Direct Liquid Coating of Chalcopyrite Light-Absorbing Layers for Photovoltaic Devices

Liquid deposition approaches for chalcopyrite films used in thin-film photovoltaic devices are reviewed. Most of the targeted materials are based on Cu-In or Cu-In-Ga sulfides and selenides (i.e., CIS or CIGS, respectively), although recently related alternative materials based on abundant and nontoxic elements such as the kesterite Cu2ZnSnS4 have been actively investigated. By direct liquid coating we refer collectively to a variety of techniques characterized by distributing a liquid or a paste to the surface of a substrate, followed by necessary thermal/chemical treatments to achieve the desired phase. The deposition media used are solutions or particle (usually submicrometer size) suspensions of metal oxide, organic and inorganic compounds, including metal chalcogenide species. The deposition techniques used are mainly printing and spin-coating, although any standard process such as spraying, dip-coating or slit casting can be applied, In contrast to other widely investigated liquid-coating methods such as chemical bath and electrodeposition, in which relatively slower solid film growth occurs during the actual deposition step, the techniques discussed in this Microreview are mainly sequential, featuring rapid formation of a precursor film with well-defined metal stoichiometry. The precursor film is then transformed by a thermal treatment, generally in a chalcogen-containing atmosphere, to the final crystalline layer. This approach permits the use of low-cost and high-throughput equipment and the deployment of large-scale production facilities with lower capital investment. Although many of the methods discussed are under laboratory development, there are already industrial start-ups employing these promising methods for future large-scale photovoltaic production.