was found to be 19.9 eV,regardless of the deposition power, showswhich isthe almost result equivalentof a high‐resolution to the theoretical survey of value. the S 2p Figure region,4b shows showing the that result the of peaks a high-resolution for the S 2p3/2 and S 2p1/2 binding energies were 162.2 and 163.2 eV, respectively [21]. The XPS analysis result survey of the S 2p region, showing that the peaks for the S 2p3/2 and S 2p1/2 binding energies were indicated162.2 and that 163.2 Cu eV, and respectively S oxidation [21 states]. The clearly XPS analysis exist on result CuS indicatedthin films that deposited Cu and at S varying oxidation degrees states ofclearly RF power. exist on CuS thin films deposited at varying degrees of RF power.

Energies 2020, 13, 688 6 of 12 Energies 2020, 13, 688 6 of 12

(a) (b) S 2p3/2 Cu 2p3/2 Cu 2p S 2p

19.9 eV S 2p1/2

Cu 2p1/2

100W

100W 80W 80W

60W 60W Intensity (arb.unit) Intensity (arb.unit) 40W 40W

925 930 935 940 945 950 955 960 155 160 165 170 175 Binding Energy (eV) Binding Energy (eV)

Figure 4. X-ray photoelectron spectroscopy (XPS) spectra of CuS thin films at different sputtering Figurepowers: 4. ( aX)‐ Curay 2pphotoelectron and (b) S 2p spectroscopy peaks. (XPS) spectra of CuS thin films at different sputtering powers: (a) Cu 2p and (b) S 2p peaks. 3.4. CuS Thin-Film Optical Properties 3.4. CuS Thin‐Film Optical Properties Figure5 shows the optical transmittance spectrum of the CuS thin film measured at wavelengths rangingFigure from 5 shows 400 to the 1200 optical nm. transmittance The transmittance spectrum spectrum of the CuS for thin the CuSfilm measured thin film depositedat wavelengths with ranging40 W RF from power 400 appeared to 1200 nm. above The the transmittance 400 to 1200 nmspectrum range, for and the a clearCuS thin peak film form deposited was observed with 40 with W RFa transmittance power appeared of approximately above the 400 less to 1200 than 18%nm range, in the visibleand a clear region peak of 450 form to 800was nm.observed For the with 60 Wa transmittancecondition, a peak of approximately of approximately less less than than 18% 6% in was the observed visible region in the rangeof 450 from to 800 450 nm. to 750For nm, the which60 W condition,was significantly a peak lowerof approximately than that of less 40 W. than A transmittance 6% was observed of approximately in the range from less than 450 to 2% 750 was nm, observed which wasin the significantly range of 500–700 lower nm than for thethat 80 of W, 40 while W. aA transmittance transmittance of lessof approximately than 1% was observed less than in the2% range was observedof 500–650 in nm the forrange the of 100 500–700 W. The nm maximal for the 80 center W, while point a of transmittance the transmittance of less spectrum than 1% was peaks observed moved infrom the 560range (at of 40 500–650 W) to 570 nm nm for (atthe 100100 W)W. asThe RF maximal power increased.center point The of the cases transmittance of the CuS thinspectrum films peaksdeposited moved at RF from powers, 560 (at except 40 W) for to the 570 40 nm W, (at were 100 observed W) as RF to power be nontransmissive increased. The in cases the near-infrared of the CuS thin(NIR) films region deposited (λ > 750 at RF nm). powers, This transmittance except for the reduction40 W, were was observed determined to be nontransmissive to be a major cause in the of nearthe increase‐infrared in (NIR) CuS thin-film region (λ thickness> 750 nm). as RFThis power transmittance increased. reduction Results in was Figure determined5, with a transmittance to be a major causeof 10%–20% of the increase in the visible in CuS (Vis.) thin‐film region thickness and nearly as RF zero power in the increased. infrared Results (IR) region, in Figure were 5, in with good a transmittanceagreement with of 10%–20% the optical in properties the visible of (Vis.) the CuSregion thin and film nearly reported zero in by the Nair’s infrared group (IR) [22 region,]. The were drop indown good of agreement the transmission with the spectrum, optical properties starting at of the the 560 CuS nm thin or film 570 nm reported wavelength by Nair’s and group extending [22]. The into dropthe NIR down region, of the is transmission consistent with spectrum, the literature starting to demonstrate at the 560 nm the or typical 570 nm metallic wavelength behavior and of extending CuS [23]. intoThe the main NIR reason region, for is the consistent great absorption with the literature of light in to the demonstrate NIR region the is typical due to metallic the nanocrystalline behavior of CuSnature [23]. of theThe films main (great reason amount for ofthe grain great boundaries), absorption andof light the high in the concentration NIR region of chargeis due carriersto the nanocrystalline(holes) derived fromnature the of crystalline the films (great structure amount [24]. Theof grain transmitted boundaries), NIR light and decreasedthe high concentration with increasing of chargethickness carriers and reached (holes) almost derived zero from for thethe thickercrystalline CuS films.structure In the [24]. visible The region,transmitted a relatively NIR light high decreasedtransmission with followed increasing by thickness an absorption and reached edges can almost be observed, zero for the indicating thicker CuS typical films. semiconductor In the visible region,behavior. a relatively In addition, high the transmission fall down of followed transmitted by an light absorption could be edges due to can the be forbidden observed, band indicating gap of typicalCuS [25 semiconductor]. behavior. In addition, the fall down of transmitted light could be due to the forbidden band gap of CuS [25].

