Aqueous Synthesis of High-Quality Cu₂ZnSnS₄ Nanocrystals and Their Thermal Annealing Characteristics

Abstract

Copper zinc tin sulfide (CZTS) nanocrystal inks are promising candidates for the development of cheap, efficient, scalable, and nontoxic photovoltaic (PV) devices. However, optimization of the synthetic chemistry to achieve these goals remains a key challenge. Herein we describe a single-step, aqueous-based synthesis that yields high-quality CZTS nanocrystal inks while also minimizing residual organic impurities. By exploiting simultaneous redox and crystal formation reactions, square-platelet-like CZTS nanocrystals stabilized by SnS and thiourea are produced. The CZTS synthesis is optimized by using a combination of inductively coupled plasma analysis, Raman spectroscopy, Fourier transform infrared spectroscopy, and synchrotron powder X-ray diffraction to assess the versatility of the synthesis and identify suitable composition ranges for achieving phase-pure CZTS. It is found that mild heat treatment between 185 and 220 °C is most suitable for achieving this because this temperature range is sufficiently high to thermalize existing ligands and ink additives while minimizing tin loss, which is problematic at higher temperatures. The low temperatures required to process these nanocrystal inks to give CZTS thin films are readily amenable to production-scale processes.

“Green” Aqueous Synthesis and Advanced Spectral Characterization of Size-Selected Cu2ZnSnS4 Nanocrystal Inks

“…However, the examples of aqueous syntheses of CZTS NCs are quite rare 53,54 typically yielding strongly agglomerated NCs and providing no reliable means for the control of the NC size and size distribution. Only recently aqueous syntheses with Sn 2 S 6 4− complex anions as capping ligands were reported to produce stable and uniform CZTS NCs 55,56 .…”

Aqueous-Based Binary Sulfide Nanoparticle Inks for Cu2ZnSnS4 Thin Films Stabilized with Tin(IV) Chalcogenide Complexes

“…In their work, however, they prepared the tin chalcogenide complex by dissolving metallic tin and elemental sulfur in ammonium sulfide solution, a rather inefficient method prone to poor reproducibility due to oxidation of metallic tin without the use of a glovebox. Ritchie et al showed that the redox process to form the tin chalcogenide complexes had poor stability, which may complicate larger scale applications [29]. Instead, a dissociative process can be used to make the tin chalcogenide complex as per the method of Krebs involving dissolution of tin(IV) sulfide into aqueous ammonium sulfide solution [25] adopted for CZTS film fabrication by Ritchie et al [29].…”

“…Ritchie et al showed that the redox process to form the tin chalcogenide complexes had poor stability, which may complicate larger scale applications [29]. Instead, a dissociative process can be used to make the tin chalcogenide complex as per the method of Krebs involving dissolution of tin(IV) sulfide into aqueous ammonium sulfide solution [25] adopted for CZTS film fabrication by Ritchie et al [29]. In both of these CZTS aqueous nanoink works [28,29], thiourea was used as a supplementary complexing-stabilizing agent [30,31,32] for the soluble copper and zinc salts.…”

“…An alternative method to synthesize CZTS is to introduce copper and zinc as binary sulfide nanoparticles (e.g., Cu x S, ZnS) instead of soluble salts that the “one-pot” methods utilize [28,29], which can be blended with SnS 2 nanoparticles and/or [Sn 2 S 6 ] 4− complex-bearing ammonium sulfide solution, offering a facile method of adjusting the elemental composition of CZTS, a critical parameter of its photovoltaic performance [34,35]. This method of synthesis also removes the need to use the toxic thiourea complexing agent.…”

“…This work demonstrates the robust nature of binary sulfide nanocrystal conversion to kesterite CZTS crystalline films using an aqueous thiostannate(IV)-stabilized nanoink method with minimal secondary phases and impurities. The binary nanoparticle mixing with the thiostannate(IV) and aqueous ammonium sulfide solution allows the formulation of highly concentrated inks compared to in situ formation of nanoparticles from thiourea-metal solutions in the one-pot route [28,29], achieving a 5-10 times higher yield. The concentration of solid weight fraction of suspended nanoparticles and dilution of ammonium sulfide in water are further modulated in order to enhance the stability of the nanoink and consequently the film microstructure, morphology and density.…”