Cu2ZnSnS4: A potential photovoltaic material for low-cost thin film solar cell
Shifting to renewable energy is the solution for meeting the ever-increasing energy demand as well as preventing the deterioration of the environment due to the use of fossil fuel-based energy sources. The Sun from which abundant energy is received on the Earth is probably the most promising renewable energy source. A solar photovoltaic (PV) device converts the solar radiation directly into electricity. Such as device is comprised of a photo-absorber material that absorbs incident photons and uses their energy for raising the chemical potential of electrons within the material. Other materials interfaced with this photo-absorber help collect these excited electrons at one end of the PV device and thereby help generate a potential difference.
Silicon has been the semiconductor of choice for the role of photo-absorber in a PV device, however, due to lower absorption coefficient (α =103 cm−1; λ < 825 nm) and brittleness, several other materials and related PV technologies have been developed. Among these, the technologies based on CdTe and Cu(In,Ga)Se2 (CIGSe) semiconductors have been commercially successful with a photoconversion efficiency of more than 20 %. However, the concerns associated with toxic element Cd and less abundant elements In and Ga have propelled the investigation for alternative semiconductors. A quaternary compound Cu2ZnSnS4 (CZTS) is a promising candidate owing to the facts that all the elements used in it are non-toxic and have relatively abundant availability; moreover, it has a high absorption coefficient (α =104 cm−1; λ < 825 nm) and close to an optimum band gap ( = 1.5 eV).
In the present talk, the challenges associated with the synthesis of CZTS thin films via solution chemistry will be discussed and ways to eliminate/control them have been suggested. First, in order to obtain a film with a desired surface morphology, a combined use of the chloride and acetate salts of copper in the precursor solution has been found to be helpful. Further to this, a capping layer of ZnS was necessary on top of the precursor film in order to obtain a CZTS thin film with controlled surface morphology. The CZTS film prepared by this strategy had a stoichiometric composition, the grain size of the order of ~ 200 nm, = 1.5 eV, and a high hole-carrier density ~ 1019 cm−3.
Further, the tuning of electrical and optical properties of the CZTS via incorporation of silver (Ag) at copper sub-lattice sites in CZTS will be discussed. Thin films of the resulting pentanary alloys (Ag1-xCux)2ZnSnS4 (0 ≤ x ≤ 1) show a remarkable change in their microstructure and electronic properties with increasing Ag content. Going from Cu2ZnSnS4 (x = 0) to Ag2ZnSnS4 (x = 1), the grain size increased from 0.2 to 2 μm which has been correlated to the formation of intermediate phase with relatively lower melting point.
Finally, optimized (Ag1-xCux)2ZnSnS4 thin films were incorporated in a PV device that resulted in a short circuit current density Jsc of 9.47 mA/cm2, open circuit voltage (Voc) of 600 mV, and a fill factor of 34 % leading to ƞ of 1.92 %.