Energies 2020, 13, 688 7 of 12 Energies 2020, 13, 688 7 of 12

20 560 570 40W 60W 80W 15 80W 100W

5 Transmittance (%) Transmittance (%)

0 400 600 800 1000 1200 1400 1600 1800 2000 Wavelength (nm)

FigureFigure 5. 5.Optical Optical transmittance transmittance spectraspectra of CuS thin films ﬁlms grown grown at at different diﬀerent sputtering sputtering powers. powers.

The band-gap energy (Eg) of the CuS thin film was calculated using the Tauc plot for absorption The band‐gap energy (Eg) of the CuS thin film was calculated using the Tauc plot for absorption α coecoefficientfficient α obtainedobtained fromfrom the transmittance transmittance spectra, spectra, and and the theresults results are shown are shown in Figure in Figure 6 [26].6 The[ 26 ]. Theband band-gap‐gap energy energy of the of theCuS CuS thin thin film film deposited deposited with witha sputtering a sputtering power power of 40 W of was 40 W 2.68 was eV. 2.68 The eV. Theband band-gap‐gap energy energy with with increasing increasing power power was was2.57 2.57eV for eV 60 for W, 60 2.56 W, eV 2.56 for eV 80 for W, 80and W, 2.47 and eV 2.47 for eV100 for 100W, W, showing showing a adecreasing decreasing trend. trend. This This tendency tendency was was determined determined to be to a be result a result of the of improvement the improvement in in crystallinity and and increased increased thickness thickness of of the the CuS CuS thin thin film film as as deposition deposition power power increased, increased, as asobserved observed throughthrough XRD XRD patterns patterns (Figure (Figure1), 1), Raman Raman spectra spectra (Figure (Figure2 2),), and and FESEM FESEM imagesimages (Figure 33).).

13 2.0x1013 40W 60W 80W 100W E = 2.68 eV E = 2.57 eV E = 2.56 eV E = 2.47 eV Eg = 2.68 eV Eg = 2.57 eV Eg = 2.56 eV Eg = 2.47 eV

13 2 13 2 1.5x10

13 1.0x1013 (eV/cm) (eV/cm) 2 2 hv) hv) 12

 12

 5.0x10 ( 5.0x10 (

0.0 2.0 2.5 2.0 2.5 2.0 2.5 2.0 2.5

Photon Energy (eV)

