Effects of various grading types of gallium-ion contents on the properties of Cu(In,Ga)Se2 films prepared via the spin coating method

Copper indium gallium diselenide (Cu(In,Ga)Se2) 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)Se2 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)Se2 films significantly increased open-circuit voltage Voc. By combining a normal and a reverse grading of gallium-ion contents, Cu(In,Ga)Se2 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.