Growth model of CuGaSe2 grains in a Cu-rich/Cu-poor bilayer process

Cu-rich Cu–Ga–Se films were prepared on a Mo coated soda-lime glass substrate by thermal codeposition and the microstructures were characterized. An increase in the grain size of CuGaSe2 (CGS) as the Cu-rich composition approached stoichiometry was observed. In the Cu-rich film, precipitated Cu–Se grains were located between the CGS grains and were connected to the film surface, which suppressed the lateral growth of the CGS grains. On the film surface, a quasi-epitaxial growth of Cu2−xSe grains was found. In order to investigate the development of the microstructure together with the phase change from Cu–Se to CGS, a small amount of Ga and Se were deposited onto Cu-rich Cu–Ga–Se films. By transmission electron microscopy, Cu1.8Se and Cu1.5Se grains were identified in the Cu-rich film. A coherent lattice structure from the CGS into the Cu1.5Se grains was found. The microstructural development of the Cu-rich Cu–Ga–Se film is discussed and models of the growth mechanism are proposed.