Single Phase Formation of SnS Competing with SnS2

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Single Phase Formation of SnS Competing with SnS and SnS for Photovoltaic Applications: Optoelectronic Characteristics of Thin-Film Surfaces and Interfaces 2

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Juran Kim, Jayeong Kim, Seokhyun Yoon, Jeong-yoon Kang, Chan Wook Jeon, and William Jo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00179 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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The Journal of Physical Chemistry

Single Phase Formation of SnS Competing with SnS2 and Sn2S3 for Photovoltaic Applications: Optoelectronic Characteristics of Thin-Film Surfaces and Interfaces Juran Kima, Jayeong Kima, Seokhyun Yoona, Jeong-yoon Kangb, Chan-Wook Jeonb, and William Joa,* a

Department of Physics and New and Renewable Energy Research Center (NREC), Ewha Womans University, Seoul, Korea 03760 b School of Chemical Engineering, Yeungnam University, Gyeongsan, Korea 38541 *E-mail: [email protected]

Abstract Tin monosulfide (SnS) is one of the most promising binary compounds for thin-film solar cells owing to its suitable optical properties and abundance in nature. However, in solar cells it displays a low open circuit voltage and power conversion efficiency owing to multiphases in the absorber layers. In this study, we investigated approximately 1.2-µm-thick SnS thin films prepared via a two-step process involving (1) the deposition of metal precursor layers and (2) sulfurization at 400 °C. To investigate the phase variations inside the thin films we employed a dimpling method to get a vicinal cross-section of the sample. Kelvin probe force microscopy, conductive atomic force microscopy, and micro-Raman scattering spectroscopy were used to characterize the local electrical and optical properties of the sample. We studied the distribution of the Sn-S polytypes in the film and analyzed their electrical performances for solar cell applications. The work functions of SnS and SnS2 were determined to be 4.3–4.9 eV and ~5.3 eV, respectively. The local current transport properties were also measured; they displayed an interesting transition in the conduction mechanism, namely from Ohmic shunt current at low voltages to space-charge-limited current at high voltages.

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Introduction Tin monosulfide (SnS) has gained much attention as a promising candidate for use as a light-absorbing layer owing to its direct band gap of 1.3 eV, which is close to the optimal value of 1.5 eV for solar cell performace.1 Moreover, the constituent elements, tin and sulfur, are non-toxic and abundant in nature.2 In addition, SnS has a high absorption coefficient of 104 cm−1, which makes it favorable for photovoltaic applications.3 Beyond these basic characteristics, SnS has a preferentially orthorhombic and layered structure; it can thus be used to create various nano-structures, such as nanoparticles,4 nanoflowers,5 nanoflakes,6 nanowires,7 nanosheets8, and more.9,10 SnS is intrinsically p-type as the tin vacancies in the lattice produce acceptor levels.11 Therefore, no extrinsic doping is required.12 Owing to its suitable properties, SnS has been employed in optoelectronics applications,13-15 lithium-ion batteries,16 electrical switching,17 gas sensors,18 and photocatalysts.9,10,19 Owing to the ease with which its stoichiometry can be controlled, there have been many attempts to fabricate SnS thin films and nanostructures using various methods, including chemical vapor deposition,20

spray

pyrolysis,11,21

thermo-decomposition,4

hydrothermal

processes,22

microwave-oven-assisted methods,6 and chemical bath deposition.23 As of 2017, the highest power conversion efficiency (PCE) of a SnS solar cell device was about 4%.24 This is much lower than the theoretical maximum PCE of 24% for a SnS single junction cell according to Prince–Loferski diagrams.25,26 Several issues regarding its low open circuit voltage (VOC) and PCE have already been addressed by others.1-3,24 Thus, it is known that the poor solar cell performance can be the result of small grain sizes, band gap misalignment near the interface, impurity of the phase, and off-stoichiometry.24 Uneven and poor morphology may also result in many defects inside the film and cause carrier recombination. Additionally, attempts have been made to improve the surface morphology and microstructure using various deposition methods and post-treatment conditions. Vidal et

