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Impact of growth temperature on the properties of SnS film prepared by thermal evaporation and its photovoltaic performance
Current Applied Physics xxx (2015) 1e5
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Impact of growth temperature on the properties of SnS film prepared by thermal evaporation and its photovoltaic performance Yu Kawano*, Jakapan Chantana, Takashi Minemoto Department of Electrical and Electronic Engineering, Ritsumeikan University, 1-1-1 Kusatsu, Nojihigashi, Shiga 525-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 December 2014 Received in revised form 3 March 2015 Accepted 26 March 2015 Available online xxx
Tin(II) sulfide (SnS) films are one of the most promising absorber materials for high efficiency solar cells without using rare metals. In this work, SnS films were deposited by the thermal evaporation on glass substrates under the variation of growth temperatures of 100e250 C. It was revealed that the SnS thin film prepared under the temperature of 100 C had relatively small crystal grains. On the other hand, the denser and larger crystal grains of the SnS films were obtained with the constant compositions, when the growth temperature increased to 225 C. With the temperature of higher than 225 C, the SnS began to be re-evaporated from the films. The highest Hall mobility of the films was obtained under the temperature of 200 C. Ultimately, the results suggested that the optimized growth temperature of SnS by the evaporation is 200 C, giving rise to compact and large crystal grains and the highest Hall mobility, thereby contributing to the 2.53%-efficient SnS thin-film solar cell. © 2015 Elsevier B.V. All rights reserved.
Keywords: Tin(II) sulfide SnS Thin-film solar cell Hall mobility
1. Introduction Recently, there are many kinds of the compound semiconductors as the absorbers of thin-film solar cells, such as CdTe [1], and Cu(In,Ga)Se2 [2], and Cu2ZnSn(Se,S)4 [3], and etc. The CdTe and Cu(In,Ga)Se2 can absorb sun light with the thickness of a few mm owing to their high absorption coefficients. However, In is a rare metal, and Cd and Se have detrimental impact on the environmental. In this work, we therefore concentrated on alternative Tin(II) Sulfide(VI) (SnS), which is abundant and non-harmful to the environment. The SnS is p-type IIeVI compound semiconductor with high potential as an absorber of single junction solar cell because it has direct band-gap energy (Eg) of around 1.3 eV, absorption coefficient of above 105 cm1 [4], and high carrier mobility of 90 cm2/V s in bulk [5]. In addition, it was reported that theoretical conversion efficiency of chalcogenide thin-film solar cell with Eg of 1.3 eV is over 25% [6], and the ideal short-circuit current density (JSC) is 35.5 mA/cm2 calculated from the standard solar spectrum with Eg of 1.3 eV [7]. The SnS films as the absorbers have been prepared by various methods such as sputtering [8], electrodeposition [9,10],
* Corresponding author. E-mail addresses: [email protected] (Y. Kawano), jakapan@fc. ritsumei.ac.jp (J. Chantana), [email protected] (T. Minemoto).
evaporation [11e14], spray pyrolysis [15,16], hot wall deposition [17,18], and pulsed-chemical vapor deposition [4]. The SnS thin-film solar cell with the highest conversion efficiency of around 5% has been recently obtained [19]. The thermal evaporation is the one of the most popular method to prepare the thin-film solar cell absorber (e.g. Cu(In,Ga)Se2). In addition, this method is able to fabricate SnS absorber films with homogeneous and large grains. In this work, we therefore fabricated the SnS absorber using the thermal evaporation and investigated the effect of growth temperature of SnS film on its cell performance to improve the film quality (i.e., compact and large SnS grains), which is appropriate for the application in the solar cell. 2. Experimental methods In this work, the SnS thin-film solar cell with the active area of 0.23 cm2 was prepared with a structure of Al/ZnO:Al2O3/ZnO/CdS/ SnS/Mo/soda-lime glass(SLG). The 800-nm-thick Mo layer as back electrode was first deposited on SLG substrate by radio frequency (RF) sputtering method under the pressure of 1.1 Pa in Ar atmosphere. The SnS thin film was then evaporated on Mo-coated SLG substrate using SnS powder with 99.9% purity as a material source. The wool fiber of SiO2 was added in the crucible with SnS powder to suppress splashed SnS grain on the absorber surface. The temperature of the crucible was controlled at 575 C. The distance between crucible and sample substrate is 20 cm. The growth temperature
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Please cite this article in press as: Y. Kawano, et al., Impact of growth temperature on the properties of SnS film prepared by thermal evaporation and its photovoltaic performance, Current Applied Physics (2015), http://dx.doi.org/10.1016/j.cap.2015.03.026
