History and prospects of the physical synthesis of kesterite for photovoltaic applications

Through the years, kesterite Cu2ZnSn(S,Se)4 thin films have been fabricated using various physical synthesis technologies. Among them, sequential stacking or co-sputtering of precursors as well as sequential or co- evaporation of elemental sources have led to the achievement of high-efficient solar cells. In this work, we provide an up-to-date overview of the physical vapor technologies used to synthesize CZT(S,Se) thin films as absorber layers for photovoltaic applications. This review starts with an enumeration of the well-known pro- cesses used for the growth of CZT(S,Se) absorber layers. A historical description of the main issues limiting the efficiency and of the experimental pathways designed to prevent or limit these issues is presented and discussed. The discussion is articulated through the transition from a one-step synthesis process consisting in a high temperature deposition to a two-step synthesis process composed of (i) the deposition of a precursor film and (ii) a thermal annealing under S (sulfurization) or Se (selenization) atmosphere. To complete the de- scription, morphological properties, such as void formation and thin films blistering are discussed in relation with the synthesis protocols used. In addition, alternative methods for kesterite layers deposition are devel- oped such as pulsed laser deposition of composite targets and monograin growth by the molten salt method, both leading to a significant progress in device efficiency. A final discussion is dedicated to the description of promising process steps aiming at further improvements of solar cell efficiencies, such as alkali doping and alloying of kesterite for bandgap grading. As a result, this work highlights the growing increase of the kesterite-based solar cell efficiencies achieved over the recent years and offers a broad and updated overview of the physical vapor deposition technologies currently applied to the fabrication of performant CZT(S,Se) absorber layers.