Mechanism on the modified sulfurization process for growing large-grained Cu2ZnSnS4 thin films

Solar Energy 196 (2020) 597–606

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Mechanism on the modified sulfurization process for growing large-grained Cu2ZnSnS4 thin films Xiaoshuang Lua, Bin Xua, Chuanhe Maa, Ye Chena, Pingxiong Yanga, Junhao Chua,b, Lin Suna, a b

T

⁎

Key Laboratory of Polar Materials and Devices (MOE), Department of Electronics, East China Normal University, Shanghai 200241, China National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

A R T I C LE I N FO

A B S T R A C T

Keywords: CZTS Magnetron sputtering SnS Sulfurization Large grains

A facile and effective sulfurization method is developed to grow the large-grained Cu2ZnSnS4 (CZTS) films. This method involves short duration of high temperature and adding SnS powder. The roles of SnS powder to promote the growth of CZTS large grains are comprehensively investigated. SnS can suppress the loss of Sn in CZTS precursor during the sulfurization process. More importantly, the reaction between SnS and S leads to the formation of SnS2 intermediate phase, which is a fluxing agent under high S atmosphere and more easily reacts with other secondary phases. Consequently, large grains of CZTS through the whole film thickness are successfully prepared with the help of SnS2. Furthermore, the CZTS solar cell with efficiency of 5.8% (active area efficiency 6.4%) without MgF2 anti-reflection coating is fabricated. The severe carrier recombination within the heterojunction interface is the major factor limiting device efficiency. The optimization for the heterojunction interface is expected to further improve efficiency.

1. Introduction As an earth abundant and low-toxic alternative to the other chalcogenide compounds, such as CuInGa(S,Se)2 (CIGSSe), Cu2ZnSnS4 (CZTS) has at present huge potential. Up to now, the high conversion efficiency of 12.6% for Cu2ZnSn(S,Se)4 (CZTSSe) solar cell has been achieved using a hydrazine solution method (Wang et al., 2014). The pure sulfide CZTS solar cell has also demonstrated an efficiency over 11% (Yan et al., 2018). However, these efficiencies lag far behind that of CIGSSe (23.35%) (Solar Frontier Achieves World Record). The large open circuit voltage (Voc ) deficit (Voc,deficit = Eg / q − Voc , Eg is the bandgap of CZTS) is one of the most important reasons for this efficiency gap, which results from many aspects, such as the crystallinity of absorber layer, homogeneous composition, defects and interface (Yan et al., 2018; Zhang et al., 2018). A poor quality absorber layer contains more small grains, which lead to more grain boundaries. The grain boundary acts as a defect in semiconductors, and provides a trap center for charge carrier recombination, thus deteriorates the device performance (Kumar et al., 2015). Undoubtedly, good quality of absorber layer is prerequisite for high Voc and efficiency of CZTS solar cells. It is essential to prepare absorber layer with large grain sizes (micron scale), which can reduce the recombination of grain boundary and improve the performance of CZTS solar cells (Guan et al., 2016). However, during the preparation of CZTS thin film, the grain ⁎

growth is easily-insufficient and thus leads to the double-layer grain distribution, that is, the upper layer is a large grains layer, and the bottom layer is a fine grains layer (Ananthoju et al., 2019; Zhang et al., 2019; Li et al., 2017). This phenomenon is more frequently to occur in CZTS prepared by sequential deposition of metallic layers using magnetron sputtering and post-sulfurization, which is caused by inhomogeneous distribution of elements and the lack of fluxing agent (Li et al., 2017; He et al., 2018; He et al., 2015; Dhakal et al., 2014). To increase the crystallinity of absorber layer, one of the common methods is the doping of K, Na, Bi or Sb etc. into CZTS films (Johnson et al., 2014; Prabhakar and Jampana, 2011; Carrete et al., 2013; Guo et al., 2015; Tong et al., 2016). The role of these dopants involves the formation of fluxing agents and hence is helpful to facilitate CZTS grain growth. Unfortunately, doping method increases the complexity of the fabrication process and is hard to completely eliminate the existence of fine-grain. Additionally, ultra-high sulfurization temperature (600–700 °C) can promote the inter-diffusion of elements and thus enhance the grain growth of CZTS (Chen et al., 2017; Kuo and Jan 2012; Yang et al., 2017), but the long duration of high-temperature sulfurization process will induce the excessive thickness of MoS2, which deteriorates device performance significantly. Excess Sn content in the precursor also has been reported to promote CZTS grain growth (Zhang et al., 2018; Sutter-Fella et al., 2015). Nevertheless, Sn-rich composition inevitably causes the formation of impurity phase or Sn-related deep

Corresponding author. E-mail address: [email protected] (L. Sun).

