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The fabrication of Cu2BaSnS4 thin film solar cells utilizing a maskant layer
Solar Energy 181 (2019) 301–307
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
The fabrication of Cu2BaSnS4 thin film solar cells utilizing a maskant layer a
a
a
a
a
a,⁎
Huafei Guo , Changhao Ma , Zhiwen Chen , Xuguang Jia , Qingfei Cang , Ningyi Yuan , ⁎ Jianning Dinga,b,
T
a
School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center for Photovoltaic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164, China Micro/Nano Science and Technology Center, Jiangsu University, Zhenjiang 212013, China
b
ARTICLE INFO
ABSTRACT
Keywords: CBTS Sn Thickness Mask layer Solar cell
Cu2BaSnS4 (CBTS) is a low cost, non-toxic material composed of abundant elements with a large optical absorption coefficient. However, in CBTS solar cells, elemental Ba reacts readily with water, which decreases the quality of the CBTS film. Therefore, in this paper, we present an easy and cost-effective approach to prevent the hydrolysis in CBTS films by adding an ultrathin Sn mask layer at the surface of the CBT precursor film. Adding a Sn mask layer can improve morphology, reduce recombination, and enhance the quality of p–n junction between CBTS and the CdS film by inhibiting the reaction between Ba and water. When the thickness of Sn mask layer was increased to 5 nm, the open circuit voltage, short current density, fill factor all increased and the conversion efficiency of the solar cell increased from 0.27% to 1.21%. However, the second phase SnS2 appeared at the surface of the CBTS film when the thickness of the Sn mask layer increased from 5 to 15 nm, which may severely impair the p–n junction resulting in deterioration of the performance of the CBTS device.
1. Introduction Kesterite (Cu2ZnSnS4; CZTS) is considered to a promising photovoltaic material for low cost and high efficiency thin-film solar cells due to its optimal optical band-gap and high absorption coefficient of > 104 cm−1; in addition, it is composed of abundant elements (Mitzi et al., 2011). Consequently, CZTS solar cells are considered as an alternative to CIGS and CdTe solar cells. CZTS solar cells produced via many synthesis routes have reached power conversion efficiencies close to or above 10% (Mitzi et al., 2013; Steinhagen et al., 2009). In particular, the highest efficiency for CZTSSe solar cell was achieved by using a hydrazine solution method (Wang et al., 2014). But up to date, the efficiency (η) of CZTS devices is much lower than those of CIGS (21.7%) (Kim and Shafarman, 2016). The main reason for the low efficiency is the low open circuit voltage (Voc), which may be due to cation antisite defects (i.e., CuZn and ZnCu). Recently, many efforts have been made to design new materials that avoid the antisite defect by using large ionicradius elements, such as Ba, to replace Zn: Cu2BaSnS4 (CBTS) is one of such new material (Donne et al., 2017; Guo et al., 2016; Miao et al., 2017; Maldar et al., 2017; Prabhakar et al., 2016; Dong et al., 2018). CBTS materials is now proposed as an alternative to CZTS due to inhibition formation of antiste defects and associated band tailing,
optimal band gap (2.0 eV) and high absorption coefficient of > 104 cm−1 (Shin et al., 2017, 2016; Ge, Roland, et al., 2017; Ge, Grice, et al., 2017; Chen et al., 2018). Up to now, many woks of CBTS solar cells has been reported. For example, Shin et al. (2016) demonstrated the first 1.62%-efficient CBTS-based solar cell (0.425 cm2 device area) using a well-known device structure analogous to CZTSSe devices. Then Shin et al. (2017) achieved a higher efficiency with CBTSSe solar cells of over 5% by adjusting the proportion of Se and S. A more recent CBTSbased device with a CdS:O buffer layer demonstrated a slightly higher power conversion efficiency (PCE) of 2.03% (0.08 cm2 device area) and a maximum Voc of 1 V, which is larger than those achieved by CIGS and CZTS solar cells (Ge, Koirala, et al., 2017). However, Ba reacts readily with water during the preparation process; consequently, the precursor film of CBT will also react readily with water prior to the sulfuriation process This problem may be solved by using high vacuum, low humidity links and storage devices, but this will greatly increase the cost of production. So the question of how to avoid or reduce the extent of reaction between the precursor film of CBT and water at a low cost is the key to developing CBTS film for potential applications in low-cost solar cells. In this work, the CBTS film was first fabricated by co-sputtering with three targets: Cu, Ba, and Sn. Then, different thicknesses of
⁎ Corresponding authors at: School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center for Photovoltaic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164, China (J. Ding). E-mail addresses: [email protected] (N. Yuan), [email protected] (J. Ding).
https://doi.org/10.1016/j.solener.2019.02.007 Received 30 November 2018; Received in revised form 4 February 2019; Accepted 5 February 2019 0038-092X/ © 2019 Published by Elsevier Ltd on behalf of International Solar Energy Society.
