Significantly Different Solubility of Li, Na and K Dopants in Cu(In,Ga)Se₂ and Cu₂ZnSn(S,Se)₄ Solar Cells

The doping of alkaline elements such as Na, K, Rb and Cs in Cu(In,Ga)Se₂ (CIGS) and Cu₂ZnSn(S,Se)₄ (CZTSSe) thin film solar cells had been intensively studied for decades and was shown to be critical for achieving high photovoltaic efficiency. However, the doping of the light alkaline element Li has been much less studied. Using the first-principles calculations, we show here that the formation energy of Li dopants is very low in CIGS and CZTSSe, while the formation energies of Na and K dopants are much higher, so the doping concentration (solubility) of Li can be very high in the CIGS and CZTS lattices at room temperature, much higher than those of Na and other heavier alkaline elements. Na and K may be doped into the CIGS and CZTSSe lattices with a high solubility at high temperature, but the dopants will diffuse out of the grains and prefer staying on the grain boundaries at room temperature because of the low solubility in the lattice. The concentration increase of the hole carriers after the alkaline doping will be discussed based on the calculated solubility.