https://doi.org/10.1016/j.solener.2019.12.063 Received 4 November 2019; Received in revised form 11 December 2019; Accepted 23 December 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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processes of S1, S2 and S3 device are ensured to be exactly the same. The CdS buffer layer (about 70 nm) was deposited by chemical bath deposition method at 65 °C. The intrinsic ZnO and ITO layers were prepared by RF and DC magnetron sputtering, respectively. Ni/Al grids were evaporated on ITO window layer as the front electrode of CZTS solar cell. The total area of solar cells is 0.23 cm2 (active area is 0.21 cm2). No MgF2 anti-reflection coating was applied. Most of the cell preparation process stems from our reported work (Li et al., 2017; He et al., 2018). For all CZTS films, grazing incidence X-ray diffraction (GIXRD) measurements were conducted by Bruker D8 Discover with CuKα radiation operated at 40 mA and 40 kV, the using incidence angle is 3°. While for initial SnS powder and the remaining powder (exp.Ⅱ), X-ray diffraction (XRD) measurements were conducted. Raman measurements were carried out using LabRAM HR Evolution Raman spectrometer with laser source of 325, 532 and 633 nm. The power of laser used in Raman measurement is 1% and the exposure time is 30 s. Scanning electron microscope (SEM) images were measured with Hitachi S-4800 SEM with an energy dispersive X-ray (EDX) analyzer. Current–voltage (J-V) characteristics of the devices were measured under AM1.5 global spectrum (AAA) with their radiance set to 100 mW/cm2. External quantum efficiency (EQE) spectra were recorded by a single source illumination system (Xenon lamp) combined with a monochromator. The Capacitance-Voltage (C-V) was performed using 30 mV and 100 kHz AC signal with DC bias from −2 to 0 V at 300 K in darkness by a Keithley 4200 semiconductor characterization system.

defects in CZTS, which deviates from the optimal composition ratio (i.e. Cu-poor and Zn-rich) and hinder the fabrication of high-efficiency solar cell. Therefore, it is still a great challenge to obtain large-grained CZTS with appropriate composition by a simple and low-cost method. In this work, on the basis of the conventional sulfurization process, we have proposed a modified sulfurization method, which combines short duration of high temperature and addition of SnS powder. The small grains layer is completely eliminated by this modified sulfurization method. As a result, the performance of CZTS solar cells is improved remarkably, due to the removal of the double-layer grain distribution and the formation of large-grained CZTS. 2. Experimental procedure The stacked metallic precursor films were deposited by sputtering 99.99% pure Zn, Sn and Cu metallic targets on the Mo coated soda-lime glass at room temperature in the sequence of Mo/Zn/Cu/Sn/Cu, more explanations for the sputtering sequence used can be found in other literature (Thota et al., 2017). The operating pressure of high purity Ar gas for Zn, Sn and Cu was 1.6 pa, 1.2 pa and 1.6 pa, respectively. Metal precursors mentioned in this work were prepared by the same sputtering process. Besides, all obtained precursors were characterized by XRF spectroscopy to ensure to have the same composition. The sputtered precursor was first annealed at 310 °C for 15 min and then sulfurized at 570 °C for 20 min in a graphite box with 1 g sulfur (S) powder, which was placed in a tube furnace under N2 atmosphere and the working pressure was 400 Torr. This obtained sample is labeled as S1. Using the same sulfurization process, sample S2 was sulfurized at 570 °C for 15 min and then the sulfurization temperature was raised to 600 °C in 10 s and kept for 5 min. While the sulfurization profile of sample S3 is exactly the same as that of S2. The only difference is the addition of 50 mg SnS powder to graphite box for sample S3. The sulfurization annealing processes of sample S1, S2 and S3 were plotted in Fig. 1. In order to investigate the mechanism of adding SnS powder to promote grain growth, another set of experiments (exp.Ⅱ) was designed and conducted. A metallic precursor was divided into five pieces, and placed into the graphite box under the corresponding sulfurization conditions (first annealed at 310 °C for 15 min and then sulfurized at 570 °C for 0 min/5min/10 min/15 min, 600 °C for 5 min after 570 °C for 15 min, respectively) to form CZTS films. The sulfurization annealing processes of five metallic precursors were plotted in Fig. S1. Both 1 g S and 50 mg SnS powder were added in the graphite box for every sulfurization round. The CZTS devices were fabricated based on S1, S2 and S3 with a configuration of typical CZTS solar cell. The subsequent fabrication