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Y. Kawano et al. / Current Applied Physics xxx (2015) 1e5
(i.e., surface temperature of substrate) was varied from 100 (not controlled) to 250 C with deposition time of 120 min. The substrate temperature was measured by thermocouple attached on the surface of the samples. The densities of the SnS film were calculated by mass of the film divided by its volume before and after the film deposition. After the deposition, the SnS film was analyzed by scanning electron microscopy (SEM). The crystallographic structure was characterized by x-ray diffraction (XRD) with Cu Ka (l ¼ 1.5405 Å) radiation operated at 45 kV and 40 mA. The resulting film was moreover investigated by energy dispersive x-ray spectroscopy, Raman spectrum, and optical properties. Next, the SnS film was lifted-off from Mo-coated SLG substrate to perform Hall effect measurement [20]. The results were compared to the SnS film on Mo-coated SLG substrate. After the preparation of SnS absorber, 50-nm-thick CdS buffer layer was deposited by chemical bath deposition. Then, ZnO as window layer with a thickness of 100 nm and ZnO:Al (Al2O3: 2 wt %-doped) as a transparent conductive oxide (TCO) layer with a thickness of 300 nm were deposited by RF sputtering method. Finally, the 250-nm-thick Al grid electrode was deposited as front electrode by thermal evaporation. The current densityevoltage (JeV) characteristics of the SnS thin-film solar cell was measured under standard test condition (100 mW/cm2, and air mass 1.5G illumination at 25 C). The external quantum efficiency (EQE) of the SnS thin-film solar cells was also measured using two sources of illumination (xenon and halogen lamps) together with a conventional lock-in detection system. 3. Results and discussion Fig. 1 demonstrates surface and cross-sectional images of SnS films prepared under the variation of the growth temperature from 100 to 225 C. The S/Sn ratios in the figure were almost constant at about 1.04 although the phase change is observed. According to Fig. 1(a) and (b) under the growth temperature of 100 C, the SnS films with the thickness of approximately 8 mm were observed with relatively small and flake-like crystal grains, similar to those of the films, prepared by spray pyrolysis in Ref. [16]. On the other hand, in Fig. 1(e)e(h) under the growth temperature from 200 to 225 C, the thicknesses of resulting SnS films drastically decrease to about 2 mm, much thinner than those of the films prepared under the temperatures of 100 and 150 C. Moreover, the SnS films with larger and non-flake-like crystal grains were observed. With further increasing the growth temperature of higher than 225 C, the SnS
Fig. 2. Density of SnS thin films as a function of the growth temperature.
film began to be re-evaporated, which will be demonstrated later. Based on the strong decrease in the thickness of the SnS film from around 8 to below 2 mm with increasing the growth temperature in Fig. 1, the film density as a function of the growth temperature was next investigated, as shown in Fig. 2. It was disclosed that the film density was increased to 4.39 g/cm3 with enhancing the growth temperature up to 225 C. Namely, the SnS crystal quality was improved (i.e., compact and large SnS grains). As a result, the decrease in the thickness of SnS films in Fig. 1 was not attributed to the re-evaporation of the films under increasing growth temperature, but caused by the denser films. The reported SnS densities are 4.6 g/cm3(film) [4], 5.05 g/cm3(bulk) [21], and 5.27 g/cm3(bulk) [22], which shows higher density than our SnS absorber. It is however considered that the film density of our SnS (i.e., crystal qualities) can be further increased by optimizing other growth conditions such as temperature of sulfurization, which will be reported elsewhere. Fig. 3 shows the XRD spectra of the SnS thin films under different growth temperatures. With increasing the growth temperature from 100 to 225 C, full width at half maximum (FWHM) values of corresponding SnS peaks were decreased, implying the increase in crystal grain size of resulting films, well consistent with
Fig. 1. Surface and cross-sectional images of SnS films prepared under the change of the growth temperatures of (a, b) 100, (c, d) 150, (e, f) 200, and (g, h) 225 C.
Please cite this article in press as: Y. Kawano, et al., Impact of growth temperature on the properties of SnS film prepared by thermal evaporation and its photovoltaic performance, Current Applied Physics (2015), http://dx.doi.org/10.1016/j.cap.2015.03.026
Y. Kawano et al. / Current Applied Physics xxx (2015) 1e5
Fig. 3. XRD patterns of SnS absorber films grown under the growth temperatures from 100 to 250 C.
3
Fig. 4. Raman scattering spectra of SnS films prepared under the growth temperature from 100 to 225 C.
