Barium Substitution in Kesterite Cu2ZnSnS4: Cu2Zn1-xBaxSnS4 Quinary Alloy Thin Films for Efficient Solar Energy Harvesting

Metal chalcogenides such as Cu(In,Ga)(S,Se)(2) (CIGS), CdTe, and Cu2ZnSnS4 (CZTS) have empowered significant development in thin-film photovoltaic performance. Nevertheless, the toxicity, scarcity of raw elements in CdTe and CIGS technologies, and unavoidable cationic disorder realized in CZTS hinder the scalability to produce highly efficient solar cells. Here, Cu2Zn1-xBaxSnS4 (CZBTS) thin films with controllable phase composition and tunable band gap were fabricated on soda-lime glass substrates by a solution-based approach. The impact of replacing zinc with barium with different compositions in CZBTS with x = 0.0-1.0 on structural, morphological, optical, and electrical properties was investigated. It was found that with an increase in x from 0 to 1.0 the band gap energy of CZBTS increased from 1.48 to 1.92 eV with a small bowing constant confirming the significant miscibility of the alloying constituents in the crystal lattice. XRD and Raman studies demonstrate the inclusion of Ba into CZTS thin films and exhibit the structural phase transition from kesterite (tetragonal) to trigonal with increasing Ba content. SEM images reveal compact, homogeneous, and densely packed grain in all the fabricated films. In addition, the Hall measurement for all the materials exhibited p-type conduction behavior and indicates that the hole concentration of CZBTS films varies inversely with Ba content. Our results suggest that the CZBTS alloy is a potentially suitable material as a top cell absorber in a tandem device for efficient solar energy harvesting.