*Article* **Fabrication of Cu2ZnSnS4 (CZTS) Nanoparticle Inks for Growth of CZTS Films for Solar Cells**

**Xianfeng Zhang 1,\* , Engang Fu 2, Yuehui Wang <sup>1</sup> and Cheng Zhang <sup>1</sup>**


Received: 8 January 2019; Accepted: 24 February 2019; Published: 2 March 2019

**Abstract:** Cu2ZnSnS4 (CZTS) is a promising candidate material for photovoltaic applications; hence, ecofriendly methods are required to fabricate CZTS films. In this work, we fabricated CZTS nanocrystal inks by a wet ball milling method, with the use of only nontoxic solvents, followed by filtration. We performed centrifugation to screen the as-milled CZTS and obtain nanocrystals. The distribution of CZTS nanoparticles during centrifugation was examined and nanocrystal inks were obtained after the final centrifugal treatment. The as-fabricated CZTS nanocrystal inks were used to deposit CZTS precursors with precisely controlled CZTS films by a spin-coating method followed by a rapid high pressure sulfur annealing method. Both the grain growth and crystallinity of the CZTS films were promoted and the composition was adjusted from S poor to S-rich by the annealing. XRD and Raman characterization showed no secondary phases in the annealed film, the absence of the detrimental phases. A solar cell efficiency of 6.2% (open circuit voltage: *V*oc = 633.3 mV, short circuit current: *J*sc = 17.6 mA/cm2, and fill factor: FF = 55.8%) with an area of 0.2 cm2 was achieved based on the annealed CZTS film as the absorber layer.

**Keywords:** Cu2ZnSnS4 solar cell; ball milling; nano-ink; annealing

## **1. Introduction**

In recent years, kesterite Cu2ZnSnS4 (CZTS) and Cu2ZnSn(S, Se)4 (CZTSSe) solar cells have drawn attention because of their promise as an absorbing layer for applications in thin-film photovoltaics owing to its low cost, nontoxicity and earth abundance of its elemental components as well as an adjustable bandgap [1–3]. One advantage of CZTS over other kinds of chalcopyrite-related solar cells is its suitability for achieving high efficiency solar cells through nonvacuum fabrication methods. Furthermore, the world record conversion efficiency of CZTSSe solar cells is currently 12.6% [4] based on a hydrazine pure solution approach. There have been several reports on the fabrication of CZTS or CZTSSe solar cells. Both vacuum methods, such as sputtering [5], coevaporation [6], epitaxial methods [7], and nonvacuum methods [8–11], have been reported. Nonvacuum methods are lower in cost and more suitable for mass production than are vacuum methods. Among nonvacuum methods, the highest conversion efficiency CZTS solar cells are based on molecular precursor solutions or nanoparticle dispersions [12,13]. Although these kinds of fabrication methods are appealing because of their low complexity, low-cost, and scalability, such methods are complicated by the need for toxic solvents or metal–organic solutions that contain large amounts of organic contaminants, which induce cracking during the following annealing process [14,15]. The use of the toxic and unstable solvent hydrazine requires all processes for ink and film preparation to be performed under an inert atmosphere. As a result, it is difficult to adapt this approach to low-cost and large-scale solar cell fabrication.

In this work, we report a simple technique for fabricating CZTS nanoparticles ink by a wet ball milling method using nontoxic ethanol and 2-(2-ethoxyethoxy) ethanol as the solvents. A similar study has been reported by Woo, et al.; an efficiency of 7% was achieved [16]. However, the use of CZTS powder to fabricate CZTS film is expected to have the following benefits. (1) The fabrication process is simpler. (2) The growth of grains will be promoted because the grain boundary meltdown temperature is lowered. (3) Stoichiometric film compositions are easier to obtain because the chemical reactions are less complicated. The use of nontoxic solvents is more cost-effective and environment friendly, which is important for practical photovoltaic applications. The ink was used to fabricate CZTS thin films (precursors) by a spin-coating method, followed by annealing the precursor in a sulfur-rich atmosphere. The commercial CZTS powder was obtained from Mitsui Kinzoku, and detailed information of its characteristics is currently unavailable. The sulfur vapor not only prevents the formation of volatile Sn–S compounds but also supplies S atoms to make the CZTS films sulfur-rich, which is a requirement for high performance solar cells. The procedure for fabricating CZTS films from CZTS powder is reported in detail in this paper.