Solar Energy 181 (2019) 301–307
H. Guo, et al.
2018). 2.2. Characterization The structure of CBTS films was performed using XRD (Ultima III, Japan). The morphology, Surface roughness and elemental composition of CBTS films was performed by SEM (JEOL 6360LA, Japan), AFM and EDS. The optical and electrical properties were determined by UV spectrophotometer (UV-2250, Japan) and Hall measurement (QT-50, Germany). The efficiency and EQE of CBTS solar cells were investigated by solar cell scan photovoltaic characterization system (Solar cell scan, Chia). 3. Result and discussion The detailed fabrication process to create CBTS films with different thicknesses of Sn maskant is illustrated in Fig. 2. Fig. S1 shows the physical configuration of a CBTS films, with and without 5 nm Sn mask layer. As shown in the Fig. 2, the CBT precursor film without a Sn mask layer has a rough surface with curved scars. In contrast, the CBT precursor film with the Sn mask layer shows a glazed surface. This indicates that the additional Sn mask layer successfully prevents the reaction between Ba and water. The XRD patterns of CBTS films with different thickness of Sn film are shown in Fig. 3(a). It can be seen for all the CBTS film with different thickness of Sn mask layer the diffraction peaks were observed at 2θ values characteristic of CBTS (JCPDS no. 30-0124). Compared to the pure CBTS sample (i.e., without a Sn mask layer), the other three samples show an additional peak at 30.718 (marked by * in Fig. 3(a)). This peak is in accordance with the (0 0 4) plane of SnS2((JCPDS no. 211231). In addition, the intensity of the (0 0 4) planes increased with increasing thickness of the Sn mask layer. Raman spectroscopy was utilized to identify the phases of CBTS film with different thicknesses of the Sn mask layer. As shown in Fig. 3(b), the main peaks of the Raman spectra of the CBTS at 256, 345, 358, and 370 cm−1 all correspond to the pure CBTS structure (Mccarthy and Brutchey, 2018). Compared to the pure CBTS sample (without an Sn mask layer), the patterns of the other three samples have additional peaks at 318 cm−1. Reference to previous work implies that the peaks are associated with the SnS2 phase, which is consistent with the XRD results (Wang et al., 2018). Xray photoelectron spectroscopy (XPS) was used to gain insight into the valence states of the material. Fig. 3(c) shows the high-resolution
Fig. 1. The structure of CBTS solar cell.
ultrathin Sn film were used to prevent the reaction between the precursor film of CBT and water from developing. The physical, electrical, and optical properties of the CBTS thin films with different thickness of Sn film were investigated in detail. Finally, a 1.21% efficiently of a CBTS solar cell based on the structure of FTO/CBTS/CdS/i-ZnO/ITO/ Ag has been obtained. 2. Experimental 2.1. Preparation of CBTS solar cells The CBTS film was deposited on FTO glasses by co-sputtering method. First, the CBT precursors were fabricated by co-sputtering with Cu, Sn and Ba target. The sputtering power is 40 W(DC), 40 W(RF) and 60 W(RF), respectively. After the deposition of CBT precursors film, different thickness (0, 5, 10 and 15 nm) of Sn layer was deposited on the surface of CBT precursors film, Then the sample were loaded into a horizontal tube furnace and annealed at 550 °C for 60 min with 25 mg of sulfur powder. The structure of CBTS solar cells was illustrated in Fig. 1. The preparation methods for CdS, ZnO, ITO and Ag are consistent with our previous reports (Guo, Ma, et al., 2018; Guo, Li, et al.,
Fig. 2. Illustration of a method for fabricating a compact CBTS film by further sputtering thin Sn film on the CBT precursor film.
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Fig. 3. (a) The XRD and (b) Raman patterns of CBTS films with different thick Sn mask layers. (c) The XPS patterns of CBTS film with 5 nm thick Sn mask layer.
spectra for the Cu 2p, Ba 3d, Sn 3d, and S 2p regions of CBTS film with 5 nm Sn mask layer. It can be seen all the splitting and peak positions observed are similar to Brutchey and Yan's (Mccarthy and Brutchey, 2018; Ge, Grice, et al., 2017) report for Cu2BaSnS4 and are corroborated by the expected values for Cu+, Ba2+, Sn4+ and S2−. Fig. 4 shows the surface SEM images of CBTS with different thicknesses of Sn mask layer. It can be seen from Fig. 4(a) that the CBTS film with no Sn mask layer has a poorly crystalline rough surface, irregularly sized grains, and obvious flaws. Fig. 4(b) shows the surface of a CBTS film with 5 nm thick Sn mask layer. It can be seen clearly that CBTS films show good crystallinity, uniformly sized grains, no pinholes, and no flaws. Fig. 4(c) and (d) show the surface of CBTS film with 10 and 15 nm thickness Sn mask layer. Both films show good crystalline and uniformly sized grains. However, in contrast to Fig. 4(b), another phase has gradually grown as the thickness of the Sn mask layer increases. Lastly, a schistose structure is evident in Fig. 4(d), which is identified as the SnS2 phase; this concurs with the previous results from XRD and Raman spectroscopy. The SEM result shows that the Sn mask layer can successfully prevent the reaction between Ba and water, which results in CBTS films with good crystallinity and uniformly sized grains. Fig. S2 shows the elemental map of a selected area of a CBTS film with a 5 nm
thick Sn mask layer, which demonstrates a uniform distribution of elements across the entire area of analysis. The elemental composition of the CBTS with different thicknesses of Sn mask layer are shown in Table 1. It can be seen the molar ratios of Cu/(Ba + Sn) and Ba/Sn of as-deposited CBTS films are about 0.91 and 1.21. It was also consistent with the reported Cu-poor and Ba-rich growth conditions (Liu et al., 2016). The Sn content increased gradually with the increase in thickness of the Sn mask layer and the proportion of Sn gradually increased to be greater than the stoichiometric composition. AFM characterization was used to gauge the surface roughness of CBTS film. Fig. 5(a) shows the AFM images of as-deposited CBTS film; an obvious flaw can be seen in the middle of the as-deposited film, which is consistent with the SEM result. However, the obvious defect gradually disappears as the thickness of the Sn mask layer increases until the film eventually becomes flat with the addition of the Sn maskant. The surface roughness of the CBTS film decreased with the increasing Sn mask layer from 0 to 5 nm, and then increased with the increasing Sn mask layer from 5 to 15 nm. Moreover, Fig. 5(d) also shows a schistose structure of SnS2 in the CBTS films with 15 nm thickness of Sn, which indicates that the SnS2 gradually appeared with the increasing thickness of Sn mask layer. The trend of CBTS films with
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Fig. 4. The surface SEM images of CBTS films with different thickness of Sn mask layers (a) 0 nm, (b) 5 nm, (c) 10 nm, (d) 15 nm.