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al. employed pulsed laser deposition to grow SnS thin films in which the Sn to S ratio differed.12 Sinsermsuksakul et al. annealed the SnS thin film in a H2S atmosphere at 400 °C to increase the grain size.24 Misfits of the electrical contacts between the SnS layer and buffer layer may also be a cause of the low PCE. In this study, we focused on the effect of the presence of Sn-S polytypes, for instance, SnS, Sn2S3, and SnS2 on the PCE. Reddy et al. and Banu et al. reported the formation and the effects of Sn-S polytypes on SnS thin-film growth.3,27 The phase transition of SnS2 occurs at a lower sulfurization temperature than Sn2S3, while SnS requires the highest sulfurization temperature. Additionally, a sulfurization temperature of 500 °C applied for 30 min is required to generate a single-phase SnS thin film from a liquid Sn precursor.27 Burton et al. investigated the enthalpy of formation for the various Sn-S polytypes28 and their electrical characteristics.29 SnS2 is known to have a large optical band gap of 2.24 eV and to exhibit n-type conductivity. Therefore, it can be considered to be a n-type buffer material that could replace conventional CdS. Sn2S3 has a band gap of 1.09 eV, which is similar to the band gap of SnS. Although phase mixtures of Sn2S3 and SnS phase do not appear to critically impact the materials' characteristics, they can affect carrier transport.29 Therefore, it is critical to determine both the phase distribution in SnS thin films and the exact location of the various Sn-S polytypes within the films. In this research, we used a dimple grinder to expose the inside of the thin films via vicinal surfaces from the Mo back-contact to the thin-film surface; thus, the phase location could be obtained as a function of depth. Further, the electrical properties of the various phases were revealed, which provides insight into how to optimize n-type buffer materials for SnS thin-film solar cells. Furthermore, for polycrystalline compound semiconductor thin films, surface potential bending at grain boundaries (GBs) can be an issue, since bending is known to have an impact on carrier transport in light-absorbing materials.30 Using scanning probe microscopy, the local electrical properties of SnS thin-film surfaces can be characterized and the most likely

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carrier behavior can be determined. Consequently, the results provide a way to determine the energy losses in SnS thin-film solar cells to develop strategies to increase device performance. We fabricated SnS thin films in a two-step process and characterized them with Kelvin probe force microscopy (KPFM), conductive atomic force microscopy (c-AFM), and micro-Raman scattering spectroscopy.

Experimental Sample preparation SnS thin films were fabricated in a two-step process that included sputtering and sulfurization. Sn precursor layers were first deposited on Mo coated soda lime glass (SLG) by direct current (dc) magnetron sputtering, employing a Sn metal target (99.99% purity). The thickness of the Sn precursor film was about 400 nm. After that, the precursor layers were sulfurized under an Ar2 atmosphere at a pressure of 100 Torr and a temperature of 400 °C. Before the sulfurization, the precursor was soaked in a sulfur atmosphere at a temperature of 240 °C. We obtained SnS light-absorbing layers with a thickness of ~1.2 µm. Subsequently, a 50-nm-thick CdS buffer layer and an 80-nm-thick i-ZnO layer were sequentially deposited on the SnS layer via a chemical bath deposition (CBD) method and dc magnetron sputtering, respectively. Further, a 500-nm-thick (Ga,Al):ZnO TCO layer was deposited by radio frequency magnetron sputtering, followed by a Ni (0.1 µm) and Ag (1.65 µm) front contact deposited via electron beam evaporation. The fabricated SnS thin-film solar cell had an efficiency of 1.65%.31 The thin-film samples were polished with a dimpling grinder to expose the vicinal crosssections. This kind of mechanical etching is generally utilized when preparing samples for transmission electron microscopy (TEM). Therefore, the SnS thin films were gently polished

