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SnS nanosheet films deposited via thermal evaporation: The effects of buffer layers on photovoltaic performance
Solar Energy Materials & Solar Cells 154 (2016) 49–56
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
SnS nanosheet films deposited via thermal evaporation: The effects of buffer layers on photovoltaic performance Farid Jamali-Sheini a,n, Mohsen Cheraghizade b, Ramin Yousefi c a
Advanced Surface Engineering and Nano Materials Research Center, Department of Physics, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran Young Researchers and Elite Club, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran c Department of Physics, Masjed-Soleiman Branch, Islamic Azad University (I.A.U), Masjed-Soleiman, Iran b
art ic l e i nf o
a b s t r a c t
Article history: Received 8 December 2015 Received in revised form 5 February 2016 Accepted 3 April 2016 Available online 5 May 2016
Tin sulfide films were synthesized on different metal (Ag, Au) buffer layers by the deposition method of thermal evaporation. X-ray studies indicated an orthorhombic phase of SnS and Sn2S3 and hexagonal phase of SnS2 for all specimens. Microscopic investigations showed sheet morphologies with high density and uniform distribution across the surface of the specimens. It was also observed that Ag-buffer layer nanosheet films had the smallest thickness. The elemental analysis indicated the desired element on the surface of the films. Optical investigations confirmed that the SnS films on the buffer layer had strong emission bands which were red and near infrared (NIR) regions. Furthermore, Raman studies presented five Raman bands, and a shift to lower value of the wavenumber was observed by using buffer layers. The I–V and Mott–Schottky plots confirmed the p-type conductivity of nanosheet films. The solar cell devices fabricated from the films of SnS nanosheets exhibited a higher efficiency and power efficiency conversion (PEC) for the specimens that had buffer layers. The results of photosensitivity also demonstrated higher photosensitivity in red and NIR regions. And finally, photocurrent results displayed a higher photocurrent and lower raise and fall times for the films with buffer layers compared to the freebuffer layer film. & 2016 Elsevier B.V. All rights reserved.
Keywords: SnS films Metal-buffer layers Nanosheets Vapor–liquid–solid growth Optical and electrical properties Photovoltaic performance
1. Introduction Tin (II) sulfide (SnS) has attracted much attention as a potential to be the material of photovoltaic cells (PVCs) due to its nearoptimal energy band gap and high optical absorption coefficient. SnS is a narrow band gap and semiconducting material with a high absorption coefficient of 105 cm 1. Its direct and indirect optical band gap energies range from 1.2–1.5 eV to 1.0–1.2 eV, respectively [1,2]. This material exhibits a two-dimensional (2D) layered structure characterized by strong intra-layer covalent bonds and weak interlayer interactions, bound together by van der Waals (vdW) forces along their crystallographic c-axis. An important consequence of this unique feature is that the material is more tolerant of the mismatches in the lattice constants and the thermal expansion coefficients between the substrates and the epilayers. Dangling bonds and material strain may be potentially eliminated between the adjacent layers and at the surfaces completely. Such a unique 2D-layered structure makes SnS a promising photovoltaic n
Corresponding author. Tel.: þ 98 613 3348420 24; fax: þ98 613 3329200. E-mail addresses: [email protected], [email protected] (F. Jamali-Sheini). http://dx.doi.org/10.1016/j.solmat.2016.04.006 0927-0248/& 2016 Elsevier B.V. All rights reserved.
material as the dangling bonds and strain at the hetero-interfaces can be significantly subdued [3]. Thus, a much wider choice of substrates can be utilized for the growth of high quality vdW. Furthermore, since the chalcogenide planes consist of saturated bonds, the heterojunctions, as a result, may be totally free of dangling bonds. This will have significant implications on the properties of the devices as the interface states typically have critical impacts on both the electrical and the optoelectronic properties of the junctions: first, the Fermi level will not be pinned at the metal/semiconductor interface; second, the trap-assisted recombination will be substantially suppressed; and lastly, the low-frequency noise will be significantly lowered. Studies on conventional semiconductor devices have shown that device degradations can often be traced back to the material defects in the devices; thus, lowering the concentration of the interface states in the material may have positive effects on the lifetime of the devices [4]. So far, a few researchers have investigated and reported the effects of different buffer layers on physical properties of SnS thin films. Using Molecular Beam Epitaxy technique, the SnS thin films were deposited on a graphene buffer layer on GaAs (100) substrate by Wang et al. [4]. They found out that further improvement in the properties of the SnS can be achieved through optimizing the
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microscope (FESEM, Quanta 200 F) was applied for morphological studies. The elemental composition was obtained with an energy dispersive X-ray spectrometer (EDS) attached to the FESEM (TeScan, Mira 3-XMU). Furthermore, Cross FESEM and EDS studies were performed through this microscope. The optical properties were investigated by means of the UV–Vis and photoluminescence (PL) analysis at room temperature. The UV–Vis spectra were recorded by a UV–Vis spectrometer (Perkin Elmer lambda-750 UV–Vis Spectrometer) over a wavelength range of 40–1200 nm. PL spectra were measured with a photoluminescence spectrometer (Perkin Elmer LS-55), using a xenon arc lamp (300 nm excitation wavelengths) as the source for PL. The Raman spectra were measured with a Raman spectrophotometer (SENTEERA-BRUKER), using an Ar ion laser with an emission wavelength of 785 nm for the Raman source.
graphene transfer process to avoid formation of folds and pinholes in the layer. Devika et al. synthesized SnS thin films on free- and Ag-coated buffer layers on glass substrate, using the resistive thermal evaporation method [2]. The results showed that the buffer layers had a strong influence on the physical properties of SnS thin films. For example, Ag-buffer layers decreased the electrical resistance and increased the optical band gaps of films in comparison with the free-buffer layer films. In addition to their unique optical properties, SnS nanosheets have an excellent potential to be applied to battery cell due to their excellent migration of electrons, optical transmittance, large reversible capacity, high rate capability, and long-term cycle life [5,6]. An attempt was made by Kang et al. to synthesize the 2-dimensional SnS nanosheets prepared via the non-catalytic, template-free, and vapor-transport synthetic route [5]. These nanosheets demonstrated good cycling performance and superior rate capabilities which were larger than the theoretical capacity of the carbon-based electrodes currently used in commercial Li ion batteries. Zhang et al. synthesized the ultra large single crystal SnS rectangular nanosheets by thermal decomposing of a single-source precursor [6]. The ultra-large SnS nanosheets exhibited fascinating electrochemical properties with a capacity of 350 mAh g 1 around 1.2 V and an appealing cycling reversibility. Although the important works have been thus far done on tin sulfide film, solar cell based on nanostructured SnS thin films on buffer layer is rather a new research field in photovoltaic applications [7–10]. Therefore, in this research work, we aim to synthesize the SnS nanosheets thin films, using thermal evaporation set-up and study the effects of different metal-buffer layers on the morphological, optical, electrical, photovoltaic, and photocurrent properties. To the best of our knowledge, the synthesis and study of SnS thin films properties under different metal-buffer layers have not been reported yet.
Two Ag electrodes were placed on the two edges of the specimens and were prepared for electrical characterization. The effective surface of the specimens exposed to the light was 0.25 cm2 for responsivity measurements. In order to check the solar cell characteristics, the measurement was carried out under 100 mW/cm2 (1.5 Air Mass) illumination from a solar simulator (solar cell simulator IIIS-200 þ, Nanosat Co., Iran). A 100-W xenon lamp served as a light source, and the intensity of the light was calibrated, using a standard silicon solar cell. For C–V plot, an LCR meter (LCR-8000G Series-Gwinstek) as well as a function generator at a frequency of 1 kHz was employed to measure the capacitance in order to obtain Mott–Schottky plot. Finally, the photoresponse (photocurrent) of specimens were measured, using an NIR source with 780 nm wavelength and 0.25 Hz chopped light illumination.
2. Experimental
3. Results and discussion
2.1. Synthesis
XRD patterns of the deposited films are shown in Fig. 1. All the specimens were crystalline and of the orthorhombic phase of SnS and Sn2S3 (JCPDS card no. 14-0620 and 14-0619) and the hexagonal phase of SnS2 (JCPDS card no. 83-1705). It can be observed
The nanostructure of SnS thin films were synthesized from SnS powder ultrasonically prepared in the previous works (20 kHz sonication frequency) [11]. First, the SiO2 substrates were cleaned by soap and then prepared ultrasonically in ethanol and deionized water. Three sets of substrate were made, one using a catalyst-free substrate and the others a thin layer of Au and Ag coated on SiO2 substrate with the thickness of 20 nm as a buffer layer. The specimens were named as Free-coated, Ag-coated, and Au-coated layers for the catalyst-free and the thin layers of Ag and Au, respectively. The synthesis was carried out in an atmospherecontrolled tube furnace (YTF 1450-30 6, Yaran Behgozin Parsa Co., Iran). 0.2 g of SnS powder was employed as the precursor material. The precursor material and substrates were placed downstream, and the precursor material was positioned at the two open ends of a quartz tube in a horizontal furnace tube. The precursor material was heated up to 800 °C. The distance of the substrate from the center of tube furnace was 23.5 cm (400 °C). High purity Ar gas was fed into the furnace at one end at about 35 sccm, while the other end was connected to a rotary pump. The growth process was allowed to proceed for 2 h. A vacuum of 10 mbar was maintained inside the tube furnace during the deposition of the layers.