the enlargement of the SnS grains in Fig. 1. Mo peaks in XRD pattern of the sample deposited under the growth temperature of 250 C were only observed, thus implying the re-evaporation of SnS from the substrate. The peak positions for SnS and Sn2S3 in XRD patterns almost the same position are located at about 31.53 for (111)-oriented SnS, 31.25 for (050)-oriented Sn2S3 and other peaks. Table 1 shows the detail of XRD peaks originated from Sn2S3 and SnS. These peaks were assigned by joint committee on powder diffraction standards (JCPDS) databases 39-354 (indexed SnS peak patterns), 33-1375 (calculated SnS peak patterns) and 30-1377 (indexed Sn2S3 peak patterns). It is therefore difficult to identify between SnS and Sn2S3 peaks by XRD measurement. In addition, XRD peaks of 50.16 and 42.96 resulting from (351)-oriented Sn2S3 and (541)-oriented Sn2S3 were not detected on this results. As a result, Raman measurement is conduced to identify them. In order to further analyze the SnS and other compounds such as Sn2S3 in the resulting films, the Raman scattering measurement was conducted. Fig. 4 depicts the Raman scattering spectra of the SnS films grown under various growth temperatures. The three peaks at 158, 186 and 211 cm1 from SnS crystal were analyzed at all SnS thin-film absorber. Under increasing the growth temperature from 100 to 150 C, the Raman spectra of the SnS films clearly demonstrated the Sn2S3 secondary phase at the peak of 302 cm1, where the Sn2S3 possesses high resistance of over 105 U cm [23], which is undesirable phase in SnS absorber for cell application. From Figs. 1 and 4, the flake-like crystal grains in SnS films under the growth temperatures of 100e150 C were considered to be Sn2S3 secondary phase. Under further increasing the growth temperature from 175 to 225 C, the Sn2S3 secondary phases in the resulting films was disappeared, which is good for cell application.
Table 1 Originated XRD peaks: (hkl) and diffracted angles from JCPDS database. JCPDS 30-1377(i) Sn2S3 (111) (050) (301) (331) (351) (222) (541) (311)
26.26 31.25 38.50 42.99 50.16 54.16 62.96 71.71
JCPDS 39-354(*) SnS (120) (111) (140) (210)
26.01 31.53 38.24 42.50
(231) 54.07 (081) 71.40
Fig. 5. (a) Carrier density, (b) Resistivity, and (c) Hall mobility of SnS absorber films on both SLG and Mo-coated SLG as a function of the growth temperature, deduced from Hall effect measurement.
Please cite this article in press as: Y. Kawano, et al., Impact of growth temperature on the properties of SnS film prepared by thermal evaporation and its photovoltaic performance, Current Applied Physics (2015), http://dx.doi.org/10.1016/j.cap.2015.03.026
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Fig. 5 shows electrical properties of SnS absorber films on both SLG and Mo-coated SLG as a function of the growth temperature. It is noted that the SnS films grown under the temperature of 100 C could not obtain electrical parameters by Hall effect measurement because the films possess too low resistance and many defects. According to Hall effect measurement, SnS films prepared under the growth temperatures of 125e225 C exhibited p-type conductivity, and the SnS films on SLG and Mo-coated SLG showed the same trend. It was found that carrier density (NA) in Fig. 5(a) was decreased with increasing the growth temperature from 125 to 225 C, thereby increasing the resistivity of the resulting films in Fig. 5(b). In the area of growth temperature from 100 to 150 C, our SnS thin film has low density with Sn2S3 mixture compound, as seen in Figs. 2 and 4. Since SnS has much lower resistivity than Sn2S3 [23], the electrical properties of the resulting film should be much influenced by SnS compound, leading to low resistivity in the resulting film in Fig. 5(b). Moreover, with further increasing growth temperature from 175 to 225 C, the Sn vacancy (acceptor) in SnS thin film is considered to be decreased because the resulting film has the denser and larger SnS crystal grains as seen in Fig. 1. As a result, the carrier density is decreased in Fig. 5(a), leading to the increase in resistivity in Fig. 5(b), which is well consistent with the result reported by other group [24]. From Fig. 5(c), the Hall mobility of the resulting film took a maximum point (6.2 cm2/Vs) under the growth temperature of 200 C. This is because the grain size increases with increasing the growth temperature up to 200 C, thus decreasing defect in the resulting films; however, SnS starts to be
Fig. 6. (a) Absorption coefficient of SnS films with different thicknesses deposited under varying Tg from 100 to 200 C and (b) calculated optical bandgap (Eg) of SnS absorbers from their absorption coefficient under the growth temperatures (Tg) of 100e200 C.
