Employing Successive Ionic Layer Adsorption and Reaction (SILAR) Method on the Fabrication of Cu3BiS3-Semiconductor-Sensitized Solar Cells

Successive ionic layer adsorption and reaction (SILAR) method is a modified version of chemical bath deposition (CDB) that serves as a low-cost and convenient on the production of ternary metal chalcogenides. This research reported the utilization of SILAR method on the fabrication of Cu3BiS3 semiconductor-sensitized solar cells. The concentration of bismuth and copper precursor were varied, namely 0.03 M and 0.1 M, whereas the precursor of sulfide was varied in the concentration of 0.02 M and 0.05 M. The variation of SILAR cycles was employed to investigate the most appropriate cycle numbers in producing Cu3BiS3, in particular 3-9 cycles, 5-15 cycles, and 6-6 cycle with the immersing time of 20 s for each. The results show that there were only two suitable peaks appeared for 3-9 cycles and 6-6 cycles, while 5-15 cycles provide the more preferable XRD patterns with the power conversion efficiency of 0.02% (Jsc of 1.75 mA/cm2; Voc of 0.04 V; FF of 29.65%). It can be said that SILAR method with higher number of cycles can be employed to fabricate Cu3BiS3; however, smaller PCE came from inappropriate structure alignment between Cu3BiS3 and metal oxide layer.


Introduction
Among the methods used to synthesize ternary metal chalcogenides, successive ionic layer adsorption and reaction (SILAR) method attracts researchers' attention recently due to its several benefits, such as its serving as an inexpensive and simple production for large area of deposition, its capacity to control the particle size of ternary metal chalcogenides by varying deposition time and number of coating cycles, its easiness to be operated at room temperature, and its flexibility in the use of the substrate [1], [2]. There are several parameters which play crucial rule on the deposition process, such as immersing time, concentration of precursor solutions, and the number of cycles [1], [3]. SILAR method is successfully employed for deposition of ternary metal chalcogenides into metal oxide semiconductor to obtain tunable quantum dots, resulting large value of power energy conversion (PCE) [4]- [7].
One of ternary metal chalcogenides that has potential application on semiconductor-sensitized solar cells is copper bismuth sulfide (Cu-Bi-S) [8], [9], which was successfully used in liquid junction quantum dots-sensitized solar cells (QD-SSCs) in the form of CuBiS2 [10] and Cu3BiS3 [11]. However, both researches were done by using chemical bath deposition (CBD) and solvothermal route, respectively; therefore, the use of SILAR method to fabricate Cu3BiS3 should yield distinct results due to its capability to tune the particle size of deposited materials.
Based on the above explanation, this study aimed to utilize SILAR method on the fabrication of Cu3BiS3 semiconductor-sensitized solar cells. X-Ray diffraction (XRD) pattern was provided to analyze the structure of resulted materials and PCE was obtained to figure out the effectiveness of SILAR method on the production of the assembled solar cells.

Method
After obtaining the metal oxide (TiO2) layer following the procedure of Rahayu et al. [4], the sample was immersed in the solution of bismuth nitrate precursor (varied in concentration of 0.03 M and 0.1 M) for 20 seconds, followed by rinsing by using DI-water for 25 s, dried at room temperature, and dipped into the solution of sodium sulfate precursor (varied in concentration of 0.02 M and 0.05 M) for 20 seconds, followed by rinsing in methanol and drying step. This process was called as one cycle of obtaining Bi-S particles into metal oxide layer. Subsequently, the sample was dipped into the solution of copper sulfide precursor (varied in concentration of 0.03 M and 0.1 M) for 30 s, followed by rinsing in the DI-water and drying at room temperature, and immersed into sodium sulfate precursor (varied in concentration of 0.02 M and 0.05 M) for 20 seconds followed by rinsing in methanol and drying step. This process was called as one cycle of obtaining Cu-S particles into metal oxide layer. The final step was annealing at 350⁰ C for 50 minutes in tube furnace. The resulted photoanode was then assembled with Pt-counter electrode using parafilm spacer and filled with polyiodide electrolyte to undergo the photochemical process. Figure 1 shows the X-ray diffraction pattern of Cu3BiS3 in the metal oxides (TiO2) layer resulted by undergoing 3 cycles for Bi-S deposition and 9 cycles of Cu-S deposition. As shown, there were only two appropriate peaks of Cu3BiS3 resulted from SILAR method compared to Cu3BiS3 produced using CBD method [12], which has cell constants a=7.697 Å, b=10.388 Å, c=6.712 Å, having the same agreement with JCPDS card (No. 43-1479) [13]. This occurred due to the lack of deposition layer of both Bi-S and Cu-S which was obtained from 3-9 cycles.

Figure 1. XRD pattern of Cu3BiS3 produced using 3-9 SILAR cycles and CBD
However, when the number of cycles increased, the respectable peaks for Cu3BiS3 appeared more than those of 3-9 cycles as given in figure 2. It shows that the more SILAR cycles, the more peaks appeared. This study also investigated different concentration used and order of the process as shown in figure 3. Since the concentration of precursor was increased, from 0.02 M to 0.05 M for sodium sulfate precursor and 0.03 M to 0.1 M for bismuth and copper precursor, the cycles were decreased to 6 cycles for Bi-S deposition and 6 cycles for Cu-S deposition. As seen, the appropriate peaks only existed for two positions. It gives information that for fabricating Cu3BiS3 using SILAR method needs large number of cycles.  Figure 3. XRD pattern of Cu3BiS3 produced using 6-6 SILAR cycles and CBD.

Photovoltaic Measurement Results
Photovoltaic results was yielded by measuring PCE using Keithley 2400 Source Meter with Oriel 150W Xe lamp using band-pass filter simulating the AM 1.5 solar spectrum and evaluate by using the formula (1) [14]

PCE = (FF × Isc × Voc)/Pin
(1) where Isc is the short-circuit current, Voc is the open-circuit voltage, FF is the fill factor, Pin is the incident light power. FF is a crucial parameter to indicate the effective output power for a solar cell as shown in the formula (2) where Popt is the maximum output power. As given in Table 1, the cycle numbers was proved to have a huge effect of PCE; it can be seen that 5-15 cycles gave the most preferable PCE of 0.02%. However, FF obtained was very low compared to 6-6 cycles. The large value of PCE of 5-15 cycles was suggested to come from the appropriate crystallinity shown in XRD pattern in Figure 2. Nevertheless, this result was much smaller than those of Yin and Jia [11] which can yield PCE of 1.281% using TiO2 nanorod arrays as their metal oxide. This could occur due to different structure of metal oxide used; as mentioned that this study employed mesoporous TiO2 as the structure of metal oxide, the surface to volume ratio is different from that of nanorod, resulting poor chemical reaction with the electrolyte.

Conclusion
To conclude, SILAR method was successfully employed to produce Cu3BiS3 Semiconductor-Sensitized Solar Cells with the PCE of 0.02%. The most suitable cycle number to produce such PCE was 5 cycles for Bi-S deposition and 15 cycles for Cu-S deposition, which also gave the best XRD pattern. The smaller PCE resulted from this study came from a mismatch structure of metal oxides.