2.3. Electrical and solar cell experiment
2.2. Characterization The deposited films were characterized, using an X-ray diffractometer (XRD, PAN Analytica). A field emission scanning electron
Fig. 1. XRD patterns of SnS nanosheet thin films.
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Fig. 2. FESEM images and EDS spectra of (a, b) Free-coated, (c, d) Ag-coated and (e, f) Au-coated buffer layer SnS nanosheet thin films.
that the intensity phase of SnS was lower for the free-coated layer compared to the others. The FESEM images of the deposited SnS films are shown in Fig. 2. The surface of the films indicates a sheet-like morphology on the entire films. The average thicknesses of the deposited SnS sheets were measured to be around 80 nm, 37 nm, and 51 nm for Free-, Ag-, and Au-coated layers, respectively. By careful observation it is clear that the thickness of the nanosheets deposited on Ag and Au (buffer) layers decreased in comparison with the uncoated layer. An elemental purity of the deposited SnS film was tested by the EDS technique in an alignment normal to the surface, and the typical spectrum of all specimens is demonstrated in Fig. 2. It is evident that using buffer (Ag/Au) layers increases atomic percentage of Sn, while it decreases S. Additionally, the Au buffer layer is more diffused in deposited layers compared to Ag buffer layer. To measure the thickness, the elemental distributions, and the growth mechanism of SnS films, a cross FESEM and EDS analysis was carried out, which is presented in Fig. 3. It can be seen that all specimens are dense and align on substrates, and that the deposited SnS films on buffer layers are denser than those on the Free-coated layer. For buffer layer deposited films, a flat layer without Ag and Au was created at first. Subsequently, the growth of SnS nanosheets began. The film thickness of SnS nanosheets on Free-coated layer was more than that of SnS nanosheets on the buffer layers. In addition, the thickness of SnS film on the Aucoated layer was greater than that of SnS film on the Ag-coated layer. It is very interesting to see that flat layers are free of the metal-buffer elements. This returns us to the growth mechanism of films. We think that the growth mechanism of SnS films in this
work is a vapor–liquid–solid (VLS) process. The main characteristics of VLS growth are as follows: the nanosheets have a metal or alloy droplet on their tips, and these droplets control their diameters and growth orientation. While the substrate temperature increased and the previous precursor evaporated the metals (Ag melting point is 961 °C and Au melting point is 1064 °C), the buffer thin films were transformed into the metal islands on the SiO2 substrate. The size of these islands was dependent on the thickness and the temperature of the buffer layers. These droplets became the preferred deposition sites for the formation of SnxSy molecules from the vapor phase. As the synthesis was performed in the higher temperature, in the eutectic point the Metal-Sn droplets began to form. These Metal-Sn droplets acted as energetically favored sites for adsorption of the incoming Sn and S vapors driven by the carrier gas. With the gradual absorption of the Sn and S vapors, when the liquid droplets reached the supersaturation concentration, a stable Sn-S phase initiated to grow via the vapor–liquid–solid (VLS) mechanism, and the formation of crystallites of SnS was observed [12,13]. Cross EDS results showed that with the growth of the films from the substrates to the upward, the atomic percentage of Sn and S decreased and increased, respectively. This meant that the mobility of the holes on top of layers was lower than that of the holes at the bottoms of films, and the bottoms of the films are more of a p-type in comparison with the upper parts. This is because of the high density of acceptor in a point where Sn atoms are more. The acceptor levels are created by the tin vacancies normally present in the lattice. An excess of tin changes the type of conductivity of SnS from p-to n-type [14].
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Fig. 3. Cross FESEM images and EDS spectra of (a, b) Free-coated, (c, d) Ag-coated and (e, f) Au-coated buffer layer SnS nanosheet thin films.
PL analysis was done to investigate the optical properties of SnS nanosheets illustrated in Fig. 4. The spectra show band emissions at 421, 481, and 757 nm for Free-coated, 378, 417, 481, and 747 nm for Ag-coated, and 385, 419, and 483 nm for Au-coated layers. It is well known that two types of band emissions are seen in the PL spectra of semiconductors; the first is an exitonic band that is known as near band edge emission (NBE), and the second is a trapped luminescence band emission that arises from deep levels of electronic band structures of semiconductors. The excitonic emission is sharp and located near absorption edge, while the trapped emission is broad and stokes-shifted [15,16]. In general, NBE emissions develop from energy levels of semiconductor materials in electronic band structures, and the trapped emissions appear from the crystal defects or the energy levels created by impurities. In this research, the band emission at 757 nm is an NBE emission, while the band emissions at 350–500 nm are trapped emissions. During the synthesis process, very different defects such as vacancies and interstitials were introduced to the structures. It is evident that the main defect was restricted to the surface, and the decrease in the size of the structures caused an increase in their effective surface area [11,13,17].
Fig. 4. PL spectra of the SnS nanosheet thin films.
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The UV–Visible absorption spectra of the SnS nanosheet films are exhibited in Fig. 5. SnS nanosheets have high absorption in the range of ultraviolet, and the absorption coefficient is about 37%, 90%, and 70% for Free-coated, Ag-coated, and Au-coated nanosheets, respectively. The absorption rises rapidly, using buffer layers. SnS nanosheets synthesized with Ag-coated layer have relatively stronger absorption intensity in the ultraviolet and Visible regions compared to the other specimens. This could be due to the fact that this specimen has the smallest size in comparison with the other specimens. As the size of SnS nanosheets decreases by using metal-catalyst, the surface to volume ratio was affected by the surface states of structures could increase. The surface states of structures could influence the intensity of absorption. The absorption range is from the ultraviolet to the Visible region, making them potentially suitable absorbing materials for photocatalyst activity. The optical band gaps of the specimens could be evaluated via Kubelka–Munk equation. Fig. 6 depicts the curves of (αhν)2 versus hν of the SnS nanosheets. The curves have a good straight-line fit with higher energy range above the absorption edge, indicating a direct optical transition near the absorption edge. Based on Fig. 6, the direct optical energy band gaps of the specimens have been a red shift, using Ag-coated and a blue shift, using Au-coated nanosheets. Besides, these energies are higher than the literature value of SnS bulk or films [18].
Raman spectra of SnS nanosheet films are shown in Fig. 7. Tin sulfide is a compound semiconductor (IV–VI) including an orthorhombic structure with eight atoms per unit cell. For orthorhombic structure, the 24 vibrational modes are represented in which 12 modes are Raman active modes (4Ag, 2B1g, 4B2g, and 2B3g), 7 modes are infrared active modes (3B1u, 1B2u, and 3B3u), and 2 modes are inactive (2Au) [19]. Raman band position and its modes for the Free-coated specimen are shown in Fig. 7. All the Raman bands of SnS nanosheet films demonstrate a shift to lower value of the wavenumber, by using buffer layers. This is due to the phonon confinement effect [1]. The intensity of Raman band decreases by using buffer layers, indicating decays in the crystallinity of the nanosheet films [20]. The I–V characteristics of SnS nanosheet films deposited by different buffer layers appear in Fig. 8. The I–V characteristics demonstrate good rectification properties with a turn-on voltage of 0.49, 0.60, and 0.54 v for Free-, Ag-, and Au-coated buffer layers, respectively, and show Schottky-like characteristics to SnS films. As it can be observed in this plot, in a constant voltage, applying buffer layers causes a lower current. This means that a higher
Fig. 5. UV–Vis spectra of the SnS nanosheet thin films.
Fig. 7. Raman spectra of the SnS nanosheet thin films.
Fig. 6. Tauc plot of SnS nanosheet thin films synthesized with (a) Free-coated, (b) Ag-coated, and (c) Au-coated buffer layers.
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Table 1 Physical parameters obtained from Mott–Schottky plot of SnS nanosheet thin films. Physical parameters/specimen carrier type Vfb: Flat band voltage (Volt) NA: Acceptor concentration (10 þ 15/cm3) Vb: Built in voltage (band bending) (Volt) W: Depletion width (mm) Nv: Density of states in the valance band (10 þ 18/cm3) Cv: Depletion Capacitance (nF/cm2)
Freecoated p
Ag-coated Au-coated p
p
0.38 5.95 0.881 4.64 1.32
0.23 9.09 0.892 3.07 2.50
0.32 6.67 0.884 4.15 1.57
6.10
9.20
6.81
Fig. 8. I–V characteristics of SnS nanosheet thin films synthesized on different metal-buffer layers.
