The path towards a high-performance solution-processed kesterite solar cell

Despite the promise of thin-film Cu(In,Ga)(S,Se)2 (CIGSSe) chalcopyrite and CdTe photovoltaic technologies with respect to reducing cost per watt of solar energy conversion, these approaches rely on elements that are either costly and/or rare in the earthʹs crust (e.g., In, Ga, Te) or that present toxicity issues (e.g., Cd), thereby potentially limiting these technologies in terms of future cost reduction and production growth. In order to develop a photovoltaic technology that is truly compatible with terawatt deployment, it is desirable to consider material systems that employ less toxic and lower cost elements, while maintaining the advantages of the chalcopyrite and CdTe materials with respect to appropriate direct band gap tunability over the solar spectrum, high device performance (e.g., >10% power conversion efficiency) and compatibility with low-cost manufacturing. In this review, the development of kesterite-based Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells, in which the indium and gallium from CIGSSe are replaced by the readily available elements zinc and tin, will be reviewed. While vacuum-deposited devices have enabled optimization within the compositional phase space and yielded selenium-free CZTS device efficiencies of as high as 6.8%, more recently a liquid-based approach has been described that has enabled deposition of CZTSSe devices with power conversion efficiency of 9.7%, bringing the kesterite-based technology into a range of potential commercial interest. Electrical characterization studies on these high-performance CZTSSe cells reveal some of the key loss mechanisms (e.g., dominant interface recombination, high series resistance and low minority carrier lifetime) that limit the cell performance. Further elucidation of these mechanisms, as well as building an understanding of long-term device stability, are required to help propel this relatively new technology forward.