re-evaporate from resulting film with the temperature of over 200 C, as confirmed in Figs. 1(f), (h) and 2, thereby increasing defect density. Fig. 6 illustrates the optical properties of the SnS films with different thicknesses prepared under several growth temperatures. In Fig. 6(a) the calculated absorption coefficients depending on film thickness were increased with increasing the growth temperatures from 100 to 200 C. In addition, under a constant growth temperature of 200 C, the secondary absorption edge is more obviously observed in the films with the thickness less than 800 nm, implying that the thinner SnS film was easily measured for the optical properties under the light with energy above 1.3 eV. As a result, at the photon energy (hn) of 1.5 eV, the secondary absorption edge was observed in the SnS films prepared under the temperature of 200 C with the thicknesses of 120e800 nm, which was also detected by other group [4]. In Fig. 6(b), it was shown that the optical Egs of the SnS films in this work were almost constant in a small range of about 1.26e1.28 eV. Fig. 7 shows JeV characteristics of the SnS thin-film solar cells with SnS absorbers grown under 175, 200 and 225 C. It is noted that CdS films cannot be formed on the SnS absorber prepared under 150 C because the secondary phase (Sn2S3) on film surface feasibly reacts with CdS solution. The cell parameters are depicted in Table 2. It was exhibited that conversion efficiency was improved with increasing the growth temperature up to 200 C, while it was deteriorated under the growth temperature of 225 C. The results were consistent with the highest Hall mobility under the growth temperature of 200 C in Fig. 5(c). Consequently, the SnS solar cell with conversion efficiency of 2.53% (JSC ¼ 26.1 mA/cm2, opencircuit voltage (VOC) ¼ 0.223 V, fill factor ¼ 0.435) was obtained. Furthermore, Fig. 8 shows (a) the ratio of internal quantum efficiency (IQE) at negative (1 V) bias to IQE at zero bias denoted by IQE(1 V)/IQE(0 V), and (b) IQE spectra under the growth temperatures of 175, 200, and 225 C. In Fig. 8(a), the IQE(1 V)/ IQE(0 V) under the temperature of 200 C was the closest to 1, implying the highest minority carrier collection in the quasineutral region owing to the highest Hall mobility, as shown in Fig. 5(c). Consequently, the Hall mobility is considered to be corresponding to the electron mobility. In Fig. 8(b), the IQE demonstrated 4 transition points at approximately 3.2, 2.4, 1.5 and 1.3 eV. The transition edges at about 3.2, 2.4, and 1.3 eV were corresponding to the Egs of the ZnO layer, CdS buffer layer and SnS absorber in the cell structure, respectively, and the IQE shoulders at about 1.5 eV (827 nm) were resulted from the secondary absorption edge, which was previously confirmed in Fig. 6(a). This IQE
Fig. 7. JeV characteristics of SnS solar cells, where the absorbers were prepared under the growth temperatures of 175, 200, and 225 C.
Please cite this article in press as: Y. Kawano, et al., Impact of growth temperature on the properties of SnS film prepared by thermal evaporation and its photovoltaic performance, Current Applied Physics (2015), http://dx.doi.org/10.1016/j.cap.2015.03.026
Y. Kawano et al. / Current Applied Physics xxx (2015) 1e5 Table 2 SnS solar cell parameters. Growth temperature ( C) Short-circuit current density (mA/cm2) Open-circuit voltage (V) Fill factor Efficiency (%) Series resistance (U cm2) Shunt resistance (U cm2)
175 17.4 0.172 0.454 1.36 0.675 157
200 26.1 0.223 0.435 2.53 0.832 213
225 19.3 0.151 0.347 1.01 0.798 41
5
SnS films grown under the temperature of higher than 150 C do not have Sn2S3 secondary phase, which is suitable for cell application. The appropriate growth temperature of SnS absorber is 200 C, contributing to dense and large crystal grain and the highest Hall mobility, thereby leading to the 2.53%-efficient SnS solar cell. The Hall mobility and cell performance demonstrate same trend because Hall mobility is corresponding to electron mobility. It is subsequently suggested that the growth temperature of the SnS film is considered to be one of the most important parameters to obtain the SnS solar cells with high conversion efficiency.
Acknowledgment This work was partly supported by Murata Science Foundation.
References
Fig. 8. (a) A ratio of internal quantum efficiency (IQE) at negative (1 V) bias to IQE at zero bias, and (b) IQE spectra under the growth temperatures of 175, 200, and 225 C. Tg denotes the growth temperature.
shoulder at 1.5 eV was also reported by other group [19,25e27]. The IQE spectrum under the temperature of 175 C showed the decrease in long wavelength region (>750 nm). This is because the SnS absorber has a short carrier diffusion length and/or narrow space charge region owing to high NA, as shown in Fig. 5(a) under the decreased growth temperature. On the other hand, the IQE spectra under the growth temperatures of 200 and 255 C were improved in the long wavelength region; however, it is not enough carrier collection for obtaining high JSC. In addition, the IQE under the temperature of 225 C was the lowest in the short wavelength area (<600 nm), feasibly attributed to the high carrier recombination near SnS absorber surface, resulting from high defect density formed by the re-evaporation of SnS under the high growth temperature. According to the relation between (hn) and (hn)* {ln(1 EQE)}2), Egs of SnS absorber was estimated [28]. The calculated Egs were in the small range of 1.28e1.32 eV, well consistent with the optical Eg, as demonstrated in Fig. 6(b). Ultimately, the results suggested that the optimized growth temperature of SnS by the thermal evaporation is 200 C, giving rise to dense and large crystal grain and the highest Hall mobility, thereby leading to the 2.53%-efficient SnS solar cell. 4. Conclusion In this work, the SnS thin-film solar cell absorbers were prepared under varying the growth temperatures of 100e250 C by the thermal evaporation method. Under increasing growth temperature up to 225 C, the SnS films become denser with larger and non-flake-like crystal grains. However, under the growth temperature of above 225 C, the SnS began to be re-evaporated from the substrates. According to the Raman scattering measurement, the
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Please cite this article in press as: Y. Kawano, et al., Impact of growth temperature on the properties of SnS film prepared by thermal evaporation and its photovoltaic performance, Current Applied Physics (2015), http://dx.doi.org/10.1016/j.cap.2015.03.026
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Impact of growth temperature on the properties of SnS film prepared by thermal evaporation and its photovoltaic performance Yu Kawano*, Jakapan Chantana, Takashi Minemoto Department of Electrical and Electronic Engineering, Ritsumeikan University, 1-1-1 Kusatsu, Nojihigashi, Shiga 525-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 December 2014 Received in revised form 3 March 2015 Accepted 26 March 2015 Available online xxx
Tin(II) sulfide (SnS) films are one of the most promising absorber materials for high efficiency solar cells without using rare metals. In this work, SnS films were deposited by the thermal evaporation on glass substrates under the variation of growth temperatures of 100e250 C. It was revealed that the SnS thin film prepared under the temperature of 100 C had relatively small crystal grains. On the other hand, the denser and larger crystal grains of the SnS films were obtained with the constant compositions, when the growth temperature increased to 225 C. With the temperature of higher than 225 C, the SnS began to be re-evaporated from the films. The highest Hall mobility of the films was obtained under the temperature of 200 C. Ultimately, the results suggested that the optimized growth temperature of SnS by the evaporation is 200 C, giving rise to compact and large crystal grains and the highest Hall mobility, thereby contributing to the 2.53%-efficient SnS thin-film solar cell. © 2015 Elsevier B.V. All rights reserved.
