Effect of Magnesium Incorporation on Solution-Processed Kesterite Solar Cells

The introduction of the alkaline-earth element Magnesium (Mg) into Cu₂ZnSn(S,Se)₄ (CTZSSe) is explored in view of potential photovoltaic applications. Cu₂Zn_1-xMg_xSn(S,Se)₄ absorber layers with variable Mg content x = 0…1 are deposited using the solution approach with dimethyl sulfoxide solvent followed by annealing in selenium atmosphere. For heavy Mg alloying with x = 0.55…1 the phase separation into Cu₂SnSe₃, MgSe₂, MgSe and SnSe₂ occurs in agreement with literature predictions. A lower Mg content of x = 0.04 results in the kesterite phase as confirmed by XRD and Raman spectroscopy. A photoluminescence maximum is red-shifted by 0.02 eV as compared to the band-gap and a carrier concentration N_CV of 1 × 10¹⁶ cm^-3 is measured for a Mg-containing kesterite solar cell device. Raman spectroscopy indicates that structural defects can be reduced in Mg-containing absorbers as compared to the Mg-free reference samples, however the best device efficiency of 7.2% for a Mg-containing cell measured in this study is lower than those frequently reported for the conventional Na doping.