FigureFigure 6. 6.Plot Plot (α (αhνhν)2)2versus versus photonphoton energy (h νν)) of of CuS CuS thin thin films films grown grown at at different different sputtering sputtering powers. powers. 3.5. Photovoltaic Performance of CuS SnS Absorber-Based Solar Cells 3.5. Photovoltaic Performance of CuS/SnS/ Absorber‐Based Solar Cells A numberA number of of studies studies have have reported reported on on the the value value and and nature nature of of the the energy energy band band gaps gaps of of covellite covellite CuS thinCuS films. thin The films. direct The band direct gaps band reported gaps reported in the literature in the literature lie in the lie range in the from range 2.05 from to 2.58 2.05 eV to [3 2.58,12,16 eV,20 ]. The[3,12,16,20]. band gaps The of CuSband thin gaps films of CuS prepared thin films in prepared this study in alsothis study ranged also from ranged 2.47 from to 2.68 2.47 eV. to 2.68 However, eV. theseHowever, band-gap these ranges band‐ aregap not ranges suitable are not for suitable light-absorbing for light‐absorbing layers for layers solar cells. for solar Therefore, cells. Therefore, a different approacha different is neededapproach than is needed directly than applying directly the applying CuS thin the filmCuS thin of this film type of this to thetype absorber to the absorber layer. R. Chierchia’slayer. R. Chierchia’s research group research fabricated group fabricated Cu2SnS3 Cu(CTS)2SnS3 thin (CTS) films thin by films a two-step by a two‐ procedurestep procedure in which in a CuSwhich thin a filmCuS wasthin depositedfilm was deposited by DC sputtering; by DC sputtering; then, the then, SnS thin the filmSnS thin was film grown was by grown RF magnetron by RF sputtering.magnetron The sputtering. CTS thin The films CTS were thin films optimized were optimized for use as for absorber use as absorber in solar in cells solar at cells two at di fftwoerent different sulfurization temperatures. The CTS thin film sulfurized at 520 °C revealed the energy band sulfurizationdifferent sulfurization temperatures. temperatures. The CTS The thin CTS film thin sulfurized film sulfurized at 520 ◦ atC 520 revealed °C revealed the energy the energy band band gap of gap of 0.92 eV, and the CTS‐based solar cell achieved power‐conversion efficiency (PCE) of 3.05% 0.92gap eV, of and 0.92 the eV, CTS-based and the CTS solar‐based cell solar achieved cell achieved power-conversion power‐conversion efficiency efficiency (PCE) of (PCE) 3.05% of [27 3.05%]. [27]. [27].This approach is one of the more efficient methods of applying covellite CuS thin films as absorbing This approach is one of the more efficient methods of applying covellite CuS thin films as layers inThis solar approach cells. CuSis one/SnS-absorber of the more layers efficient of 1methods um thickness of applying were covellite deposited CuS by thin RF magnetron films as absorbing layers in solar cells. CuS/SnS‐absorber layers of 1 um thickness were deposited by RF sputtering.absorbing CuSlayers thin in solar films cells. with CuS/SnS a thickness‐absorber of 500 layers nm wereof 1 um grown thickness on glass were substrates deposited at by a roomRF

Energies 2020, 13, 688 8 of 12 EnergiesEnergies2020 2020, 13, ,13 688, 688 8 of8 12 of 12 magnetron sputtering. CuS thin films with a thickness of 500 nm were grown on glass substrates at a magnetron sputtering. CuS thin films with a thickness of 500 nm were grown on glass substrates at a temperatureroom temperature with various with various sputtering sputtering powers powers of 40–100 of 40–100 W. To prepare W. To prepare the CuS the/SnS CuS/SnS absorber absorber layer, RF roomlayer, temperatureRF power and with substrate various temperature sputtering werepowers adjusted of 40–100 to 60 W. W andTo prepare 300 °C and the aCuS/SnS 500 nm thickabsorber SnS power and substrate temperature were adjusted to 60 W and 300 ◦C and a 500 nm thick SnS thin layer,thin film RF powerwas deposited and substrate on the temperature CuS samples, were respectively. adjusted to Figure 60 W and7 shows 300 °C the and optical a 500 transmittance nm thick SnS film was deposited on the CuS samples, respectively. Figure7 shows the optical transmittance of thinof CuS/SnS film was absorber deposited layers on thegrown CuS at samples, a substrate respectively. temperature Figure of 300 7 shows °C in thethe 400–2000optical transmittance nm spectral CuS/SnS absorber layers grown at a substrate temperature of 300 C in the 400–2000 nm spectral region. ofregion. CuS/SnS The absorbertransmission layers spectrum grown at in a Figuresubstrate 7 istemperature clearly distinguished of◦ 300 °C in from the 400–2000that in Figure nm spectral 5. The The transmission spectrum in Figure7 is clearly distinguished from that in Figure5. The absorption region.absorption The edge transmission shifted to spectruma wavelength in Figure near 800 7 is nm, clearly and thedistinguished spectra range from was that also in extended Figure 5. to The the edge shifted to a wavelength near 800 nm, and the spectra range was also extended to the 2000 nm absorption2000 nm wavelength, edge shifted including to a wavelength the NIR nearregion. 800 Transmittance nm, and the spectra in the rangeNIR region was also (λ =extended 780–2000 to nm) the λ wavelength,2000for the nm CuS/SnS wavelength, including thin films including the decreased NIR the region. NIR from region. Transmittance50% to Transmittance 48% on average in the in NIRas the the NIR region deposition region ( =( λ power780–2000= 780–2000 of the nm) CuSnm) for theforthin CuS the film/SnS CuS/SnS increased thin films thin from films decreased 40 decreased to 100 from W. from 50% 50% to 48% to 48% on average on average as the as the deposition deposition power power of of the the CuS CuS thin filmthin increased film increased from 40 from to 100 40 to W. 100 W. 100 100 100W 100W80W 80 80W60W 80 60W40W 40W 60 60

