The two main types of thin film solar cells are the CdTe i.e. Cadmium Telluride, and the CIGS category namely Copper Indium Gallium, Sulphide. But there are drawbacks to both these technologies. While Cadmium is regarded as hugely toxic, Indium and Tellurium are considered as scarce elements in nature and very volatile in cost. In addition, one can note that Tellurium is as scarce as gold. Indium is not that scarce but has a demand for multiple other uses. One can also note the price volatility of both elements between 2003 and 2009.
A new alternative these days is the thin film solar based on a CZTS semiconductor. The constituents of this semiconductor are copper, zinc, tin and sulfur, that are not scarce but abundant in the earth's crust and don’t have the issue of toxicity which happens to be a drawback with the CdTe solar cells. A plus point with CZTS is also linked to its near ideal properties for solar photovoltaics. In addition, it happens to be a strong absorber and has a band gap of around 1.4eV.
Recently, quaternary compound Cu2ZnSn(S/Se)4 has been studied as a potential photovoltaic material, that has similarities with CIGS (Cu(In,Ga)Se2) in material properties and the relative availability of raw materials. CZTS is a compound semiconductor in the form (I)2(II)(IV)(VI)4 with a high absorption coefficient (>104 cm−1) and a band gap of around 1.45 eV. In addition, theoretically, efficiencies of around 32% have been calculated for CZTS thin film solar cells.
Basic Structure of CZTS
Molybdenum is very stable in harsh reactive conditions and temperatures hence a thin film of it having thickness of 500–700 nm is deposited on glass substrate and acts as a back contact.
The absorber layer, p-type CZTS thin film is deposited on Mo layer, having thickness in the range of 1-2 micrometers.
To form a p-n junction, CdS layer having thickness of 50–100 nm is coated on CZTS layer. However, the CZTS surface is uneven and cannot be completely covered by CdS thin film.
To avoid leakage, 50–90 nm intrinsic ZnO thin film is coated on CdS layer. Then transparent conducting oxide thin film is deposited and acts as the front contact.
Fabrication of CZTS Devices
In vacuuming method all the elements are simultaneously incorporated using effective techniques called as a) Cosputtering and b) Coevaporation followed by c) Annealing. The incorporation of Chalcogens can occur in all steps.
Mostly procedures like electro-deposition, sol-gel methods, and spray coating are used in non-vacuum category. Non-vacuum domain has a big plus that comes with their scalability and being cheap which makes them complaint with even large-scale processes. In this category the highest efficiency of CZTSSe cell (12.6%) has been achieved. Some other promising methods in this category are nanoparticles, electroplating and the Electrochemical deposition.