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employing a sub-micron alumina slurry to yield a nearly undamaged surface. The grinding wheel was applied at 4 rotations per minute for 10 min. After polishing, the samples were rinsed with distilled water to remove any residues. The dimpled surface was nearly damagefree, since electrical signals could be obtained during KPFM and c-AFM measurements. To avoid undesirable impurities and secondary phases, the dimpled SnS thin films were etched in a 10% KCN solution for 3 min. Following solution etching, the samples were rinsed with distilled water for several seconds. Sample characterization Micro-Raman scattering spectra were obtained at room temperature using a McPherson 207 spectrometer equipped with a nitrogen-cooled charge-coupled device array detector. The samples were excited with a coherent 514 nm wavelength laser with a power of 1.5 mW that was focused to a 1–2-µm-diameter spot. The absorption coefficient (α) was obtained by transmittance and reflectance measurements using a Lambda UV/Vis/IR spectrometer (Perkin-Elmer) for a wavelength range of 250–800 nm. Cross-sectional TEM (JEM-2100F) measurements were conducted to investigate thin-film formation and crystallinity. We also obtained high resolution TEM (HRTEM) images along with the fast Fourier transform (FFT) patterns derived from these HRTEM images. Selected area electron diffraction (SAED) patterns were acquired simultaneously. KPFM was used to measure the local surface potential, and the surface work function was calculated by obtaining the absolute work function value of the metal tip with Eq. (1).32 The work function was determined by sampling the surface potential. Pt/Ir-coated silicon tips were used to measure the surface potential of the thin-film samples. The tips were calibrated with highly oriented pyrolytic graphite (HOPG), which has a work function (ΦHOPG) of 4.6 ± 0.1 eV.33 The absolute work function of the tip was calculated according to Eq. (1). From the contact potential difference (VCPD), the surface work function of the sample was thus

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obtained. VCPD was measured in a non-contact mode by applying an alternating current voltage with an amplitude of 1.0 V.

=

(1)

As the dimple surface was sloped and flat, it was difficult to measure the surfaces. However, the AFM system used includes a function to flatten the sample during the KPFM measurements.33 c-AFM and current–electric field (I–E) curves were measured under contact mode. As with KPFM, we used Pt/Ir-coated tips and applied external electric fields ranging from −10 to 10 V/cm. The metal-coated tip was connected to ground and the current was detected using a single terminal.

Results and Discussion According to the literature, as the annealing temperature is increased from 150 to 300 °C, the formation of SnS2 and Sn2S3 phases can be observed.3,23,34 Reddy et al. have reported that single phase SnS thin films can be obtained for a sulfurization temperature around 300 to 350 °C.34 Additionally, Minemura et al. have shown that for a higher sulfurization temperature, both small and large area SnS thin films have a single phase.2 Generally, at lower temperatures, the formation of SnS2 and Sn2S3 is more favorable than that of SnS.11 Figure 1(a) shows the X-ray diffraction pattern result of the SnS thin films grown using the two-step process. The intensity of the (040)-oriented peak is larger than that of the (101)-oriented peak. Figure 1(b) indicates that the crystal structure is that of orthorhombic SnS. When the b-axis is perpendicular to the substrate, a (040)-oriented peak appears. Conversely, the (101)-oriented peak indicates that the b-axis is parallel to the substrate. 6


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Therefore, we were able to determine how the SnS phase is oriented in the thin films. The result also shows an average pattern over a relatively large sample area. The overlap of the peaks between SnS and SnS2 can be seen; further, some peaks of undetermined origin also appeared. Therefore, Raman scattering were also employed to accurately assign the peaks. As expected for the thin film annealed at 400 °C, the SnS phase was found to be dominant in the grown film. Figure 2(a) shows a cross-sectional TEM image of a Pt/SnS/Mo/SLG sample. We measured [A] the surface area and [B] the middle parts of the thin films (as indicated in Fig. 2). Both areas include intra-grains (IGs) and GBs. The HRTEM images and FFT patterns (insets) of the [A] and [B] areas are shown in Fig. 2(b) and (c), respectively. GBs are indicated by yellow dotted lines. The surface shows two different phases neighboring each other (Fig. 2(b)). The left grain in Fig. 2(b) appears to contain the SnS2 phase since its FFT pattern indicates sulfur atoms arranged into a hexagonal network.8,9,19,35,36 Interestingly, SnS grains with different crystal structures were observed in [B] (Fig. 2(c)). The left grain and its FFT patterns indicate orthorhombic (α-) SnS, while the other grain indicates cubic (π-) SnS.10 The layered SnS structures were also observable via the SAED pattern in the IG area (Fig. 3(a)). Figure 3(b) shows a GB area that includes a SnS2 grain (in the red circle) whose SAED pattern has a hexagonal shape. Table 1 shows the optimized cell parameters of Sn-S polytypes reported by Skeleton et al.37,38 Our TEM results do not fit precisely with the theoretical parameters since the thin film is polycrystalline and it can be affected by the cutting direction. A surface scanning electron microscopy (SEM) image of the dimpled SnS thin film is shown in Fig. 4. It is generally thought that polycrystalline absorbing layers with a large grain size have a low recombination rate for photo-generated carriers, which leads to better carrier transport behavior.39 Indeed, Sinsermsuksakul et al. have reported that SnS thin films with a