Fig. 10. J–V curves of the SnS nanosheet thin films and solar-based cells synthezised on different metal-buffer layers.
Fig. 9. Mott–Schottky plot of the SnS nanosheet thin films synthezised on different metal-buffer layers at frequency of 1 kHz.
resistance and a lower conductivity were obtained when the buffer layers were used. In addition, this means that a higher concentration of hole carriers can be accomplished [21]. Thus, drops in p-type properties can be achieved via using of buffer layers. Probably the most accurate method to determine the flat-band potential of a semiconductor is by measuring the capacitance of the cell containing the semiconductor as a function of applied voltage. For non-intrinsic semiconductors, this can be obtained through Mott–Schottky equation [22]. In Mott–Schottky relation [23] 1 2 kT ð1Þ ¼ V V fb 2 e C scl eε0 εs N A where Cscl is the capacitance of the space charge layer, e is the charge of electron, ε0 is the permittivity of Free space, εs is the static permittivity of the semiconductor, NA is acceptor concentration, V is applied voltage, Vfb is flat band potential, k is Boltzmann constant, and T is Kelvin temperature. This equation is only valid for a space charge region in which the majority carrier density is depleted with respect to the bulk density. In such a case, the space charge layer is
called a depletion layer [22]. The negative slope of Mott–Schottky plot confirms the p-type conductivity of nanosheet films [24]. In order to obtain the flat band potential of junction, we determined the intercept of linear plot at 1/C2 ¼0. Fig. 9 presents the typical Mott–Schottky plot drawn, using the capacitance at frequency of 1 kHz in a system for different voltages. Likewise, the acceptor concentration (NA) was calculated from the slope of this plot via the following relation [25]: NA ¼
2
ε0 εs e Slope
ð2Þ
The flat band potential and the acceptor concentration along with other physical parameters are shown in Table 1. Since the acceptor concentrations are commonly attributed to Sn vacancies [26], it is evident that buffer layer films have higher acceptor concentration, indicating that buffer layers increase Sn vacancies. Built-in voltage is calculated by the following relation [27]: ! N A np V b ¼ V t ln ð3Þ n2i where Vt is thermal voltage (25.9 mV at 300 K), np is electron density, and ni is intrinsic carrier concentration. The width of the depletion layer (W) was calculated via this equation [24] 2 ε0 εs V b W¼ ð4Þ e NA
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Table 2 Photovoltaic parameters of SnS nanosheet thin films as solar cells. Specimens
Voc (v) Jsc (mA/ cm2)
Free-coated 0.27 Ag-coated 0.36 Au-coated 0.33
2.56 5.60 4.28
Fill factor Efficiency (%) PEC
0.42 0.48 0.35
0.29 0.97 0.63
Rss (Ω/ Rsh (Ω/ cm2) cm2)
1.45 43 4.83 19 3.14 21
289 302 291
The effective density of states in the valence band (Nv) is given in [27] 3=2 2 π mdh kT ð5Þ Nv ¼ 2 2 h where mdh is the effective mass of the holes in valence band and h is Plank constant. Fig. 10 shows the J–V curves of solar cells fabricated from the SnS nanosheet films. The results prove that the solar cell with Agbuffer layer has higher energy conversion efficiency, fill factor, and power efficiency conversion (PEC). These improvements are due to the increases in the charge mobility and effective reductions in the charge recombination [28]; consequently, the short current density (Jsc) increases, and the open circuit voltage (Voc) increases later because of injecting the electrons from the conduction band of SnS to the Ag electrode [29]. In addition, other solar cell parameters such as the series and shunt resistance (Rss & Rsh) were calculated for solar cell fabricated from the SnS nanosheet films through the relations below [30] dV Rss ¼ ð6Þ dI I ¼ 0 Rsh ¼
dV dI
Fig. 11. Responsibility characteristics of SnS nanosheet thin films synthesized on different metal buffer layers. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
V ¼0
ð7Þ
Their quantities are given in Table 2. It can be seen that the series resistance decreases, whereas the shunt resistance increases by utilizing buffer layers. The series resistance is closely related to the intrinsic resistance, morphology, and thickness of the semiconductor layer. Moreover, the shunt resistance is correlated with the amount and the character of the impurities and defects in the active semiconductor layer because impurities and defects cause charge recombination and leakage current [31]. All solar cell parameters of SnS nanosheet films are displayed in Table 2. As a result, it seems that by obtaining the thickness of the films from the FESEM cross images, more thickness films have more series resistance. Therefore, by decreasing the thickness of films via using buffer layers, the series resistance drops. According to the results of the cross EDS and Raman, the defects in films increased with buffer layers. More shunt resistance of films with the buffer layers is probably related to the more defects in these films. Spectral response study is important in the sensitivity investigation of the films in order to identify the recombination center and diagnose, consequently, the problems that have influenced the lower conversion efficiency. The photosensivity of the fabricated device under zero bias was investigated as a function of wavelength, and is presented in Fig. 11. The device responsivity in the red and near infrared (NIR) regions of the electromagnetic spectrum is high, even with zero bias. Correspondingly, this result reveals that Ag- and Au-coated buffer layer SnS films have high sensitivity in NIR region, and that Au-buffer layer SnS nanosheet films have responsivity in the red region. This is in a good agreement with the results of UV–Vis measurements. The decay of the photocurrent towards the shorter wavelength region may be due to the large amount of the surface recombination of the photo-generated minority carriers. Similarly, the sharp decay
Fig. 12. Photoresponse of SnS nanosheet thin films synthesized on different metalbuffer layers.
of photocurrent towards the longer wavelength region is attributed to the poor light absorption of short wavelength by the photoelectrode [32]. Photocurrent studies act as a good method to investigate the steady state and the transient response of fabricated devices from thin films semiconductors. This has also given us information regarding the surface trapping, generation, combination, and recombination of the carrier in different times [33,34]. Fig. 12 demonstrates a typical photocurrent of SnS nanosheet films under the illuminations of pulse NIR light. The inset here shows time raise and fall of specimens. It can be perceived that the SnS nanosheet film which is Ag-coated layer has the lowest raise and fall times and the most photocurrent amplitude in comparison with others. This result agrees well with the spectral response of specimens in pervious sections and suggests that the Ag-coated buffer layer is better than the Au-coated one in photosensivity applications of SnS films. The absence of current transient in our response indicated the minimum existence of surface trap and recombination center and/or un-recordable data.
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4. Conclusion SnS nanosheet films were synthesized on SiO2 substrate without/with different metal-buffer layers, using a thermal evaporation set-up. The X-ray results confirmed Ag- and Au-buffer resuscitation SnS phases in nanosheet thin films. In addition, it was verified that the metal-buffer layers were a key parameter in determining the size, band gap, electrical and photovoltaic performance of the deposited SnS nanosheets. Finally, this synthesis route was found out to be a useful technique for the fabrication of SnS-based optical devices such as solar cell and high speed NIR sensor.
Acknowledgments F. Jamali-Sheini and R. Yousefi gratefully wish to acknowledge the financial support of Islamic Azad University, Ahvaz and Masjed-Soleiman Branches in this research work. F. Jamali-Sheini also expresses his thanks to Advanced Surface Engineering and Nano Materials Research Center, Islamic Azad University, Ahvaz Branch, Ahvaz, Iran, for their instrumentation support.