Keywords: Tin(II) sulfide SnS Thin-film solar cell Hall mobility
1. Introduction Recently, there are many kinds of the compound semiconductors as the absorbers of thin-film solar cells, such as CdTe [1], and Cu(In,Ga)Se2 [2], and Cu2ZnSn(Se,S)4 [3], and etc. The CdTe and Cu(In,Ga)Se2 can absorb sun light with the thickness of a few mm owing to their high absorption coefficients. However, In is a rare metal, and Cd and Se have detrimental impact on the environmental. In this work, we therefore concentrated on alternative Tin(II) Sulfide(VI) (SnS), which is abundant and non-harmful to the environment. The SnS is p-type IIeVI compound semiconductor with high potential as an absorber of single junction solar cell because it has direct band-gap energy (Eg) of around 1.3 eV, absorption coefficient of above 105 cm1 [4], and high carrier mobility of 90 cm2/V s in bulk [5]. In addition, it was reported that theoretical conversion efficiency of chalcogenide thin-film solar cell with Eg of 1.3 eV is over 25% [6], and the ideal short-circuit current density (JSC) is 35.5 mA/cm2 calculated from the standard solar spectrum with Eg of 1.3 eV [7]. The SnS films as the absorbers have been prepared by various methods such as sputtering [8], electrodeposition [9,10],
* Corresponding author. E-mail addresses: [email protected] (Y. Kawano), jakapan@fc. ritsumei.ac.jp (J. Chantana), [email protected] (T. Minemoto).
evaporation [11e14], spray pyrolysis [15,16], hot wall deposition [17,18], and pulsed-chemical vapor deposition [4]. The SnS thin-film solar cell with the highest conversion efficiency of around 5% has been recently obtained [19]. The thermal evaporation is the one of the most popular method to prepare the thin-film solar cell absorber (e.g. Cu(In,Ga)Se2). In addition, this method is able to fabricate SnS absorber films with homogeneous and large grains. In this work, we therefore fabricated the SnS absorber using the thermal evaporation and investigated the effect of growth temperature of SnS film on its cell performance to improve the film quality (i.e., compact and large SnS grains), which is appropriate for the application in the solar cell. 2. Experimental methods In this work, the SnS thin-film solar cell with the active area of 0.23 cm2 was prepared with a structure of Al/ZnO:Al2O3/ZnO/CdS/ SnS/Mo/soda-lime glass(SLG). The 800-nm-thick Mo layer as back electrode was first deposited on SLG substrate by radio frequency (RF) sputtering method under the pressure of 1.1 Pa in Ar atmosphere. The SnS thin film was then evaporated on Mo-coated SLG substrate using SnS powder with 99.9% purity as a material source. The wool fiber of SiO2 was added in the crucible with SnS powder to suppress splashed SnS grain on the absorber surface. The temperature of the crucible was controlled at 575 C. The distance between crucible and sample substrate is 20 cm. The growth temperature
http://dx.doi.org/10.1016/j.cap.2015.03.026 1567-1739/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Kawano, et al., Impact of growth temperature on the properties of SnS film prepared by thermal evaporation and its photovoltaic performance, Current Applied Physics (2015), http://dx.doi.org/10.1016/j.cap.2015.03.026
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Y. Kawano et al. / Current Applied Physics xxx (2015) 1e5
(i.e., surface temperature of substrate) was varied from 100 (not controlled) to 250 C with deposition time of 120 min. The substrate temperature was measured by thermocouple attached on the surface of the samples. The densities of the SnS film were calculated by mass of the film divided by its volume before and after the film deposition. After the deposition, the SnS film was analyzed by scanning electron microscopy (SEM). The crystallographic structure was characterized by x-ray diffraction (XRD) with Cu Ka (l ¼ 1.5405 Å) radiation operated at 45 kV and 40 mA. The resulting film was moreover investigated by energy dispersive x-ray spectroscopy, Raman spectrum, and optical properties. Next, the SnS film was lifted-off from Mo-coated SLG substrate to perform Hall effect measurement [20]. The results were compared to the SnS film on Mo-coated SLG substrate. After the preparation of SnS absorber, 50-nm-thick CdS buffer layer was deposited by chemical bath deposition. Then, ZnO as window layer with a thickness of 100 nm and ZnO:Al (Al2O3: 2 wt %-doped) as a transparent conductive oxide (TCO) layer with a thickness of 300 nm were deposited by RF sputtering method. Finally, the 250-nm-thick Al grid electrode was deposited as front electrode by thermal evaporation. The current densityevoltage (JeV) characteristics of the SnS thin-film solar cell was measured under standard test condition (100 mW/cm2, and air mass 1.5G illumination at 25 C). The external quantum efficiency (EQE) of the SnS thin-film solar cells was also measured using two sources of illumination (xenon and halogen lamps) together with a conventional lock-in detection system. 