Elsevier

Current Applied Physics

Volume 21, January 2021, Pages 89-95
Current Applied Physics

Effect of precursor concentration and post-annealing temperature on (040) oriented tin sulfide thin films deposited on SLG/Mo substrates by spin coating

https://doi.org/10.1016/j.cap.2020.10.009Get rights and content

Abstract

SnS is a layered material that crystallizes in an orthorhombic structure. This hinders the formation of a dense, pinhole-free morphology. The present study demonstrated the deposition of SnS thin films on soda-lime glass (SLG) and SLG/Mo substrates by spin-coating approach. The developed films were subsequently applied for the fabrication of a thin-film solar cell. The effect of the annealing temperature on the structural, optical, and morphological properties of the deposited SnS films was analyzed. The precursor concentrations and the annealing temperature played a critical role in determining the phase composition and morphological characteristics of the SnS thin films. TFSC with SLG/Mo/SnS/CdS/i-ZnO/AZO/Al configuration was fabricated using the optimal precursor ratio, i.e., Sn:S = 1:1.2, and this device showed a photoconversion efficiency of 0.076%. The reasons for the poor performance of the device were addressed in detail, and the scope for future research to optimize the device performance was elucidated.

Introduction

Orthorhombic tin sulfide (α-SnS) is one of the most promising p-type chalcogenides for applications in thin-film devices such as thin-film solar cells (TFSCs) [1,2], water splitting [3], batteries [4,5], supercapacitors [6], and sensors [7]. The suitability of SnS for photovoltaic applications is attributed to its optimal optical bandgap of ~1.32 eV (indirect ~1.07 eV; direct ~1.3–1.43 eV) and high optical absorption coefficient of >104 cm−1 in the visible spectrum [[8], [9], [10], [11]]. Furthermore, SnS is an earth-abundant material with a lower toxicity as compared to that of copper indium gallium sulfur selenide (CIGSSe) and cadmium telluride (CdTe) absorbers [12,13]. The long-term stability of SnS is superior to that of organic and lead iodide perovskites that exhibit instability in contact with oxygen and water [2]. SnS-based TFSCs possess excellent absorber properties and a high theoretical Shockley–Queisser (S-Q) limit of ~32%. However, a maximum photo conversion efficiency (PCE) of only ~4.36% and ~4.8% has been achieved for the SnS-based TFSCs in the substrate (Glass/Mo/SnS/SnO2/Zn(O,S):N/ZnO/ITO) and superstrate (FTO/TiO2/SnS/Au) configuration, respectively [2,14]. Therefore, the PCE of SnS-based TFSCs is substantially lower than that of the highly efficient CIGSSe- (PCE ~23.35%) and copper zinc tin sulfo selenide (CZTSSe)- (PCE ~12.6%) based TFSCs [15,16].

The poor performance of the SnS TFSCs is attributed to the low quality of SnS thin films, the presence of unavoidable secondary phases (SnS2 and Sn2S3), non-ideal buffer layers, point defects, and grain boundaries [2,8,17,18]. The presence of multiple oxidation states, i.e., Sn2+ and Sn4+, inhibits the formation of single-phase SnS. Furthermore, the uncontrolled stoichiometry induces the generation of unwanted defects in SnS during the deposition. There has been extensive research on the fabrication of high-quality single-phase SnS absorber layers by several vacuum-based and chemical deposition techniques including atomic layer deposition (ALD) [2], vapor-transport deposition (VTD) [12,19], thermal evaporation [13], chemical vapor deposition (CVD) [20], and reactive sputtering [21]. ALD, VTD, and thermal evaporation have proven to be the most effective techniques for fabricating efficient (PCE > 3.0%) SnS TFSCs. The SnS absorbers that are deposited using these techniques typically possess a pure single phase and exhibit a (111) preferred orientation with a columnar or cube-like morphology. However, it has been reported that most of the chemical or solution-based techniques produce α-SnS films with a (040) preferred orientation [14]. It is believed that structural changes in α-SnS with the different deposition techniques originate from the varied morphologies of α-SnS and the diverse kinetic deposition routes. There has been limited research on the structural and morphological inhomogeneities in the SnS thin films, which arise owing to the different deposition techniques. Furthermore, the morphological variation owing to the different deposition techniques and annealing temperatures significantly affects the texture coefficient, preferred orientation, and surface energy of the film, thereby affecting the performance of the solar cell devices [19]. Recently, solution-processed techniques such as spin coating and spray pyrolysis have attracted significant attention for the fabrication of SnS absorbers. Ding et al. and Yun et al. achieved the highest PCE of 3% (in 2018) and 4.8% (in 2019), respectively, for solution-processed SnS absorbers [14,22]. Furthermore, the morphologies of the SnS absorbers fabricated by solution-processed methods are different than that of the SnS absorbers fabricated by physical deposition techniques. Therefore, conducting a detailed research on solution-processed SnS thin films and understand their properties is crucial to fabricate highly efficient TFSCs.