3. Results and discussion Fig. 2(a) presents GIXRD patterns of sample S1, S2 and S3. Compared with the PDF card of kesterite CZTS (JCPDS 26-0575), all the diffraction peaks of our CZTS films match almost perfectly with those of the kesterite phase. Three strong GIXRD peaks at 28.53°, 47.33° and 56.18°, which could be corresponding to the (1 1 2), (2 2 0) and (3 1 2) planes of kesterite structure of CZTS (Li et al., 2017; He et al., 2015). The GIXRD peak at 40.4° is the peak of Mo substrates (JCPDS 65-7442). Besides, no obvious impurity phases are detected (e.g. Cu2-xS, SnS and SnS2). We can observe that the diffraction peaks of samples S2 and S3 are higher and sharper than those of sample S1, which may indicate the improvement in the crystallinity of the CZTS thin films. In order to further estimate the crystallinity, the full width at half maximum (FWHM) of (1 1 2) diffraction peak are calculated in the inset of Fig. 2(a). FWHM of (1 1 2) peak decreases slightly as the sulfurizing temperature increases from 570 °C (Sample S1) up to 600 °C (Sample S2). When adding SnS powder in the sulfurization process (Sample S3), FWHM of (1 1 2) peak further decreases, which indicates an enhancement of the crystal size. However, it should be noted that distinguishing some secondary phases (e.g. Cu2SnS3 and ZnS) from the CZTS phase by analyzing GIXRD patterns is difficult, for their lattice constant and lattice structure are almost identical. As a supplementary method to GIXRD, Raman scattering is also a powerful tool to identify the structure and the phase of CZTS thin films. Raman spectra of sample S1, S2 and S3 using different excited lasers (e.g. 325, 532, 633 nm) are shown in Figs. 2(b) and S2. As shown in Fig. S2(a), no ZnS related Raman peaks (347 cm−1) can be observed using 325 excitation laser sources. There are obviously the main Raman scattering peaks of CZTS at 338 cm−1 (A1 model), while the other weak peaks at 167 cm−1, 238 cm−1, 287 cm−1, 348 cm−1, 367 cm−1 and 374 cm−1 are also distinguished using 532 nm wavelength. All of those peaks agree well with the reported values of CZTS (Sousa et al., 2014). No characteristic peak of other possible secondary phases, e.g., Cu2S, SnS2 appears in the Raman spectra. At 633 nm wavelength, no Cu2SnS3 phases (265 and 305 cm−1) are identified (Fig. S2(b)). The Raman spectrum and the GIXRD pattern characterizations indicate that no secondary phases present in the films. On the other hand, the inset of

Fig. 1. Sulfurization profiles used to prepare S1, S2 and S3 CZTS thin films. 598

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Fig. 2. (a) GIXRD patterns of sample S1, S2 and S3 (grazing angle = 3°), inset of (a) is the plot of FWHM values of (1 1 2) diffraction peak of three samples; (b) Raman spectra of CZTS thin films, inset of (b) is the plots of FWHM and intensity of the 338 cm−1 peak.

growth is accelerated due to further increase of sulfurization temperature. Furthermore, S3 has a smoother surface with the grain size of 1–2.5 µm and the microstructure becomes homogeneous, as shown in Fig. 3(c). More importantly, the double-layer grain distribution was completely eliminated, and the absorber layer consists of large grains that have the same size as the thickness of CZTS thin films. The whole absorber layer displays continuous densely packed large grains and no obvious voids on the bottom of the absorber layer. This tendency agrees well with the GIXRD and Raman analysis above as well. It is worth noting that Fig. 3(c) and (f) display the typical crystal morphology and size distribution of S3 sample. Large-scale surface and cross-section SEM images are given in Fig. S3(a) and (b) of Supporting Information. In the cross-section SEM images (Fig. 3(d)–(f)), it can be estimated that the thickness of MoS2 formed at CZTS/Mo interface of the three samples is about 150 nm, 170 nm and 90 nm, respectively. In contrast with the S1, the thicker MoS2 forms and lots of voids are observed in S2, which originates from higher sulfurization temperature. However, MoS2 layer in S3 sample is the thinnest, which reveals that the large grains running through the absorber layer may be beneficial to inhibit the formation of MoS2.

Fig. 2(b) plots the values of FWHM and the intensity of the Raman A1 mode peak (338 cm−1). As shown in the inset of Fig. 2(b), FWHM decreases from 13 cm−1 (S1) to 9 cm−1 (S3) and the intensity is stronger, which indicates the improvement of crystallinity of CZTS. This is consistent with the GIXRD results. SEM is the most intuitive technique to observe the crystal size and distribution of CZTS absorber layer. The surface and cross-section images of all CZTS absorber layers are shown in Fig. 3(a)–(f). As shown in the surface morphologies of samples, when sample S1 was prepared under the relatively lower temperature (570 °C) sulfurization process, the grain growth is insufficient and the grain size is smaller. In Fig. 3(d), S1 exhibits a much rougher morphology and typical double-layer structure that fine grains on the bottom and large grains on the top, respectively. The incomplete growth of grains may be attributed to insufficient thermal energy transferred from the substrate during conventional sulfurizing process (Chen et al., 2017). For sample S2, the sulfurization temperature was raised to 600 °C in a short time and kept for 5 min, it is obvious that the grain size of CZTS film is enhanced, as shown in Fig. 3(b, e). In addition, the thickness of the small-grain layer is reduced, and the whole absorber layer is close to complete large grains except for some voids on the bottom. It implies that the grain

Fig. 3. SEM images for surface morphologies of CZTS films (a) S1; (b) S2; (c) S3 and their corresponding cross-sections (d) S1; (e) S2; (f) S3. 599

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Table 1 Element composition ratio of CZTS thin films determined from EDX analysis. (Average ratio error ≈0.01). Sample ID