References [1] S. Sohila, M. Rajalakshmi, C. Ghosh, A.K. Arora, C. Muthamizhchelvan, Optical and Raman scattering studies on SnS nanoparticles, J. Alloy. Compd. 509 (2011) 5843–5847, http://dx.doi.org/10.1016/j.jallcom.2011.02.141. [2] M. Devika, N.K. Reddy, K. Ramesh, H.R. Sumana, K.R. Gunasekhar, E.S.R. Gopal, et al., The effect of substrate surface on the physical properties of SnS films, Semicond. Sci. Technol. 21 (2006) 1495–1501, http://dx.doi.org/10.1088/ 0268-1242/21/10/024. [3] S.F. Wang, W.K. Fong, W. Wang, C. Surya, Growth of highly textured SnS on mica using an SnSe buffer layer, Thin Solid Films 564 (2014) 206–212, http: //dx.doi.org/10.1016/j.tsf.2014.06.010. [4] W. Wang, K.K. Leung, W.K. Fong, S.F. Wang, Y.Y. Hui, S.P. Lau, et al., Molecular beam epitaxy growth of high quality p-doped SnS van der Waals epitaxy on a graphene buffer layer, J. Appl. Phy. 111 (2012) 093520, http://dx.doi.org/ 10.1063/1.4709732. [5] J.-G. Kang, J.-G. Park, D.-W. Kim, Superior rate capabilities of SnS nanosheet electrodes for Li ion batteries, Electrochem. Commun. 12 (2010) 307–310, http://dx.doi.org/10.1016/j.elecom.2009.12.025. [6] Y. Zhang, J. Lu, S. Shen, H. Xu, Q. Wang, Ultralarge single crystal SnS rectangular nanosheets, Chem. Commun. 47 (2011) 5226, http://dx.doi.org/10.1039/ c0cc05528j. [7] F. Bella, C. Gerbaldi, C. Barolo, M. Grätzel, Aqueous dye-sensitized solar cells, Chem. Soc. Rev. 44 (2015) 3431–3473, http://dx.doi.org/10.1039/c4cs00456f. [8] F. Bella, A. Chiappone, J.R. Nair, G. Meligrana, C. Gerbaldi, Effect of different green cellulosic matrices on the performance of polymeric dye-sensitized solar cells, Chem. Engi. Trans. 41 (2014) 211–214, http://dx.doi.org/10.3303/ CET1441036. [9] J. Li, C. Xue, Y. Wang, G. Jiang, W. Liu, C. Zhu, Cu2SnS3 solar cells fabricated by chemical bath deposition–annealing of SnS/Cu stacked layers, Sol. Energy Mater. Sol. Cells 144 (2016) 281–288, http://dx.doi.org/10.1016/j. solmat.2015.09.017. [10] Z. Tong, K. Zhang, K. Sun, C. Yan, F. Liu, L. Jiang, et al., Modification of absorber quality and mo-back contact by a thin bi intermediate layer for kesterite cu2ZnSnS4 solar cells, Sol. Energy Mater. Sol. Cells. 144 (2016) 537–543, http://dx.doi.org/10.1016/j.solmat.2015.09.066. [11] F. Jamali-Sheini, R. Yousefi, N. Ali Bakr, M. Cheraghizade, M. Sookhakian, N.M. Huang, Highly efficient photo-degradation of methyl blue and band gap shift of SnS nanoparticles under different sonication frequencies, Mater. Sci. Semicond. Proc. 32 (2015) 172–178, http://dx.doi.org/10.1016/j. mssp.2014.12.059. [12] F. Jamali Sheini, D.S. Joag, M.A. More, J. Singh, O.N. Srivasatva, Low temperature growth of aligned ZnO nanowires and their application as field emission cathodes, Mater. Chem. Phys. 120 (2010) 691–696, http://dx.doi.org/10.1016/j. matchemphys.2009.12.022.
[13] S.R. Suryawanshi, S.S. Warule, S.S. Patil, K.R. Patil, M.A. More, Vapor–liquid– solid growth of one-dimensional tin sulfide (SnS) nanostructures with promising field emission behavior, ACS Appl. Mater. Interfaces 6 (2014) 2018–2025, http://dx.doi.org/10.1021/am405039j. [14] M. Ristov, G. Sinadinovski, I. Grozdanov, M. Mitreski, Chemical deposition of TIN(II) sulphide thin films, Thin Solid Films 173 (1989) 53–58, http://dx.doi. org/10.1016/0040-6090(89)90536-1. [15] J. Gan, X. Lu, T. Zhai, Y. Zhao, S. Xie, Y. Mao, et al., Vertically aligned in 2O3 nanorods on FTO substrates for photoelectrochemical applications, J. Mater. Chem. 21 (2011) 14685, http://dx.doi.org/10.1039/c1jm11774b. [16] W. Chen, Z.G. Wang, Z.J. Lin, L.Y. Lin, New observation on the luminescence of CdS clusters in zeolite-y, Solid State Commun. 101 (1997) 371–375, http://dx.doi.org/ 10.1016/s0038-1098(96)00530-3. [17] F. Niknia, F. Jamali-Sheini, R. Yousefi, Photocurrent properties of undoped and Pb-doped SnS nanostructures grown using electrodeposition method, J. Electron. Mater. 44 (2015) 4734–4739, http://dx.doi.org/10.1007/ s11664-015-4079-8. [18] H. Zhu, D. Yang, Y. Ji, H. Zhang, X. Shen, Two-dimensional SnS nanosheets fabricated by a novel hydrothermal method, J. Mater. Sci. 40 (2005) 591–595, http://dx.doi.org/10.1007/s10853-005-6293-x. [19] H.R. Chandrasekhar, R.G. Humphreys, U. Zwick, M. Cardona, Infrared and Raman spectra of the IV–VI compounds SnS and SnSe, Phys. Rev. B 15 (1977) 2177–2183, http://dx.doi.org/10.1103/physrevb.15.2177. [20] S.K. Panda, A. Antonakos, E. Liarokapis, S. Bhattacharya, S. Chaudhuri, Optical properties of nanocrystalline SnS2 thin films, Mater. Res. Bull. 42 (2007) 576–583, http://dx.doi.org/10.1016/j.materresbull.2006.06.028. [21] N. Sato, M. Ichimura, E. Arai, Y. Yamazaki, Characterization of electrical properties and photosensitivity of SnS thin films prepared by the electrochemical deposition method, Sol. Energy Mater. Sol. Cells 85 (2005) 153–165, http://dx.doi.org/10.1016/j.solmat.2004.04.014. [22] A.J. Nozik, R. Memming, Physical chemistry of semiconductor liquid interfaces, J. Phys. Chem. 100 (1996) 13061–13078, http://dx.doi.org/10.1021/ jp953720e. [23] W.J. Basirun, M. Sookhakian, S. Baradaran, Z. Endut, M.R. Mahmoudian, M. Ebadi, et al., Graphene oxide electrocatalyst on MnO2 air cathode as an efficient electron pump for enhanced oxygen reduction in alkaline solution, Scientific Rep. 5 (2015) 9108, http://dx.doi.org/10.1038/srep09108. [24] B. Subramanian, C. Sanjeeviraja, M. Jayachandran, Cathodic electrodeposition and analysis of SnS films for photoelectrochemical cells, Mater. Chem. Phys. 71 (2001) 40–46, http://dx.doi.org/10.1016/s0254-0584(00)00526-5. [25] V. Damodara Das, L. Damodare, A study of band-bending and barrier height variation in thin film n-cdSe0.5Te0.5 photoanode/polysulphide junctions, Solid State Commun. 99 (1996) 723–728, http://dx.doi.org/10.1016/0038-1098 (96)00195-0. [26] M. Stellmacher, J. Nagle, J.F. Lampin, P. Santoro, J. Vaneecloo, A. Alexandrou, Dependence of the carrier lifetime on acceptor concentration in GaAs grown at low-temperature under different growth and annealing conditions, J. Appl. Phys. 88 (2000) 6026, http://dx.doi.org/10.1063/1.1285829. [27] S.M. Sze, K.K. Ng, Physics of semiconductor devices, 3rd ed., Wiley-Blackwell (an imprint of John Wiley & Sons Ltd), United Kingdom, 2006. [28] Z. Wang, S. Qu, X. Zeng, J. Liu, C. Zhang, F. Tan, et al., The application of SnS nanoparticles to bulk heterojunction solar cells, J. Alloy. Compd. 482 (2009) 203–207, http://dx.doi.org/10.1016/j.jallcom.2009.03.158. [29] H. Tsukigase, Y. Suzuki, M.-H. Berger, T. Sagawa, S. Yoshikawa, Synthesis of SnS nanoparticles by SILAR method for quantum dot-sensitized solar cells, J. Nanosci. Nanotechnol. 11 (2011) 1914–1922, http://dx.doi.org/10.1166/ jnn.2011.3582. [30] Z. Wang, S. Qu, X. Zeng, J. Liu, F. Tan, Y. Bi, Organic/inorganic hybrid solar cells based on SnS/SnO nanocrystals and MDMO-PPV, Acta Mater. 58 (2010) 4950–4955, http://dx.doi.org/10.1016/j.actamat.2010.05.022. [31] M.-S. Kim, B.-G. Kim, J. Kim, Effective variables to control the fill factor of organic photovoltaic cells, ACS Appl. Mater. Interfaces 1 (2009) 1264–1269, http://dx.doi.org/10.1021/am900155y. [32] B. Subramanian, C. Sanjeeviraja, M. Jayachandran, Photoelectrochemical characteristics of brush plated tin sulfide thin films, Sol. Energy Mater. Sol. Cells 79 (2003) 57–65, http://dx.doi.org/10.1016/s0927-0248(02)00366-5. [33] J. Chao, Z. Xie, X. Duan, Y. Dong, Z. Wang, J. Xu, et al., Visible-light-driven photocatalytic and photoelectrochemical properties of porous SnSx(x¼ 1, 2) architectures, CrystEngComm 14 (2012) 3163, http://dx.doi.org/10.1039/ c2ce06586j. [34] M.T.S. Nair, P.K. Nair, Simplified chemical deposition technique for good quality SnS thin films, Semicond. Sci. Technol. 6 (1991) 132–134, http://dx.doi. org/10.1088/0268-1242/6/2/014.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
SnS nanosheet films deposited via thermal evaporation: The effects of buffer layers on photovoltaic performance Farid Jamali-Sheini a,n, Mohsen Cheraghizade b, Ramin Yousefi c a
Advanced Surface Engineering and Nano Materials Research Center, Department of Physics, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran Young Researchers and Elite Club, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran c Department of Physics, Masjed-Soleiman Branch, Islamic Azad University (I.A.U), Masjed-Soleiman, Iran b
art ic l e i nf o
a b s t r a c t
Article history: Received 8 December 2015 Received in revised form 5 February 2016 Accepted 3 April 2016 Available online 5 May 2016
Tin sulfide films were synthesized on different metal (Ag, Au) buffer layers by the deposition method of thermal evaporation. X-ray studies indicated an orthorhombic phase of SnS and Sn2S3 and hexagonal phase of SnS2 for all specimens. Microscopic investigations showed sheet morphologies with high density and uniform distribution across the surface of the specimens. It was also observed that Ag-buffer layer nanosheet films had the smallest thickness. The elemental analysis indicated the desired element on the surface of the films. Optical investigations confirmed that the SnS films on the buffer layer had strong emission bands which were red and near infrared (NIR) regions. Furthermore, Raman studies presented five Raman bands, and a shift to lower value of the wavenumber was observed by using buffer layers. The I–V and Mott–Schottky plots confirmed the p-type conductivity of nanosheet films. The solar cell devices fabricated from the films of SnS nanosheets exhibited a higher efficiency and power efficiency conversion (PEC) for the specimens that had buffer layers. The results of photosensitivity also demonstrated higher photosensitivity in red and NIR regions. And finally, photocurrent results displayed a higher photocurrent and lower raise and fall times for the films with buffer layers compared to the freebuffer layer film. & 2016 Elsevier B.V. All rights reserved.