3. Results and discussion Fig. 1 demonstrates surface and cross-sectional images of SnS films prepared under the variation of the growth temperature from 100 to 225 C. The S/Sn ratios in the figure were almost constant at about 1.04 although the phase change is observed. According to Fig. 1(a) and (b) under the growth temperature of 100 C, the SnS films with the thickness of approximately 8 mm were observed with relatively small and flake-like crystal grains, similar to those of the films, prepared by spray pyrolysis in Ref. [16]. On the other hand, in Fig. 1(e)e(h) under the growth temperature from 200 to 225 C, the thicknesses of resulting SnS films drastically decrease to about 2 mm, much thinner than those of the films prepared under the temperatures of 100 and 150 C. Moreover, the SnS films with larger and non-flake-like crystal grains were observed. With further increasing the growth temperature of higher than 225 C, the SnS
Fig. 2. Density of SnS thin films as a function of the growth temperature.
film began to be re-evaporated, which will be demonstrated later. Based on the strong decrease in the thickness of the SnS film from around 8 to below 2 mm with increasing the growth temperature in Fig. 1, the film density as a function of the growth temperature was next investigated, as shown in Fig. 2. It was disclosed that the film density was increased to 4.39 g/cm3 with enhancing the growth temperature up to 225 C. Namely, the SnS crystal quality was improved (i.e., compact and large SnS grains). As a result, the decrease in the thickness of SnS films in Fig. 1 was not attributed to the re-evaporation of the films under increasing growth temperature, but caused by the denser films. The reported SnS densities are 4.6 g/cm3(film) [4], 5.05 g/cm3(bulk) [21], and 5.27 g/cm3(bulk) [22], which shows higher density than our SnS absorber. It is however considered that the film density of our SnS (i.e., crystal qualities) can be further increased by optimizing other growth conditions such as temperature of sulfurization, which will be reported elsewhere. Fig. 3 shows the XRD spectra of the SnS thin films under different growth temperatures. With increasing the growth temperature from 100 to 225 C, full width at half maximum (FWHM) values of corresponding SnS peaks were decreased, implying the increase in crystal grain size of resulting films, well consistent with
Fig. 1. Surface and cross-sectional images of SnS films prepared under the change of the growth temperatures of (a, b) 100, (c, d) 150, (e, f) 200, and (g, h) 225 C.
Please cite this article in press as: Y. Kawano, et al., Impact of growth temperature on the properties of SnS film prepared by thermal evaporation and its photovoltaic performance, Current Applied Physics (2015), http://dx.doi.org/10.1016/j.cap.2015.03.026
Y. Kawano et al. / Current Applied Physics xxx (2015) 1e5
Fig. 3. XRD patterns of SnS absorber films grown under the growth temperatures from 100 to 250 C.
3
Fig. 4. Raman scattering spectra of SnS films prepared under the growth temperature from 100 to 225 C.
the enlargement of the SnS grains in Fig. 1. Mo peaks in XRD pattern of the sample deposited under the growth temperature of 250 C were only observed, thus implying the re-evaporation of SnS from the substrate. The peak positions for SnS and Sn2S3 in XRD patterns almost the same position are located at about 31.53 for (111)-oriented SnS, 31.25 for (050)-oriented Sn2S3 and other peaks. Table 1 shows the detail of XRD peaks originated from Sn2S3 and SnS. These peaks were assigned by joint committee on powder diffraction standards (JCPDS) databases 39-354 (indexed SnS peak patterns), 33-1375 (calculated SnS peak patterns) and 30-1377 (indexed Sn2S3 peak patterns). It is therefore difficult to identify between SnS and Sn2S3 peaks by XRD measurement. In addition, XRD peaks of 50.16 and 42.96 resulting from (351)-oriented Sn2S3 and (541)-oriented Sn2S3 were not detected on this results. As a result, Raman measurement is conduced to identify them. In order to further analyze the SnS and other compounds such as Sn2S3 in the resulting films, the Raman scattering measurement was conducted. Fig. 4 depicts the Raman scattering spectra of the SnS films grown under various growth temperatures. The three peaks at 158, 186 and 211 cm1 from SnS crystal were analyzed at all SnS thin-film absorber. Under increasing the growth temperature from 100 to 150 C, the Raman spectra of the SnS films clearly demonstrated the Sn2S3 secondary phase at the peak of 302 cm1, where the Sn2S3 possesses high resistance of over 105 U cm [23], which is undesirable phase in SnS absorber for cell application. From Figs. 1 and 4, the flake-like crystal grains in SnS films under the growth temperatures of 100e150 C were considered to be Sn2S3 secondary phase. Under further increasing the growth temperature from 175 to 225 C, the Sn2S3 secondary phases in the resulting films was disappeared, which is good for cell application.