S1 S2 S3

Top

Middle

Bottom

Cu/(Zn + Sn)

Zn/Sn

S/metals

Cu/(Zn + Sn)

Zn/Sn

S/metals

Cu/(Zn + Sn)

Zn/Sn

S/metals

0.85 0.87 0.83

1.12 1.13 1.17

1.08 1.05 1.06

0.82 0.85 0.84

1.17 1.21 1.20

1.02 1.01 1.04

0.65 0.79 0.81

1.71 1.35 1.21

1.01 1.03 1.03

further higher temperature (600 °C) process (Chen et al., 2017; Liu et al., 2016). It should be noted that as shown in Table 1, the ratio of Zn/Sn at the bottom of S2 (about 1.35) is far less than that of S1 (about 1.71), but this does not mean that the Sn loss of S1 is more serious. As discussed above, the insufficient diffusion of Zn element results in Znricher phenomenon at the bottom of S1. As a result, the higher ratio of Zn/Sn at the bottom of S1 mainly results from the enrichment of Zn element rather than the loss of Sn element. In contrast, the EDX analysis results of sample S3 demonstrate that the element compositions of Cu, Zn, and Sn in the CZTS large grain layer are quite homogeneous at different vertical positions. Even at the bottom of the film, there was no serious Zn-rich phenomenon. The ratios of Zn/Sn are all about 1.2 from the surface to the bottom, revealing that the added SnS powder suppress the loss of Sn successfully. The formation and decomposition reactions of CZTS are as follows (Scragg et al., 2011):

To better understand the formation mechanism of double-layer grain distribution of S1 and the relationship between grain growth and element composition distribution, the absorber films of three samples were analyzed by EDX. Considering the electron beam limited penetration depth of top-view configuration, cross-sectional EDX measurement was performed. Three observation square areas at different depths were selected for each sample. The different positions of cross-section measurements for each sample can be seen in Fig. S4(a)–(i) of Supporting Information. In order to ensure the reliability of the composition data, three different horizontal positions are selected at the same depth of each sample film. The final data in the Table 1 are the average values of these three positions. The results of compositional analyses of all CZTS thin films are listed in Table 1. For S1, from top to bottom, the value of Cu/Zn + Sn decreases from 0.85 to 0.65, which means that the relative content of Cu decreases from the surface to the bottom of the film. Whereas the value of Zn/Sn increases from 1.12 to 1.71, implying that the relative content of Zn increases gradually from top to bottom. Hence, the bottom layer composed of smaller grains is seriously Zn-rich and Cu-poor in comparison with the top layer. Considering that the sequence of our precursors is Mo/Zn/Cu/Sn/Cu, the insufficient diffusion of Zn and Cu elements during sulfurization is mainly responsible for it. Furthermore, the loss of Sn during the sulfurization maybe contributes to higher Zn/Sn ratio (Chen et al., 2017; Liu et al., 2016). On the other hand, as shown in Table 1, the values of S/metals change slightly from the surface to the bottom of S1 film, which indicates that the fine grains at the bottom is not caused by poor S. The richness of Zn element may also result in the formation of ZnS at the bottom, which hinders the grain growth. This observation is in good agreement with other literatures (Zhang et al., 2018; He et al., 2018). Moreover, our previous work has pointed out that Cu-rich composition has a positive effect on grain growth and can promote the formation of large grains (Li et al., 2017). Kaur et al. have reported that appropriate content of zinc is necessary to regulate Sn loss in the form of SnS, thus suppress Cu-S based secondary phases. Therefore, the increase in Zn/Sn ratio from the surface to the bottom can lead to unreactive Cu2-xS phase, which hinders the uniform distribution of components (Kaur et al., 2018). As a result, too high Zn and too low Cu content in the bottom layer are the main origin of the double-layer grain distribution. By the above analysis, we can understand why the grain growth of S2 has been further improved. The high temperature process at 600 °C provides more thermal energy to promote the diffusion of metal elements and the reaction between secondary phases, thus facilitates the grain growth. As can be seen from Table 1, compared with S1, the situation of Zn-richer at the bottom of absorber film has been alleviated to a certain extent, the element distribution of S2 sample is more uniform. However, the ratio of Zn/Sn in the bottom is still 1.35, which is higher than the appropriate ratio (~1.2). Besides, it should be noted that for S1, the voids appear between the large grain layer and the small grain layer as shown in Fig. 3(d), which indicates the decomposition of CZTS and the volatilization of SnS begin at the bottom of the large grain layer, this may be due to the enrichment of zinc at the bottom. While for S2 (Fig. 3(e)), the voids appear between MoS2 layer and CZTS layer. This is because S2 almost has no small grains layer, the decomposition of CZTS starts from the bottom of whole absorber layer, and the size of voids is much bigger than that of S1, which result from more serious loss of Sn element via the formation of SnS vapor as a result of the

Cu2S (s) + ZnS (s) + SnS(s) + 1/2S2 (g) ⇋ Cu2ZnSnS4 (s)

(1)