Keywords: SnS films Metal-buffer layers Nanosheets Vapor–liquid–solid growth Optical and electrical properties Photovoltaic performance
1. Introduction Tin (II) sulfide (SnS) has attracted much attention as a potential to be the material of photovoltaic cells (PVCs) due to its nearoptimal energy band gap and high optical absorption coefficient. SnS is a narrow band gap and semiconducting material with a high absorption coefficient of 105 cm 1. Its direct and indirect optical band gap energies range from 1.2–1.5 eV to 1.0–1.2 eV, respectively [1,2]. This material exhibits a two-dimensional (2D) layered structure characterized by strong intra-layer covalent bonds and weak interlayer interactions, bound together by van der Waals (vdW) forces along their crystallographic c-axis. An important consequence of this unique feature is that the material is more tolerant of the mismatches in the lattice constants and the thermal expansion coefficients between the substrates and the epilayers. Dangling bonds and material strain may be potentially eliminated between the adjacent layers and at the surfaces completely. Such a unique 2D-layered structure makes SnS a promising photovoltaic n
Corresponding author. Tel.: þ 98 613 3348420 24; fax: þ98 613 3329200. E-mail addresses: [email protected], [email protected] (F. Jamali-Sheini). http://dx.doi.org/10.1016/j.solmat.2016.04.006 0927-0248/& 2016 Elsevier B.V. All rights reserved.
material as the dangling bonds and strain at the hetero-interfaces can be significantly subdued [3]. Thus, a much wider choice of substrates can be utilized for the growth of high quality vdW. Furthermore, since the chalcogenide planes consist of saturated bonds, the heterojunctions, as a result, may be totally free of dangling bonds. This will have significant implications on the properties of the devices as the interface states typically have critical impacts on both the electrical and the optoelectronic properties of the junctions: first, the Fermi level will not be pinned at the metal/semiconductor interface; second, the trap-assisted recombination will be substantially suppressed; and lastly, the low-frequency noise will be significantly lowered. Studies on conventional semiconductor devices have shown that device degradations can often be traced back to the material defects in the devices; thus, lowering the concentration of the interface states in the material may have positive effects on the lifetime of the devices [4]. So far, a few researchers have investigated and reported the effects of different buffer layers on physical properties of SnS thin films. Using Molecular Beam Epitaxy technique, the SnS thin films were deposited on a graphene buffer layer on GaAs (100) substrate by Wang et al. [4]. They found out that further improvement in the properties of the SnS can be achieved through optimizing the
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microscope (FESEM, Quanta 200 F) was applied for morphological studies. The elemental composition was obtained with an energy dispersive X-ray spectrometer (EDS) attached to the FESEM (TeScan, Mira 3-XMU). Furthermore, Cross FESEM and EDS studies were performed through this microscope. The optical properties were investigated by means of the UV–Vis and photoluminescence (PL) analysis at room temperature. The UV–Vis spectra were recorded by a UV–Vis spectrometer (Perkin Elmer lambda-750 UV–Vis Spectrometer) over a wavelength range of 40–1200 nm. PL spectra were measured with a photoluminescence spectrometer (Perkin Elmer LS-55), using a xenon arc lamp (300 nm excitation wavelengths) as the source for PL. The Raman spectra were measured with a Raman spectrophotometer (SENTEERA-BRUKER), using an Ar ion laser with an emission wavelength of 785 nm for the Raman source.
graphene transfer process to avoid formation of folds and pinholes in the layer. Devika et al. synthesized SnS thin films on free- and Ag-coated buffer layers on glass substrate, using the resistive thermal evaporation method [2]. The results showed that the buffer layers had a strong influence on the physical properties of SnS thin films. For example, Ag-buffer layers decreased the electrical resistance and increased the optical band gaps of films in comparison with the free-buffer layer films. In addition to their unique optical properties, SnS nanosheets have an excellent potential to be applied to battery cell due to their excellent migration of electrons, optical transmittance, large reversible capacity, high rate capability, and long-term cycle life [5,6]. An attempt was made by Kang et al. to synthesize the 2-dimensional SnS nanosheets prepared via the non-catalytic, template-free, and vapor-transport synthetic route [5]. These nanosheets demonstrated good cycling performance and superior rate capabilities which were larger than the theoretical capacity of the carbon-based electrodes currently used in commercial Li ion batteries. Zhang et al. synthesized the ultra large single crystal SnS rectangular nanosheets by thermal decomposing of a single-source precursor [6]. The ultra-large SnS nanosheets exhibited fascinating electrochemical properties with a capacity of 350 mAh g 1 around 1.2 V and an appealing cycling reversibility. Although the important works have been thus far done on tin sulfide film, solar cell based on nanostructured SnS thin films on buffer layer is rather a new research field in photovoltaic applications [7–10]. Therefore, in this research work, we aim to synthesize the SnS nanosheets thin films, using thermal evaporation set-up and study the effects of different metal-buffer layers on the morphological, optical, electrical, photovoltaic, and photocurrent properties. To the best of our knowledge, the synthesis and study of SnS thin films properties under different metal-buffer layers have not been reported yet.
Two Ag electrodes were placed on the two edges of the specimens and were prepared for electrical characterization. The effective surface of the specimens exposed to the light was 0.25 cm2 for responsivity measurements. In order to check the solar cell characteristics, the measurement was carried out under 100 mW/cm2 (1.5 Air Mass) illumination from a solar simulator (solar cell simulator IIIS-200 þ, Nanosat Co., Iran). A 100-W xenon lamp served as a light source, and the intensity of the light was calibrated, using a standard silicon solar cell. For C–V plot, an LCR meter (LCR-8000G Series-Gwinstek) as well as a function generator at a frequency of 1 kHz was employed to measure the capacitance in order to obtain Mott–Schottky plot. Finally, the photoresponse (photocurrent) of specimens were measured, using an NIR source with 780 nm wavelength and 0.25 Hz chopped light illumination.
2. Experimental
3. Results and discussion
2.1. Synthesis
XRD patterns of the deposited films are shown in Fig. 1. All the specimens were crystalline and of the orthorhombic phase of SnS and Sn2S3 (JCPDS card no. 14-0620 and 14-0619) and the hexagonal phase of SnS2 (JCPDS card no. 83-1705). It can be observed
The nanostructure of SnS thin films were synthesized from SnS powder ultrasonically prepared in the previous works (20 kHz sonication frequency) [11]. First, the SiO2 substrates were cleaned by soap and then prepared ultrasonically in ethanol and deionized water. Three sets of substrate were made, one using a catalyst-free substrate and the others a thin layer of Au and Ag coated on SiO2 substrate with the thickness of 20 nm as a buffer layer. The specimens were named as Free-coated, Ag-coated, and Au-coated layers for the catalyst-free and the thin layers of Ag and Au, respectively. The synthesis was carried out in an atmospherecontrolled tube furnace (YTF 1450-30 6, Yaran Behgozin Parsa Co., Iran). 0.2 g of SnS powder was employed as the precursor material. The precursor material and substrates were placed downstream, and the precursor material was positioned at the two open ends of a quartz tube in a horizontal furnace tube. The precursor material was heated up to 800 °C. The distance of the substrate from the center of tube furnace was 23.5 cm (400 °C). High purity Ar gas was fed into the furnace at one end at about 35 sccm, while the other end was connected to a rotary pump. The growth process was allowed to proceed for 2 h. A vacuum of 10 mbar was maintained inside the tube furnace during the deposition of the layers.