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large grain size are effective for photoconversion applications.24 However, the grain size in our SnS thin film was relatively small, and voids were observed, and so the deposition conditions need to be optimized further. The composition at each point was measured using energy dispersive X-ray spectrometry (EDX). As the penetration depth of X-rays is much greater than for the visible light used for the micro-Raman scattering measurements, the EDX spectra were able to reveal the Mo atomic percentage across the sample surface (Table 2). Near the sample surface, the Mo atomic percentage was lower than in the center of the film, and the Sn-S ratio shows stoichiometric values, indicating that the SnS phase was dominant in the thin film. Figure 5(a) shows an optical image of the SnS thin-film sample (top-down view). Four sampling points were chosen (as shown in Fig. 5), and the phase distribution at these points was measured by KPFM and micro-Raman scattering spectroscopy. So that almost the same exact points could be used for both the micro-Raman scattering and KPFM measurements, an optical microscope was used to align the KPFM measurements. As it was possible to observe the point excited by the laser during the micro-Raman scattering measurement, the KPFM measurements could be aligned so that the area close to the spot illuminated by the laser was scanned. Figure 5(b) could be used to determine the depth profiles obtained from the microRaman scattering. An excitation laser with a wavelength of 514 nm was employed at the surface of the SnS light-absorbing layer to investigate the phase distribution, including the presence of secondary phases, even though these were only present in small quantities. The SnS thin film had an absorption coefficient of ~105 cm−1, so that the 514 nm laser was expected to reach a penetration depth of 160 nm.3 In the spectra, α-SnS peaks (Ag: 95, 189, and 218 cm−1; B3g: 160 cm−1; and B2g: 287 cm−1) were observed at all points.40 Moreover, a π-SnS peak at 123 cm−1 was also detected, although its mode has not yet been determined.41 At point 4, which is near the surface, a SnS2 peak (Ag: 312 cm−1) could be observed.7 The

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results agree relatively well with the HRTEM results. Although the α-SnS phase is dominant in the sample, a small amount of π-SnS was also detected. Figure 6(a)–(d) show the surface topographies exposed by dimpling etching. Point 1 is the nearest to the Mo back-contact, whereas point 4 is nearest to the thin-film surface. Figure 6(e)–(h) show the surface potential maps that correspond to the topographies in Fig. 6(a)–(d). Because of the mechanical etching, some scratches and small pits, resulting from voids and weak adhesion, were expected. However, the images do not indicate any damage to the thin films: the surface potential could be detected at all points and the work function could also be calibrated. Measurements of the work function for a dimpled sample have already been demonstrated using other materials.33 Such work function measurements are possible with a Kelvin probe as it allows for the detection of potential differences by measuring the phase variations of a sample. After etching, the exposed surfaces were significantly flatter. It should be noted that surface potential maps can be affected by the topography. For example, the results show slightly smaller values along the scratched lines. However, this does not affect the phase distribution because the variation of the surface potential owing to the topographical variation was too small to be distinguished as a different phase. Conversely, at point 4 (Fig. 6(d) and (h)), the surface potential varied considerably regardless of any topographical changes. According to equation (1), inordinately large surface potentials (such as those indicated Fig. 6 with a line from A to B) indicate the existence of phases other than SnS. As shown in Fig. 7, a variation in VCPD occurs along the GBs; this is similar to the behavior in kesterite and chalcogen-based materials. Additionally, line profiles (Fig. 7(a) and (b)) were obtained from the maps in Fig. 6(d) and (h) for the sections in the yellow boxes that are considered to be GBs. The line from A to B indicates the VCPD variation near the SnS2containing area, while the line from C to D shows the variation near the SnS area.

