Effects of various grading types of gallium-ion contents on the properties of Cu(In,Ga)Se 2 films prepared via the spin coating method

Copper indium gallium diselenide (Cu(In,Ga)Se 2 ) films with different gallium-ion contents were prepared via a spin coating method followed by a selenization process. Increasing the gallium-ion contents of the prepared films decreased the lattice constants and increased the band gaps. Secondary ion mass spectroscopy analysis revealed that Cu(In,Ga)Se 2 films with different band gap grading types were successfully fabricated. Increasing the contents of gallium ions near the substrate resulted in an increase in the short-circuit current density. On the other hand, increasing in the contents of gallium ions near the surface of the Cu(In,Ga)Se 2 films significantly increased open-circuit voltage V oc . By combining a normal and a reverse grading of gallium-ion contents, Cu(In,Ga)Se 2 films with double grading types of gallium-ions contents exhibited superior electrical properties. The conversion efficiency and fill factor of solar cells with a double grading type of gallium-ion contents reached 6% and 58.7%, respectively. ? 2018 IOP Publishing Ltd.