Kesterite, Cu2ZnSn(S,Se)4, for earth abundant photovoltaics: can we make single phase thin films, and does it matter?

In order for thin film photovoltaic devices to contribute to future renewable energy generation they must consist of earth abundant materials and be cheap to manufacture. At the heart of thin film photovoltaic devices lies the p-type semiconductor absorber layer whose job is to convert incoming solar radiation into electrical charge carriers that are subsequently separated across the p-n junction. The leading earth abundant thin film absorber layer is Kesterite, Cu2Zn(Sn,Ge)(S,Se)4, which has produced laboratory scale power conversion efficiencies of around 12 %, close to the necessary efficiency for thinking about industrial production.
Here, the current status of Kesterite technology will be reviewed in terms of desired and achieved properties and the synthetic challenges which must be overcome to improve overall device efficiency. Amongst the key semiconductor properties for thin film absorber layers are band gap, doping density and minority carrier life. Additionally, attention to layer integrity and the presence of detrimental secondary phases is of equal importance.
The quaternary Kesterite has a rather small single phase region and multiple secondary phases are possible. Also, under normal synthesis conditions it releases two volatile species that must be taken into account for any controllable synthesis. Given these challenges, and with a perspective to depositing films on a meter squared basis, a close examination of Kesterite layer growth strategies is required.