Kesterite: New Progress Toward Earth-Abundant Thin-Film Photovoltaic

Abstract Among the emerging thin-film photovoltaic (PV) materials formed by earth-abundant and nontoxic elements, the so-called kesterite (Cu2ZnSn(S,Se)4—CZTSSe; Cu2ZnSnS4—CZTS; Cu2ZnSnSe4—CZTSe) has become in the last years the most relevant and promising technology. Similar in many aspects to the most mature chalcopyrite (Cu(In,Ga)Se2—CIGS), first innovations on kesterite have been driven by the knowledge gathered from CIGS, in a kind of “copy-paste” strategy, which has contributed to large extent to a fast initial progress. Nevertheless, CZTSSe is probably the most complex inorganic thin-film PV material developed up to now, and currently researchers are facing the challenge of understanding its peculiarities and complexities, looking for technological solutions that will help overcome the current limitations of this technology, where clearly the low open circuit voltage (or large voltage deficit) of the solar cell devices remains their Achilles heel. This chapter reviews some of the most relevant progresses of kesterite in the last years, with the objective of identifying the main possible reasons for the voltage deficit. The chapter is structured in five parts, including an introduction highlighting the main characteristics of kesterite, and dividing the problematics depending on the architecture of the solar cells into front contact, absorber, and back contact issues. A detailed review and analysis of each part of the solar cell is presented in the next three sections. Finally, a section of conclusions and future perspectives is developed with the aim of inspiring new strategies and developments to solve the high voltage deficit challenge, moving toward devices with conversion efficiencies beyond 20%.