electricity properties of CBTS films. Hall measurement was used to gauging the mobility, carrier concentration and resistivity of CBTS films with different thickness of Sn mask layer. The trend and detailed value are shown in Fig. 7(a) and Table 2. It can be seen the mobility and carrier concentration of CBTS film increase first and then decreased. A higher mobility of 1.36 cm2 v−1 s−1 is obtained from the CBTS film with a 5 nm Sn mask layer. Fig. 7(b) show the cross-sectional SEM images of CBTS film with 5 nm thick Sn mask, it can be seen the CBTS films has well crystallinity and larger grain sizes. No pinholes were found in the all area. The J-V curves of CBTS solar cells is shown in Fig. 7(c) and detail photovoltaic parameters summarizes in Table 3. It can be seen with the adding of 5 nm thickness Sn mask layer the efficiency of CBTS solar cells has been increased from 0.45% to 1.21%. The improvement is mainly due to the increase in Voc, Jsc and FF. We believe that the increase in PCE is mainly due to the improved quality of the masked CBTS film, which is apparent in the SEM and AFM results and the reduction in surface recombination. However, the PCE of the CBTS solar cells decreased with the increase in thickness of Sn mask layer from 5 nm to 15 nm. The visible reduction in the power conversion efficiency of the CBTS samples with 5 nm to 10 nm thickness of Sn masking is due to the decrease in Voc, Jsc, and FF. We believe the main reason for the decreasing of PCE is the presence of the second phase of SnS2 sheet, which will result in a decreasing quality of p-n (CBTS/CdS) junction and rectifying behaviour. The mechanism to describe such behavior is proposed as follows: Firstly, SnS2 can form a p–n junction with CBTS instead of n-type CdS. But the mismatch between SnS2 and CBTS is higher than the mismatch between CdS and CBTS, which will cause incoherent interfaces with high recombination velocities (Vaibhav and Srinadh, 2016; Yin et al., 2015). Secondly, the Jsc of CBTS solar cells will be decreased due to the high resistivity of SnS2 which will prevent the collection of carriers (Chawla and Clemens, 2012). Thirdly, the existence of the SnS2 thin film deforms the window layer, leaving a hollow area in the device, which will perturb the structure and cause severe recombination (Larsen et al., 2015; Wang et al., 2018;
Table 1 The element composition of CBTS film with different thickness of Sn mask layer. CBTS films Sn thickness (nm)
Cu (at%)
Ba (at%)
Sn (at%)
S (at%)
Cu/Ba + Sn
Ba/Sn
0 5 10 15
22.99 22.11 22.65 22.08
13.72 13.22 13.13 13.31
11.31 12.84 13.55 14.03
51.98 51.83 50.67 5058
0.91 0.82 0.84 0.80
1.21 1.02 0.96 0.94
different thicknesses of Sn mask layer is depicted in Fig. S3. Fig. 6(a) shows the absorption spectra of the CBTS thin films with different thickness of Sn mask layer. It can be seen all the CBTS films show high absorption in the range of visible light, although the absorption of CBTS film decreased with the increasing thickness of the Sn mask layer, especially once it had increased to 15 nm; the absorption of CBTS is lower than that of the pure CBTS film. Fig. 6(b) shows the values of band-gaps that were determined for CBTS films with different thickness of Sn mask layer. As shown in the figure, the band-gap values of CBTS first decreased from 1.94 to 1.88 eV as the thickness of the Sn mask layer increased from 0 to 5 nm, then increased from 1.88 eV to 1.97 eV as the thickness of the Sn mask layer increases from 5 to 15 nm. The obviously wavy of CBTS films can be seen in the range of 500–600 nm, it was the band gap cutoff of CBTS film which result from the band tailing and the optical transitions involving one valence and two conduction bands (Shin et al., 2016; Gokmen et al., 2013; Ge, Grice, et al., 2017). Photoluminescence (PL) measurements (Fig. S4) were used to confirm the optical band gap of the CBTS film with 5 nm Sn mask layer. It can be seen a PL peak emitted at 670 nm (closing to 1.88 eV) is consistent with UV − vis spectroscopy result. In order to investigated the influence of Sn masking layer on the
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Fig. 5. The AFM images of CBTS films with different thickness of Sn mask layers (a) 0 nm, (b) 5 nm, (c) 10 nm, (d) 15 nm.
