Synthesis and Comparison of Properties of Single-Crystalline Bi13S18X2, and Fabrication of Solar Cells By Physical Vapor Deposition
Authors: Li Sen , Xu Linfeng , Feng Qi 
Published: December 2020

Introduction

As the global demand for energy is expected to grow rapidly, securing clean energy is essential for sustainable economic growth. Solar energy is the most promising clean energy source for the next-generation. One of the grand challenges in the field of photovoltaic materials is to find suitable materials which can produce high powder conversion efficiency, environmental-friendly and low cost. Recently, some V–VI–VII compounds (V = Sb, Bi; VI = O, S, Se; VII = Cl, Br, I) have attracted much attention as potential electronic materials. These compounds exhibit a variety of physical characteristics, including high photoconductivity, ferroelectric photovoltaic effects, piezoelectricity, electrooptical effects, and structural diversity.1 However, only a few of studies have been reported on Bi13S18X2 (X = Cl, Br, I) (BSX) compounds.2 Traditionally, these bismuth sulfohalogenides are synthesized using a vapor transport process and flux process under high temperature conditions over 600 oC. Herein, we reported a facile solvothermal process for the synthesis of bismuth sulfohalogenides Bi13S18X2 and systematic characterization of their photoelectric behavior as light absorbers for solar cells. A BSX-based solar cells, fabricated by PVD method, demonstrated a champion PCE of 1.15% for the first time, revealing the potential of BSX as a new type of light absorber material for solar cells.

Experimental

Typically, Bi13S18I2 (BSI) was synthesized by solvothermal reaction of BiI3 (2 mmol), MAI (MA = CH3NH3
+, 3 mmol), and a desired amount of CH4N2S in 5 mL of ethylene glycol solvent at desired temperature for 12 h. Similarly, Bi13S18Br2 (BSB) was synthesized by solvothermal reaction of BiBr3 (2 mmol), MABr (3 mmol), and a desired of CH4N2S under the same conditions. For Bi13S18Cl2 (BSC), BiCl3 (2 mmol) and a desired amount of CH4N2S were solvothermally treated in 5 mL of acetone solvent at desired temperature for 12 h. After reaction, the black solid products were filtered, washed with ethanol or acetone, and dried at 60 oC for 12 h.
The TiO2 electrode was prepared on a FTO glass surface to obtain a FTO/TiO2 electrode. A FTO/TiO2/BSX electrode was fabricated by PVD of BSX on the FTO/TiO2 electrode using a thermal evaporator under vacuum conditions at current of 18 A for 30 seconds, where 200 mg of BSX bulk was used and FTO/TiO2 electrode was placed 8.0 cm above the BSX source. The obtained FTO/TiO2/BSX electrode was heated at desired temperatures under N2 atmosphere. An electrolyte solution of I3
-/I- redox couple was used as the hole-transporting material to constructed FTO/TiO2/BSI/(I-/I3
- solution)/Pt solar cells.

Results and discussion

Fig. 1(A) shows the XRD patterns of solvothermally synthesized BSX samples at 200 oC for 12h, which are single phases with high crystallinity. BSX are black crystals with a rod-like particle morphology. UV–vis-NIR spectrum results show that BSX has strong light absorption spanning from visible to near IR spectrum; the corresponding bandgaps of BSI, BSB and BSC are 0.87, 0.89 and 0.90 eV, respectively. The result suggests the potential application of BSX as a promising light absorber for solar cells. Fig. 1(B) shows the XRD patterns of FTO/BSX electrodes after heat treatment at 250 oC for 12 under N2 atmosphere. The XRD patterns suggest that single phases of crystalline BSX film can be obtained by heat treatment the PVD-fabricated films. Fig. 1(C) shows the cross-sectional and top-view FE-SEM images of FTO/TiO2/BSI electrode. A high quality BSI film, with a thickness of 350 nm was obtained. Fig. 1(D) exhibits the J-V curves of the BSX-based solar cells. The preliminary solar cells study results demonstrate PCE values of 0.67, 1.15 and 0.81% for BSI, BSB and BSC-based solar cells, respectively, revealing the potential of the BSX light absorber for a new type of solar cells that can be fabricated by a simple PVD method.

References

H. Koc, S. Palaz, A. M. Mamedov and E. Ozbay, Ferroelectrics, 2017, 511, 22-34.

R. Groom, A. Jacobs, M. Cepeda, R. Drummey and S. E. Latturner, Chemistry of Materials, 2017, 29, 3314-3323.

Figure 1