(608d) Synthesis and Characterization of Cu3SbS4 Nanoparticles for Solution-Based Thin Film Solar Cells

The most efficient inorganic, thin film-based solar cells use CdTe or CuInGaSe₂ (CIGS) as absorber layers, both of which rely on toxic (Cd) and/or scarce elements (In, Te). The search for more sustainable processing of solar technologies has led to the identification of many Earth abundant and non-hazardous copper chalcogenide absorber materials. The Cu-Sb-S (CAS) system is promising in that it can be obtained in four major phases including CuSbS₂ (chalcostibite), Cu₁₂Sb₄S₁₃ (tetrahedrite), Cu₃SbS₃ (skinnerite), and Cu₃SbS₄ (famatinite), and the bandgaps for these compositions can vary from 0.5 to 2.0 eV.¹ The band-gap range and the high absorption coefficients (~ 10⁵ cm^-1 for Cu₃SbS₄ and 4x10⁵ cm^-1 for bulk CuSbS₂) make these materials of interest as a p-type absorber for thin-film solar cells.²

There have been several attempts to prepare CAS thin films to fabricate single junction solar cells, where fairly low efficiencies between 0.46 % for sputtered thin film Cu₃SbS₄ solar cells and 3.22 % for CuSbS₂ have been obtained.^3,4 One of the challenges for the Cu-Sb-S system is the poor thermal stability of CAS particles. Decomposition of the most promising phases, such as Cu₃SbS₄ and CuSbS₂, during thermal treatments can significantly change the films electrical and optical properties, which then limits the solar cell efficiency.⁵There are still many strategies that need to be investigated for the CAS absorber layers to reduce film defects and film decomposition, which should increase efficiencies for CAS-based solar cell devices.

Typically higher efficiencies are obtained for photovoltaic devices prepared using vacuum deposition techniques. This solid state approach can be difficult since the fine control over the final film composition is challenging for CAS materials, since many phases can coexist due to their nearly identical thermodynamic stability.¹ However, for CAS and Cu₂ZnSnS₄(CZTS) the highest efficiencies have been obtained for solution processed devices. For these systems, the use of nanoparticle inks to prepare thin film based solar cells may be a cost-effective approach for producing higher performance solar cells.

There are many challenges related to the solution-based synthesis of uniform CAS nanoparticles with controlled properties. We have used the hot-injection method to produce CAS, since the method provides a high level of supersaturation of reagents for a very short instant of time causing rapid formation of nuclei.⁶ This nucleation step and the use of proper capping agents lead to the formation of monodisperse and uniform nanoparticles. In this study, we have synthesized colloidal famatinite (Cu₃SbS₄) nanoparticles. X-ray diffraction results indicated that optimization of the synthesis conditions allows selective formation of the Cu₃SbS₄ phase without other impurities. Thin films were deposited by spin-coating nanoparticle inks, and their optical and electrical properties were studied. Optimization of post-deposition annealing conditions was performed to obtain large grain films of the desired phase. Characterization of the CAS nanoparticles and films was performed using scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and UV-Vis-NIR spectroscopy to evaluate effects of synthesis conditions on the nanoparticle properties and also to evaluate the optimization of films obtained by post-deposition annealing steps. The goal of these studies is to determine the most effective method to enhance CAS film optical and electrical properties to achieve high efficiency CAS thin film solar cells.

References

1. Ramasamy, K., et al., "Selective Nanocrystal Synthesis and Calculated Electronic Structure of All Four Phases of Copperâ??Antimonyâ??Sulfide," Chemistry of Materials, 26, pp. 2891â??2899 (2014).

2. Van Embden, J. and Tachibana, Y., "Synthesis and characterisation of famatinite copper antimony sulfide nanocrystals," Journal of Materials Chemistry, 22, pp. 11466-11469 (2012).

3. Franzer, N. D., et al., "Study of RF sputtered Cu₃SbS₄ thin-film solar cells," Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40^th, pp. 2326â??2328 ( 2014).

4. Banu, S., et al., "Fabrication and characterization of cost-efficient CuSbS₂ thin film solar cells using hybrid inks," Solar Energy Materials and Solar Cells, 151, pp. 14â??23 (2016).

5. Welch, A. W., et al., "Self-regulated growth and tunable properties of CuSbS₂ solar absorbers," Solar Energy Materials and Solar Cells, 132, pp. 499â??506 (2015).

6. Kwon, S. G. and Hyeon, T., "Formation Mechanisms of Uniform Nanocrystals via Hot-Injection and Heat-Up Methods," Small, 7, pp. 2685â??2702 (2011).