Fig. 6. (a) UV–vis–NIR absorption spectrum and (b) band gap of the CBTS film with different thickness of Sn mask layer.
Khalate et al., 2018). The EQE is shown in Fig. 7(d), the calculated Jsc values obtained by integrating the product of the EQEs are 2.5, 5.01, 4.03 and 3.81 mA/cm−2 which is in good agreement with the measured Jsc values. In addition, it can be seen an obviously improvement of EQE due to the adding of Sn mask layer, especially the thickness of Sn mask layer is 5 nm. This can be attributed to the increasing quality of CBTS film with a 5 nm thick Sn mask layer.
to the surface of the CBT precursor film. Subsequently, devices made with the masked film exhibit better PCE, improving from 0.45% to 1.21%, by adding a 5 nm Sn mask layer at the surface of CBT precursor film, which is attributed to improved morphology, reduced recombination, and the enhanced quality of the p–n junction. However, the PCE of CBTS solar cells decreased when the thickness of Sn mask layer increased from 5 to 15 nm, which may be caused by the existence of SnS2 thin films. We demonstrated that the use of a 5 nm Sn mask layer can easily increase the efficiencies of CBTS solar cells. We hope these results can serve as guiding principles for preparing high-quality CBTS thin films
4. Conclusion In summary, we present an easy and cost-effective approach to overcome hydrolysis in CBTS films by adding an ultrathin Sn mask layer
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Fig. 7. (a) The electrical properties of CBTS films with different thickness of Sn mask layers. (b) The cross-sectional SEM images of CBTS solar cells with 5 nm Sn mask layer. (c) and (d) The J-V characteristics and EQE of illuminated CBTS solar cell with different thickness of Sn mask layer.
Higher Education Institutions, the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 14KJA430001).
Table 2 The detailed electrical properties of CBTS thin film with different thickness of Sn mask layer. CBTS Sn thickness (nm)
Resistivity (Ω·cm)
Mobility (cm2/Vs)
Density (cm−3)
Type of carrier
0 5 10 15
1020.75 106.07 239.43 489.4
0.07 2.75 1.36 0.97
8.65 × 1012 3.75 × 1014 7.29 × 1013 1.31 × 1013
P P P P
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.02.007. References Chawla, V., Clemens, B., 2012. Effect of composition on high efficiency CZTSSe devices fabricated using co-sputtering of compound targets. In: Proceedings of the 38th Photovoltaic Specialists Conference (PVSC), pp. 2990–2992. Chen, Z., Sun, K., Su, Z., Liu, F., Tang, D., Xiao, H., Shi, L., Jiang, L., Hao, X., Lai, Y., 2018. Solution-processed trigonal Cu2BaSnS4 thin-film solar cells. ACS Appl. Energy Mater. 7, 3420–3427. Dong, C., Ashebir, G., Qi, J., 2018. Solution-processed Cu2FeSnS4 thin films for photovoltaic application. Mater Lett. 214, 287–289. Donne, A., Marchionn, S., Acciarri, M., Cernuschi, F., Binetti, S., 2017. Relevant efficiency enhancement of emerging Cu2MnSnS4 thin film solar cells by low temperature annealing. Sol. Energy 149, 125–131. Ge, J., Roland, P., Koirala, P., 2017a. Employing over layers to improve the performance of Cu2BaSnS4 thin film based photoelectrochemical water reduction devices. Chem. Mater. 29, 916–920. Ge, J., Grice, C., Yan, Y., 2017b. Cu-based quaternary chalcogenide Cu2BaSnS4 thin films acting as hole transport layers in inverted perovskite CH3NH3PbI3 solar cells. J. Mater. Chem. A 5, 2920–2928. Ge, J., Koirala, P., Grice, C., 2017c. Oxygenated CdS buffer layers enabling high opencircuit voltages in earth-abundant Cu2BaSnS4 thin-film solar cells. Adv. Energy Mater. 7, 1601803. Guo, H.F., Li, Y., Fang, X., Zhang, K.Z., Ding, J.N., Yuan, N.Y., 2016. Co-sputtering deposition and optical-electrical characteristic of Cu2CdSnS4 thin films for use in solar cells. Mater. Lett. 162, 97–100. Gokmen, T., Gunawan, O., Todorov, K., Mitzi, B., 2013. Band tailing and efficiency limitation in kesterite solar cells. Appl. Phys. Lett. 103, 103506.