40 40 Transmittance (%) 20 Transmittance (%) 20

0 0400 600 800 1000 1200 1400 1600 1800 2000 400 600 800 1000 1200 1400 1600 1800 2000 Wavelength (nm) Wavelength (nm) Figure 7. Optical transmittance spectra of CuS/tin sulfide (SnS) absorber layers grown at a substrate Figure 7. Figuretemperature 7.Optical Optical of 300 transmittance transmittance °C as function spectra spectra of various ofof CuSCuS/tin sputtering/tin sulfide sulfide powers (SnS) (SnS) forabsorber absorber CuS thin layers layers films. grown grown at ata substrate a substrate temperaturetemperature of of 300 300◦ C°C as as function function of of various various sputtering powers powers for for CuS CuS thin thin films. films. The band gap for the CuS/SnS absorber layers is shown in Figure 8. As the sputtering power for The band gap for the CuS/SnS absorber layers is shown in Figure8. As the sputtering power for depositingThe band the gapCuS for thin the film CuS/SnS was increased absorber fromlayers 40 is Wshown to 100 in W,Figure the 8.band As thegap sputtering of the CuS/SnS power thin for depositingdepositingfilms was the changed the CuS CuS thin fromthin film film1.58 was waseV increased to increased 1.57 eV, from showingfrom 40 40 W Wa to slight to 100 100 W, difference W, the the band band of gap 0.01 gap of eV. theof theThe CuS CuS/SnS results/SnS thin show thin films wasfilmsthat changed when was changed fromthe covellite 1.58 from eV CuS to1.58 1.57 eVfilm eV, to was 1.57 showing stackedeV, showing a slight together dia slightff erencewith difference the of 0.01SnS eV. film,of 0.01 The and resultseV. given The show resultsappropriate that show when thethatprocess covellite when conditions, CuS the filmcovellite wasthe CuS stackedCuS film film togethercould was bestacked withsufficiently the together SnS applied film, with and asthe givenan SnS absorbing appropriatefilm, and layer given process of the appropriate solar conditions, cell. theprocessIn CuS order film conditions, to could apply be thea suCuS CuSfficiently thinfilm couldfilm applied in be solarsufficiently as an cells, absorbing appliedphotovoltaic layer as an of cellsabsorbing the solarwith layer cell.a glass/Mo/absorber of In the order solar to cell. apply a CuSIn(CuS/SnS)/CdS/i order thin filmto apply in solar‐ZnO/ITO/Al a cells,CuS photovoltaicthin structure film in were cellssolar fabricated, with cells, a glassphotovoltaic as /shownMo/absorber in cellsFigure (CuSwith 9. The/ SnS)a effectglass/Mo/absorber/CdS of/i-ZnO sputtering/ITO/ Al structure(CuS/SnS)/CdS/ipowers werefor the fabricated, deposition‐ZnO/ITO/Al as of shown CuS structure thin in films Figure were on fabricated,9 .the The performance e ffect as shown of sputtering of CuS/SnSin Figure powers absorber9. The effect for‐based the of deposition sputtering solar cells of CuSpowerswas thin investigated. films for the on deposition the performance of CuS thin of CuS films/SnS on the absorber-based performance solarof CuS/SnS cells was absorber investigated.‐based solar cells was investigated.