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Furthermore, histograms of the VCPD values were made using several measurements at each point (Fig. 7(c) and (d)). The surface potential values varied slightly depending on the scanned area (each area scanned was relatively small (5 × 5 µm2)). Thus, we scanned several areas and analyzed them to obtain definitive results. To observe the surface potential bending at the surface, VCPD was re-calculated on a 0 mV basis. The results show that about 72% of IGs had smaller VCPD values than the standard 0 mV, while around 74% of GBs had a larger VCPD. This verifies the downward band bending near the GBs on the SnS thin-film surface, which should assist carrier transport. SnS2 is known to be a n-type semiconductor, and thus the band bending induced by a metal tip should be the opposite of that induced in SnS, which is a p-type semiconductor. By making use of these opposite band bending directions, electron and holes could be effectively separated. However, the parasitic SnS2 phase in SnS thin films could also create unwanted p-n junctions in the material and hinder the electrons and holes from separating. A distorted lattice and unsaturated chalcogen-bonds near the GBs can also cause band bending.42 To date, other light-absorbing materials such as Cu2ZnSn(S,Se)443,44 and Cu(In,Ga)Se245 and other textures for thin-film solar cells have been explored. Electron– hole pair recombination is a critical issue for solar cell PCE, and therefore better ways of separating the electrons and holes would prove beneficial for enhancing solar cell performance. The presence of downward band bending at the GBs means that they can become main current flow paths by collecting electrons into the GBs and simultaneously repelling holes to the IGs Figure 8 shows the surface work function distribution as a function of the depth profile. The location of the points probed is shown in Fig. 5(a), and the surface work function was calculated from the surface potential maps in Fig. 6. The work function was measured by detecting the differences in the work function between the metal tip and the sample. To eliminate the difference, a lock-in amplifier applies a dc bias (Vdc). This way, the surface

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potential values were determined and VCPD could be set. The values differ depending on the phases present on the surface. That is why comparing the work function can be used as a method to identify the phase distribution on the sample surface. The area at point 1 near the Mo back-contact comprises a relatively uniform SnS phase. This may be because the S reaction is favorable from the back-contact. During sulfurization, Mo reacts with S, resulting in the formation of MoS2. Then, Sn will gradually react with S, forming SnS phase. SnS2 has a larger band gap than SnS,29 and the work function of SnS2 (ΦSnS2 = 5.36 ± 0.45 eV) is higher than that of SnS (ΦSnS = 4.4–4.9 eV). Reported values of the work function of SnS2 range from 5.2 to 5.3 eV,46,47 while that of SnS is about 4.9 eV; these values were all measured using KPFM.48 Therefore, the other phase on the surface, which was mentioned in Fig. 6(d) and (h), could be SnS2. The results of the micro-Raman scattering and HRTEM measurements can thus be used to determine the presence of SnS2 on the surface. The summarized results of the micro-Raman scattering and work function distribution profiles as a function of depth are presented in Table 3. The presence of SnS2 phase in a SnS thin film should have a deleterious effect on SnS solar cells. This is because SnS2 has n-type conductivity, while SnS is a p-type semiconductor.49 Further, Burton et al. reported that SnS2 phase in SnS thin films can form metallic type IIb heterojunctions with SnS.29 Consequently, it is difficult to make a depletion layer between the light-absorbing layer and the buffer layer. Despite its high carrier mobility and the attractive band gap properties, the presence of Sn2S3 in SnS thin films may hinder charge transport owing to the formation of a type II junction between the two phases.29 Therefore, the presence of Sn2S3 n-type secondary phase could cause SnS solar cells to have a low VOC, resulting in a low PCE. Our local surface investigation revealed small amounts of SnS2 at the sampled points near the surface of the sample. Even though the amounts found were small, the presence of the SnS2 phase may affect the properties of the junction formed