Table 3 Photovoltaic parameters of the CBTS solar cells. CBTS film solar cells Sn thickness (nm)
Voc (mV)
Jsc (mA/cm−2)
FF (%)
Eff (%)
0 5 10 15
370 562 521 430
2.56 4.98 4.17 3.9
29 43 38 36
0.27 1.21 0.82 0.59
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 91648109, 51335002, 51572037, 51272033), the Priority Academic Program Development of Jiangsu
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Mitzi, D.B., Gunawan, O., Todorov, T.K., 2013. Prospects and performance limitations for Cu–Zn–Sn–S–Se photovoltaic technology. Trans. R. Soc. A 371, 20110432. Prabhakar, R., Su, Z., Xin, Z., 2016. Photovoltaic effect in earth abundant solution processed Cu2MnSnS4 and Cu2MnSn(S, Se)4 thin films. Sol. Energy Mater. Sol. Cells 157, 867–873. Shin, D., Tong, Z., Xuan, H., 2017. Earth-abundant chalcogenide photovoltaic devices with over 5% efficiency based on a Cu2BaSn(S, Se)4 absorber. Adv. Mater. 29, 1601945. Shin, D., Saparov, B., Zhu, T., 2016. BaCu2Sn(S, Se)4: earth-abundant chalcogenides for thin-film photovoltaics. Chem. Mater. 13, 4771–4780. Steinhagen, C., Panthani, M.G., Akhavan, V., Goodfellow, B., Koo, B., Korgel, B.A., 2009. Synthesis of Cu2ZnSnS4 nanocrystals for use in low-cost photovoltaics. J. Am. Chem. Soc. 131, 12554–12555. Vaibhav, A., Srinadh, K., 2016. Status of CZTS thin films and future prospect of CZTS nanocrystal synthesis by three step hybrid technology. Int. Adv. Res. J. Sci. Eng. Tech. 3, 2393–8021. Wang, W., Chen, G., Cai, H., 2018. The effects of SnS2 secondary phases on Cu2ZnSnS4 solar cells: a promising mechanical exfoliation method for its removal. J. Mater. Chem. A 6, 2995–3004. Wang, W., Winkler, M., Gunawan, O., Gokmen, T., Todorov, K., Zhu, Y., Mitzi, D., 2014. Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency. Adv. Energy Mater. 4, 1301465. Yin, L., Cheng, G., Feng, Y., Li, Z., Yang, C., Hao, X., 2015. Limitation factors for the performance of kesterite Cu2ZnSnS4 thin film solar cells studied by defect characterization. RSC Adv. 5, 40369.
307
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
The fabrication of Cu2BaSnS4 thin film solar cells utilizing a maskant layer a
a
a
a
a
a,⁎
Huafei Guo , Changhao Ma , Zhiwen Chen , Xuguang Jia , Qingfei Cang , Ningyi Yuan , ⁎ Jianning Dinga,b,
T
a
School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center for Photovoltaic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164, China Micro/Nano Science and Technology Center, Jiangsu University, Zhenjiang 212013, China
b
ARTICLE INFO
ABSTRACT
Keywords: CBTS Sn Thickness Mask layer Solar cell
Cu2BaSnS4 (CBTS) is a low cost, non-toxic material composed of abundant elements with a large optical absorption coefficient. However, in CBTS solar cells, elemental Ba reacts readily with water, which decreases the quality of the CBTS film. Therefore, in this paper, we present an easy and cost-effective approach to prevent the hydrolysis in CBTS films by adding an ultrathin Sn mask layer at the surface of the CBT precursor film. Adding a Sn mask layer can improve morphology, reduce recombination, and enhance the quality of p–n junction between CBTS and the CdS film by inhibiting the reaction between Ba and water. When the thickness of Sn mask layer was increased to 5 nm, the open circuit voltage, short current density, fill factor all increased and the conversion efficiency of the solar cell increased from 0.27% to 1.21%. However, the second phase SnS2 appeared at the surface of the CBTS film when the thickness of the Sn mask layer increased from 5 to 15 nm, which may severely impair the p–n junction resulting in deterioration of the performance of the CBTS device.