SnS (s) ⇋ SnS (g)

(2)

The formation and decomposition of CZTS always exist during high temperature sulfurization process, while the Cu2S and ZnS are not volatile substances but SnS is. Therefore, there should be SnS in the sulfurization atmosphere to make the above reaction (1) toward to the right direction. In fact, the existence of S atmosphere alone is not enough to make CZTS stable. Lack of additional SnS in sulfurization atmosphere leads to more serious decomposition of CZTS. Therefore, the surfaces of S1 and S2 samples are rougher and there are voids at the bottom. Conversely, adding SnS powder makes the surface of sample S3 smoother and the composition distribution more uniform. Note that even after a long duration of high-temperature sulfurization for sample S3, some black powder (SnS) always remains in the graphite box. So as to further explore how the added SnS powder further promote the growth of CZTS grains, exp.Ⅱ was conducted. XRD measurement was carried out on the initial SnS powder and the remaining powder after different sulfurization time and temperature. In Fig. 4(a), as expected, XRD diffraction peaks of added SnS powder almost coincide with PDF card of SnS (JCPDS 39-0354). While SnS2 peaks at 15.03° (JCPDS 23-0677) appear in the diffraction peaks of the remaining powders after 570 °C sulfurization for 0 to 15 min, and other peaks of SnS2 such as (1 1 0) (at 49.96°) and (1 1 1) (at 52.45°) also appear when sulfurization time is 0 and 5 min at 570 °C, but only SnS powders remain after 600 °C sulfurization, reflecting the SnS2 as an intermediate phase. In addition, some peaks of Sn2S3 (JCPDS 14-0619) as an intermediate phase also appear during sulfurization process. On the other hand, as it is observed in Fig. 4(b), the GIXRD peaks of CZTS film become stronger with increasing sulfurization time and temperature, which suggests that the crystallinity of CZTS film becomes better. No peaks of SnS2 are detected in the samples after 0 min or 5 min sulfurization at 570 °C, possibly due to its poor crystallinity in absorber layer. However, when the metallic precursor is sulfurized at 570 °C for 10 min, a strong diffraction peak of SnS2 is observed. And the peak is weakened after the sulfurization of 570 °C lasts for 15 min, then disappear completely after sulfurized at 600 °C for 5 min. From the above analysis, we speculate that some SnS has been converted into SnS2, which indicates that the following reactions occur during the sulfurization process (Scragg et al., 2011): 600

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Fig. 4. (a) XRD patterns of the added SnS powder and remaining powder in graphite box; (b) GIXRD patterns of CZTS films after different sulfurization times (grazing angle = 3°).

SnS (g) + 1/2S2 (g) ⇋ SnS2 (g)

(3)

2SnS (g) + 1/2S2 (g) ⇋ Sn2S3 (s)

(4)

Sn2S3 (s) + 1/2S2 (g) ⇋ 2SnS2 (s)

(5)

which also explains why the bottom of the S3 film is not Zn rich in comparison with S1 and S2. Finally, SnS2 disappears due to evaporation and reaction in the CZTS film. As a result, we obtain a vertical-penetration CZTS thin film with micron-scale grain sizes after sulfurization. Three thin-film solar cells were fabricated based on sample S1, S2, and S3 (named as cell S1, S2, and S3, respectively). Fig. 6 exhibits the illuminated J–V curve for cell S3. Table 2 shows the detailed solar cell performance parameters extracted from J-V curves. Series resistance (Rs ), ideality factor (A) and the shunt conductance (G), reverse saturation current density (J0) can be calculated using the method reported by Hegedus and Shafarman (2004). The details and plots of parameter extraction of cell S3 can be seen in Fig. S5(b)–(d) of Supporting Information. The device parameters are calculated with the total device area (0.23 cm2), which includes the areas shaded by Al grid electrode. The cross-section SEM images and photography of cell S3 are given in Fig. S6(a)–(b) of Supporting Information. In Table 2, it can be clearly seen that cell S1 exhibits relatively poor conversion efficiency. As discussed above, the fine grains and voids on the bottom may lead to high Rs (Li et al., 2017). The high Rs reduces short-circuit current density (Jsc ) and results in a drop in efficiency (Eff.). The Voc and Jsc of cell S2 and S3 increase gradually with the growth of absorber grains. The enhanced Jsc and Voc is the main reason for the performance improvement of champion cell S3. Compared to the cell S1, the Jsc and Voc of the cell S3 increases from 13.04 mA/cm2 and 569.1 mV to 18.31 mA/cm2 and 631.1 mV, respectively, thus the efficiency increases from 3.7% to 5.8% (the active area efficiency is 6.4%). Comparing these device parameters, we can reveal that the increase of Jsc results partially from the decrease of Rs . Fig. 7(a) and (b) shows the statistical devices parameters (Jsc , Voc , FF and Eff.) of three CZTS solar cells, each dataset is obtained from six solar cells. It is evident that the

