The CZTSSe thin-films deposited by sputtering method have different thicknesses in the NaF layer: 0 (a), 15 (b), and 30 nm (c). The CZTSSe thin-films contain ~4% of Na. The efficiency of the CZTSSe solar cell is almost similar at ~3%. The grain size of the CZTSSe thin-films increases by adding NaF, as compared to the thin-films without an NaF layer, as shown in Figure 2. The CZTSSe thin-films without NaF presented a grain size of 1–2 μm, and that with an NaF layer was of 4–5 μm. The surface uniformity also improved by adding an NaF layer.
Figure 3 shows the topographic and the surface potential images of the CZTSSe thin-films with and without an NaF layer. The topographic images and the line profile results showed that the height of GBs in the CZTSSe thin-films with the NaF layer were reduced from 400 nm to 100 nm. We also obtained the root mean square (RMS) roughness of the CZTSSe to be 177 nm, and the films with the NaF layer had a lower roughness of 106 nm (15 nm NaF) and 112 nm (30 nm NaF). Thus, adjustments of the NaF layer thickness were observed to influence the grain growth and the formation of a uniform surface. Figures 3(d)–(f) show the surface potential images taken through KPFM measurements. The yellow region represents the positive surface potential values, and the blue region represents the negative potential values. Figures 3(e) and (f) exhibit the images of the potential mapping, and the potential near the GBs with an NaF layer increase relative to that without NaF [Figure 3(d)]. In the CZTSSe films with NaF, the ratio of the yellow regions, especially that near the GBs, is larger than that of pure CZTSSe thin-films.
Figure 4(a)–(c) presents the line profile of the topographic, and the surface potential images are marked with a red line, as shown in Figure 3. Figures 4(d)–(f) show the total potential distribution of the CZTSSe thin-films with respect to the thickness of the NaF layers. In Figures 4(a)–(c), the CZTSSe thin-films show that a negative potential value of about −100 mV at the half region of the GBs and a positive potential of about 110 mV at the IGs, as shown in Figure 4(a). However, the potential of the CZTSSe films with the NaF layer showed a fully positively charged potential of about 120 mV (15 nm NaF) and 380 mV (30 nm NaF) at the GB regions and a negatively charged potential of about −200 mV (15, 30 nm NaF) at the IG regions. The potential distribution was found to be related to the downward band bending because the positive potential at GBs led to a reduction in the work function near the GBs in CZTSSe.
The positive potential distribution was higher as the NaF layer became thicker for the CZTSSe films, as shown in Figure 4(f). Thus, the potential value was the highest for the 30 nm NaF layer CZTSe film. In previous studies, many groups had reported benign behaviors of the GBs in polycrystalline CIGS thin-film solar cells [20]. Jiang et al. confirmed the presence of a positively charged potential (smaller work function) at the GBs, as compared to that at the IGs, with a formation of a local built-in potential near the GBs. This can explain how electrons are drawn to the GBs and how holes are repelled from the GB. This is beneficial for carrier collection, and it also suppresses recombination at the GBs. Therefore, the local built-in potential on GBs in the CIGS thin-films plays a critical role to achieve the desired device performance and efficiency [14],[15].
Meanwhile, the GBs of the CZTSSe thin-films exhibit similar electrical properties to those of GBs in CIGS thin-films. A higher positive surface potential was measured at the GBs relative to that measured at the IGs, and a current flowed through the GBs of CZTSSe thin-films. This could improve the minority carrier collection near the GBs, and thus, the GBs are one of the essential factors that can enhance the high conversion efficiency of CZTSSe thin-film solar cells [21].
Furthermore, our group reported the presence of the positive potential ratio at the GBs and a negative potential ratio at the IGs in CZTSSe thin-film, and these are closely related to device properties, especially to the J
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. The high conversion efficiency of the GBs of the CZTSSe thin-film consists of dominant, positively charged GBs and negatively charged IGs. This leads to a downward band bending at the GBs and an upward band bending at the IGs, which helps in carrier collection and suppresses the recombination at GBs that are limited to downward band-bended GBs. Thus, the J
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and the shunt resistance are enhanced [16], and therefore we tried to characterize the effects of the GB on the CZTSSe film with respect to the presence of an NaF layer.
Figure 5 shows a histogram from the statistical analysis of the potential value, focusing on the GBs and the IGs in the CZTSSe with and without NaF. The results are based on the line profiles of the regions of near GBs, and the histograms exhibit a positive potential distribution at the GBs as shown in Figures 5(a)–(c) and a negative potential distribution ratio at the IGs as shown in Figures 5(d)–(f). The CZTSSe samples exhibit a lower positive potential ratio of 66% at the GBs and also a lower negative potential ratio of 30% at the IGs. However, the CZTSSe with NaF indicates a higher positive potential ratio of 77% (15 nm of NaF) and 94% (30 nm of NaF) at the GBs and a negative potential ratio of 45% (15 nm of NaF) and 60% (30 nm of NaF) at the IGs. The average potential at the GBs in the CZTSSe thin-film is of 74 mV, and that of CZTSSe with a 15 nm NaF thin-film is of 93 mV. The highest average potential of about ~160 mV is shown in the 30 nm NaF thin-films. Therefore, we conclude that the average potential at the GBs increases by adding NaF.
Figure 6(a) displays the schematics of a band diagram near the GBs in the CZTSSe thin-films, and Figure 6(b) shows the CZTSSe thin-films with a 30 nm NaF layer. The higher thickness of the NaF layer enhances the positively charged (i.e. lower work function) GBs in the CZTSSe layer. In previous studies, the Na on the surface was observed to cause surface dipoles, and thus the dipoles may increase the potential near the GBs in the CIGS surface due to the higher Na concentration around the GBs than around the IGs [13]. Thus, Na could enhance the positively charged GBs in the CIGS thin-film surface, which is a phenomenon consistent with our results. The potential at the GB increases by almost 100 meV with the NaF layer in the CZTSSe absorber, as shown in Figure 6(b). Moreover, the ratio of the positively charged potential at the GBs and the negatively charged potential at the IG increased with the NaF layer in the CZTSSe thin-films. The electronic properties of the material should be able to efficiently collect the electrons and the holes, helping in the separation around GBs where an NaF layer was added to the CZTSSe. However, it is essential to understand the defect states and the Na passivation through the GBs in CZTSSe layer in order to identify the influence of Na in the CZTSSe solar cells.