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between the SnS absorber and the buffer layers, increasing the interface recombination rate and hindering carrier transport. Thus, appropriate post-treatment methods to completely remove the secondary phases should be investigated in future studies. We made a junction between the SnS thin film and a Pt-coated silicon tip. Figure 9(a)– (e) show the I–E curves obtained using c-AFM, which was operated in contact mode. As the film thickness differed at each measured point, an electric field (E [V/cm]) was applied during the c-AFM measurement. Figure 9(f)–(j) show a logI–logE curve under reverse bias measured using c-AFM. As previously reported, under reverse bias the dark current is governed by the shunt current, which has a linear relationship to the applied electric field.50 However, in Fig. 9(f)–(j) it can be seen that the logI–logE curves in the reverse bias region display a non-Ohmic response as well as the traditional Ohmic shape. In thin-film solar cells, the space-charge-limited current (SCLC) model can be used to explain the presence of the non-Ohmic shunt current through the GBs.51 Generally, SCLC can be seen in semiconductors with symmetric contacts, where the barrier for electron injection is relatively high compared with the barrier for hole injection.51 Not only is the shunt current Ohmic in chalcogenide thinfilm solar cells, but SCLC can be considered to be a possible reason for the observed reverse current. This is because of the non-homogeneous electronic properties of the thin-film layer, which may originate from, for example, nanodomains, percolating dislocations, or GBs.51 We also determined the slope of the linear responses in the logI–logE curves. In the reverse bias region, the slope of the Ohmic linear response was around 1, and that of the non-Ohmic response was about 2–2.3. Based on the relationships listed below, the non-Ohmic response is in agreement with the SCLC model. (Ohmic) ∝

(2)

(SCLC) ∝

(3)

(Schottky) ln 12

!

∝ √


(4)

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Under the reverse E field, the Ohmic model for the shunt current has a linear response, which can also be observed in our data. The possible causes of shut current are reduced resistivity near GBs and pinhole defects in the light-absorbing layer.50 As the external E field is increased in the reverse direction, non-Ohmic shunt current is observed from around −3.2 V/cm. This can be explained using the SCLC model for thin-film solar cells as being caused by diode breakdown. Toward the Mo back-contact, the breakdown bias increased, i.e., the lower the amount of defect phase inside the thin film, the lower the current leakage and current density loss. For SnS thin-film solar cells, short-circuiting is also a critical issue with regard to improving PCE. Malfunctioning heterojunctions and disorder defects near the absorber surface would hinder the formation of p-n junctions with an n-type buffer layer, which can cause current leakage via electron–hole recombination. Accordingly, by manufacturing desirable single-phase thin films, the electrical parameters (e.g., short-circuit current density (JSC)) of such a solar cell device can be enhanced. With an applied forward bias E field, the I–E curves display Schottky diode I–V characteristics, which occur at the junction between the semiconductor and metal. In Fig. S2, the model Schottky curve and the ln − √$ curves are both shown. The curves with the real data are similar to the model Schottky curve, but weak diode characteristics were also observed near the surface, which is the origin of the deviation from the model. Charge carrier recombination governed by transport or leakage current may explain the disagreement at the low E fields.50 The small amount of SnS2 phase on the surface may also affect the I–E curves as it may act as a defect level. Additionally, SnS2’s larger band gap may affect the properties of the junction formed with the metal-coated tip. With regard to current flow, point 1 has the highest conductivity, and in the middle parts points 2 and 3 show a slightly lower conductivity than point 1. The I– E curves of SnS (Fig. 9(d)) and SnS2 (Fig. 9(e)) were obtained at the surface. The sampling point in the SnS region displayed curves with a similar shape to those from the three points 13