In this study, SnS thin films were prepared by spin coating. Spin coating results in the development of homogeneous thin films with easily controllable thicknesses [23]. Here, we conducted a detailed investigation of the structural, morphological, and optical properties of the spin-coated SnS thin films on soda-lime glass (SLG) and SLG/Mo substrates at different post-annealing temperatures and using different concentrations of thioacetamide (TAA) in the precursor solution. The observed outcomes were significantly different for the different substrates, precursor concentrations, and post-annealing conditions, indicating diverse effects on the properties of the SnS thin films. The ratio of Sn and S in the precursor solution was found to be the controlling parameter for the film thickness. Although the films prepared by physical deposition techniques show (120) and (021) orientations, the spin-coated SnS thin films on the Mo substrates showed an (040) orientation. The films that were deposited using a precursor ratio of 1:1.2 (Sn:S) exhibited the formation of a highly pure SnS phase with a horizontally aligned compact plate-like morphology. These characteristics are suitable for the fabrication of highly efficient TFSCs. A TFSC with a structure of SLG/Mo/SnS/CdS/i-ZnO/AZO/Al was fabricated using the optimal precursor ratio (Sn:S), and the device exhibited a PCE of 0.076%.

Section snippets

Deposition of SnS thin-films

SnS thin films were formed on SLG (1 × 1 in, iTASCO, Taewon Science Co., Ltd., thickness = 0.7 mm) and 1-μm-thick SLG/Mo substrates (1 × 1 in, Abrisa Technologies, thickness = 0.7 mm) by spin coating. Prior to the deposition, the glass substrates were ultrasonically cleaned for 15 min in soap solution, deionized (DI) water, acetone, ethanol, and isopropanol (IPA), in sequence. The Mo substrates were cleaned using IPA and DI water for 15 min and dried under an N2 flow. The glass substrates were

Results and discussion

The plan-view and cross-sectional FE-SEM images of the SnS films, including the films that were deposited using different TAA concentrations, were observed, and the surface morphology and thickness of the as-deposited and annealed SnS thin films were analyzed. Fig. 2(a)−(d) show the plan-view FE-SEM images of the as-deposited (or pre-annealed) and the post-annealed films (300 °C, 350 °C, and 400 °C) on the SLG/Mo substrate. The as-deposited and annealed films exhibited randomly oriented

Conclusion

In this study, we demonstrated the effect of the spin-coating parameters on the structural, optical, and morphological properties of SnS films, with a highly preferred (040) orientation, deposited on the SLG/Mo substrates. All the deposited SnS thin films exhibited a plate-like surface morphology. The film that was deposited using a precursor ratio (Sn:S) of 1:1.2 showed the formation of a highly pure SnS phase with a horizontally aligned plate-like structure after annealing at 300 °C. Our

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government (2018R1A2B6002268) and the Human Resources Development (20164030201310) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea.

References (33)

  • P. Sinsermsuksakul et al.

    Overcoming efficiency limitations of SnS-based solar cells

    Adv. Energy Mater.

    (2014)
  • M.F. Afsar et al.

    Two-dimensional SnS nanoflakes: synthesis and application to acetone and alcohol sensors

    RSC Adv.

    (2017)
  • J. Vidal et al.

    Band-structure, optical properties, and defect physics of the photovoltaic semiconductor SnS

    Appl. Phys. Lett.

    (2012)
  • J. Vidal et al.

    Structural and electronic modification of photovoltaic SnS by alloying

    J. Appl. Phys.

    (2014)
  • L.A. Burton et al.

    Phase stability of the earth-abundant tin sulfides SnS, SnS2, and Sn2S3

    J. Phys. Chem. C

    (2012)
  • D. Lim et al.

    Kinetically controlled growth of phase-pure SnS absorbers for thin film solar cells: achieving efficiency near 3% with long-term stability using an SnS/CdS heterojunction

    Adv. Energy Mater.

    (2018)
  • Cited by (2)

    View full text