And the SnS2 intermediate products evaporate at a high temperature due to its high vapor pressure, which may plays a key role as a fluxing agent under high S atmosphere and promotes the grain growth of CZTS (Guan et al., 2016). Based on the above results, we propose a model to elucidate the role of adding SnS powder on CZTS growth, as displayed in Fig. 5. Initially, the metal precursor with S and SnS powder are added in the graphite box. During the process of sulfurization, the atmosphere contains both SnS(g) and S(g), and some SnS(g) react with S(g) to form the intermediate product SnS2(g). Simultaneously, the metal precursor reacts with S(g) to form CZTS small grains. Sn2S3(s) and SnS2(s) appear in the powder, then both of them eventually turn into SnS2 (g) or SnS(g). Under high S(g) atmosphere, the gaseous SnS2 is converted into a liquid fluxing agent (Sharma and Chang, 1986), and penetrates readily into the grain boundaries of CZTS small grains. Consequently, the small grains merge with another and become entirely large-grained CZTS throughout the film with the aid of short-duration high temperature sulfurization (600 °C). Moreover, SnS2 can also participate in the following reactions (Hergert and Hock, 2007): Cu2

− xS

+ SnS2 → Cu2SnS3

Cu2SnS3 + ZnS → Cu2ZnSnS4

(6) (7)

Therefore, the liquid-phase SnS2 facilitates the sufficient reaction between the secondary phases and promotes the formation of CZTS,

Fig. 5. Schematic diagram of the formation process of large-grained CZTS. 601

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CZTS thin films, which are determined from the plot of [hυ × ln(1 − EQE)]2 againsthυ . Eg is estimated to 1.48, 1.47 and 1.42 eV for cell S1, S2, and S3, respectively, which are in accordance with the reported bandgap of CZTS (≈1.5 eV) 2. The decrease of band gap may be due to the uniform composition distribution of S3 compared with S1 and S2, and the absence of Zn-rich at the bottom, as shown in Table 1. In spite of the good crystallinity of S3 absorber layer, the PCE of cell S3 is relatively lower than the world-record efficiency of CZTS cell (~11%) 2. To further clarify its dominant carrier recombination mechanism, temperature-dependent J–V measurements were conducted for cell S3. The temperature measurement range is 100–320 K. Fig. 9 (a) shows the illuminated (AM 1.5) J-V characteristic of cell S3 with various temperature. The temperature-dependent photovoltaic performance is shown in Fig. 9(b) and (c). The efficiency of cell S3 decreases quickly at low temperature (3.04% at 100 K), whereas Voc increases steadily with the decrease in temperature (798.4 mV at 100 K). Thus, this efficiency decay comes mainly from the decrease in FF and Jsc with reducing temperature. The decline in FF and Jsc should be attributed to the increased Rs , the apparent rise of the Rs at low temperature is primarily due to the carrier freeze-out effect, which arises from the lack of a shallow acceptor in the CZTS absorber. The similar behavior has also been observed in other CZTS-based solar cells. (Mitzi et al., 2011; Todorov et al., 2013) As shown in Fig. 9(d), Voc increases with decreasing temperature in the whole measurement range. The relationship between Voc and temperature (T ) is expressed by the following equation:

Fig.6. J-V curve for S3 cell under illuminated (AM1.5) condition.

cell parameters including Jsc , Voc , FF and Eff. of the corresponding cells have increased from S1 to S3. These results reflect that the growth of the large-grained absorber film is very critical to the high device performance. However, it should be pointed out that compared to the reported CZTS-based solar cells (Wang et al., 2014; Yan et al., 2018; Hegedus and Shafarman, 2004) (The A value is as low as about 1.3 for the record IBM device), there are very high A values (> 2) for our cells, as shown in Table 2. This implies the severe carrier recombination within the heterojunction interface, or that the tunneling enhanced interface recombination is the major limiting factor (Chen et al., 2014). J0 value of cell S3 is 1.8×10-2 mA/cm2, which is the smallest among the samples. But it is still much larger than the values reported in the literature (J0 = 8.3 × 10−4, J0 = 6.8 × 10−8 mA/cm2). (Yan et al., 2018; Liu et al., 2016) Large J0 values are also related to many defects of CZTS/ CdS junction interface region. Moreover, all FF of our cells are lower (~50%), which is more attributed to higher G values and higher series resistances compared with literature (Yan et al., 2018). Poor quality of heterojunction interface is very likely to produce shunt paths and leads to high G. It should emphasized that no any interface treatment (i.e. etching, post-annealing, etc.) has been conducted to our cells. Therefore, interface optimization for lowering the defect density of CZTS/CdS interface are expected to further boost the device efficiency in our next work. EQE spectra of the champion solar cells in each groups of cell S1, S2, and S3 and Eg of these solar cells extracted from EQE spectra are shown in Fig. 8. Without the antireflection layer, the maximum quantum efficiency of cell S1, S2 and S3 is 64%, 73% and 82% at about 560 nm, respectively. Actually, EQE response of champion cell S3 is significantly higher than that of champion cell S2 and champion cell S1 in the whole wavelength region, indicating the superiority of the excellent crystalline quality and higher Jsc (as listed in Table 2). Near the blue spectra region, EQE spectra show a steep decay, revealing the light loss due to the absorption of CdS buffer layer (Liu et al., 2016). By integrating the EQE spectra from 300 to 1000 nm, the calculated Jsc of cell S1, S2 and S3 are 13.89, 15.92 and 19.04 mA/cm2 respectively, which are very close to the values obtained from J-V curves. Inset of Fig. 8 shows Eg of the