1. Introduction Kesterite (Cu2ZnSnS4; CZTS) is considered to a promising photovoltaic material for low cost and high efficiency thin-film solar cells due to its optimal optical band-gap and high absorption coefficient of > 104 cm−1; in addition, it is composed of abundant elements (Mitzi et al., 2011). Consequently, CZTS solar cells are considered as an alternative to CIGS and CdTe solar cells. CZTS solar cells produced via many synthesis routes have reached power conversion efficiencies close to or above 10% (Mitzi et al., 2013; Steinhagen et al., 2009). In particular, the highest efficiency for CZTSSe solar cell was achieved by using a hydrazine solution method (Wang et al., 2014). But up to date, the efficiency (η) of CZTS devices is much lower than those of CIGS (21.7%) (Kim and Shafarman, 2016). The main reason for the low efficiency is the low open circuit voltage (Voc), which may be due to cation antisite defects (i.e., CuZn and ZnCu). Recently, many efforts have been made to design new materials that avoid the antisite defect by using large ionicradius elements, such as Ba, to replace Zn: Cu2BaSnS4 (CBTS) is one of such new material (Donne et al., 2017; Guo et al., 2016; Miao et al., 2017; Maldar et al., 2017; Prabhakar et al., 2016; Dong et al., 2018). CBTS materials is now proposed as an alternative to CZTS due to inhibition formation of antiste defects and associated band tailing,
optimal band gap (2.0 eV) and high absorption coefficient of > 104 cm−1 (Shin et al., 2017, 2016; Ge, Roland, et al., 2017; Ge, Grice, et al., 2017; Chen et al., 2018). Up to now, many woks of CBTS solar cells has been reported. For example, Shin et al. (2016) demonstrated the first 1.62%-efficient CBTS-based solar cell (0.425 cm2 device area) using a well-known device structure analogous to CZTSSe devices. Then Shin et al. (2017) achieved a higher efficiency with CBTSSe solar cells of over 5% by adjusting the proportion of Se and S. A more recent CBTSbased device with a CdS:O buffer layer demonstrated a slightly higher power conversion efficiency (PCE) of 2.03% (0.08 cm2 device area) and a maximum Voc of 1 V, which is larger than those achieved by CIGS and CZTS solar cells (Ge, Koirala, et al., 2017). However, Ba reacts readily with water during the preparation process; consequently, the precursor film of CBT will also react readily with water prior to the sulfuriation process This problem may be solved by using high vacuum, low humidity links and storage devices, but this will greatly increase the cost of production. So the question of how to avoid or reduce the extent of reaction between the precursor film of CBT and water at a low cost is the key to developing CBTS film for potential applications in low-cost solar cells. In this work, the CBTS film was first fabricated by co-sputtering with three targets: Cu, Ba, and Sn. Then, different thicknesses of
⁎ Corresponding authors at: School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center for Photovoltaic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164, China (J. Ding). E-mail addresses: [email protected] (N. Yuan), [email protected] (J. Ding).
https://doi.org/10.1016/j.solener.2019.02.007 Received 30 November 2018; Received in revised form 4 February 2019; Accepted 5 February 2019 0038-092X/ © 2019 Published by Elsevier Ltd on behalf of International Solar Energy Society.
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2018). 2.2. Characterization The structure of CBTS films was performed using XRD (Ultima III, Japan). The morphology, Surface roughness and elemental composition of CBTS films was performed by SEM (JEOL 6360LA, Japan), AFM and EDS. The optical and electrical properties were determined by UV spectrophotometer (UV-2250, Japan) and Hall measurement (QT-50, Germany). The efficiency and EQE of CBTS solar cells were investigated by solar cell scan photovoltaic characterization system (Solar cell scan, Chia). 3. Result and discussion The detailed fabrication process to create CBTS films with different thicknesses of Sn maskant is illustrated in Fig. 2. Fig. S1 shows the physical configuration of a CBTS films, with and without 5 nm Sn mask layer. As shown in the Fig. 2, the CBT precursor film without a Sn mask layer has a rough surface with curved scars. In contrast, the CBT precursor film with the Sn mask layer shows a glazed surface. This indicates that the additional Sn mask layer successfully prevents the reaction between Ba and water. The XRD patterns of CBTS films with different thickness of Sn film are shown in Fig. 3(a). It can be seen for all the CBTS film with different thickness of Sn mask layer the diffraction peaks were observed at 2θ values characteristic of CBTS (JCPDS no. 30-0124). Compared to the pure CBTS sample (i.e., without a Sn mask layer), the other three samples show an additional peak at 30.718 (marked by * in Fig. 3(a)). This peak is in accordance with the (0 0 4) plane of SnS2((JCPDS no. 211231). In addition, the intensity of the (0 0 4) planes increased with increasing thickness of the Sn mask layer. Raman spectroscopy was utilized to identify the phases of CBTS film with different thicknesses of the Sn mask layer. As shown in Fig. 3(b), the main peaks of the Raman spectra of the CBTS at 256, 345, 358, and 370 cm−1 all correspond to the pure CBTS structure (Mccarthy and Brutchey, 2018). Compared to the pure CBTS sample (without an Sn mask layer), the patterns of the other three samples have additional peaks at 318 cm−1. Reference to previous work implies that the peaks are associated with the SnS2 phase, which is consistent with the XRD results (Wang et al., 2018). Xray photoelectron spectroscopy (XPS) was used to gain insight into the valence states of the material. Fig. 3(c) shows the high-resolution
Fig. 1. The structure of CBTS solar cell.
ultrathin Sn film were used to prevent the reaction between the precursor film of CBT and water from developing. The physical, electrical, and optical properties of the CBTS thin films with different thickness of Sn film were investigated in detail. Finally, a 1.21% efficiently of a CBTS solar cell based on the structure of FTO/CBTS/CdS/i-ZnO/ITO/ Ag has been obtained. 2. Experimental 2.1. Preparation of CBTS solar cells The CBTS film was deposited on FTO glasses by co-sputtering method. First, the CBT precursors were fabricated by co-sputtering with Cu, Sn and Ba target. The sputtering power is 40 W(DC), 40 W(RF) and 60 W(RF), respectively. After the deposition of CBT precursors film, different thickness (0, 5, 10 and 15 nm) of Sn layer was deposited on the surface of CBT precursors film, Then the sample were loaded into a horizontal tube furnace and annealed at 550 °C for 60 min with 25 mg of sulfur powder. The structure of CBTS solar cells was illustrated in Fig. 1. The preparation methods for CdS, ZnO, ITO and Ag are consistent with our previous reports (Guo, Ma, et al., 2018; Guo, Li, et al.,
Fig. 2. Illustration of a method for fabricating a compact CBTS film by further sputtering thin Sn film on the CBT precursor film.