2.3. Electrical and solar cell experiment
2.2. Characterization The deposited films were characterized, using an X-ray diffractometer (XRD, PAN Analytica). A field emission scanning electron
Fig. 1. XRD patterns of SnS nanosheet thin films.
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Fig. 2. FESEM images and EDS spectra of (a, b) Free-coated, (c, d) Ag-coated and (e, f) Au-coated buffer layer SnS nanosheet thin films.
that the intensity phase of SnS was lower for the free-coated layer compared to the others. The FESEM images of the deposited SnS films are shown in Fig. 2. The surface of the films indicates a sheet-like morphology on the entire films. The average thicknesses of the deposited SnS sheets were measured to be around 80 nm, 37 nm, and 51 nm for Free-, Ag-, and Au-coated layers, respectively. By careful observation it is clear that the thickness of the nanosheets deposited on Ag and Au (buffer) layers decreased in comparison with the uncoated layer. An elemental purity of the deposited SnS film was tested by the EDS technique in an alignment normal to the surface, and the typical spectrum of all specimens is demonstrated in Fig. 2. It is evident that using buffer (Ag/Au) layers increases atomic percentage of Sn, while it decreases S. Additionally, the Au buffer layer is more diffused in deposited layers compared to Ag buffer layer. To measure the thickness, the elemental distributions, and the growth mechanism of SnS films, a cross FESEM and EDS analysis was carried out, which is presented in Fig. 3. It can be seen that all specimens are dense and align on substrates, and that the deposited SnS films on buffer layers are denser than those on the Free-coated layer. For buffer layer deposited films, a flat layer without Ag and Au was created at first. Subsequently, the growth of SnS nanosheets began. The film thickness of SnS nanosheets on Free-coated layer was more than that of SnS nanosheets on the buffer layers. In addition, the thickness of SnS film on the Aucoated layer was greater than that of SnS film on the Ag-coated layer. It is very interesting to see that flat layers are free of the metal-buffer elements. This returns us to the growth mechanism of films. We think that the growth mechanism of SnS films in this
work is a vapor–liquid–solid (VLS) process. The main characteristics of VLS growth are as follows: the nanosheets have a metal or alloy droplet on their tips, and these droplets control their diameters and growth orientation. While the substrate temperature increased and the previous precursor evaporated the metals (Ag melting point is 961 °C and Au melting point is 1064 °C), the buffer thin films were transformed into the metal islands on the SiO2 substrate. The size of these islands was dependent on the thickness and the temperature of the buffer layers. These droplets became the preferred deposition sites for the formation of SnxSy molecules from the vapor phase. As the synthesis was performed in the higher temperature, in the eutectic point the Metal-Sn droplets began to form. These Metal-Sn droplets acted as energetically favored sites for adsorption of the incoming Sn and S vapors driven by the carrier gas. With the gradual absorption of the Sn and S vapors, when the liquid droplets reached the supersaturation concentration, a stable Sn-S phase initiated to grow via the vapor–liquid–solid (VLS) mechanism, and the formation of crystallites of SnS was observed [12,13]. Cross EDS results showed that with the growth of the films from the substrates to the upward, the atomic percentage of Sn and S decreased and increased, respectively. This meant that the mobility of the holes on top of layers was lower than that of the holes at the bottoms of films, and the bottoms of the films are more of a p-type in comparison with the upper parts. This is because of the high density of acceptor in a point where Sn atoms are more. The acceptor levels are created by the tin vacancies normally present in the lattice. An excess of tin changes the type of conductivity of SnS from p-to n-type [14].
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Fig. 3. Cross FESEM images and EDS spectra of (a, b) Free-coated, (c, d) Ag-coated and (e, f) Au-coated buffer layer SnS nanosheet thin films.
PL analysis was done to investigate the optical properties of SnS nanosheets illustrated in Fig. 4. The spectra show band emissions at 421, 481, and 757 nm for Free-coated, 378, 417, 481, and 747 nm for Ag-coated, and 385, 419, and 483 nm for Au-coated layers. It is well known that two types of band emissions are seen in the PL spectra of semiconductors; the first is an exitonic band that is known as near band edge emission (NBE), and the second is a trapped luminescence band emission that arises from deep levels of electronic band structures of semiconductors. The excitonic emission is sharp and located near absorption edge, while the trapped emission is broad and stokes-shifted [15,16]. In general, NBE emissions develop from energy levels of semiconductor materials in electronic band structures, and the trapped emissions appear from the crystal defects or the energy levels created by impurities. In this research, the band emission at 757 nm is an NBE emission, while the band emissions at 350–500 nm are trapped emissions. During the synthesis process, very different defects such as vacancies and interstitials were introduced to the structures. It is evident that the main defect was restricted to the surface, and the decrease in the size of the structures caused an increase in their effective surface area [11,13,17].
Fig. 4. PL spectra of the SnS nanosheet thin films.
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The UV–Visible absorption spectra of the SnS nanosheet films are exhibited in Fig. 5. SnS nanosheets have high absorption in the range of ultraviolet, and the absorption coefficient is about 37%, 90%, and 70% for Free-coated, Ag-coated, and Au-coated nanosheets, respectively. The absorption rises rapidly, using buffer layers. SnS nanosheets synthesized with Ag-coated layer have relatively stronger absorption intensity in the ultraviolet and Visible regions compared to the other specimens. This could be due to the fact that this specimen has the smallest size in comparison with the other specimens. As the size of SnS nanosheets decreases by using metal-catalyst, the surface to volume ratio was affected by the surface states of structures could increase. The surface states of structures could influence the intensity of absorption. The absorption range is from the ultraviolet to the Visible region, making them potentially suitable absorbing materials for photocatalyst activity. The optical band gaps of the specimens could be evaluated via Kubelka–Munk equation. Fig. 6 depicts the curves of (αhν)2 versus hν of the SnS nanosheets. The curves have a good straight-line fit with higher energy range above the absorption edge, indicating a direct optical transition near the absorption edge. Based on Fig. 6, the direct optical energy band gaps of the specimens have been a red shift, using Ag-coated and a blue shift, using Au-coated nanosheets. Besides, these energies are higher than the literature value of SnS bulk or films [18].
Raman spectra of SnS nanosheet films are shown in Fig. 7. Tin sulfide is a compound semiconductor (IV–VI) including an orthorhombic structure with eight atoms per unit cell. For orthorhombic structure, the 24 vibrational modes are represented in which 12 modes are Raman active modes (4Ag, 2B1g, 4B2g, and 2B3g), 7 modes are infrared active modes (3B1u, 1B2u, and 3B3u), and 2 modes are inactive (2Au) [19]. Raman band position and its modes for the Free-coated specimen are shown in Fig. 7. All the Raman bands of SnS nanosheet films demonstrate a shift to lower value of the wavenumber, by using buffer layers. This is due to the phonon confinement effect [1]. The intensity of Raman band decreases by using buffer layers, indicating decays in the crystallinity of the nanosheet films [20]. The I–V characteristics of SnS nanosheet films deposited by different buffer layers appear in Fig. 8. The I–V characteristics demonstrate good rectification properties with a turn-on voltage of 0.49, 0.60, and 0.54 v for Free-, Ag-, and Au-coated buffer layers, respectively, and show Schottky-like characteristics to SnS films. As it can be observed in this plot, in a constant voltage, applying buffer layers causes a lower current. This means that a higher
Fig. 5. UV–Vis spectra of the SnS nanosheet thin films.
Fig. 7. Raman spectra of the SnS nanosheet thin films.
Fig. 6. Tauc plot of SnS nanosheet thin films synthesized with (a) Free-coated, (b) Ag-coated, and (c) Au-coated buffer layers.
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Table 1 Physical parameters obtained from Mott–Schottky plot of SnS nanosheet thin films. Physical parameters/specimen carrier type Vfb: Flat band voltage (Volt) NA: Acceptor concentration (10 þ 15/cm3) Vb: Built in voltage (band bending) (Volt) W: Depletion width (mm) Nv: Density of states in the valance band (10 þ 18/cm3) Cv: Depletion Capacitance (nF/cm2)
Freecoated p
Ag-coated Au-coated p
p
0.38 5.95 0.881 4.64 1.32
0.23 9.09 0.892 3.07 2.50
0.32 6.67 0.884 4.15 1.57
6.10
9.20
6.81
Fig. 8. I–V characteristics of SnS nanosheet thin films synthesized on different metal-buffer layers.
