Abstract
Cu2ZnSnS4 (CZTS) is a promising semiconductor material for photovoltaic applications, with excellent optical and electronic properties while boasting a nontoxic, inexpensive, and abundant elemental composition. Previous high-quality CZTS thin films often required either vacuum-based deposition processes or the use of organic ligands/solvents for ink formulation, which are associated with various issues regarding performance or economic feasibility. To address these issues, an alternative method for depositing CZTS thin films using an aqueous-based nanoparticle suspension is demonstrated in this work. Nanoparticles of constituent binary sulfides (CuxS and ZnS) are stabilized in an ink using tin(IV)-based, metal chalcogenide complexes such as [Sn2S6]4−. This research paper provides a systematic study of the nanoparticle synthesis and ink formulation via the enabling role of the tin chalcogenide complexing power, the deposition of high-quality CZTS thin films via spin coating and annealing under sulfur vapor atmosphere, their structural characterization in terms of nanocrystal phase, morphology, microstructure, and densification, and their resultant optoelectronic properties.
Highlights
Cu2ZnSnS4 (CZTS) in the kesterite crystal structure [1] is a promising absorber layer material for next-generation thin film solar cells, featuring desirable optoelectronic properties and a composition made of earth-abundant and nontoxic elements, the latter in contrast to current commercialized thin film photovoltaics such as CuIn1−x GaxSe2 (CIGS) and CdTe [2,3,4]
A reduction in the number of layers deposited via spin coating is recommended for more concentrated inks
A novel, nanoparticle-based process was developed for the deposition of CZTS absorber layers, making use of binary metal sulfides stabilized in a thiourea-free, aqueous nanoink with tin(IV) chalcogenide complexes
Summary
Cu2ZnSnS4 (CZTS) in the kesterite crystal structure [1] is a promising absorber layer material for next-generation thin film solar cells, featuring desirable optoelectronic properties and a composition made of earth-abundant and nontoxic elements, the latter in contrast to current commercialized thin film photovoltaics such as CuIn1−x GaxSe2 (CIGS) and CdTe [2,3,4]. 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., CuxS, ZnS) instead of soluble salts that the “one-pot” methods utilize [28,29], which can be blended with SnS2 nanoparticles and/or [Sn2S6]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 removes the need to use the toxic thiourea complexing agent. Optical properties are characterized, showing good absorption coefficient and bandgap for CZTS films prepared using this technique
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