2.0x1012 12 40W 60W 80W 100W 2.0x10 Eg 40W= 1.58 eV Eg 60W= 1.58 eV Eg 80W= 1.57 eV Eg 100W= 1.58 eV Eg = 1.58 eV Eg = 1.58 eV Eg = 1.57 eV Eg = 1.58 eV 1.6x1012 2 1.6x1012 2

1.2x1012 1.2x1012 (eV/cm) 2 8.0x1011 (eV/cm) 2 8.0x1011 hv)  hv)

( 11

 4.0x10 ( 4.0x1011

0.0 0.0 1.3 1.4 1.5 1.6 1.7 1.3 1.4 1.5 1.6 1.7 1.3 1.4 1.5 1.6 1.7 1.3 1.4 1.5 1.6 1.7 1.3 1.4 1.5 1.6 1.7 1.3 1.4 1.5 1.6 1.7 1.3 1.4 1.5 1.6 1.7 1.3 1.4 1.5 1.6 1.7 Photon Energy (eV) Photon Energy (eV)

FigureFigure 8. 8.Plot Plot ( α(αhνhν)2)2 versusversus photonphoton energy (h νν)) of of CuS/SnS CuS/SnS absorber absorber layers layers grown grown at ata substrate a substrate Figure 8. Plot (αhν)2 versus photon energy (hν) of CuS/SnS absorber layers grown at a substrate temperaturetemperature of of 300 300◦C °C as as function function of of various various sputteringsputtering powers for for CuS CuS thin thin films. ﬁlms. temperature of 300 °C as function of various sputtering powers for CuS thin films.

Energies 2020, 13, 688 9 of 12 EnergiesEnergies 2020 2020, ,13 13, ,688 688 99 ofof 1212

FigureFigure 9. 9. CuS/SnS CuS/SnSCuS/SnS absorber absorber absorber-based‐based‐based solar solar cell. cell.

FigureFigure 10 1010 shows shows shows the the the current current current density density density-voltage‐‐voltagevoltage (J–V) (J–V) (J–V) characteristics characteristics characteristics of of the the of fabricated fabricated the fabricated thin thin‐‐filmfilm thin-film solar solar‐‐ cellcellsolar-cell device device devicebased based on basedon the the CuS/SnS on CuS/SnS the CuS absorber absorber/SnS absorber layer. layer. In In layer. order order Into to orderinvestigate investigate to investigate how how the the deposition howdeposition the deposition power’s power’s effectpower’seffect onon e CuSffCuSect on thinthin CuS‐‐filmfilm thin-film growthgrowth growth affectedaffected aff ected CuS/SnSCuS/SnS CuS / SnSabsorberabsorber absorber-based‐‐basedbased solarsolar solar-cell‐‐cellcell performance,performance, performance, thethe the J–VJ–V characteristicscharacteristics of of of all all all samples samples samples were were were measured, measured, measured, and and and results results results are are are shown shown shown in in Table inTable Table 2. 2. The 2The. The efficiency efficiency e fficiency of of the the of solarsolarthe solar cell cell manufactured manufactured cell manufactured with with a witha CuS CuS a thin thin CuS film film thin deposited deposited film deposited at at the the power atpower the powercondition condition condition of of 40 40 W W of was was 40 0.19%, W 0.19%, was whilewhile0.19%, efficiencyefficiency while effi waswasciency 0.25%0.25% was andand 0.25% 0.38%0.38% and forfor 0.38% 6060 andand for 8080 60 W,W, and respectively.respectively. 80 W, respectively. TheThe 100100 WW The powerpower 100 condition Wcondition power showedshowedcondition the the showed highest highest theefficiency efficiency highest of of e 0.39%.ffi 0.39%.ciency As As of shown shown 0.39%. in in As Figure Figure shown 10, 10, inopen open Figure‐‐circuitcircuit 10, voltage open-circuitvoltage (V (Vococ),), voltageshort short‐‐circuitcircuit (Voc), 2 2 currentcurrentshort-circuit densitydensity current (J(Jscsc),), density andand fillfill (J factorscfactor), and (FF)(FF) fill factor werewere (FF)measuredmeasured were measured toto bebe 115115 to mV,mV, be 115 9.819.81 mV, mA/cmmA/cm 9.81 mA2, , andand/cm 35%,,35%, and respectively.respectively.35%, respectively. TheThe reasonreason The reason forfor thisthis for increased thisincreased increased efficiencyefficiency efficiency cancan can bebe be attributedattributed attributed toto to thethe the structuralstructural structural andand and optical optical propertyproperty improvements improvements of of the the CuS CuS thin thinthin film filmfilm as asas power powerpower increased, increased,increased, as asas discussed discusseddiscussed earlier. earlier.

