Polymorphs of Copper Zinc Tin Sulfide: Optoelectronic Properties and Detection Using Raman

Despite its growing popularity as a solar material, there is no comprehensive reference to aid with determining various polymorphs of copper zinc tin sulfide (CZTS) in the lab. Understanding the diverse electronic, optical and vibrational properties of these polymorphs is crucial to further enhance the conversion efficiency of CZTS solar cells. Here, findings are presented that suggest the effective hole mass in a monoclinic polymorph, that is likely to form along with the more commonly reported tetragonal variations, is about 10 times heavier than that of its tetragonal counterparts, thereby affecting key attributes such as carrier transport and concentration. Among tetragonal polymorphs, the so-called primitive mixed CuAu (PMCA) arrangement is likely to undergo a direct-indirect gap transition due to internal strains. Optical absorption between different polymorphs could also vary by up to 90% in the visible spectrum. Finally, accurate Raman footprint of each polymorph is predicted to help establish a definitive method for their detection. From a modeling perspective, Becke-Lee-Yang-Parr with spin-orbit coupling is shown to be a superior yet 100X faster alternative to hybrid functionals when applied to studying electronic structure of CZTS. Copper zinc tin sulfide samples are often labelled as Kesterite without in-depth characterization. In principle, other polymorphs with similar level of energy may also emerge. These polymorphs exhibit vastly different optical and transport properties. Raman spectroscopy could be an effective method for detecting of each polymorph.image (c) 2024 WILEY-VCH GmbH