Fig. 10. J–V curves of the SnS nanosheet thin films and solar-based cells synthezised on different metal-buffer layers.
Fig. 9. Mott–Schottky plot of the SnS nanosheet thin films synthezised on different metal-buffer layers at frequency of 1 kHz.
resistance and a lower conductivity were obtained when the buffer layers were used. In addition, this means that a higher concentration of hole carriers can be accomplished [21]. Thus, drops in p-type properties can be achieved via using of buffer layers. Probably the most accurate method to determine the flat-band potential of a semiconductor is by measuring the capacitance of the cell containing the semiconductor as a function of applied voltage. For non-intrinsic semiconductors, this can be obtained through Mott–Schottky equation [22]. In Mott–Schottky relation [23] 1 2 kT ð1Þ ¼ V V fb 2 e C scl eε0 εs N A where Cscl is the capacitance of the space charge layer, e is the charge of electron, ε0 is the permittivity of Free space, εs is the static permittivity of the semiconductor, NA is acceptor concentration, V is applied voltage, Vfb is flat band potential, k is Boltzmann constant, and T is Kelvin temperature. This equation is only valid for a space charge region in which the majority carrier density is depleted with respect to the bulk density. In such a case, the space charge layer is
called a depletion layer [22]. The negative slope of Mott–Schottky plot confirms the p-type conductivity of nanosheet films [24]. In order to obtain the flat band potential of junction, we determined the intercept of linear plot at 1/C2 ¼0. Fig. 9 presents the typical Mott–Schottky plot drawn, using the capacitance at frequency of 1 kHz in a system for different voltages. Likewise, the acceptor concentration (NA) was calculated from the slope of this plot via the following relation [25]: NA ¼
2
ε0 εs e Slope
ð2Þ
The flat band potential and the acceptor concentration along with other physical parameters are shown in Table 1. Since the acceptor concentrations are commonly attributed to Sn vacancies [26], it is evident that buffer layer films have higher acceptor concentration, indicating that buffer layers increase Sn vacancies. Built-in voltage is calculated by the following relation [27]: ! N A np V b ¼ V t ln ð3Þ n2i where Vt is thermal voltage (25.9 mV at 300 K), np is electron density, and ni is intrinsic carrier concentration. The width of the depletion layer (W) was calculated via this equation [24] 2 ε0 εs V b W¼ ð4Þ e NA
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Table 2 Photovoltaic parameters of SnS nanosheet thin films as solar cells. Specimens
Voc (v) Jsc (mA/ cm2)
Free-coated 0.27 Ag-coated 0.36 Au-coated 0.33
2.56 5.60 4.28
Fill factor Efficiency (%) PEC
0.42 0.48 0.35
0.29 0.97 0.63
Rss (Ω/ Rsh (Ω/ cm2) cm2)
1.45 43 4.83 19 3.14 21
289 302 291
The effective density of states in the valence band (Nv) is given in [27] 3=2 2 π mdh kT ð5Þ Nv ¼ 2 2 h where mdh is the effective mass of the holes in valence band and h is Plank constant. Fig. 10 shows the J–V curves of solar cells fabricated from the SnS nanosheet films. The results prove that the solar cell with Agbuffer layer has higher energy conversion efficiency, fill factor, and power efficiency conversion (PEC). These improvements are due to the increases in the charge mobility and effective reductions in the charge recombination [28]; consequently, the short current density (Jsc) increases, and the open circuit voltage (Voc) increases later because of injecting the electrons from the conduction band of SnS to the Ag electrode [29]. In addition, other solar cell parameters such as the series and shunt resistance (Rss & Rsh) were calculated for solar cell fabricated from the SnS nanosheet films through the relations below [30] dV Rss ¼ ð6Þ dI I ¼ 0 Rsh ¼
dV dI
Fig. 11. Responsibility characteristics of SnS nanosheet thin films synthesized on different metal buffer layers. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
V ¼0
ð7Þ
Their quantities are given in Table 2. It can be seen that the series resistance decreases, whereas the shunt resistance increases by utilizing buffer layers. The series resistance is closely related to the intrinsic resistance, morphology, and thickness of the semiconductor layer. Moreover, the shunt resistance is correlated with the amount and the character of the impurities and defects in the active semiconductor layer because impurities and defects cause charge recombination and leakage current [31]. All solar cell parameters of SnS nanosheet films are displayed in Table 2. As a result, it seems that by obtaining the thickness of the films from the FESEM cross images, more thickness films have more series resistance. Therefore, by decreasing the thickness of films via using buffer layers, the series resistance drops. According to the results of the cross EDS and Raman, the defects in films increased with buffer layers. More shunt resistance of films with the buffer layers is probably related to the more defects in these films. Spectral response study is important in the sensitivity investigation of the films in order to identify the recombination center and diagnose, consequently, the problems that have influenced the lower conversion efficiency. The photosensivity of the fabricated device under zero bias was investigated as a function of wavelength, and is presented in Fig. 11. The device responsivity in the red and near infrared (NIR) regions of the electromagnetic spectrum is high, even with zero bias. Correspondingly, this result reveals that Ag- and Au-coated buffer layer SnS films have high sensitivity in NIR region, and that Au-buffer layer SnS nanosheet films have responsivity in the red region. This is in a good agreement with the results of UV–Vis measurements. The decay of the photocurrent towards the shorter wavelength region may be due to the large amount of the surface recombination of the photo-generated minority carriers. Similarly, the sharp decay
Fig. 12. Photoresponse of SnS nanosheet thin films synthesized on different metalbuffer layers.
of photocurrent towards the longer wavelength region is attributed to the poor light absorption of short wavelength by the photoelectrode [32]. Photocurrent studies act as a good method to investigate the steady state and the transient response of fabricated devices from thin films semiconductors. This has also given us information regarding the surface trapping, generation, combination, and recombination of the carrier in different times [33,34]. Fig. 12 demonstrates a typical photocurrent of SnS nanosheet films under the illuminations of pulse NIR light. The inset here shows time raise and fall of specimens. It can be perceived that the SnS nanosheet film which is Ag-coated layer has the lowest raise and fall times and the most photocurrent amplitude in comparison with others. This result agrees well with the spectral response of specimens in pervious sections and suggests that the Ag-coated buffer layer is better than the Au-coated one in photosensivity applications of SnS films. The absence of current transient in our response indicated the minimum existence of surface trap and recombination center and/or un-recordable data.
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4. Conclusion SnS nanosheet films were synthesized on SiO2 substrate without/with different metal-buffer layers, using a thermal evaporation set-up. The X-ray results confirmed Ag- and Au-buffer resuscitation SnS phases in nanosheet thin films. In addition, it was verified that the metal-buffer layers were a key parameter in determining the size, band gap, electrical and photovoltaic performance of the deposited SnS nanosheets. Finally, this synthesis route was found out to be a useful technique for the fabrication of SnS-based optical devices such as solar cell and high speed NIR sensor.
Acknowledgments F. Jamali-Sheini and R. Yousefi gratefully wish to acknowledge the financial support of Islamic Azad University, Ahvaz and Masjed-Soleiman Branches in this research work. F. Jamali-Sheini also expresses his thanks to Advanced Surface Engineering and Nano Materials Research Center, Islamic Azad University, Ahvaz Branch, Ahvaz, Iran, for their instrumentation support.