Voc =

EA AkT ⎛ J00 ⎞ ln − q q ⎝ JL ⎠ ⎜

⎟

(1)

where EA , A , k , J00 and JL are the activation energy of the dominant recombination mechanism, the ideality factor, Boltzmann constant, reverse saturation current prefactor and the photocurrent, respectively (Turcu et al., 2002). The value of EA is the intercept of the linear extrapolation of the temperature-dependent Voc curve when the temperature T is 0 K (Hegedus and Shafarman, 2004; Turcu et al., 2002). In Fig. 9(d), we observe that cell S3 has an activation energy of EA = 0.87 eV, which is very close to the value of Yan et al. (EA = 0.89 eV, CZTS prepared by magnetron sputtering) (Yan et al., 2018), significantly lower than the absorber band gap of 1.42 eV. Similar results were found in other research groups, and the reported EA are consistently lower than the CZTS band gap by about 0.3–0.6 eV (Yan et al., 2018; Tajima et al., 2015; Wang et al., 2010; Ericson et al., 2014). Low EA value reflects that the dominant recombination path is not located in the CZTS bulk, but at the interface of the heterojunction (Crovetto and Hansen, 2017). A cliff-like conduction band offset (CBO) between CZTS and its typical heterojunction partner CdS (buffer layer) is frequently invoked to explain this phenomenon. Therefore, the interface recombination is dominant in our samples, possibly due to both high density of heterojunction interface defects and a nonideal band alignment. Temperature-dependent J-V measurement also reveals that severe carrier recombination within the heterojunction interface region limits the overall performance of cell S3. Fig. 9(e) and (f) shows dark J-V characteristic of cell S3 with various temperature and the temperature-dependent Rs values extracted from dark J–V data, respectively. A simple circuit model is used as displayed in the inset of Fig. 9(e), where DSC, DBC, R 0 represent the main solar cell diode, a back contact diode, a background series resistance,

Table 2 Device parameters of S1, S2 and S3 cells. Samples

Voc (mV)

Jsc (mA/cm2)

FF. (%)

Eff. (%)

RS,

S1 S2 S3

569.1 594.9 631.2

13.04 15.87 18.31

49.1 49.8 50.2

3.7 4.7 5.8

6.35 4.68 3.87

602

L

(Ωcm2)

GL (mS/cm2)

Rsh (Ω cm2)

A

J0L (mA cm−2)

3.31 2.94 2.43

302.1 340.1 411.5

4.28 3.84 2.96

7.6x10-2 4.3x10-2 1.8x10-2

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Fig. 7. Statistical box charts of S1, S2 and S3 cells: (a) Jsc ; (b) Voc ; (c) FF; (d) Eff. 6 devices for each sample are selected.

height, R 0 is the background series resistance which is quite small and approximately constant thus can be neglected at lower temperature (Gunawan et al., 2010; Zhao and Persson, 2011). Additionally, from ln(Rs T ) versus 1/T plot (the inset of Fig. 9(f)), we fit Eq. (2) to the low 1/T region (from 220 K to 320 K) of the curve and obtain ΦB of 92 meV for cell S3, which is lower than the values (127–325 meV) reported by other groups (Wang et al., 2010; Ge et al., 2019). A thinner MoS2 interfacial layer (about 90 nm) in cell S3 (as shown in Fig. 3(f)) should be responsible for the lower barrier height. C-V measurements were performed to extract the free carrier density and the depletion width ( Xd ) of cell S1, S2 and S3, so as to evaluate the effects of absorber crystallinty on these junction parameters. Fig. 10(a) demonstrates C-V sweep of cell S1, S2 and S3 from −2 to 0 V. All measured capacitances decrease gradually with the increase of reverse bias voltage, which indicates that the reverse bias increases the width of depletion layer in CZTS device, consequently reducing the capacitance of p–n junction (Su et al., 2015). Fig. 10(b) exhibits the C-V derived space-charge density (Ncv ) as a function of profiling position for CZTS devices. Xd of three devices at 0 V can be calculated from:

Fig. 8. EQE spectra of the champion solar cells of S1, S2, and S3. Inset: Eg of these solar cells are determined from the plot of [hυ × ln(1 − EQE)]2 againsthυ.