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Fig. 3. (a) The XRD and (b) Raman patterns of CBTS films with different thick Sn mask layers. (c) The XPS patterns of CBTS film with 5 nm thick Sn mask layer.
spectra for the Cu 2p, Ba 3d, Sn 3d, and S 2p regions of CBTS film with 5 nm Sn mask layer. It can be seen all the splitting and peak positions observed are similar to Brutchey and Yan's (Mccarthy and Brutchey, 2018; Ge, Grice, et al., 2017) report for Cu2BaSnS4 and are corroborated by the expected values for Cu+, Ba2+, Sn4+ and S2−. Fig. 4 shows the surface SEM images of CBTS with different thicknesses of Sn mask layer. It can be seen from Fig. 4(a) that the CBTS film with no Sn mask layer has a poorly crystalline rough surface, irregularly sized grains, and obvious flaws. Fig. 4(b) shows the surface of a CBTS film with 5 nm thick Sn mask layer. It can be seen clearly that CBTS films show good crystallinity, uniformly sized grains, no pinholes, and no flaws. Fig. 4(c) and (d) show the surface of CBTS film with 10 and 15 nm thickness Sn mask layer. Both films show good crystalline and uniformly sized grains. However, in contrast to Fig. 4(b), another phase has gradually grown as the thickness of the Sn mask layer increases. Lastly, a schistose structure is evident in Fig. 4(d), which is identified as the SnS2 phase; this concurs with the previous results from XRD and Raman spectroscopy. The SEM result shows that the Sn mask layer can successfully prevent the reaction between Ba and water, which results in CBTS films with good crystallinity and uniformly sized grains. Fig. S2 shows the elemental map of a selected area of a CBTS film with a 5 nm
thick Sn mask layer, which demonstrates a uniform distribution of elements across the entire area of analysis. The elemental composition of the CBTS with different thicknesses of Sn mask layer are shown in Table 1. It can be seen the molar ratios of Cu/(Ba + Sn) and Ba/Sn of as-deposited CBTS films are about 0.91 and 1.21. It was also consistent with the reported Cu-poor and Ba-rich growth conditions (Liu et al., 2016). The Sn content increased gradually with the increase in thickness of the Sn mask layer and the proportion of Sn gradually increased to be greater than the stoichiometric composition. AFM characterization was used to gauge the surface roughness of CBTS film. Fig. 5(a) shows the AFM images of as-deposited CBTS film; an obvious flaw can be seen in the middle of the as-deposited film, which is consistent with the SEM result. However, the obvious defect gradually disappears as the thickness of the Sn mask layer increases until the film eventually becomes flat with the addition of the Sn maskant. The surface roughness of the CBTS film decreased with the increasing Sn mask layer from 0 to 5 nm, and then increased with the increasing Sn mask layer from 5 to 15 nm. Moreover, Fig. 5(d) also shows a schistose structure of SnS2 in the CBTS films with 15 nm thickness of Sn, which indicates that the SnS2 gradually appeared with the increasing thickness of Sn mask layer. The trend of CBTS films with
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Fig. 4. The surface SEM images of CBTS films with different thickness of Sn mask layers (a) 0 nm, (b) 5 nm, (c) 10 nm, (d) 15 nm.
electricity properties of CBTS films. Hall measurement was used to gauging the mobility, carrier concentration and resistivity of CBTS films with different thickness of Sn mask layer. The trend and detailed value are shown in Fig. 7(a) and Table 2. It can be seen the mobility and carrier concentration of CBTS film increase first and then decreased. A higher mobility of 1.36 cm2 v−1 s−1 is obtained from the CBTS film with a 5 nm Sn mask layer. Fig. 7(b) show the cross-sectional SEM images of CBTS film with 5 nm thick Sn mask, it can be seen the CBTS films has well crystallinity and larger grain sizes. No pinholes were found in the all area. The J-V curves of CBTS solar cells is shown in Fig. 7(c) and detail photovoltaic parameters summarizes in Table 3. It can be seen with the adding of 5 nm thickness Sn mask layer the efficiency of CBTS solar cells has been increased from 0.45% to 1.21%. The improvement is mainly due to the increase in Voc, Jsc and FF. We believe that the increase in PCE is mainly due to the improved quality of the masked CBTS film, which is apparent in the SEM and AFM results and the reduction in surface recombination. However, the PCE of the CBTS solar cells decreased with the increase in thickness of Sn mask layer from 5 nm to 15 nm. The visible reduction in the power conversion efficiency of the CBTS samples with 5 nm to 10 nm thickness of Sn masking is due to the decrease in Voc, Jsc, and FF. We believe the main reason for the decreasing of PCE is the presence of the second phase of SnS2 sheet, which will result in a decreasing quality of p-n (CBTS/CdS) junction and rectifying behaviour. The mechanism to describe such behavior is proposed as follows: Firstly, SnS2 can form a p–n junction with CBTS instead of n-type CdS. But the mismatch between SnS2 and CBTS is higher than the mismatch between CdS and CBTS, which will cause incoherent interfaces with high recombination velocities (Vaibhav and Srinadh, 2016; Yin et al., 2015). Secondly, the Jsc of CBTS solar cells will be decreased due to the high resistivity of SnS2 which will prevent the collection of carriers (Chawla and Clemens, 2012). Thirdly, the existence of the SnS2 thin film deforms the window layer, leaving a hollow area in the device, which will perturb the structure and cause severe recombination (Larsen et al., 2015; Wang et al., 2018;
