Non-uniform distribution of sulfur vapor and its influence on Cu2ZnSnS4 thin film solar cells
Introduction
Cu2ZnSnS4 (CZTS) thin film solar cell has attracted wide interest because of its excellent properties which facilitate the cost-effective, large-scale and eco-friendly production (Katagiri, 2005, Kushwaha, 2017). Up until now, the pure sulfide CZTS solar cells have achieved a record efficiency of 11% by heterojunction heat treatment (Green et al., 2018, Yan et al., 2018). For those CZTS thin film solar cells with efficiency over 8%, a classical two-step method was widely used, which required a precursor first and then processed with sulfurization (Shin et al., 2013, Sun et al., 2016, Yan et al., 2018). The sulfurization process plays an important role in the performance of solar cell devices. Researchers have done a lot of work on the sulfurization process to improve device efficiency. Sulfurization temperature of 550–590 °C was commonly used for the fabrication of high efficiency CZTS thin film solar cells (Emrani et al., 2013, Jiang et al., 2014, Shin et al., 2013). Low pressure during sulfurization process was suggested to improve the device performance (Zhang et al., 2014). A static sulfurization process provided a higher sulfur partial pressure which could suppress the loss of Sn, the generation of secondary phases (Liu et al., 2016).
In order to achieve cost-effective and large-scale production, the sulfurization process needs to be carried out in a wide area, therefore, achieving uniform sulfurization is critical to device performance. However, few studies have reported on the relations between the diffusion uniformity of sulfur vapor in a closed chamber and the performance of CZTS thin film solar cells.
For all we know, sulfur powders was often used as the initial sulfur source in the sulfurization process (He et al., 2015, Jiang et al., 2014, Li et al., 2017, Liu et al., 2016, Sun et al., 2016, Yan et al., 2018, Zhang et al., 2014). Sulfur powers were heated in furnace to transform solid state to gas state. Sulfur vapor consists of different gas molecules such as S2, S3, S4, S5, S6, S7, S8 and so on (Berkowitz and Marquart, 1963). Those sulfur molecules have different molecular masses and different diffusion velocities. When the sulfur vapor diffuses in a sealed chamber, the sulfur vapor and the sulfur partial pressure presents a non-uniform distribution along the chamber which could influence the sulfurization process. Another factor makes the distribution more complex: with a ramping rate of 10 °C/min in the chamber, the division and integration between parent ions and fragments proceed at different temperatures. Reaction (2) symbolically points out one of these reactions (Rau et al., 1973). The dynamic equilibrium of sulfur species is hard to achieve in a temperature ramping zone.
To investigate the influence of non-uniform distribution of sulfur vapor on the properties of CZTS thin films and the device performance of CZTS thin film solar cells, the experiment is described and analyzed as below. Fig. 1 shows the schematic diagram of the sulfurization furnace in which the sulfur source is placed in the left side and several CZTS samples are sequentially placed in the right side. The chamber of the sulfurization process is sealed and kept at a static state. As the furnace heats up, the sulfur begins to melt, and the sulfur vapor starts to diffuse in the chamber. Therefore, reaction (1) will start. CZTS precursors will react with the sulfur molecules. Under different sulfur partial pressure, the CZTS thin films and the solar cells based on the thin films will demonstrate different aspects.
The results have shown that there is an optimal position where could improve the crystallinity, reduce the formation of secondary phases, and improve the efficiency of solar cells. But the best efficiency of solar cells was restrained at 3.49%, because of the Sn loss which was revealed from the EQE result.
Section snippets
Experimental details
The CZTS precursor layers were deposited on Mo-coated soda lime glass by co-sputtering of Cu, ZnS and SnS2 targets. The working powers for Cu, ZnS and SnS2 targets were 40 W, 80 W, and 92 W respectively, aiming at a Cu/(Zn + Sn) ratio of 0.85 and Zn/Sn ratio of 1.1. The targets have a diameter of 3 in. and 4 N purity. At the time of sputtering, a quartz crucible containing sulfur powders was heated to 135 °C for evaporation. The thickness of the precursors was fixed at 1 μm by adjusting the
Results and discussion
Fig. 3 shows the XRD patterns of CZTS thin films sulfurized at different positions from S1 to S7. From S1 to S7, peaks located at 28.53°, 29.75°, 33.11°, 47.42° and 56.19° are corresponding to (1 1 2), (1 0 3), (2 0 0), (2 2 0) and (3 1 2) planes of kesterite CZTS (JCPDS 26–0515) respectively. Meanwhile, the intensity of these peaks gets stronger and the full width at half maximum of (1 1 2) become narrower, indicating that the crystallinity of CZTS thin films is improved. What can be confirmed
Conclusion
For the sulfurization in a long diffusion range, the sulfur vapor showed a non-uniform distribution which had a significant influence on the properties of the CZTS thin film solar cells and the device performance of solar cells. In the position close to the sulfur source the grain size of CZTS thin films was quite small and the associated PCE of solar cells was limited. With the extension of distance between sulfur source and CZTS sample, the grain size increased and the device efficiency of
Acknowledgments
This work was financially supported by the National Science Foundation of China (Grant no. 61275058, 51772019). It was also supported by the Key Laboratory of Luminescence and Optical Information of China in Beijing Jiaotong University.
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