Growth of Bridgman ingots of CuGa xIn 1− xSe 2 for solar cells

Abstract

Ingots containing single crystals of the quaternary alloys CuGa x In 1− x Se 2 were grown by a vertical Bridgman method for compositions with x=0.2 and x=0.3. With stoichiometric Cu:(Ga+In):Se starting proportions in the ratio 1:1:2, respectively, all the ingots were p-type. X-ray powder diffraction and the observation, in ingot pieces, of cleavage in the unique {10 1} planes confirmed the crystal structure to be single-phase chalcopyrite. The lattice a-spacing decreased with increase of gallium content, as expected. Photovoltaic cells fabricated from wafers cut from the ingots showed the shift in quantum efficiency edge towards shorter wavelengths with an increase of x from 0 to 0.3. A Mott–Schottky plot for a cell made with CuGa 0.2In 0.8Se 2 material yielded an apparent hole concentration of the order of 10 16 cm −3. The quaternary ingots were found, in general, to be more brittle than ternary material grown by the same Bridgman procedures.