Table 1 The element composition of CBTS film with different thickness of Sn mask layer. CBTS films Sn thickness (nm)
Cu (at%)
Ba (at%)
Sn (at%)
S (at%)
Cu/Ba + Sn
Ba/Sn
0 5 10 15
22.99 22.11 22.65 22.08
13.72 13.22 13.13 13.31
11.31 12.84 13.55 14.03
51.98 51.83 50.67 5058
0.91 0.82 0.84 0.80
1.21 1.02 0.96 0.94
different thicknesses of Sn mask layer is depicted in Fig. S3. Fig. 6(a) shows the absorption spectra of the CBTS thin films with different thickness of Sn mask layer. It can be seen all the CBTS films show high absorption in the range of visible light, although the absorption of CBTS film decreased with the increasing thickness of the Sn mask layer, especially once it had increased to 15 nm; the absorption of CBTS is lower than that of the pure CBTS film. Fig. 6(b) shows the values of band-gaps that were determined for CBTS films with different thickness of Sn mask layer. As shown in the figure, the band-gap values of CBTS first decreased from 1.94 to 1.88 eV as the thickness of the Sn mask layer increased from 0 to 5 nm, then increased from 1.88 eV to 1.97 eV as the thickness of the Sn mask layer increases from 5 to 15 nm. The obviously wavy of CBTS films can be seen in the range of 500–600 nm, it was the band gap cutoff of CBTS film which result from the band tailing and the optical transitions involving one valence and two conduction bands (Shin et al., 2016; Gokmen et al., 2013; Ge, Grice, et al., 2017). Photoluminescence (PL) measurements (Fig. S4) were used to confirm the optical band gap of the CBTS film with 5 nm Sn mask layer. It can be seen a PL peak emitted at 670 nm (closing to 1.88 eV) is consistent with UV − vis spectroscopy result. In order to investigated the influence of Sn masking layer on the
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Fig. 5. The AFM images of CBTS films with different thickness of Sn mask layers (a) 0 nm, (b) 5 nm, (c) 10 nm, (d) 15 nm.
Fig. 6. (a) UV–vis–NIR absorption spectrum and (b) band gap of the CBTS film with different thickness of Sn mask layer.
Khalate et al., 2018). The EQE is shown in Fig. 7(d), the calculated Jsc values obtained by integrating the product of the EQEs are 2.5, 5.01, 4.03 and 3.81 mA/cm−2 which is in good agreement with the measured Jsc values. In addition, it can be seen an obviously improvement of EQE due to the adding of Sn mask layer, especially the thickness of Sn mask layer is 5 nm. This can be attributed to the increasing quality of CBTS film with a 5 nm thick Sn mask layer.
to the surface of the CBT precursor film. Subsequently, devices made with the masked film exhibit better PCE, improving from 0.45% to 1.21%, by adding a 5 nm Sn mask layer at the surface of CBT precursor film, which is attributed to improved morphology, reduced recombination, and the enhanced quality of the p–n junction. However, the PCE of CBTS solar cells decreased when the thickness of Sn mask layer increased from 5 to 15 nm, which may be caused by the existence of SnS2 thin films. We demonstrated that the use of a 5 nm Sn mask layer can easily increase the efficiencies of CBTS solar cells. We hope these results can serve as guiding principles for preparing high-quality CBTS thin films
4. Conclusion In summary, we present an easy and cost-effective approach to overcome hydrolysis in CBTS films by adding an ultrathin Sn mask layer
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Fig. 7. (a) The electrical properties of CBTS films with different thickness of Sn mask layers. (b) The cross-sectional SEM images of CBTS solar cells with 5 nm Sn mask layer. (c) and (d) The J-V characteristics and EQE of illuminated CBTS solar cell with different thickness of Sn mask layer.
Higher Education Institutions, the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 14KJA430001).
Table 2 The detailed electrical properties of CBTS thin film with different thickness of Sn mask layer. CBTS Sn thickness (nm)
Resistivity (Ω·cm)
Mobility (cm2/Vs)
Density (cm−3)
Type of carrier
0 5 10 15
1020.75 106.07 239.43 489.4
0.07 2.75 1.36 0.97
8.65 × 1012 3.75 × 1014 7.29 × 1013 1.31 × 1013
P P P P
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Table 3 Photovoltaic parameters of the CBTS solar cells. CBTS film solar cells Sn thickness (nm)
Voc (mV)
Jsc (mA/cm−2)
FF (%)
Eff (%)
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370 562 521 430
2.56 4.98 4.17 3.9
29 43 38 36
0.27 1.21 0.82 0.59
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 91648109, 51335002, 51572037, 51272033), the Priority Academic Program Development of Jiangsu
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