inside the thin film. However, the curves from the SnS2 sample point showed different characteristics. In the forward bias region, the conductivity was lower than for the other points, and in the reverse bias region (Fig. 9(j)) they showed a relatively sharp slope, which may affect the current density. This is in agreement with the materials' optical properties; we measured the absorption of the SnS and SnS2 thin film, see Fig. S1. In the absorption results, SnS2 has a larger optical band gap energy (2.11 eV) than SnS (1.68 eV). Accordingly, the approximate values of the band gap (Eg) and the work function of SnS (Eg = 1.68 eV and Φ = 4.78 eV, respectively) and SnS2 (Eg = 2.11 eV and Φ = 5.36 eV, respectively) could be obtained. These values are based on the results from the surface, which contains areas of SnS2 phase. Thus, the band diagram could be constructed (Fig. 10). Ref. [29] by Burton et al. was used for the electron affinity (χ) values of SnS and SnS2 (χSnS = 3.59 eV and χSnS2 = 4.22 eV, respectively). Consequently, a larger external E field was required to produce the same current flow at the sampling point close to the surface compared with for the other sampling points.30 Indirect semiconductors in which weak phonons are responsible for the absorption properties require an appreciable material thickness for complete light absorption. For instance, the typical thickness of an absorbing layer in a c-Si solar cell is ~200 µm. However, for these SnS thin films, which have a thickness of about 1.2 µm, the absorption is dominated by direct, allowed optical transitions. For thin-film solar cell applications, with a typical film thickness of 1 µm, the absorption coefficient needs to be about 2–3 × 104 cm−1 for complete photon absorption.12 Accordingly, the SnS thin films with areas of SnS2 on the surface display a lower conductivity, which can cause a misalignment of the band gap and reduced carrier transport on the surface and at the interfaces.

Conclusion SnS thin films were successfully fabricated using sputtering and sulfurization methods.

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Using cross-sectional TEM results, we were able to confirm the presence of SnS phase inside the thin films and confirm the presence of other phases near the surface area, which we propose contains SnS2. With optical probes, the existence of the SnS2 secondary phase near the thin-film surface was verified. We demonstrated that KPFM measurements can be used to determine the surface work function value of the sample; the work function of the SnS was 4.3–4.9 eV and that of the SnS2 phase was ~5.3 eV. Both the KPFM and micro-Raman spectra showed that the SnS phase dominated across the entire depth of the thin film while a small amount of SnS2 phase existed at the surface. The presence of the SnS2 phase, which can deteriorate the junction properties, may result in a reduction of VOC. The I–E curves near the surface also differed from those in the film’s center; a higher bias voltage was required at the surface for the same current to be generated compared with for the sampling points in the film’s center. This investigation of the film’s local properties allowed us to determine the phase distribution in the film and further suggest that high-quality SnS light-absorbing layers could be used to create low-cost and highly efficient SnS thin-film photovoltaic devices.

Supporting Information Optical absorption coefficient of SnS and SnS2, and ln − √$ curves as a function of depth, which were fitted by Schottky model

Acknowledgement This work was supported by the Technology Development Program to solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (2016M1A2A2936753 and 2016M1A2A29366784), by the Basic Science Research

Program

through

the

NRF

funded

by

the

Ministry

of

Education

(2017R1D1A1B03034293), and by the DGIST R&D Program of the Ministry of Science and 15



ICT (17-BD-05).

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Tables and captions Table 1. Lattice parameters and space groups of Sn-S polytypes. Optimized lattice parameters of SnS (orthorhombic) and SnS (cubic); the SnS2 and Sn2S3 values quoted here were reported by Skelton et al.37 (Published by the PCCP Owner Societies)

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Table 2. SEM-EDX atomic percentage at each sampling point.

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Table 3. Raman spectra of the depth profiles of the dimpled SnS thin films and work function depth profiles of the dimpled SnS thin films.

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Figures and captions

Figure 1. (a) X-ray diffraction pattern of SnS thin films and (b) schematic of the crystal structure of orthorhombic SnS. As expected, since the sample was sulfurized at 400 °C, the thin film appears to be single-phase. SnS2 peaks were observed. However, the SnS (040) peak and SnS2 (101) peak overlapped. The JCPDS files referred to were those for orthorhombic SnS (39-0354), cubic SnS (77-3356), P-3m1 SnS2 (23-0677), and Pnma Sn2S3 (75-2183).

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Figure 2. (a) Cross-sectional TEM image of the Pt coated SnS thin film. Areas [A] and [B] marked in the panel both contain grain boundaries. HRTEM images of two grains each in (b) area [A] and (c) area [B] as defined in panel (a). The insets in panels (b) and (c) show the FFT patterns obtained from the images. The FFT pattern in area [A] near the surface shows both SnS2 (left grain) and SnS (right grain) signatures. Conversely, the FFT pattern in area [B] displays patterns that correspond to SnS grains with different crystal structures: the pattern indicates an orthorhombic structure for the grain on the left-hand-side of the image and a cubic structure for the grain on the right-hand-side.