Table 1 Originated XRD peaks: (hkl) and diffracted angles from JCPDS database. JCPDS 30-1377(i) Sn2S3 (111) (050) (301) (331) (351) (222) (541) (311)
26.26 31.25 38.50 42.99 50.16 54.16 62.96 71.71
JCPDS 39-354(*) SnS (120) (111) (140) (210)
26.01 31.53 38.24 42.50
(231) 54.07 (081) 71.40
Fig. 5. (a) Carrier density, (b) Resistivity, and (c) Hall mobility of SnS absorber films on both SLG and Mo-coated SLG as a function of the growth temperature, deduced from Hall effect measurement.
Please cite this article in press as: Y. Kawano, et al., Impact of growth temperature on the properties of SnS film prepared by thermal evaporation and its photovoltaic performance, Current Applied Physics (2015), http://dx.doi.org/10.1016/j.cap.2015.03.026
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Y. Kawano et al. / Current Applied Physics xxx (2015) 1e5
Fig. 5 shows electrical properties of SnS absorber films on both SLG and Mo-coated SLG as a function of the growth temperature. It is noted that the SnS films grown under the temperature of 100 C could not obtain electrical parameters by Hall effect measurement because the films possess too low resistance and many defects. According to Hall effect measurement, SnS films prepared under the growth temperatures of 125e225 C exhibited p-type conductivity, and the SnS films on SLG and Mo-coated SLG showed the same trend. It was found that carrier density (NA) in Fig. 5(a) was decreased with increasing the growth temperature from 125 to 225 C, thereby increasing the resistivity of the resulting films in Fig. 5(b). In the area of growth temperature from 100 to 150 C, our SnS thin film has low density with Sn2S3 mixture compound, as seen in Figs. 2 and 4. Since SnS has much lower resistivity than Sn2S3 [23], the electrical properties of the resulting film should be much influenced by SnS compound, leading to low resistivity in the resulting film in Fig. 5(b). Moreover, with further increasing growth temperature from 175 to 225 C, the Sn vacancy (acceptor) in SnS thin film is considered to be decreased because the resulting film has the denser and larger SnS crystal grains as seen in Fig. 1. As a result, the carrier density is decreased in Fig. 5(a), leading to the increase in resistivity in Fig. 5(b), which is well consistent with the result reported by other group [24]. From Fig. 5(c), the Hall mobility of the resulting film took a maximum point (6.2 cm2/Vs) under the growth temperature of 200 C. This is because the grain size increases with increasing the growth temperature up to 200 C, thus decreasing defect in the resulting films; however, SnS starts to be
Fig. 6. (a) Absorption coefficient of SnS films with different thicknesses deposited under varying Tg from 100 to 200 C and (b) calculated optical bandgap (Eg) of SnS absorbers from their absorption coefficient under the growth temperatures (Tg) of 100e200 C.
