Solar energy is a renewable energy source and is increasingly used as an alternative to fossil fuels. To meet the growing demand for solar cell devices, the development of high-performance light-absorbing materials is highly desirable. Among the available materials, tin monosulfide (SnS) has attracted significant research attention owing to its abundance and excellent properties. The existing SnS solar cells mainly comprise of p-n heterojunction with p-type SnS and n-type contact layers.. Unfortunately, they exhibit a significantly lower conversion efficiency of 5% at maximum, which is much lower than the theoretical expectation for the p-n homojunction SnS solar cell of 25%. The lower conversion efficiency, which can be attributed to lower photovoltage due to unsuitable band offset and defect formation at the interface, has hindered their applications in fabricating high-performance solar cells.
To overcome the issues, homojunction SnS solar cells comprising both n- and p-type SnS could realize high conversion efficiencies. This, however, requires the use of n-type SnS, which has remained a challenge to obtain. Previously, fabrications of n-type SnS have been tried via three routes, among which aliovalent doping at the sulfur site with halogens has been identified as a sole and promising practical approach for obtaining n-type SnS crystals with great application potential. Consequently, and unlike polycrystalline materials, single SnS crystals are better for achieving well-defined p-n interfaces. Nevertheless, the reported single crystals are smaller in size and unsuitable for fabrication using various thin-film deposition techniques. Therefore, much larger single crystals of n-type SnS are suitable for developing p-n homojunction devices because several larger p-n homojunctions can be easily fabricated on single n-type crystals using the available deposition techniques.
To this note, Tohoku University scientists: Dr. Sakiko Kawanishi and Dr. Issei Suzuki led the team and synthesized large single crystals of n-type SnS with an exposed (100)-plane and diameter greater than 16 mm. in particular, the team explored the effects of halogen addition to the flux. Their research work is currently published in the research journal, Crystal Growth and Design.
In their approach, the research team fabricate two different single crystals from a molten Sn-based flux. One involved growing of n-type Cl-doped and Br-doped SnS crystals, while the other involved fabricating single crystals of undoped SnS to study the effects of adding halogen to the flux. The resulting crystals were characterized using various methods such as X-ray diffraction and electronic measurements to determine their electronic and optical properties as well as crystalline qualities.
Results showed that the grown crystals reached a maximum diameter and thickness of 24 mm and 1 mm for Br-doped SnS and 16 mm and 0.7 mm for Cl-doped SnS, respectively. Unlike the small and lamellar undoped SnS, Cl-doped and Br-doped single crystals reported significantly improved lateral and vertical growth, attributed to the addition of SnCl2 and SnBr2 halogen sources to the flux. Similarly, a high crystal quality was observed as indicated by the X-ray rocking curves and back-reflection Laue patterns. Furthermore, both the Cl- and Br-doped SnS single crystals exhibited degenerate n-type conductivities with relatively high electrical conductivity at room temperature. The same was confirmed with the photoelectron spectroscopy measurements.
In summary, the study reported the successful growth of large single crystals on n-type SnS from Cl and Br added flux. The addition of the halogen sources enhanced the growth of the larger crystals both in the lateral direction along the (100)-plane and vertical direction. The maximum sizes of the crystals also increased significantly. Based on the electronic structure and electrical properties, the obtained crystals were confirmed to be degenerated n-type conductors. In a statement to Advances in Engineering, the authors said that larger n-type SnS crystals are potential candidates for the fabrication of SnS homojunction solar cells with a high conversion efficiency
Kawanishi, S., Suzuki, I., Ohsawa, T., Ohashi, N., Shibata, H., & Omata, T. (2020). Growth of Large Single Crystals of n-Type SnS from Halogen-Added Sn Flux. Crystal Growth & Design, 20(9), 5931-5939.