References [1] S. Sohila, M. Rajalakshmi, C. Ghosh, A.K. Arora, C. Muthamizhchelvan, Optical and Raman scattering studies on SnS nanoparticles, J. Alloy. Compd. 509 (2011) 5843–5847, http://dx.doi.org/10.1016/j.jallcom.2011.02.141. [2] M. Devika, N.K. Reddy, K. Ramesh, H.R. Sumana, K.R. Gunasekhar, E.S.R. Gopal, et al., The effect of substrate surface on the physical properties of SnS films, Semicond. Sci. Technol. 21 (2006) 1495–1501, http://dx.doi.org/10.1088/ 0268-1242/21/10/024. [3] S.F. Wang, W.K. Fong, W. Wang, C. Surya, Growth of highly textured SnS on mica using an SnSe buffer layer, Thin Solid Films 564 (2014) 206–212, http: //dx.doi.org/10.1016/j.tsf.2014.06.010. [4] W. Wang, K.K. Leung, W.K. Fong, S.F. Wang, Y.Y. Hui, S.P. Lau, et al., Molecular beam epitaxy growth of high quality p-doped SnS van der Waals epitaxy on a graphene buffer layer, J. Appl. Phy. 111 (2012) 093520, http://dx.doi.org/ 10.1063/1.4709732. [5] J.-G. Kang, J.-G. Park, D.-W. Kim, Superior rate capabilities of SnS nanosheet electrodes for Li ion batteries, Electrochem. Commun. 12 (2010) 307–310, http://dx.doi.org/10.1016/j.elecom.2009.12.025. [6] Y. Zhang, J. Lu, S. Shen, H. Xu, Q. Wang, Ultralarge single crystal SnS rectangular nanosheets, Chem. Commun. 47 (2011) 5226, http://dx.doi.org/10.1039/ c0cc05528j. [7] F. Bella, C. Gerbaldi, C. Barolo, M. Grätzel, Aqueous dye-sensitized solar cells, Chem. Soc. Rev. 44 (2015) 3431–3473, http://dx.doi.org/10.1039/c4cs00456f. [8] F. Bella, A. Chiappone, J.R. Nair, G. Meligrana, C. Gerbaldi, Effect of different green cellulosic matrices on the performance of polymeric dye-sensitized solar cells, Chem. Engi. Trans. 41 (2014) 211–214, http://dx.doi.org/10.3303/ CET1441036. [9] J. Li, C. Xue, Y. Wang, G. Jiang, W. Liu, C. Zhu, Cu2SnS3 solar cells fabricated by chemical bath deposition–annealing of SnS/Cu stacked layers, Sol. Energy Mater. Sol. Cells 144 (2016) 281–288, http://dx.doi.org/10.1016/j. solmat.2015.09.017. [10] Z. Tong, K. Zhang, K. Sun, C. Yan, F. Liu, L. Jiang, et al., Modification of absorber quality and mo-back contact by a thin bi intermediate layer for kesterite cu2ZnSnS4 solar cells, Sol. Energy Mater. Sol. Cells. 144 (2016) 537–543, http://dx.doi.org/10.1016/j.solmat.2015.09.066. [11] F. Jamali-Sheini, R. Yousefi, N. Ali Bakr, M. Cheraghizade, M. Sookhakian, N.M. Huang, Highly efficient photo-degradation of methyl blue and band gap shift of SnS nanoparticles under different sonication frequencies, Mater. Sci. Semicond. Proc. 32 (2015) 172–178, http://dx.doi.org/10.1016/j. mssp.2014.12.059. [12] F. Jamali Sheini, D.S. Joag, M.A. More, J. Singh, O.N. Srivasatva, Low temperature growth of aligned ZnO nanowires and their application as field emission cathodes, Mater. Chem. Phys. 120 (2010) 691–696, http://dx.doi.org/10.1016/j. matchemphys.2009.12.022.
[13] S.R. Suryawanshi, S.S. Warule, S.S. Patil, K.R. Patil, M.A. More, Vapor–liquid– solid growth of one-dimensional tin sulfide (SnS) nanostructures with promising field emission behavior, ACS Appl. Mater. Interfaces 6 (2014) 2018–2025, http://dx.doi.org/10.1021/am405039j. [14] M. Ristov, G. Sinadinovski, I. Grozdanov, M. Mitreski, Chemical deposition of TIN(II) sulphide thin films, Thin Solid Films 173 (1989) 53–58, http://dx.doi. org/10.1016/0040-6090(89)90536-1. [15] J. Gan, X. Lu, T. Zhai, Y. Zhao, S. Xie, Y. Mao, et al., Vertically aligned in 2O3 nanorods on FTO substrates for photoelectrochemical applications, J. Mater. Chem. 21 (2011) 14685, http://dx.doi.org/10.1039/c1jm11774b. [16] W. Chen, Z.G. Wang, Z.J. Lin, L.Y. Lin, New observation on the luminescence of CdS clusters in zeolite-y, Solid State Commun. 101 (1997) 371–375, http://dx.doi.org/ 10.1016/s0038-1098(96)00530-3. [17] F. Niknia, F. Jamali-Sheini, R. Yousefi, Photocurrent properties of undoped and Pb-doped SnS nanostructures grown using electrodeposition method, J. Electron. Mater. 44 (2015) 4734–4739, http://dx.doi.org/10.1007/ s11664-015-4079-8. [18] H. Zhu, D. Yang, Y. Ji, H. Zhang, X. Shen, Two-dimensional SnS nanosheets fabricated by a novel hydrothermal method, J. Mater. Sci. 40 (2005) 591–595, http://dx.doi.org/10.1007/s10853-005-6293-x. [19] H.R. Chandrasekhar, R.G. Humphreys, U. Zwick, M. Cardona, Infrared and Raman spectra of the IV–VI compounds SnS and SnSe, Phys. Rev. B 15 (1977) 2177–2183, http://dx.doi.org/10.1103/physrevb.15.2177. [20] S.K. Panda, A. Antonakos, E. Liarokapis, S. Bhattacharya, S. Chaudhuri, Optical properties of nanocrystalline SnS2 thin films, Mater. Res. Bull. 42 (2007) 576–583, http://dx.doi.org/10.1016/j.materresbull.2006.06.028. [21] N. Sato, M. Ichimura, E. Arai, Y. Yamazaki, Characterization of electrical properties and photosensitivity of SnS thin films prepared by the electrochemical deposition method, Sol. Energy Mater. Sol. Cells 85 (2005) 153–165, http://dx.doi.org/10.1016/j.solmat.2004.04.014. [22] A.J. Nozik, R. Memming, Physical chemistry of semiconductor liquid interfaces, J. Phys. Chem. 100 (1996) 13061–13078, http://dx.doi.org/10.1021/ jp953720e. [23] W.J. Basirun, M. Sookhakian, S. Baradaran, Z. Endut, M.R. Mahmoudian, M. Ebadi, et al., Graphene oxide electrocatalyst on MnO2 air cathode as an efficient electron pump for enhanced oxygen reduction in alkaline solution, Scientific Rep. 5 (2015) 9108, http://dx.doi.org/10.1038/srep09108. [24] B. Subramanian, C. Sanjeeviraja, M. Jayachandran, Cathodic electrodeposition and analysis of SnS films for photoelectrochemical cells, Mater. Chem. Phys. 71 (2001) 40–46, http://dx.doi.org/10.1016/s0254-0584(00)00526-5. [25] V. Damodara Das, L. Damodare, A study of band-bending and barrier height variation in thin film n-cdSe0.5Te0.5 photoanode/polysulphide junctions, Solid State Commun. 99 (1996) 723–728, http://dx.doi.org/10.1016/0038-1098 (96)00195-0. [26] M. Stellmacher, J. Nagle, J.F. Lampin, P. Santoro, J. Vaneecloo, A. Alexandrou, Dependence of the carrier lifetime on acceptor concentration in GaAs grown at low-temperature under different growth and annealing conditions, J. Appl. Phys. 88 (2000) 6026, http://dx.doi.org/10.1063/1.1285829. [27] S.M. Sze, K.K. Ng, Physics of semiconductor devices, 3rd ed., Wiley-Blackwell (an imprint of John Wiley & Sons Ltd), United Kingdom, 2006. [28] Z. Wang, S. Qu, X. Zeng, J. Liu, C. Zhang, F. Tan, et al., The application of SnS nanoparticles to bulk heterojunction solar cells, J. Alloy. Compd. 482 (2009) 203–207, http://dx.doi.org/10.1016/j.jallcom.2009.03.158. [29] H. Tsukigase, Y. Suzuki, M.-H. Berger, T. Sagawa, S. Yoshikawa, Synthesis of SnS nanoparticles by SILAR method for quantum dot-sensitized solar cells, J. Nanosci. Nanotechnol. 11 (2011) 1914–1922, http://dx.doi.org/10.1166/ jnn.2011.3582. [30] Z. Wang, S. Qu, X. Zeng, J. Liu, F. Tan, Y. Bi, Organic/inorganic hybrid solar cells based on SnS/SnO nanocrystals and MDMO-PPV, Acta Mater. 58 (2010) 4950–4955, http://dx.doi.org/10.1016/j.actamat.2010.05.022. [31] M.-S. Kim, B.-G. Kim, J. Kim, Effective variables to control the fill factor of organic photovoltaic cells, ACS Appl. Mater. Interfaces 1 (2009) 1264–1269, http://dx.doi.org/10.1021/am900155y. [32] B. Subramanian, C. Sanjeeviraja, M. Jayachandran, Photoelectrochemical characteristics of brush plated tin sulfide thin films, Sol. Energy Mater. Sol. Cells 79 (2003) 57–65, http://dx.doi.org/10.1016/s0927-0248(02)00366-5. [33] J. Chao, Z. Xie, X. Duan, Y. Dong, Z. Wang, J. Xu, et al., Visible-light-driven photocatalytic and photoelectrochemical properties of porous SnSx(x¼ 1, 2) architectures, CrystEngComm 14 (2012) 3163, http://dx.doi.org/10.1039/ c2ce06586j. [34] M.T.S. Nair, P.K. Nair, Simplified chemical deposition technique for good quality SnS thin films, Semicond. Sci. Technol. 6 (1991) 132–134, http://dx.doi. org/10.1088/0268-1242/6/2/014.