1212 40W 40W

) 60W ) 2 10 60W 2 10 80W 80W 100W 88 100W

Current density (mA/cm density Current 0 Current density (mA/cm density Current

-2-2 -20-20 0 0 20 20 40 40 60 60 80 80 100 100 120 120 VoltageVoltage (mV) (mV)

FigureFigure 10. 10. Current Current density density density-voltage‐voltage‐voltage (J–V) (J–V) characteristics characteristics of of CuS/SnS CuS/SnS CuS/SnS absorber absorber absorber-based‐based‐based solar solar cells cells under under AMAM 1.5 1.5 illumination. illumination.

Energies 2020, 13, 688 10 of 12

Table 2. Current density-voltage (J–V) characteristics of CuS/SnS absorber-based solar cells evaluated from solar simulator as function of CuS sputtering powers (Ps). PCE—power conversion eﬃciency.

2 Ps (W) Voc (mA) Jsc (mA/cm ) FF (%) PCE (%) 40 119 5.15 31 0.19 60 116 6.39 33 0.25 80 109 10.15 34 0.38 100 115 9.81 35 0.39

4. Conclusions Covellite CuS thin films were fabricated using RF magnetron sputtering with different sputtering powers. The effects of sputtering powers on the structural, compositional, morphological, and optical properties of CuS thin films were investigated in detail. The CuS thin films deposited at varying RF powers showed preferred growth on the (110) plane corresponding to the diffraction angle of 2 θ 48.1 , ≈ ◦ and diffraction peak intensity for the (110) plane showed an increasing tendency proportional to power. 1 The peaks for the two bands shown in the Raman spectrum (265 and 474 cm− ) accurately matched the typical Raman spectrum values for covellite CuS. FESEM image analysis revealed that CuS crystal growth was in a direction vertical to the substrate, and there was a maximal thickness increase of approximately 330 nm, along with gradual increases in grain size and grain-boundary spacing. In the XPS study, two strong peaks for the Cu 2p3/2 and Cu 2p1/2 binding energies corresponding to 2+ the Cu state were observed at 932.1 and 952 eV, respectively. Peaks for the S 2p3/2 and S 2p1/2 binding 2 energies corresponding to the S − state were observed at 162.2 and 163.2 eV, respectively. The results above confirmed that CuS thin films were successfully fabricated with a pure covellite CuS phase, with no existing impurity phase. Transmittance of the CuS thin film deposited at the RF power of 40 W was approximately 18% in the wavelength range from 450 to 850 nm, while transmittance of the CuS thin film deposited at 100 W was less than 1% in the wavelength range from 500 to 650 nm. CuS thin-film transmittance and transmission range decreased as RF power increased. As the absorption edge moved towards the long wavelength range, the band-gap energy decreased from 2.68 (at 40 W) to 2.47 eV (at 100 W). We investigated the properties of a thin-film solar cell, fabricated by stacking the SnS thin film on a CuS thin film of equal thickness to compose the buffer layer, window layer, and electrode on top of the CuS/SnS-absorber. The results showed that the open-circuit voltage (Voc), short-circuit 2 current density (Jsc), and fill factor (FF) were 115 mV, 9.81 mA/cm , and 35%, respectively. The highest conversion efficiency of 0.39% was exhibited by the solar-cell sample incorporating a CuS thin film deposited at the RF power of 100 W.

Author Contributions: Conceptualization, D.S. and D.H.; methodology, D.H., J.H.P., and C.S.S.; software, D.S. and S.L.; validation, D.H., J.H.P., and Y.G.S.; formal analysis, S.L. and W.-J.C.; investigation, D.S., D.R.K., and S.L.; resources, C.S.S., Y.K., and Y.G.S.; data curation, W.-J.C. and D.R.K.; writing—original-draft preparation, D.S. and D.H.; writing—review and editing, Y.G.S. and D.H.; visualization, D.R.K. and S.L.; supervision, D.H., C.S.S., and Y.G.S. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173010013140); and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2017R1C1B5018403); and by the National Research Foundation (NRF-2018R1A5A1025594) of the Ministry of Science and ICT. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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