Xd =

respectively. It can be clearly observed in Fig. 9(f) that Rs increases gradually with decreasing temperature, particularly at lower temperature region. The relationship between Rs and the back-contact barrier height is described by Eq. (2) as follows:

Rs = R 0 +

k Φ exp ⎛ B ⎞ qA∗ T ⎝ kT ⎠

Ncv =

Aε0 εs C

(3)

2 qε0 εs A2

dC −2 dV

(4)

where C and A are the measured junction capacitance and the area of the device (0.23 cm2), respectively, ε0 is the free space permittivity (8.85 × 10−14 F/cm), and εs is the relative dielectric constant of the active layer (εs of CZTS = 7 (Zhao and Persson, 2011; Yan et al., 2016).

(2)

where A∗ is the effective Richardson constant and ΦB is the barrier 603

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Fig. 9. (a) J-V characteristic of cell S3 with various temperature under illuminated (AM1.5) condition; (b) Temperature dependence of cell S3 parameters Voc and Eff.; (c) Temperature dependence of cell S3 parameters FF, Jsc ; (d) Temperature dependence of Voc of cell S3; (e) Dark J-V characteristic of cell S3 with various temperature; (f) Temperature-dependent Rs values extracted from dark J–V data. Inset of (e): circuit model of the back contact blocking diode. Inset of (f): barrier height extracted from ln(Rs T ) versus 1/1000T .

sample were carried out. The details can be seen in Fig. S7(a) and (b) of Supporting Information. In theory, higher p-type doping densities lower Fermi energy level (EF) of CZTS and increase Voc of solar cells. Cell S3 has higher p-type doping density than cell S1 and S2, which contributes to relatively higher Voc and eff.

As shown in Fig. 10(b), the Xd of cell S1, S2 and S3 are estimated to be 397, 295 and 176 nm, respectively. The values of Xd for the CZTS device decreases gradually from cell S1 to S3. It is due to the change of ptype doping densities. The apparent doping density profiles extracted from C-V measurements of cell S1, S2 and S3 are 1.3 × 1016, 1.5 × 1016 and 3.9 × 1016 cm−3, respectively. The CZTS devices with high charge density generally show small depletion width, which is consistent with these previous reports. (He et al., 2018; Heath et al., 2004; Duan et al., 2013) In order to prevent the randomness of C-V data of a single device, data statistics about Xd and Ncv of six devices on each

4. Conclusions We find a facile and effective sulfurization method for promoting the grain growth of CZTS, and a vertical-penetration CZTS film with 604

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Fig. 10. (a) C– V curves for cell S1, S2 and S3. (b) Apparent doping density profile calculated from C-V measurements using a 100 kHz signal.

thoroughly explores the roles of SnS and chemical mechanism on growing large-grained CZTS films deeply. Besides, the firstly proposed model can elucidate well the formation of large-grained CZTS film.

large grains is obtained. The combination of short-time high temperature and adding SnS powder can promote grain growth significantly. The roles of SnS powder to faciliate the growth of large CZTS grains are explored in detail. SnS can not only inhibit the loss of Sn in CZTS precursor during sulfurization process, but also react with S to form SnS2 intermediate product. SnS2 play as a fluxing agent and react with other secondary phases sufficiently, and thus obtain large CZTS grains running through the whole absorber layer. By optimizing the quality of the absorber layer, the device performances are improved. The Jsc raises to 18.3 mA cm−2 and the solar cell with 5.8% efficiency is fabricated. The relatively low efficiceny is mainly due to the severe heterojunction interface recombination. Besides, an unstable back contact and the formation of MoS2 also yield reduced performance in the final device. Interface optimization are expected to further boost the device efficiency in our future work. This work initiates a facile route to eliminate the CZTS fine grains, and the growth of large grains penetrating from the top to the bottom of the film is realized.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant Nos. 61574057 and 61874045) , the Fundamental Research Funds for the Central Universities and ECNU Public Platform for Innovation. Appendix A. Supplementary material

5. Novelty statement

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.12.063.

The double –layer grain distribution, i.e., the upper layer is large grains and the bottom is small grains, usually exists in CZTS absorbers prepared by all kinds of physical and chemical methods. This phenomenon is more frequently to occur in CZTS prepared by sequential deposition of metallic layers using magnetron sputtering and post-sulfurization, which is caused by inhomogeneous distribution of elements. Although some previous studies have shown that increasing temperature can promote grain growth, adding SnS during sulfurization process can inhibit the decomposition of CZTS, the fabrication of large-grained CZTS films with optimal and homogeneous composition still very hard. In this paper, we first develop a facile and effective sulfurization method for stacked metallic precursor films, which combines short duration of high temperature and adding SnS powder. In this way, we completely eliminate the phenomenon of double-layer grain distribution and large grains of CZTS through the whole film thickness are successfully prepared. More importantly, previous studies have only interpreted the roles of added SnS as compensation for loss of Sn and inhibiting the decomposition of CZTS. But in this paper, through designed experiments and data analysis, we found that the added SnS has other functions. The reaction between SnS and S leads to the formation of SnS2 intermediate phase, which is a fluxing agent under high S atmosphere and more easily reacts with other secondary phases. This is the key factor to promote the growth of CZTS grain. The specific mechanism was not mentioned in any other previous related literatures. This paper

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