#### **2. Experiment Details**

#### *2.1. Sample Preparation*

Figure 1a–c illustrates the process for fabricating CZTS nanoparticle ink. In the ball milling system, a 1-mm ball, 50-μm ceramic balls, and CZTS powder were mixed together in the mill pot. A 5-mL portion of ethanol was added to improve the wet milling effect. Figure 1a shows a schematic diagram of the milling system. The milling pots were rotated along their own axis together with the base plate. The milling process was performed for 40 h. After ball milling, the whole mixture was strained through a filter screen to obtain particles smaller than 32 μm, and nontoxic ethanol and 2-(2-ethoxyethoxy) ethanol were used to wash the milling ball to increase nanoparticle recovery, as shown in Figure 1b. Through this procedure, the milling balls and large particles of CZTS (>32 μm) were removed whereas a mixture of relatively small CZTS particles (<32 μm) and the solvents were retained. We used 2-(2-ethoxyethoxy) ethanol as a dispersion agent to prevent coagulation of the nanoparticles, and ethanol was used to reduce the viscosity of the solvent and promote precipitation of large particles during the following centrifugation. The resulting solution was then ultrasonically processed to disperse the particles in the solvents for 1 h.

**Figure 1.** Fabrication process of Cu2ZnSnS4 (CZTS) nanoparticle ink: (**a**) schematic of ball milling machine, (**b**) filtration of CZTS particles smaller than 32 μm, (**c**) nanoparticle ink of CZTS.

The as-prepared mixture of CZTS and solvents was first centrifuged at a low speed 1500 rpm to remove particles over several μm in size. The precipitate was disposed of and the upper layer of the solution was decanted for further centrifugal treatment. The aforementioned processes were repeated three times at a higher speed of 6000 rpm and the nanoparticles were obtained. The nanoparticle ink was obtained with a concentration of 200 mg/mL by adjusting the quantity of ethanol. The nanoparticle ink was then used to fabricate CZTS precursors by spin-coating. Figure 2a shows a schematic diagram of the spin-coating system. The substrate was rotated at a speed of 2000 rpm and the CZTS ink was dripped on at a speed of 5 μL/min. The final CZTS precursor film showed a thickness of 1–1.5 μm. Finally, the precursors were annealed in a sulfur-rich atmosphere to improve the grain size and crystallinity. The sulfurization process was conducted by sealing the precursor and powdered sulfur into a vacuum quartz tube with a length of 15 cm, which was placed in the annealing furnace (FP410, Yamato Company, Tokyo, Japan), as shown in Figure 1b. The furnace was heated to 600 ◦C within 15 min and the vapor pressure of sulfur was approximately 0.1 atm. The annealing process was performed for 20 min after the system achieved 600 ◦C. Then the sample was allowed to cool to room temperature naturally.

**Figure 2.** Fabrication process of CZTS films: (**a**) fabrication of CZTS precursor by spin-coating and (**b**) the sulfur vapor annealing process.

A typical structure of a CZTS solar cell is shown in Figure 3. The as-grown CZTS film was used as the absorbing layer. A CdS layer with a thickness of 50 nm was fabricated by a chemical bath deposition method as the buffer layer. Intrinsic ZnO with a thickness of 100 nm and B-doped ZnO with a thickness of 400 nm were then sputtered as the window layer. To measure the performance of the solar cell, an Al grid was evaporated as the front electrode.


**Figure 3.** Structure of CZTS solar cell.