re-evaporate from resulting film with the temperature of over 200 C, as confirmed in Figs. 1(f), (h) and 2, thereby increasing defect density. Fig. 6 illustrates the optical properties of the SnS films with different thicknesses prepared under several growth temperatures. In Fig. 6(a) the calculated absorption coefficients depending on film thickness were increased with increasing the growth temperatures from 100 to 200 C. In addition, under a constant growth temperature of 200 C, the secondary absorption edge is more obviously observed in the films with the thickness less than 800 nm, implying that the thinner SnS film was easily measured for the optical properties under the light with energy above 1.3 eV. As a result, at the photon energy (hn) of 1.5 eV, the secondary absorption edge was observed in the SnS films prepared under the temperature of 200 C with the thicknesses of 120e800 nm, which was also detected by other group [4]. In Fig. 6(b), it was shown that the optical Egs of the SnS films in this work were almost constant in a small range of about 1.26e1.28 eV. Fig. 7 shows JeV characteristics of the SnS thin-film solar cells with SnS absorbers grown under 175, 200 and 225 C. It is noted that CdS films cannot be formed on the SnS absorber prepared under 150 C because the secondary phase (Sn2S3) on film surface feasibly reacts with CdS solution. The cell parameters are depicted in Table 2. It was exhibited that conversion efficiency was improved with increasing the growth temperature up to 200 C, while it was deteriorated under the growth temperature of 225 C. The results were consistent with the highest Hall mobility under the growth temperature of 200 C in Fig. 5(c). Consequently, the SnS solar cell with conversion efficiency of 2.53% (JSC ¼ 26.1 mA/cm2, opencircuit voltage (VOC) ¼ 0.223 V, fill factor ¼ 0.435) was obtained. Furthermore, Fig. 8 shows (a) the ratio of internal quantum efficiency (IQE) at negative (1 V) bias to IQE at zero bias denoted by IQE(1 V)/IQE(0 V), and (b) IQE spectra under the growth temperatures of 175, 200, and 225 C. In Fig. 8(a), the IQE(1 V)/ IQE(0 V) under the temperature of 200 C was the closest to 1, implying the highest minority carrier collection in the quasineutral region owing to the highest Hall mobility, as shown in Fig. 5(c). Consequently, the Hall mobility is considered to be corresponding to the electron mobility. In Fig. 8(b), the IQE demonstrated 4 transition points at approximately 3.2, 2.4, 1.5 and 1.3 eV. The transition edges at about 3.2, 2.4, and 1.3 eV were corresponding to the Egs of the ZnO layer, CdS buffer layer and SnS absorber in the cell structure, respectively, and the IQE shoulders at about 1.5 eV (827 nm) were resulted from the secondary absorption edge, which was previously confirmed in Fig. 6(a). This IQE
Fig. 7. JeV characteristics of SnS solar cells, where the absorbers were prepared under the growth temperatures of 175, 200, and 225 C.
Please cite this article in press as: Y. Kawano, et al., Impact of growth temperature on the properties of SnS film prepared by thermal evaporation and its photovoltaic performance, Current Applied Physics (2015), http://dx.doi.org/10.1016/j.cap.2015.03.026
Y. Kawano et al. / Current Applied Physics xxx (2015) 1e5 Table 2 SnS solar cell parameters. Growth temperature ( C) Short-circuit current density (mA/cm2) Open-circuit voltage (V) Fill factor Efficiency (%) Series resistance (U cm2) Shunt resistance (U cm2)
175 17.4 0.172 0.454 1.36 0.675 157
200 26.1 0.223 0.435 2.53 0.832 213
225 19.3 0.151 0.347 1.01 0.798 41
5
SnS films grown under the temperature of higher than 150 C do not have Sn2S3 secondary phase, which is suitable for cell application. The appropriate growth temperature of SnS absorber is 200 C, contributing to dense and large crystal grain and the highest Hall mobility, thereby leading to the 2.53%-efficient SnS solar cell. The Hall mobility and cell performance demonstrate same trend because Hall mobility is corresponding to electron mobility. It is subsequently suggested that the growth temperature of the SnS film is considered to be one of the most important parameters to obtain the SnS solar cells with high conversion efficiency.
Acknowledgment This work was partly supported by Murata Science Foundation.
References
Fig. 8. (a) A ratio of internal quantum efficiency (IQE) at negative (1 V) bias to IQE at zero bias, and (b) IQE spectra under the growth temperatures of 175, 200, and 225 C. Tg denotes the growth temperature.
shoulder at 1.5 eV was also reported by other group [19,25e27]. The IQE spectrum under the temperature of 175 C showed the decrease in long wavelength region (>750 nm). This is because the SnS absorber has a short carrier diffusion length and/or narrow space charge region owing to high NA, as shown in Fig. 5(a) under the decreased growth temperature. On the other hand, the IQE spectra under the growth temperatures of 200 and 255 C were improved in the long wavelength region; however, it is not enough carrier collection for obtaining high JSC. In addition, the IQE under the temperature of 225 C was the lowest in the short wavelength area (<600 nm), feasibly attributed to the high carrier recombination near SnS absorber surface, resulting from high defect density formed by the re-evaporation of SnS under the high growth temperature. According to the relation between (hn) and (hn)* {ln(1 EQE)}2), Egs of SnS absorber was estimated [28]. The calculated Egs were in the small range of 1.28e1.32 eV, well consistent with the optical Eg, as demonstrated in Fig. 6(b). Ultimately, the results suggested that the optimized growth temperature of SnS by the thermal evaporation is 200 C, giving rise to dense and large crystal grain and the highest Hall mobility, thereby leading to the 2.53%-efficient SnS solar cell. 4. Conclusion In this work, the SnS thin-film solar cell absorbers were prepared under varying the growth temperatures of 100e250 C by the thermal evaporation method. Under increasing growth temperature up to 225 C, the SnS films become denser with larger and non-flake-like crystal grains. However, under the growth temperature of above 225 C, the SnS began to be re-evaporated from the substrates. According to the Raman scattering measurement, the
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Please cite this article in press as: Y. Kawano, et al., Impact of growth temperature on the properties of SnS film prepared by thermal evaporation and its photovoltaic performance, Current Applied Physics (2015), http://dx.doi.org/10.1016/j.cap.2015.03.026