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Figure 3. SAED pattern of (a) SnS intra-grain (IG) section and (b) grain boundary (GB, blue dotted line) near the sample surface. Insets show TEM images. The IG section shown in panel (a) has a layered structure with an orthorhombic crystal structure. Panel (b) shows the GB area near the surface. As a few grains are visible in the TEM images, the SAED pattern is a mix of a number of patterns. However, the grain pattern circled in red displays a SnS2 SAED pattern.

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Figure 4. Surface SEM image of the dimpled SnS thin film. Five points including the Mo back-contact were measured using EDX. Since X-rays can penetrate inside the thin film, Mo could be detected even at point 4. The closer the sample surface, the more the Mo atomic percentage decreased and the more similar the ratio between Sn and S was. This means that the SnS phase is dominant in the thin film. The atomic percentage at each of the five points is shown in Table 2.

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Figure 5. (a) Top-down optical image of the dimpled SnS thin films. The SnS thin-film sample was prepared using a dimpling grinder. (b) Cross-sectional micro-Raman scattering spectra of the SnS thin film. The wavelength of the laser was 514 nm. The sample shows SnS peaks (95, 123, 160, 189, 218, and 287 cm−1) at all points. No Sn2S3 phase (306 cm−1) could be detected at all, while a small amount of SnS2 phase (312 cm−1) was found near the sample surface (at point 4).

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Figure 6. Panels (a)–(d) show the AFM surface topography as a function of depth and panels (e)–(h) show the surface potential maps at each point. Owing to the mechanical dimpling, scratches can be seen; however, these do not indicate that the sample is damaged. It can be seen that the sample is not damaged as the VCPD difference, which depends on the phase difference, can be detected even though the thin film was etched. Owing to the mechanical etching voids inside the thin films have been exposed on the sample surface.

31



Figure 7. VCPD line profiles corresponding to the surface mapping data in Fig. 6(a) A to B (SnS2 containing area) and (b) C to D (SnS containing area). As SnS2 has n-type conductivity, the surface potential bending is in the opposite direction to that of SnS. Panel (c) shows a histogram of VCPD near the GBs and panel (d) near the IGs. The results show that about 72% of IG areas have smaller VCPD values than expected, while 74% of GBs have larger than expected values.

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Figure 8. Work function distribution according the sampling points at various depths within the SnS thin films. The first sampling point 1 was furthers from the surface and point 4 was closest. The inside of the thin film shows a SnS work function peak, while the area near the surface also shows a SnS2 work function peak.

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Figure 9. Panels (a)–(e) show the I–E curves of the four sampling points of the SnS thin films described in Fig. 5(a). The curves show Schottky diode I–V characteristics under forward bias because of the Schottky junction between the semiconductor and the metal tip. Point 1 near the Mo back-contact displays the largest conductivity, since it shows a higher current at a lower bias. Points 2 (b), 3 (c), and 4 (d), which are in the middle part of the thin

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film, display similar conductivities. At point 4 (e), which contained an area with SnS2 phase, a higher bias was required to achieve the same current as at the other sampling points. The reverse bias curves show shunt current that is a mix of two types of shunt currents. Panels (f)–(j) show the mixed shunt current on a logarithmic scale: the Ohmic shunt current and surface-charge-limited-current (SCLC) can be distinguished from one another. The slope of the Ohmic current is a linear response, i.e. 1, while that of the SCLC is approximately 2. The change in the type of shunt current can be thought of as being caused by diode breakdown.

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Figure 10. Schematic band structure of (a) SnS and (b) SnS2 as determined from absorption and KPFM measurements. The electron affinity values were taken from Ref. [29]. As reported, the band gap of SnS2 is larger than that of SnS, and SnS2 presents n-type conductivity. Near the thin-film surface the work functions were measured to be 4.78 and 5.36 eV for SnS and SnS2, respectively.

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TOC Graphic

Different areas of a dimpled SnS thin film were examined to determine the phase distribution and the electrical characteristics at different depths.

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Single Phase Formation of SnS Competing with SnS2

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